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CoA Report No
Kluy DELFT
THE COLLEGE OF AERONAUTICS
CRANFIELD
AVIATION FUEL PROBLEMS AT HIGH ALTITUDES
AND HIGH AIRCRAFT SPEEDS
by
T H E C O L L E G E O F A E R O N A U T I C S - C R A N F I E L D
Aviation Fuel Problems
a t High A l t i t u d e s and High A i r c r a f t Speeds
E. M. Goodger, M.Sc. (Eng.), Ph.D., A.M.I.Mech.E,, A.F.R.Ae.S., F . I n s t . P e t .
SUIMART
Much usefvil data has appeared over recent jrears concerning the problems incurred by continued increases i n operational a l t i t u d e s and
a i r c r a f t speeds. This r e p o r t i s an éittempt t o c o r r e l a t e a r e p r e s e n t a t i v e amount of t h e s e data, and t o present them i n a form s u i t a b l e both for
general infonnation and for project design reference. Frequent references are made t o the l i t e r a t u r e as guides t o a d d i t i o n a l information.. Some of the work has formed the b a s i s of r e s e a r c h a c t i v i t i e s a t Cranfield.
Note; Imperial Gallons are used throughout.
1 Imperial Gallon = 1,20094 United S t a t e s Gallons 1 United S t a t e s Gallon = 0,83268 Imperial Gallons.
2
-C0I^!TE^ÏÏ'S
1. Introduction 2. Fuel System Capacity 3 . Lav7 Temperature Problems
3»1. I c e Porraation
3.2. Prevention of F i l t e r I c i n g 3 . 3 . Fuel Freezing
3.4. Prevention of ¥ax Formation 4. High Temperature Problems
4 . 1 . Fuel Quality Control 4. 2. Aircrafi; Design F a c t o r s 5 . LovT Pressure Problems
5 . 1 . Air Release 5.2. Fuel B o i l i n g 6. Inflammability Problems
6 . 1 , Inflammability Beyond Equilibrium 6.2, Preventive Meas\jres 7. Conclusions References Appendix Tables Figures
1. Introduction
A i r c r a f t operation a t high a l t i t u d e s and f orvvard speeds s e t s severe problems regarding fuel q u a l i t y , v/ith r e s u l t i n g conplications i n a i r c r a f t design, and l i m i t a t i o n s i n perfcrraance. The c o r r e l a t i o n of u p - t o - d a t e information on fuel behaviour under these conditions should prove helpful a t t h i s s t a g e , both tayards t h e appreciation of these problems, and as a reference v/ork for project design purposes
The f i n a l s p e c i f i c a t i o n of a v i a t i o n fuel p r o p e r t i e s i s e s s e n t i a l l y a compromise, in view of the many c o n f l i c t i n g requirements. D e t a i l s of current M i n i s t r y of Supply s p e c i f i c a t i o n s , given in t a b l e 1, i n d i c a t e the tvro b a s i c types of a v i a t i o n f u e l , namely gasolines for p i s t o n engines, and kerosines for gas t u r b i n e s a.nd other continuous-flow combustors. The r a p i d grov/th of the gas t u r b i n e engine i n an era geared t o the qiiantity production of gasoline would, i n the event of an emergency, have r e s u l t e d i n an acute shortage of kerosine fuel, This l e d t o the adoption of wide-cut g a s o l i n e , produced from gasoline and kerosine components, which i s used l a r g e l y in s e r v i c e a i r c r a f t . A kerosine of high f l a s h point i s specified for naval a i r c r a f t c a r r i e r s , in order t o meet shipboa.rd safety requirements. An a d d i t i o n a l s p e c i a l i s e d fuel i s t h e low freezing kerosine. American
c i v i l and service s p e c i f i c a t i o n s are designated by t h e symbols ASTM and JP r e s p e c t i v e l y , and t h e i r equivalence (vri.th a few minor differences) t o B r i t i s h fuels i s indicated i n t a b l e 1.
The proportion of each fuel d i r e c t l y obtainable from t h e parent cinde o i l can be gauged roughly from the extent of t h e d i s t i l l a t i o n temperature range. I t v/ill be sho\m t h a t c e r t a i n of the p r o p e r t i e s necessary f o r high-speed higli-altit-ude f l i g h t impose l i m i t s upon the
d i s t i l l a t i o n range of t h e pairticular f u e l , and hence upon i t s a v a i l a b i l i t y . I n order t o compare f u e l p r o p e r t i e s , i t i s convenient t o s e l e c t some
represento-tive property t o form a b a s i s of coniparison. Specific g r a v i t y provid.es such a b a s i s , and the v a r i a t i o n s with s p e c i f i c g r a v i t y i n a number of r e l e v a n t p r o p e r t i e s a r e shovm i n P i g . 1. I t i s i n t e r e s t i n g t o note t h a t , i n modem turbine-povrored a i r c r a f t , fuel qiiality requirements a r e set t o a very large degree by the a i r c r a f t fuel system r a t h e r than the engine.
2, Fuel System Capacity
The fuel load, expressed as a f r a c t i o n of take-off weight, r,anges frcm about 3% in t h e l i g h t piston-engined a i r c r a f t , t o about 45?^ i n t h e long-range j e t t r a n s p o r t . I n a l l airborne v e h i c l e s , mass i s a v i t a l f a c t o r , so tha.t the ma.ximum energy content i s reqi.iired per pound of f u e l caxried. I n the case of piston-enginod a i r c r a f t , the engine exerts an o v e r - r i d i n g requirement upon fvél t y p e , and gasoline fuels of high
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antiknook quality are e s s e n t i a l . However, reference t o F i g , 1 shows t h a t these fuels also e x h i b i t the highest heating v a l u e / l b , f cr a l l the petroleum-based l i q u i d fuels a v a i l a b l e . Hence, gasolines s a t i s f y both engine and a i r c r a f t - l o a d i n g requirements.
With t h e advent of t h e high-speed txirbojet a i r c r a f t , t h e heating v a l u e / g a l l o n has become equally s i g n i f i c a n t , since aerodynamic design s t i p u l a t e s t h i n wing s e c t i o n s , and these r e s t r i c t fuel stov/age space, P i g . 1 shows t h a t heating value/gallon increases with s p e c i f i c g r a v i t y ,
so t h a t t h e volume-limited t u r b o j e t a i r c r a f t i s more siiited t o a heavy fuel i n t h e petroleum range. The use of gas o i l has been examined by Shaxp (Ref. 2) who f i n d s tlTat, -viiien compared with kerosine, t h e capacity pay load range i s reduced by 2 t o 7^ due t o a n t i - f r e e z e p r e c a u t i o n s , but the extreme range ( f u l l tanks and reduced payload) may be increased by about 8^.
Refrigeration p r i o r t o f u e l l i n g has been proposed as one means of increasing t h e fuel load i n a given volumetric capajcity. This system would be expensive and i n f l e x i b l e , but has been used for d i s t a n c e record purposes (Ref. 3 ) , where an a d d i t i o n a l h^o mass v/as supplied t o an a i r c r a f t by p r e - c h i l l i n g from 80 t o 20 F , (2? t o - 7 C ) with s o l i d COp. For gasoline, t l i i s r e p r e s e n t s a r i s e i n s p e c i f i c gravity frcm 0,70 t o 0.73 approximately (Fig. 2 ) ,
Generally, both forms of expressing energy content are important f o r high-speed a i r c r a f t equipped with a i r - b r e a t h i n g propulsive u n i t s , and t h i s has l e d t o the d e r i v a t i o n of a 'porfoimance index' expressed as the r a t i o between the products of these two heating values f o r t h e fuel i n question and f o r a v i a t i o n k e r o s i n e , i . e .
Performance index = CHU/lb. x CHU/^allon ^
g
where K for a v i a t i o n kerosine i s 840 x 1 0 approximately, i n c o n s i s t e n t Tjnits. P i g . 3 shows performance index values of t y p i c a l petrole\:cn-•foased f u e l s , together with some 'chemical' f u e l s of current i n t e r e s t ,
3 . Low Temperature Problems
S t a t i s t i c a l data a r e now a v a i l a b l e regarding the frequency of occurrence of low atmospheric temperatures, and P i g . 4 i n d i c a t e s the minimum ambient temperatures recorded during winter months within the g r e a t e r p a r t of the northern hemisphere. The ICAN temperature curve i s included f o r comparison. Informa.tion has also r e c e n t l y been made a v a i l a b l e on t h e r a t e a t which f u e l cools a f t e r take-off. For a given r a t e of climb, t h i s depends upon t h e thermal insijlation of the fuel t a n k s , i , e, v/hether the tanks are independent u n i t s f i t t e d i n s i d e or outside t h e a i r c r a f t , or formed from compartments i n t h e airframe s t r u c t u r e , with or without f l e x i b l e l i n i n g . The effectiveness of various thermal i n s u l a t i n g m a t e r i a l s i s given by Sharp (Ref, 2 ) .
• • , Fuel temperature curves, based on r e s u l t s obtained during Comet f l i g h t t e s t s , are shovm i n F i g . 5. These i n d i c a t e t h a t the cooling r a t e of fuel i s l e s s than t h e r a t e of reduction in ambient temperature and t h a t the fuel temperature s t a b i l i s e s a t a l e v e l approximately 25 C above ambient v/hen t h e f l i g h t speed i s about 465 m.p.h. This i s s l i g h t l y g r e a t e r than t h a t expected f ran k i n e t i c h e a t i n g , " and i s due t o i n c i d e n t a l effects such as t h e proximity of engines and warm a i r d u c t s .
I n general, the minimum fuel teraperat^ure l i k e l y t o be met in o i v i l a v i a t i o n i s considered to be -55 C, e.g, -80 C ambient plus 25 C of k i n e t i c heating a t 500 m,p.h,, although the worst case i s represented by t h e moderate-speed h i g h - a l t i t u d e a i r c r a f t such as t h e B r i t a n n i a in vAiich fuel temperatures as lov/ as -58 C have been recorded (Ref. 5 ) . Reduction i n fi-iel temperature leads progressively t o the forma,tion of i c e
and wax, and lovror operating a l t i t u d e s must be accepted on the r a r e occasions v/hen t h e minimum temperatures obtain.
3.1 . I c e Formation
Water i s invajr'iably present in f u e l s due t o contact v/ith the atmosphere di;iring storage and during inward venting on descent. Some v/ater dissolves in the f u e l , a.nd Fig. 6 shows the v a r i a t i o n v/ith temperature of water s o l u b i l i t y . Any a d d i t i o n a l free v/ater e x i s t s as a suspension, u l t i m a t e l y s e t t l i n g i n the tank bottom, and promoting corrosion. Cooling causes a p r e c i p i t a t i o n of excess dissolved v/ater; a reduction from 15 t o -10 C, for example, r e l e a s e s about -^ p i n t of water from 1000 gallons of f u e l . Slov/ cooling normally perniits the p r e c i p i t a t e d water d r o p l e t s t o reach the surface and escape t o atmosphere. I n r a p i d c h i l l i n g , on tlie other hand, the p r e c i p i t a t e d water agglomerates and adds t o the free v/ater content.
Cooling below 0 C causes the free water t o freeze into ice c r y s t a l s . Supercooling may occur down t o -60 C, but impact or contamination then causes i n s t a n t freezing of v/ater and some hydrocarbon hydrates. Slow-cooling r e s u l t s i n l a r g e r c r y s t a l s v/ith a g r e a t e r tendency t o settlement. Rapidly cooled v/ater d r o p l e t s and c r y s t a l s of l e s s than 5/^ s i z e , hov/ever, have a very slov/ s e t t l i n g r a t e , and a r e c a r r i e d forward t o t h e flovvmeter and low-pressure f i l t e r s . A loose netv/ork of ine p a r t i c l e s b u i l d s up on the f i l t e r surfaces, r e s u l t i n g i n an increased pressure d i f f e r e n t i a l and, eventually, corriplete blockige. Bypa-ssing the blocked f i l t e r i s not reconmended, since the small guard f i l t e r f i t t e d t o the engine fuel pump can becane ice-blocked d e s p i t e the presence of engine heat.
6
-3,2, Prevention of F i l t e r I c i n ^
The standard t e s t f o r water tolerance (maximum volume of water which can be dissolved by a d r i e d sample of fuel) serves as a measure of the concentration of water-soluble c o n s t i t u e n t s , such as alcohol, and ensures t h e s u i t a b i l i t y of fuel s u p p l i e s . F u e l l i n g techniques are c o n t r o l l e d r i g i d l y as a precaution a g a i n s t the d e l i v e r y of free water. These c o n t r o l s include the use of e f f i c i e n t water separators i n the d i s t r i b u t i o n system, and the p e r i o d i c checking of water settlement i n the ground storage t a n k s , together with draining as necessary a f t e r adequate s e t t l i n g time,
Free v/ater concentrations dovm t o about 0,02^ w, a r e deteotable v i s u a l l y (see Ref. 8 ) , but r e c e n t l y a sinrple water d e t e c t i n g method has been introduced for f i e l d use, capable cf i n d i c a t i n g the presence of free water a t concentrations as low a s 0.003^ w, (Ref, 9 ) . This f i g u r e v/as selected on t h e b a s i s t h a t current airborne f u e l h e a t e r s can accept no more than a t o t a l water concentration of about 0 . 0 ^ w., and t h a t about 0,0l2j^ w, can be expected t o e x i s t i n solution under i n i t i a l f u e l l i n g conditions. This leaves a maxim\jm permissible free-water concentration of 0,006^ w., which i s now d e t e c t a b l e by t h e new Shell method of taking a sanple through chemically t r e a t e d paper f i t t e d on t h e end cf a p l a s t i c syringe,
Since the p o s s i b i l i t y e x i s t s of unsafe water concentrations, p r e v e n t a t i v e measures must be t a k e n t o avoid f i l t e r blockage by i c e . I n a r c t i c
operation, t h e fuel can be allowed t o cool below 0 0 i n t h e storage t a n k s , and the i c e c r y s t a l s removed by means of l a r g e e f f i c i e n t f i l t e r s dtcring a i r c r a f t r e f u e l l i n g . R e f r i g e r a t i o n p r i o r t o f u e l l i n g i s not reoonmended, as o u t l i n e d i n paragraph 2.
3 , 2 . 1 , Anti-Freeze Agents
One method of i c e p r o t e c t i o n i s t h e use of a n t i - f r e e z e m a t e r i a l s t o r e t a i n the l i q u i d phase a t low temperatures. Solid a n t i - f r e e z e agents could be contained i n a replaceable c a n i s t e r w i t h i n t h e f u e l system, whereas l i q u i d agents could be e i t h e r i n j e c t e d i n t o the fuel stream a t a given f i l t e r pressure d i f f e r e n t i a l , or added i n i t i a l l y t o t h e bulk f u e l . Shell (Refs. 10 and 11) found such s o l i d s a s ohromittm t r i o x i d e , zirlc c h l o r i d e , calcitim c h l o r i d e , and calcium n i t r a t e t o have successful a n t i - f r e e z e a c t i o n but t o a t t a c k metals and/or rubbers,
The l i g h t alcohols (methanol, ethanol, and isopropanol) a c t as a n t i - f r e e z e l i q m d s , and f i l t e r s can b e de-iced within a few seconds a f t e r i n j e c t i o n . The bulk a d d i t i o n of 0.4^ v. of methanol t o kerosine with a given f r e e -water concentration has been found t o prevent i c e formation a t temperatures above -50 C. Ethanol and iso-propanol a r e l e s s e f f e c t i v e ,
A methanol concentration of 0 . 1 ^ v, i s siifficient i n t h e complete absence of free vra.ter and, i n any case, a concentration g r e a t e r than about 0.^5% V, woiold reduce the f l a s h point of t y p i c a l kerosine below
i n s o l u b l e i n kerosine, so t h a t e f f i c i e n t mixing i s required i n i t i a l l y , Furthermore, the separation of aqueous metlianol from the bulk fuel leads
t o corrosion and d e t e r i o r a t i o n cf adhesive m a t e r i a l s , and t o s h o r t
-c i r -c u i t i n g of -c e r t a i n tj'pes of e l e -c t r o n i -c fuel -contents gauges. I n f e l t f i l t e r s , aqueous methanol tends t o accumula.te, and any subsequent use of non-additive fuel dissolves av/ay t h e methanol and could lead t o sudden i c e blockage. B r i t i s h Petroleum has now produced an a n t i - f r e e z e f l u i d v/hich avoids the above-mentioned d i f f i c u l t i e s , and has proved s a t i s f a c t o r y
down t o -35 C i n a 0,5% v, a d d i t i o n t o a kerosine containing 0,027% v, t o t a l water (See ref. 8 ) .
3 . 2 . 2 . Fuel Heating
An a l t e r n a t i v e approach i s t o heat t h e fuel immediately upstream of the f i l t e r vi^en the pressure d i f f e r e n t i a l becomes excessive. This method has proved t o be r e l i a b l e , the heat being provided by hot a i r
tapped from the l a s t stage of the compressor of a main gas turbine engine. Automatic heat s e l e c t i o n can be arranged and, i n p r a c t i c e , t h e fuel can be heated through 40 0 i n about 30 seconds. Extraneous heating e f f e c t s due to t h e proximity cf wing de-icing systems have been found h e l p f u l , but such systems are not jret capable of giving complete or continuous p r o t e c t i o n t o the f u e l f i l t e r .
3 . 2 . 3 . F i l t e r Design
An a d d i t i o n a l mea.sure of p r o t e c t i o n l i e s i n the design of the
f i l t e r . Shel r e s u l t s (Ref, 12) shov/ b e n e f i t s from preliminary f i l t r a t i o n with a 100 mesh gavize s i t u a t e d upstream of t h e low-pressure f i l t e r , and t h e suggestion i s made a l s o t h a t the f i l t e r area necessary t o cope with the water passed during the longest anticipa.ted f l i g h t maynot be g r e a t l y i n excess of standard areas c u r r e n t l y used. Research i s a l s o i n progress with s i n t e r e d Bietal f i l t e r s , and v/ith hydrophobic surface treatments. 3 . 2 . 4 . Air Dehydration
Shell research (Ref, 12) suggests that dissolved air collects around water droplets and assists their passage to the surface v/here they vaporise. This effect is noted v/hen the fuel is stirred vigorously, and the introduction of dry air to the agita.ted contents of a fuel tank appears to be a possible solution of filter ice-blookage.
3,3. Fuel Freezing
On cooling, fuels continue to behave as Newtonian fluids (i,e, viscosity independent of shear stress), and their viscosity increases in the normal v/ay until an a.dditional rise occurs due to the precipitation of v/ax crystals (Pig. 8). This condition is represented by the freezing point (the temperature at which crystals diappear on warming, the sample having been chilled v/ith stirring), which is approximately equal to the cloud point (the temperature at which crystals appear, the sample being
8
-c h i l l e d v/ithout s t i r r i n g ) , Sin-ce fuels a r e m?jx±ures of many d i f f e r e n t hydrocarbons, there i s no s i n g l e freezing p o i n t , and t h e quantity of s o l i d m a t e r i a l increases on f u r t h e r cooling. This produces a s l u r r y of f u e l a.nd wspc, v/hioh rema.ins mobile lontil apparent s o l i d i f i c a t i o n eventually s e t s i n a t lov/t5r tempera.tures. This condition i s represented by the pour point (the tonperature 5 F above t h a t a t which no movement of t h e
surface occiors vAien held v e r t i c a l l y , the sample being c h i l l e d v/ithout s t i r r i n g ) , although punping i s s t i l l p o s s i b l e below the pour point i f s u f f i c i e n t force i s applied.
I n the semi-solid condition, f u e l s exhibit t h e property of thixotropy; t h a t i s , mecha.nical a g i t a t i o n causes a reversion t o the normal Nev/tonian f l u i d s t a t e (see photographs i n Ref. 1 3 ) . Continued s t i r r i n g prevents t h e c r y s t a l s from i n t e r l o c k i n g t o form a wax: ma^trix, and the nomval v i s c o s i t y i n c r e a s e only i s found on further cooling.
3 , 4 . Prevention of Wax F c r n n t i a n
With hydrocarbon f u e l s , v/ax formation a t lov/ temperatures i s i n e v i t a b l e . Nevertheless, c e r t a i n steps can be taJcen by fuel s u p p l i e r s and a i r c r a f t
designers t o combat t h i s d i f f i c u l t y . I t has become c l e a r t h a t no c u r r e n t l a b o r a t o r y t e s t can p r e d i c t t h e minim^um temperature of pumpability.
The lowest tenperatiire a t which t h e contents of a fuel tank can be evacuated i s found t o l i e betv/een about 3 and 15 C° belov/ t h e conventional freezing p o i n t , depending upon fuel t y p e . A more representantive t e s t technique i s ,
t h e r e f o r e , necessary, and Strav/son (Ref. 5) gives d e t a i l s of the new Thornton Cold Flow Test i n which t h e quantity of f u e l ranaining in a chamber i s Kieasured a f t e r flov/ h/.is been permitted t o t a k e place i n t o a
lov/er chamber over a c o n t r o l l e d p e r i o d ( e . g , 10 seconds) and at a c o n t r o l l e d lov/ tanpera.t\jre. The interconnecting o r i f i c e i s l a r g e , so t h a t the q u a n t i t y of fuel escaping depends p r i m a r i l y upon t h e y i e l d value of the xvax matrix ra.ther than the app.arent v i s c o s i t y . A hold-up i n excess of, say, 30% Can be taken as a flow f a i l u r e . The d i f f e r e n t low temperature events for a t y p i c a l kerosine a r e represented i n Fig.
9-3 . 4 . 1 . Fuel Quality Control
The minimum operating fuel tttipera^ture expected has been seen to
be -35 C, Ga.solines and -wide-cut g a s o l i n e s meet t h i s condition s a t i s f a c t o r i l y with a s p e c i f i e d freezing point of -60°C, but kerosine (D.Eng.R.D.2482,
Avtur) i s spe-cificd dovm t o -40 C only. This s i t u a t i o n l e d i n 1955 t o t h e sijpply, and in 1957 t o t h e s p e c i f i c a t i o n , of a low freezing kerosine
(D.Eng,R.D,2494, Avtur/50) v/ith a freezing point not above -50 C. (See Table 1 ) , This requirement i s met by l i m i t i n g t h e proportions of hea.vy f r a c t i o n s , and Pig. 10 shows t h e e f f e c t of f i n a l b o i l i n g point on freezing p o i n t . Hence, as i n d i c a t e d i n paragraph 1, ajitifreeze r e q u i r e -ments s e t a l i m i t upon f u e l a v a i l a b i l i t y ,
3 . 4 . 2 . Fuel Additives
The pour-point depressant type of a d d i t i v e can be very e f f e c t i v e i n t h e case of heavy hydrocarbons, such as l u b r i c a t i n g o i l s , but l i t t l e effect has been noticeable v/hen applied t o kerosine. Strav/son (Ref, 5) suggests t h a t t h i s i s due l a r g e l y t o t h e i n s e n s i t i v i t y of t h e pour point t e s t s , and r e p o r t s a reduction of 15 C i n t h e pimpability l i m i t of a kerosine^ as measured by the Thornton Cold Plow method, when 1% of an a d d i t i v e was incorporated. The a c t i o n of promising additives v/as seen t o vary v/idely between fuel types.
3 . 4 . 3 . Fuel Hea.ting
Fuel in the pipelines can be heated to prevent blockage, as discussed in paragraph 3.2,1., and Sharp (Ref. 2) has made an assessment of the effect of fuel tank heating requirements upon aircraft economics.
3 . 4 . 4 . Taiilc InsiiLation
Fuel freezing within t h e tanks can be a l l e v i a t e d by means of thermal i n s u l a t i o n (See Ref. 2 ) . P e n a l t i e s of weight and bulk make t h i s system u n a t t r a c t i v e , but i t i s i n t e r e s t i n g t o note t h a t the thermal conductivity
of frozen kerosine i s s i m i l a r t o t h a t of rubber, and t h a t t h e s o l i d i f i e d Layer growing on the tank v/alls provides a s i g n i f i c a n t i n s u l a t i n g effect (Ref. 1 3 ) . The r a t e of growth of t h e s o l i d i f i e d layer i s shov/n i n F i g . 1 1 , The f u e l i n t h e l a y e r i s recovered e a s i l y when t h e aoribient temperature r i s e s , but the condition i s serious i n the case of vrax b u i l d up in s t a t i c fuel l i n e s .
3 . 4 . 5 . Fuel Agitation
I t may become possible t o exploit the t h i x o t r o p i c nature of hydrocarbon fuels a s a means of depressing the minimum, operating temperature t o well below t h e pour p o i n t . S h e l l (Ref. 13) found t h a t a combina.tion of tank rocking and booster pump r e c y c l i n g lowered the p\jiinpability l i m i t of fuels by 8 t o 11 C , Hov/ever, development work would probably be necessary for
er^ch individual design of fuel tank, 4. High Temperatirre Problems
I n t h e case of s t a t i o n a r y fuel t a n k s subjected t o high ambient teinperatures for prolonged periods, v a p o r i s a t i o n may accoxont for a
s i g n i f i c a n t loss of t h e more v o l a t i l e f r a c t i o n s . Although the q u a n t i t a t i v e l o s s might not be s e r i o u s , fuel q u a l i t y may be affected t o t h e extent of d i f f i c u l t stajrting under subsequent low temperature conditions.
The much higher l e v e l s of temperatijre incurred by k i n e t i c heating at high a i r c r a f t speeds give r i s e t o very severe problems of b o i l i n g
10
-v a r i a t i o n of a i r stagnation tanperatures -v/ith f l i g h t speed. The -vapour pressure curves given i n F i g . 13 show the marked r i s e a.t t h e higher
temperatures. The vapour pressure cf kerosine, for exajnple, which i s almost n e g l i g i b l e fapproxij^iately 0,15 p . s , i . ) a.t t h e st.andard t e s t temperature of 100 F , (37.8 C), r i s e s to no l e s s than 25 p , s , i , a.t a temperature of 200 C, corresponding t o a Mach number of 2,0 a t 10,000 f t a l t i t u d e . The problem i s i n t e n s i f i e d v/hen fuel i s used a.s a convenient heat sink for purposes of cooling engine o i l and a i r c r a f t equipment at supersonic a.ircraft speeds, the surrounding a i r being too hot t o act as a coolant,
I n recent y e a r s , the elevated temperatures a t supersonic speeds have proved t o be s u f f i c i e n t t o cause oxidation and degradation of
the fuel r e s u l t i n g i n t h e formation of an insoluble sediment which tends t o r e s t r i c t i o n and blockage of the fuel flow (see photographs i n Ref, 17).
At higher l e v e l s of temperature ( > 250 c ) , s u f f i c i e n t thermal
energy may be present for the fuel oxidation r e a c t i o n s t o lead t o spontaneous i g n i t i o n . The standard ASTtvT l a b o r a t o r y t e s t c o n s i s t s of assessing the
minimum temperature at which fuel d r o p l e t s v/ill i g n i t e spontaneously when introduced i n t o a heated f l a s k of a i r a t atmospheric p r e s s u r e . The more complex molecules are more ea.silj'- ruptured v/hen exposed t o therm£'l a c t i v i t y , and P i g . 14 shows the general reduction in i g n i t i o n l e v e l v/ith increase i n s p e c i f i c gra.vity< I g n i t i o n temperatures vary i n v e r s e l y v/ith p r e s s u r e . I n the event of spontaneous i g n i t i o n temperature becom.ing a l i m i t i n g
s p e c i f i c a t i o n requirement, fuels of lov/ specific g r a v i t y v/ill be r e q u i r e d , and the use of i n h i b i t i n g a d d i t i v e s may be necessary together v/ith a
l i m i t imposed upon tank p r e s s u r i s a t i o n .
Purging the oxygen from the tank free space v/ould prevent spontaneous i g n i t i o n , but fuel molecules are l i a b l e t o crack i n t o l i g h t molecules and carbon i f the temperature r i s e s miuch above the normal d i s t i l l a t i o n l i m i t of 370 C.
4.1 . Fuel Quality Control
Fuel degradation i s a r e c e n t problem, and no e x i s t i n g t e s t technique has been found s u i t a b l e for the p r e d i c t i o n of thermal s t a b i l i t y in a i r c r a f t fuel systems. In p a r a l l e l v/ith t e s t s conducted on a f u l l - s c a l e mock-up
fuel system, Esse (Ref. 17) a r e developing a flow t e s t apparatus incorporating a fuel heater and a heated f i l t e r . This i s knovm as the ERDCO r i g , and
therma.l s t a b i l i t y i s assessed on the time r e q u i r e d t o reach a c e r t a i n pressu:;'e d i f f e r e n t i a l across t h e f i l t e r . A similar t e s t technique, known as t h e CFR Fuel Coker t e s t , i s quoted i n some American fuel s p e c i f i c a t i o n s .
The development of assessment techniques of t h i s kind, has made p o s s i b l e the determination of t h e most s u i t a b l e types of f u e l , and t h e most sa.tisfactory processes of r e f i n i n g . Segregation of thermally sta.blo fuel stocks thus becomes a p o s s i b i l i t y , although t h i s causes
a d d i t i o n a l complica.tions and expense. American s p e c i f i c a t i o n s now include a t h e r m a l l y - s t a b l e wide-cut f u e l (jP 6) for gas-turbine operation at
Mach 2 , 0 , and a t h e r m a l l y - s t a b l e heavy kerosine of 0,9 s p e c i f i c g r a v i t y (RJ l ) s u i t e d t o a high-speed ramjet a i r c r a f t .
The influence of a d d i t i v e s upon thermal s t a b i l i t y i s being i n v e s t i g a t e d , but .experience w i t h conventional oxidation i n h i b i t o r s has shovm an increased tendency t o deposit formation. I n t h e case of one experimental a.dditive
(Ref. 17), no chemical difference was found betv/een d e p o s i t s , but t h e
p h y s i c a l nature v/a.s changed from small gLmmy p a r t i c l e s t o l a r g e r c r y s t a l l i n e s t r u c t u r e s v/hich had l e s s tendency t o f i l t e r blockage.
4 . 2 . A i r c r a f t Design F a c t o r s
The remarks applied t o the insula.tion of a i r c r a f t fuel tanks for t h e prevention of heat l o s s and f u e l freezing apply equally here, t h e object in t h i s case being the prevention of heat gain (see Ref. 16). Fuel cooling i n f l i g h t i s d i f f i c u l t , i n view of t h e statements made e a r l i e r . The
p r o v i s i o n of refrigera.ting equipment vri.thin the a i r c r a f t i s not impracticable, but the cooling services could not be expected t o extend much beyond t h e needs of t h e crev/ and c e r t a i n items of v i t a l e l e c t r o n i c equipment. Esso r e s u l t s (Ref. 17) suggest tha.t degradation i s i n h i b i t e d by c o n t r o l l i n g the contact between fuel a n i oxygen. This e n t a i l s t h e remova.l of dissolved a i r , and the provision cf i n e r t gas blanketing i n t h e fuel tanlcs.
5. Low Pressure Problems
Ambient pressure f a l l s a t a l t i t u d e , as indicated by t h e ICAN curve in F i g . 15. I n Q- f r e e l y vented tank, these pressures a r e exerted on the surface of the l i q u i d f u e l , and reduction i n pressure leads progressively t o t h e r e l e a s e of dissolved a i r and t o f u e l b o i l i n g . • 5 . 1 . Air Release
Hydrocarbon fuels contain a small q u a n t i t y of dissolved atmospheric gases, v/hich are released s l a v l y on climbing t o lov/>-pres3ure a l t i t u d e s ( e , g , 1 f t , v m i n u t e r e l e a s e d frcm 100 gallons of gasoline a t a climb r a t e of 10,000 f t . / m i n u t e ) . Since the s o l u b i l i t y of oxygen i s g r e a t e r
than tlTat of nitrogen, the r e l e a s e d ' a i r ' i s oxygen-rich (see paragraph 6 , 1 ) , The volume of a i r involved i s not great (Pig, 16), and can normally be
handled v/ithout d i f f i c u l t y . However, supersaturation can occur, v/ith consequent foaming when a g i t a t e d . The r e l e a s e d a i r i s s a t u r a t e d v/ith f u e l vapour, but the loss of vapour caused by a i r r e l e a s e i s not serious.
5.2. Fuel Boiling
Fuel conmences to boil v/hen the vapour pressure exerted by the
fuel reaches the level of the imposed pressure Since fuels are mixtures of many different hydrocarbons, there is no single boiling point, and the vapour pressure is the mean of those exerted by the individ-ual components.
t
12
-As t h e imposed pressure f a l l s , progressive b o i l i n g occurs u n t i l the vapour pressure of t h e lea.st v o l a t i l e component i s reached. Fuel b o i l i n g i s resTJOnsible for serious l o s s of fuel through tanl-c vents, and for vapour lock i n t h e p i p e l i n e s .
I f the imposed pressure i s held constant, b o i l i n g ceases when t h e mean vapour pressure of the remaining components f a l l s below t h i s l e v e l . The f u e l i s then said t o have 'v/eathered•. Since va.porisation involves the absorption of l a t e n t h e a t , t h e b o i l i n g process r e s u l t s in a cooling effect upon the remaining canponents, which reduces t h e i r mean vapour pressure and so eases t h e s i t u a t i o n . Values of l a t e n t heat and thermal capacity do not d i f f e r grea.tly betv/een hydrocarbon f u e l s , and a tcjaperature drop of 1,7 C /% v/eight l o s s i s found t o be rea^sonably common
(Ref. 19).
5 . 2 . 1 , Boiling A l t i t u d e s
The vapour pressures of the components, and hence of the f u e l ,
are a function of temperature, so t h a t the b o i l i n g a l t i t u d e i s determined by t h e temperature of t h e f u e l , a s shov/n i n F i g . 17. The a l t i t u d e v a r i a t i o n
of vapour pressure f o r a v i a t i o n ga.soline i s included i n F i g . 15. The r a p i d climb case i s represented by the constant fuel tempera.ture l i n e of 15 0, and compared v/ith t h e slow climb case where t h e fuel t e n p e r a t u r e follows the ICAM v a l u e s . I n pra.ctice, conditions would probably l i e
somev\rhere betv/een t h e s e two curves, biassed tov.rards the constant temperature l i n e due t o t h e low r a t e of cooling. Boiling commences a.t the i n t e r s e c t i o n p o i n t , and would cease i f the curves crossed again a t a higlier a l t i t u d e . 5 . 2 . 2 . Boiling Losses
Smith (Ref. 20) gives curves for 100/130 Avga.s shov/ing t h e values of fuel b o i l i n g losses obta.ined by calciiLation (Fig. 1 8 ) . The reduction in the extent of l o s s due t o self?-oooling i s c l e a r l y evident. As shown i n t h e f i g u r e , t h e authors compare t h e i r r e s u l t s v/ith those obtained from a f l i g h t t e s t with an a i r c r a f t using s i m i l a r fuel, Losses of 7% '">'•. are seen t o be p o s s i b l e a t an a l t i t u d e of 50,000 f t , v/ith an i n i t i a l
fuel temperature of 15 C, and as great as 2C% w. with an i n i t i a l temperat\ire of 50 C. Derry et a l (Ref, 19) have checked the following expression :
-W = X ( H - H ^ ) ,
where Y/ = % w. fuel l o s s a t a l t i t u d e H,
H, = b o i l i n g a l t i t u d e , in. thousands of f e e t ,
and found t h a t X = 1,970/(S + 1.937), where S i s t h e slope of t h e ASTM d i s t i l l a t i o n curve betv/een t h e 5% and Yil% recovery p o i n t s , i n P//^,
The v; ^ae of X was found t o vary fran 0.53 for IOO/13O Avgas t o 0,2 for Avtag. Curves of b o i l i n g losses for turbine fuels are given by Shollard (Ref,21)
for i n i t i a l fuel temperatures up t o 70"C. In view of t h e elevated temperatures r e s u l t i n g from k i n e t i c h e a t i n g . Shell has r e c e n t l y produced experimental
r e s \ i l t s f o r t h e b o i l i n g l o s s e s of a v i a t i o n turbine fuels subjected t o tetiperature l e v e l s above 200 C. P i g . 19 sliov/s these l o s s e s f o r Avtag, Avtur, and Avcat.
5 , 2 , 3 . Preventive Measures
Fuel vapo\ir pressure i s the fundamental property c o n t r o l l i n g b o i l i n g at a l t i t u d e , so t h a t s e l e c t i o n of a low vapour pressure fuel i s e s s e n t i a l for high a l t i t u d e f l i g h t , Avtur (vapour pressure approximately 0,15 p , s . i . a t 1 0 0 ^ ) i s , t h e r e f o r e , more a t t r a c t i v e than Avtag ( 3 . 0 p . s . i , ) or Avgas
( 7 . 0 p , s . i . ) . Since a low vapour pressure requiranent s e t s a l i m i t on t h e proportion of v o l a t i l e f r a c t i o n s permissible, a l t i t u d e b o i l i n g i s seen t o be another of those problems v/hich r e s t r i c t d i s t i l l a t i o n range, and hence f u e l a v a i l a b i l i t y
The next l o g i c a l approach i s t o increase the tank pressure as high as p r a c t i c a b l e above ambient i n order t o delay t h e onset of fuel b o i l i n g u n t i l g r e a t e r a l t i t u d e s are reached. Strength and weight considerations normally l i m i t t h e degree of p r e s s u r i s a t i o n t o about 3 or 4 p . s . i . The effect of p r e s s u r i s a t i o n on b o i l i n g a l t i t u d e i s included i n P i g . 17. I t i s i n t e r e s t i n g t o note tliat a s i g n i f i c a n t degree of self p r e s s u r i s a t i o n i s incurred i f t h e s i z e of t h e tanlc venting system i s inadequate. Arklay (Ref. 23) found t h a t a 2 i n . diameter vent h o l e , v/ith no e x t e r n a l p i p e , i s s u f f i c i e n t l y small t o c r e a t e a pressure d i f f e r e n t i a l of 1 p . s . i . i n a 300 gallon Avgas tank climbing a t 3,OCX) f t . / m i n u t e . Derry et a l (Ref, 19) found t h a t t h e t h e o r e t i c a l l i n e a r speed of the vapour escaping through a 2 i n , dieimeter vent a t 60,000 f t . a l t i t u d e was no l e s s than 1300 m.p.h, for 100 gallons of 7 p . s , i , vapour pressure fuel at 10,000 f t . / m i n u t e
r a t e of climb. The authors a l s o give d e t a i l s of the extent of r e f r i g e r a t i o n necessary t o prevent b o i l i n g a t 60,000 f t . a l t i t u d e , A 2 p . s . i , vapour pressiAre f u e l , f o r exanple, r e q u i r e s the e x t r a c t i o n of approximately 25,000 C.H.U, per 1000 g a l l o n s .
Vent design i s also important from considerations of foaming and slugging. Sudden r e l e a s e s of dissolved a i r or of vapour can p r o j e c t foam, or even slugs of l i q u i d f u e l , i n t o the vent pipe, considerably increasing t h e o v e r a l l l o s s of fuel. Exploratory t e s t s c a r r i e d out by Derry show t h e s e phenomena t o be more prone v/ith fvll tanlcs, but t o be
reduced by t h e presence of a film of adsorbed gas on the inner surface of t h e tank. D e t a i l s of tank p r e s s u r i s a t i o n equipment are given i n Ref. 24, Research i s also i n progress on the condensation of fuel tanl: vapours.
14
-6. Inf lammabil,.ity Problems
Within c e r t a i n ranges of temperature and p r e s s u r e , the air-vapour mixture produced above the l i q u i d fuel in a i r c r a f t fuel t a n k s w i l l support
combustion, and v/ill b u m explosively on t h e a d d i t i o n of the necessary energy for l o c a l i g n i t i o n . This energy may be provided i f t h e tank i s pierced by incendiajry m i s s i l e s , o r by m e t a l l i c p a r t i c l e s which a r e hot, or which spark v/hen s t r i k i n g i n t e r n a l b a f f l e s . The v a r i a t i o n s i n fuel temperature and p r e s s u r e , discussed i n e a r l i e r paragraphs, give r i s e t o changes i n t h e inflammability range for each a v i a t i o n f u e l ,
The condition leading t o tank explosion a r e s i m i l a r t o those obtaining during a l a b o r a t o r y determination of f l a s h point (the tempercituTe l e v e l t o v/hich the sample of l i q u i d f u e l must be r a i s e d a t atmospheric pressure i n order t o provide s u f f i c i e n t vapour t o f l a s h momentarily whesn exposed t o a naked flame). Since t h e f l a s h point may be taken as t h e weak inflammability temperature l i m i t , i t i s an i n d i c a t i o n of v o l a t i l i t y ,
so t h a t flash p o i n t s a.re expected t o show a close r e l a t i o n s h i p v/ith tonperature l e v e l s of d i s t i l l a t i o n . This i s apparent i n P i g , 1, t h e l i g h t e r f u e l s
having a f l a s h point v/ell below ambient, and hence not measurable under standard conditions of t e s t . The c l o s e r e l a t i o n s h i p betv/een f l a s h p o i n t and the 10% d i s t i l l a t i o n temperature i s shown c l e a r l y i n F i g , 20.
T]ie v/eaJc inflammable mixture obtaining a t t h e f l a s h point i s a f u e l vapour concentration of about 1,3% v . , and. t h i s holds reasonably constant over t h e whole range of hydrocarbon a.viation f u e l s . A oonplementary l i m i t i n g condition i s v i s u a l i s e d v/hen j u s t s u f f i c i e n t oxygeai i s a v a i l a b l e f o r a momentary f l a s h , and t h i s occurs a t a r i c h fuel vapour concentration of about 7,0/o V, for t h e hydrocarbon a v i a t i o n f u e l s . These two l i n i t s , t h e r e f o r e , encompass a mixture range cf inflammability v/hich, on t h e temperature scaJ.e, i s about 30 0 ° , The 'upper' fla.sh p o i n t s , which are not normally measured i n the l a b o r a t o r y t e s t , are included i n Fig, 1, I t i s seen t h a t fuels of about 3.0 p . s , i , vapour pressure are inflammable under ambient c o n d i t i o n s , whereas those of higher vapo\jr pressure are too r i c h t o i g n i t e , and those of lower vapo-ur pressure are too v/eak,
Mixture l i m i t s cf inf Laramability show l i t t l e v a r i a t i o n v/ith reduced pressure, but the temperat-ure liiTdts reduce p r o g r e s s i v e l y since vaporisation takes place more e a s i l y . Eventuallj'- a. press^ure l e v e l i s reached
(approximately 200 mm Hg, =: 32,600 f t . a l t i t u d e ) a.t v/hioh t h e inflammable mixture range begins t o shrink, t h e movement of t h e r i c h l i m i t being
p a r t i c u l a r l y marked. At a p r e s s u r e of about 50 ram Hg, (= 6l ,500 f t . a l t i t u d e ) , depending upon t h e energy of t h e i g n i t i n g agent, t h e mixttire range reduces t o zero. Similar trends are found i n t h e inflanmable temperature l i m i t s for hydrocarbon f u e l s ; those shovm i n P i g , 21 are derived from nimerous f u l l - s c a l e s t a t i c t e s t s i n a i r c r a f t f u e l t a n k s . These curves shov/ t h a t the v u l n e r a b i l i t y of a i r c r a f t f u e l tanks t o explosion depends upon t h e i n i t i a l fuel t o n p e r a t u r e , t o g e t h e r vrith t h e r a t e of climb and subsequent behaviour i n t h e a i r . Kerosine, f o r example, would be the safer f u e l t o \ise when operating with a lew ground temperature and r a t e cf climb, such
as a heavy transpor't a i r c r a f t i n t h e a r c t i c , and gasoline preferable for a high-speed a i r c r t f t i n t h e t r c p i c s . This conclusion i s i n d i r e c t opposition t o tha,t based upon heating values. Operatiana.l fuel temperature curves have been oonpared v/ith inflammable tmpera.ture zones by Ogston (Ref. 27), as i n d i c a t e d i n Fig. 22.
Inspection of the lov/er l i m i t values a t sea lesvel i n F i g . 21 shews them t o be about 'J C below the r e s p e c t i v e f l a s h points determined i n the laboratory. This disagreement i s due t o differences i n geometry and s c a l e , and t o heterogeneity of air-vapour mixtures v/ithin t h e free space
(See F i g . 5 of Ref. 28). The conventional f l a s h point i s , t h e r e f o r e , no p r e c i s e c r i t e r i o n of explosion s a f e t y i n an a i r c r a f t fuel tank.
Lightning wo-uld present a considerable hazard in the event of
inadeqi.Tate bonding, Operationa.l experience shews tha.t li,glitning s t r i k e s occur mainly a t sharply curved si.irfaces, as a.t t h e t i p s of wings and t a i l p l a n e s (See Fig., 7 of Ref. 28), By t e s t i n g model f u e l tanks v/ith a r t i f i c i a l - l i g h t n i n g - g e n e r a t o r f a c i l i t i e s , Robb et a l (Ref. 29) show t h a t i g n i t i o n of fuel i s l i k e l y only v/hen the tank v/all i s punctured, and t h a t aluminium-alloy \7alls in excess of 0,081 in, thickness are not normally punctured when exposed t o r e p r e s e n t a t i v e discharges. Lightning s t r i k e s of g r e a t e r magnitude, or dama.ge of g r e a t e r confinement, would s t i l l be dangerous, A major proportion of s t r i k e s i n s e r v i c e i s shovm t o have occurred v/ithin the tempera.ture range -10 t o + 10 0, and t h e lev/ a l t i t u d e range of 6,000 t o 14,000 f t . P i g . 21 shov/s Avtag t o be t h e most hazardous fuel under these conditions,
S t a t i c e l e c t r i c i t y p r e s e n t s another p o s s i b l e source of i g n i t i o n . The charges b u i l d up e i t h e r by t h e jpassage of v/ater or dust p a r t i c l e s over t h e surface of the a i r c r a f t , or by t h e proximity of e l e c t r i c a l l y charged clouds. Again, bonding provid,es an adequa.te safeguard, except i n t h e remote p o s s i b i l i t y of s t a t i c charges being generated due t o motion of the f u e l v/ithin t h e tanlc.
6 , 1 , Inflamnability Beyond Equilibrium Limits
The tQ-perature l i m i t boundaries of Pig. 21 enclose the equilibrium danger a r e a s , but explosion i s a i s c possible beyond both l i n i t s under the following circumstances, T/hen l i q u i d fuels are sprayed i n t o t h e a.ir, the g r e a t l y increased surfa-ce area r e s u l t s i n a correspondingly increased r a t e of vaporisai.tion, and a greater effective v o l a t i l i t y . Explosion thus also becomes p o s s i b l e beyond the low temperature side of the normal boundary curve, and may occtir v/hen a m i s s i l e e n t e r s a tank and. causes t h e l i q u i d fuel t o splash. Explosion on t h e high tem]?eratuire side of t h e s e boundaries i s possible wiien t h e oxygen content of t h e a i r i n t h e mixture i s increased. On climbing, oxygen-rich 'a.ir' i s r e l e a s e d (see paragra.ph 5 . 1 ) , t h e oxygen content being about 33% v, instea.d of the normal atmospheric value of 21% V. This displaces t h e boundary curve b o d i l y through about 10 0 i n the high tenperature d i r e c t i o n (See Ref. 30). On diving, the i n g r e s s of atmoshperic a i r through the tank vents provides explosive mixt^ures a t
16
-temperatures higlicr than t h e noimi^l bound'iry v a l u e s . This follows a s the fuel vapour, normally too r i c h t o burn, diff\ises r e l a t i v e l y slov/ly i n t o t h e incoming a i r , and c r e a t e s near t h e vent a i-x3gian of mixture whose s t r e n g t h gradates i n t o the v/eaj,c exr[)losive r;\ngo. Mien atmospheric a i r i s used a s t h e prossijrising f l u i d t o prevent fuel b o i l i n g aLt a l t i t u d e , the lower e f f e c t i v e a l t i t u d e v/ithin t h e tank brings t h e operating condition down tOT/ards t h e exi^losive range during operation on the high t e n p e r a t u r e s i d e of the boundajry. These four effect are i l l u s t r a t e d for a v i a t i o n kerosine in P i g . 23. I t i s evident, therefore, t h a t the many f a c t o r s involved make i t d i f f i c u l t t o find completely safe operating conditions f o r any a v i a t i o n f u e l ,
6, 2, P.r e vent ive Mea sure s
I t i s appa.rent t h a t seme kind of e f f e c t i v e and continuous p r o t e c t i o n a.gainst tank explosion i s very d e s i r a b l e , p a r t i c u l a r l y v/hen f l y i n g within the explosion bounclaries. The fuel q u a l i t y required depends upon f l i g h t speed; a high f l a s h point i s needed for low-speed h i g h - a l t i t u d e f l i g l i t , and a low f l a s h point v/hen f l i g h t speed i s s u f f i c i e n t t o incur appreciable
k i n e t i c hearting. Armour-plated fuel tanlcs a r e impracticable, but two main approaches e x i s t , i . e . suppressing i n c i p i e n t explosions, and ptirging
oxygen frcm the f r e e spa.ce. 6 , 2 . 1 . Ex|.olosion Suppression
I n a l l i g n i t i o n prcxjcsses, a f i n i t e p>eriod of time ela.pses betv/een t h e a p p l i c a t i o n of energy and t h e i n i t i a t i o n of flame. I f t h e delay period i s adequate, the i n c i p i e n t explosion can be sensed and then suppressed, The upper curve i n P i g , 24 shows the normal growth i n pressure inmediately p r i o r t o an explosion of a f u e l - a i r mixture i n a confined space. The lower curve i n d i c a t e s t h e pressure l e v e l s rea.ched a t t h e i n s t a n t s vdien t h e d e t e c t o r operates and vdien the suppressant f l u i d i s discharged i n t o t h e tanlc c o n t e n t s . In e x i s t i n g systems, t h e t o t a l p r e s s u r e r i s e i s l i m i t e d t o about 3 p . s . i . only (Ref. 31). The incipiejnt explosion may be detected by means of e i t h e r a pressiJire-sensitive capsule, or a p h o t o c e l l . Fuel
i t s e l f nv?.y be used as t h e suppressant f l u i d since the r e s u l t i n g enrichment precludes combustion. I n t h i s ca.se, t h e tank contents arc not contaminated.
6,2, 2, I n e r t Gas Pu-rgin|3;
A pirerequisite t o i g n i t i o n i s an adequate concentration of oxygen, rnd t e s t s show th-^.t i g n i t i o n i s not p o s s i b l e v/hen the oxygen concentration of the ' a i r ' i n an a i r - f u e l mixture f a l l s frcm the normal atmospheric value of 21% v. to about 1 ^0 v . , i . e . s t i l l considerably above zero
(see Ref. 30). I t i s p r a c t i c a b l e , t h e r e f o r e , t o carry gaseous nitrogen in crder t o purge the contents of t h e tank vapour space, and to red.uce t h e i r ojqygen concentration below t h e danger l i m i t . Additional nitrogen
i s needed on the r e l e a s e of oxygen-rich a i r on climbing, and t o deal with t h e inccming a i r on descent. Hence, n i t r o g e n purging o f f e r s a means of continuous explosion safety a t any a l t i t u d e or condition of f l i g h t , but
i t e n t a i l s considerable weight p e n a l t i e s t o cover a f l i g h t of any appreciable d u r a t i o n . A recent development of B r i t i s h Oxygeai Aro Equipment L t d . , i s a purging systan comprising a hif^i-va.cuurii insiiLatecl container s t o r i n g l i q u i d nitrogen, v/hich i s injected i n t o the conpressor bleed a i r used t o p r e s s u r i s e the fuel tanks (Ref. 32).
An a l t e r n a t i v e source of purging f l u i d , which offers great a t t r a c t i o n s of low voight and a v a i l a b i l i t y , i s t h e combustion efflux from the main propulsive engines. I n t h e gas t u r b i n e engine, the mixture strength i n the r)rimary zone of t h e combustion clambers should be very neajrly s t o i c h i o -m e t r i c , and t h e oxygen concentration n e g l i g i b l e . Tests have shown t h a t fuel-cooled probes located in fixed p o s i t i o n s i n the i r i m a r y zone w i l l sarrple gases v/ith an oxygen c o n c e n t r a t i i n not exceeding 6% v. over a wide range of engine opertiting conditions (Ref. 33), The sanpled gases, cooled and d r i e d , are found t o be suita.ble a s a purging f l u i d over a l l conditions of f l i g h t , v i t h t h e p o s s i b l e exception of diving v/ith t h e nnin engine i d l i n g . I n vie?w of t l i i s l i m i t a t i o n , i t may be more s a t i s f a c t o r y t o
provide a separate ccmbustor v/ithin t h e a i r c r a f t for pxarposes of producing i n e r t pijrging gases. A ver^/ close c o n t r o l must be maintained over the oxygen concentration of t h e output gases, but t h e d u t i e s of such a system could, be combined v/ith those of D.n a i r c r a f t heater.
7. Conclusions
I t i s evident from the foregoing discussions t h a t fuel requirements c o n f l i c t , and t h a t a compromise must be dravm v/hen s e t t i n g l i m i t s for fuel s p e c i f i c a t i o n s . The fuel requirements, together v/ith a d d i t i o n a l measures of p r o t e c t i o n , can be c o r r e l a t e d by considering s e p a r a t e l y the conditions obtaining a t high a l t i t u d e , and a t high a i r c r a f t speed, as i n Tables 2A and 2B, aixl by means of a composite dia^grajn as i n F i g . 25. The e f f e c t s of a i r c r a f t operation under these two conditions may be suinraarised as follov/s :
-( i ) High A l t i t u d e F l i g l i t . This inci.irs problems of f i l t e r bloclcage by i c e and v/ax, fuel fcajning and b o i l i n g l o s s e s , and p o s s i b i l i t i e s of tank explosion v/ith high f l a s h point (kerosine) fuels a t lov/ a i r c r a f t speeds. The lov/ ambient tanperatures a.sist i n preventing thermal degrad.atian and b o i l i n g l o s s i n high-speed a i r c r a f t ,
( i i ) High Speed g l i g h t . This incurs problems of fuel stovrige spiace, fuel b o i l i n g l o s s , f i l t e r blockage by thermal degrada-tion products, and p o s s i b i l i t i e s of tank explosion with low f l a s h point (gasoline) f u e l s , and at higher tcmporatvires, v/ith high specific g r a v i t y f u e l s . The k i n e t i c heating a s s i s t s i n preventing f i l t e r blockage by ice and v/ax.
I n the hydrocarbon range, an a p p l i c a t i o n i s seen t o be appropriate t o each s i g n i f i c a n t fuel proi:)erty. Using s p e c i f i c g r a v i t y again as the main d i s t i n g u i s h i n g property, t h e p i c t u r e appears as follows :
18
-( i ) Lov/ Specific Gravity Hysrocarbon Fuels
High hea.ting values per pound A l l a i r c r a f t a p p l i c a t i o n s Low freezing points Lov/ speed, high a l t i t u d e High vr.pour pressures Low speed, lov/ aititxjde Low f l a s h p o i n t s High speed, high a l t i t u d e ( i i ) Higji Specific Gravity Hylrocarbon Fuels
High heating values per gallon High speed
High freezing p o i n t s High speed, low a l t i t i j d e Lov/ vapour pressures High speed, high a l t i t u d e High f l a s h p o i n t s Lew speed, high a l t i t u d e
With regard to t h e additional mea.sures for combating these problems, t h e follovring suiimarising notes show c e r t a i n techniques t o be e f f e c t i v e i n dealing v/ith more than one problem :
-(a) Fuel Additives. These offer p o s s i b i l i t i e s of suppressing f i l t e r blockage due to i c e , v/ax and thermal degradation products, and a l s o t h e p o s s i b i l i t i e s of tank freezing, and ta.nk explosion due t o
spontaneous i g n i t i o n at very high f l i g h t speeds v/ith p r e s s u r i s e d tanks, (b) Fuel Hea.ting. Although inposing p e n a l t i e s of v/eight and ccmplicati on,
t h i s i s a p r a c t i c a b l e system of preventing f i l t e r blockage by i c e and v/ax. I t i s achieved by means of v. heat exchanger fed with hot a i r from the oonpressor of a. main ga.s turbine engine.
(c) Fuel Cooling. This i s a diffici.ilt and expensive p r o c e s s , but i t would permit t h e c a r r i a g e of a g r e a t e r mass of f u e l i n a given volumetric capacity, and the f i l t r a t i o n of i c e c r y s t a i s during f u e l l i n g . I t v/ould a l s o a.ssist i n the prevention cf b o i l i n g l o s s e s , and of f i l t e r blocka.ge by thermal degr-adation products.
(d) Fuel A g i t a t i o n . This a s s i s t s i n preventing f i l t e r bloclcage by ice and v/ax, a.nd l o s s of fuel by foaming and slugging i n i t i a t e d by sudden a i r relea.se. I t a l s o a s s i s t s thermal s t a . b i l i t y a t high a i r c r a f t speeds. I t could be a,.chieved by continued r e c y c l i n g
v/ith t h e booster ptmps, i n conjunction v/ith s u i t a b l e b a f f l e s i n the tanks,
(e) Fuel Pressurisa.tion. This prevents b o i l i n g l o s s e s a.t high a l t i t u d e and a t high a.ircraf t speeds, and can be achieved by tapping the oonpressor of a main gas-tvu-bine engine. I f an i n e r t gas i s used as the press\jrising f l u i d , t h i s renders the tanlcs continuo\;isly safe from explosion under a l l conditions, and a s s i s t s therma.l s t a b i l i t y . The standard p r a c t i c e of f i t t i n g booster pumps at t h e tank o u t l e t s minimises problems of vapour lock i n t h e p i p e l i n e s .
(f) Tank Insula.tion, This prevents f i l t e r blockage by ice and v/ax, and t h e excessive b u i l d up of wax on the tank v/alls, i n low-speed a i r c r a f t a t Mgh a l t i t u d e . I t also improves thermal s t a b i l i t y in high-speed a i r c r a f t .
(g) Explosion Suppression, This reduces t h e p o s s i b i l i t y of tank explosion under a l l conditions, but i s l i m i t e d to a s i n g l e opera.tion during a given f l i g h t .
The demands for higher and f a s t e r f l i g h t can be expected t o become more p r e s s i n g , and the magnitude of t h e problems outlined above t o be correspondingly increased. One exception may be the problem of f i l t e r blockage by ice and v/ax, since ambient tem.peratures are not l i k e l y to f a l l f u r t h e r , whereas a i r c r a f t speeds w i l l continue t o r i s e , and t h e effect of k i n e t i c heating t o increase. An overall r e s u l t v/ill be an increased t r e n d from hydrocarbon t o s p e c i a l l y t a i l o r e d 'chemical' f u e l s .
References 1. 2.
3.
4.
5.
6,
Goodger, Sharp, J Anon, Goodger, Strawson E.M, .G, E.M, , H, Walker, J.E. 7. Anon,Petroleum and Performance i n Intomal-Ccmbustian Engineering. Buttervvorths S c i e n t i f i c P u b l i c a t i o n s , L t d . , London, 1953.
Heavy Fuels for Turbines ? Shell Avia.tion News. March, 1953.
S h e l l Avia.tion News. May, 1949.
Designing for Boron Fuels. Engineering M a t e r i a l s and Design, Early 1959.
The Pumpability of Aviation Turbine Fuels a t Lov/ Temperatures. I n s t i t u t e of Petroleum Meeting. 3/l 2/58.
Fuel vSystems for Turbine-Engined A i r c r a f t . Journal Royal Aeronautical Society, Vol, 3^, 1952, p . 657.
Data Sheets - Fuels and Lubricants.
I n s t i t u t e of Petroleimi - Royal Aeronautical Society. Mc/aian, D.T., and Fuel System I c i n g , Air B,P,4.
20 -References (Continued) 9. Anon. 10, Sharp, J,G, 11, Davies, C,B, 12. Davies, P.L, 13. Strawson, H. 1 4 . B a s s , E , L . , Lubbock, I . , and Y/'illiams, C, G. 1 5 . Makowski, J , and Y/hitney, V.L. 1 6 . R e n d e l , D. 1 7 . Crampton, A , B , , Gleason, W,W,, and Y/ieland, E,R, 1 8 , Goodger, E.M. 1 9 . D e r r y , L . D . , Evans, E . B . , F a u l k n e r , B . A , , and J e l f s , E.C.G, 20. Smith, M., and O ' P n r r e l l , M, 2 1 . She H a r d , A D , 22, D a l e , H,L.
The S h e l l D e t e c t o r . S h e l l Avia.tion News, F e b r u a r y , 1957. F u e l s f o r Gas-Turbine A e r o - E n g i n e s . A i r c r a f t E n g i n e e r i n g . V o l . 2 3 , 1 9 5 1 , P . 2 . D e v e l o p i n g A v i a t i o n F u e l s and L u b r i c a n t s , J o u r n a l Royal A e r o n a u t i c a l S o c i e t y , V o l . 5 7 , 1953, p . 7 0 0 . F i l t e r I c i n g . S h e l l A v i a t i o n News, August, 1 9 5 3 . Using T u r b i n e F u e l s a t Low T e m p e r a t u r e s . S h e l l A v i a t i o n Nev/s. December, 1955.
The Gas T u r b i n e and i t s F u e l s . S h e l l A v i a t i o n
NGV/S. J u n e , 1951 .
P e r s o n n e l and Equipment Cooling i n S u p e r s o n i c A i r p l a n e s , S h e l l A v i a t i o n Neivs. F e b r u a r y , 1955.
Thermal Problems of High Performance P l i g h t . A i r c r a f t E n g i n e e r i n g , Vol, 26, 1954, p . 2 2 0 , Thermal S t a b i l i t y - A New F r o n t i e r f o r J e t F u e l s , Esso A i r Yforld. S e p t a m b e r / O c t o b e r , 1 9 5 5 . S p o n t a n e o u s - I g n i t i o n D a t a of Hydrocarbons and A v i a t i o n F l u i d s , C o l l e g e of A e r o n a u t i c s Nr.te No. 68, September, 1 9 5 7 .
Vapour and A i r R e l e a s e from A v i a t i o n F u e l s , J o u r n a l of t h e I n s t i t u t e of P e t r o l e u m , V o l , 3 8 ,
1952.
A v i a t i o n F u e l s . Modern Petrole\.Tm Technology. 2nd E d i t i o n , I n s t i t u t e of P e t r o l e u m , 1954. Vapovxr E v o l u t i o n C h a r a c t e r i s t i c s of A v i a t i o n
T u r b i n e F u e l s . S h e l l A v i a t i o n Nev/s, F e b r u a r y , 1952. Vapour P r e s s u r e s of A v i a t i o n T u r b i n e F u e l s .
R e f e r e n c e s ( C o n t i n u e d ) 2 3 . 24. 2 5 . 26, 27. 28, 29. 3 0 . 3 1 . 3 2 . 33. A r k l e y , W.H,, end B i g g , F , J , /inon. Williajns, C,G, Anon. Ogston, A.R. Yfeise, C A. Robb, J . D . , H i l l , E . L . , Nevmon, M.M,, and Stahmann, J . R . Goodger, E.M, Anon. Anon. Goodger, E,M,, Cadman, P , , and M u r c h i e , I , T , A .
V e n t i n g of F u e l Tanks of High Performance A i r c r a f t . R. A. E. T e c h . Not e No. M, E, 2 5 ,
Novanber, 1948.
Tank Vent L i t e r a t u r e , Teddington C o n t r o l s L t d , , Cafn Coed, Near M e r t h y r T y d f i l , Glam.
F u e l s and L u b r i c a n t s f o r Aero Ga.s T i ï r b i n e s , J o i n t Meeting I n s t i t u t e of Petrole\jm - Royal A e r o n a u t i c a l S o c i e t y , 1 2 / 2 / 4 7 , C.R.C. Handbook. 1946 E d i t i o n . C o o r d i n a t i n g R e s e a r c h C o u n c i l I n c . Nev/ York. O p e m t i o n n l S a f e t y Asjjects of J e t F u e l s . E s s o A i r World, September/October, 1956, An Airframe M a n u f a c t t i r e r ' s V i e v p o i n t on C i v i l A i r c r a f t Tirrbine F u e l , Esso A i r World, Marcly' A p r i l , 1955.
L i g h t n i n g Hazards t o A i r c r a f t F u e l Tanlcs, N . A . C A . Tech. Note 4326. Scpternber, 1958.
E x p l o s i v e L i m i t s i n A i r c r a f t F u e l Tanks, P e t r o l e u m . March, 1955.
E x p l o s i o n S u p p r e s s i o n L i t e r a t u r e , G r a v i n e r Maniofacturing Co, L t d . , 29, S t . J.ames's S t r e e t , London, S, W, 1,
L i q u i d N i t r o g e n L i t e r a t u r e . B r i t i s h Oxygen /a-'o Equixoment L t d . , B r i d g e v / a t e r Ho^ase,
C l e v e l a n d Row, S t . J a m e s ' s , London, S, W, 1 . P r o t e c t i o n of A i r c r a f t F u e l Tanks a.gainst E x p l o s i o n Hazards u s i n g I n e r t Combustion P r o d u c t s . C o l l e g e of A e r o n a u t i c s R e p o r t No. 8 5 . O c t o b e r , 1 9 5 5 .
22
-/JTENDIX
ST-'J^mTOD L.''J30I^-TaRY ITilSTS QUOTED
P.ara, 1. 1. 2. 3 . 2 , 3.3. 3.3. 3.3. 3.3. 4. 4. 6. Property D i s t i l l a t i o n Specifio Gravity C a l o r i f i c Value Water Tolerance Kinematic V i s c o s i t y Freezing Point Cloud Point Pour Point Vapour Pressure Autogenous I g n i t i o n Temg-iorature Flash Point I . P . " No, 123/55 59/55 12/53 98/44 71/55
^Wkh
15/55 15/55 69/55 -33/55 A,S.T,M.* No, D86-53 D1298-54 D240-50 D1094-53 D445-53 D91 0-53 D97-47 D97-47 D323-52 D286-30 D56-523B These t e s t procedures arc nominally equivalent, but certa.in minor differences e x i s t .
B r i t i s h t e s t techniques are d e t a i l e d in t h e crurrent e d i t i o n of "Standard Methods for Testing Petroleum and i t s Products". The I n s t i t u t e of Petroleum,
26, Portland Pla.ce, London, Y/.1,, and/jnerican teclmiques in t h e current e d i t i o n of "ASTM Standards on Petroleum Products and. Lubricants". American Society for Testing M a t e r i a l s , 191 6, Race S t r e e t , Philadelphia. 3 , P a . ,
United S t a t e s of /juerioa..
D.Eng.R.D. No. & d a t e U . S . E q u i v a l e n t S p e o i f i c G r a v i t y 1 D i s t i l l a t i o n f . t . p . R e s i d u e , % L o s s , % Lower H e a t i n g Value B.Th.U. p e r l b . Water T o l e r a n c e ml. F r e e z i n g P o i n t °C Vapour P r e s 3 \ i r e a t 1 0 0 ^ , i n p . s . i . P l a s h P o i n t °F Kinematic V i s c o s i t y Aromatic C o n t e n t 1 B r a n i n e Number T o t a l S u l p h u r , wt.jS T o t a l A c i d i t y mgm. KOH/gn. f u e l E x i s t e n t Gun/l 00 ml. A c c e l . G u n / l 0 0 ml. 2i,B5 1 / 1 2 / 5 4 -Not L i m i t e d < lOf. &;:^40g a t 75°C -f. 50fo a t 105°C < 9 C ^ ^ a t 1 3 5 C ( 1 0 ^ 5 0 ) ^ < 135 0 ° > 170°C > 1 . 5 > 1 . 5 < 1 8 , 7 0 0 ( ^ 1 8 , 9 0 0 f o r 115/145) 2 Not a b o v e -éO < 5 . 5 a n d > 7 . 0 -> 0 . 0 5 ^ ; ; ^ 3 . 0 mgn. :^-6. 0 ngm. 2486 l / l ? / 5 4 A.S.T.M. Type B (JP4) 0.751 t o . 0,802 '^20fc a t 143°C < 5 0 ^ a t 188°C < 90^c a t 243°C > 1 . 5 > 1 . 5 < 1 8 , 4 0 0 1 Not above - 6 0 < 2 , 0 a n d > 3 . 0 -> 255.'. V o l . — H > 5 > 0 , 4 0 > 0 . 1 0 : h 7 . 0 mgn. : ^ 1 4 . 0 mgn. 2482 1 / 3 / 5 7 A.S.T.M. Type A 0 . 7 7 5 t o 0.825 < 20^0 a t 200°C > 3 0 0 . 0 ° C > 2 . 0 >^.5 -<: 1 8 , 3 0 0 1 Not above - 4 0 -< 1 0 0 :|» 6 c s . a t 0°P : h 2Qf^ V o l . ^ 5 : h o . 2 0 ^ : ^ 3 . 0 mgm. ;f- 6. 0 mgm. 2488 l / l ? / 5 5
JP 5 1
0 . 7 8 8 t o 0 . 8 4 5 1 1 0 ^ a t + ' 204°C > 288°C : ^ 1 . 5 < 1 8 , 3 0 01 1
Not above -hO
-
1
< 1 4 0 :t>'l6.5 OS. a t - 3 0 ° ? 1 7^ 25% V o l . 1 ^ '^ 0 . 2 ^
1
^ • o . i o :::K7.0mgpn, > 1 4 . 0 mgn J Notes f l ) I n c l u d e s 5 g r a d e s ; 73/80 ( b o t h c o l o u r l e s s ) , 91/96 ( b l u e ) , 100/130 ( g r e e n ) , & 115/145 ( p i J r p l e ) .( 2 ) A v t u r / 5 0 , D.Eng.R D.2494 i s i d e n t i c a l t o 2482 b\it f r e e z i n g p o i n t not above -50°C. J . P , 1 f r e e z i n g p o i n t n o t above - 6 0 * ^ . J . P . 6 i s t h e r m a l l y s t a b l e .
TABEE 2è FDEL COIiSIDERATION IN HEGH-AIITITUDE F U G I g • - ' — ' - — - " l
oorciTiafTs
Low ambient ternperature Low ambient p r e s s u r e Lo57 ambient t e n p e r a t u r e end p r e s s u r e EBFECT D i s s o l v e d w a t e r p r e c i p i t a t e s . F r e e w a t e r f r e e z e s . F i l t e r blcxskage. F u e l f r e e z e s p r ogre s s i v e l y . Tank c o n t e n t s f a i l t o flow. F i l t e r b l o j k a g e . D i s s o l v e d a i r r e l e a s e d . Foaming. F u e l b o i l s . Vapour l o s s . Vapour l o c k . L e v e l of e x p l o s i o n tcanperature r a n g e r e d u c e s . FUEL EEQUIREf,IEI^ Water c o n t e n t minimised, ( N e g l i g i b l e flifference between f u e l s ) . Low f r e e z i j i g p o i n t (Low S . G , ) .Low pour p o i n t (Low S.G.)
( N e g l i g i b l e d i f f e r e n c e betv/een f u e l s ) . Lov7 va-poiur p r e s s u r e (High S . G . ) . High f l a s h p o i n t (Higli S . G . ) . AüDl'i'iÜR4L l^AEASURES P r e - r e f r i g e r a t i o n and f i l t r a t ion. A n t i - f r e e z e a g e a t s . Fi.iel h e a t i n g . High-speed f l i g h t , Upstream mesh f i l t e r . I n c r e a s e d f i l t e r a r e a . F u e l a g i t a t i o n w i t h d r y a i r . F u e l t a n k h e a t i n g . High speed f l i g h t . Tank i n s u l a t i o n . F u e l a g i t a t i o n ( t h i x o t r o p y ) . F u e l a g i t a t i o n . Tanlc p r e s s u r i s a t i o n . B o o s t e r pimp. E x p l o s i o n s i j p p r c s s i o n . I n e r t gas p u r g i n g .
; CONDITIONS T h i n w i n g s e c t i o n s Kinetic h j e a t i n g EB'i^'Eai" F u e l s t o w a g e c a p a c i t y r e d u c e d . V a p o u r p r e s s u r e s r i s e s a p p r e c i a b l y . V a p o u r l o s s . S e d i m e n t f o r m s . F i l t e r b l o c k a g e . F u e l t e n p e r a t u r e r e a c h e s e x p l o s i o n r a n g e . F u e l t e n p e r a t u r e a p p r o a c h e s s p o n t a n e o u s -i g n -i t -i o n l e v e l . FUEL SSQUllffiMSNT H i g h h e a t i n g v a l u e p e r g a l l e n . ( H i g h S . G . , a n d c h e m i c a l f u e l s ) . Low v a p o u r p r e s s i j r e ( H i ^ h S . G . ) , T h e r m a l s t a b i l i t y . ( H i g h p a j r a f f i n i c , a n d l a v a r o m a t i c ^ c o n t e n t ) . Low f l a s h p o i n t (LovT S . G . ) . H i g h s p o n t a n e o u s -i g n -i t -i c s n t e n p e r a t u r e ( L a y S . C r . ) . iffiDmONAL lffi./iJ3UEES E x t e r n a l t a n k s . P r e — c h i l l i p g . Tank p r e s s u r i s a t i o n . S e g r e g a t i o n o f s a t i s f a c t o r y f u e l s t o c k s . S e l e c t i o n of s u i t a b l e r e f i n n . n g p r o c e s s e s . A d d i t i v e s . Tainlc i n s u l a t i o n . F u e l c o o l i n g . Removal of d i s s o l v e d a i r ( a g i t a t i o n ) . I n e r t g a s b l a n k e t i n g . E x p l o s i o n s u p p r e s s i o n . I n e r t g a s p u r g i n g . I g n i t i o n s u p p r e s s i o n a d d i t i v e s .
5 Ü I O J S --MASS SASIS Vl o o VAPOUR PRESSURE DISTILLATION UMIT DISTILLATION RANGES u l O O . •85 ö 3 u 6 - 8 0 a U P P E R ( R I C H ) _ _ _ " * IBFLAMMABLE RANGE I AVTI I AVCAT 0 7 AVTAg 3 O'9 O » SPECIFIC S R A V m
FIG I. TYPICAL VALUES SHOWING TRENDS IN FUEL PROPERTIES (REF I.)
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FIG. 7 EFFECT OF MENTHANOL ON FLASH POINT OF KEROSINE.
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