MANAGEMENT OF TECHNICAL DOCUMENTS
FOR PRESSURE EQUIPMENTS ALONG THEIR
LIFETIME IN MAJOR ACCIDENT HAZARD
ESTABLISHMENTS
Bragatto
P. A., Pittiglio
P., Ansaldi
S.
ISPESL Dipartimento Insediamenti Produttivi ed Interazione con l’ Ambiente Monte Porzio, Roma, Italy
CAD PDM consultant Monte Compatri, Roma, Italy
Abstract: The understanding of actual deterioration mechanisms and the rate at which
deterioration actually occur is essential to reconsider, in plant lifetime, equipment inspection planning. A lot of information along pressure equipments lifetime should be collected and used to improve the initial safety assessment. In major accident hazard establishments, such as hydrocarbon and chemical process plant, an integrated system aimed to follow all equipments from design to testing, from inspections to maintenance and repair.
1. Introduction
The class of pressurized equipment cover pressurized vessels, pressurized storage tanks, and heat exchangers. In a typical major accident hazard establishment, such as a hydrocarbon or chemical process plant, there are hundreds of these equipments. Many of them are critical for overall plant safety. A leakage in pressure equipment can cause health and safety hazards, including poisonings, suffocations, fires, and explosion hazards. Rupture failures can be much more catastrophic and can cause considerable damage to life and property. Furthermore other failures and their consequences have to be considered in pressure equipment such as heat exchanger tube failure, pressure relief device failure. For these reasons the safe design, installation, operation, and maintenance of pressure equipments in accordance with the appropriate codes and standards are essential to safety in major accident hazard establishments.
Various codes, standards and governmental regulations cover the design, the testing and the inspection for pressure equipment. In past decades, in main industrialized countries, national regulator bodies developed their own codes, such as BS5500 in UK and ASME in US. In Italy a set of rules (ISPESL VSR code) had been developed in Seventies for
pressure vessels stability verification. In 1997 European Commission issued Directive 97/23/CE, which doesn’t enforce any national code, but recommends using comprehensive methods, which are known to incorporate adequate safety margins against all relevant failure modes in a consistent manner. In order to assure the containment of internal and external pressure under design conditions, ISPESL-VSR rules define minimal thickness of shells and heads and state conditions on all vessel details. ISPESL-VSR code defines also testing and approval procedure, furthermore it is integrated by a material code and an inspection code. Inspection code defines control methods for maintaining the mechanical integrity of pressure equipment items and minimizing the risk of loss of containment due to deterioration.
2. Equipment safety and plant safety assessment
Equipments in process plants may deteriorate and fail. Major deterioration mechanisms observed in the hydrocarbon and chemical process industry are internal and external thinning, stress corrosion cracking, and metallurgical and mechanical deterioration. Thinning includes general corrosion, localized corrosion, pitting, and other mechanisms that cause loss of material from internal or external surfaces [1].
For each equipment, or equipment group, a sound risk understanding is essential to drive integrity controls and prioritize inspections. Risk assessment basically determines what incident could occur in the event of an equipment failure, and how likely the incident could happen. Detailed methods, such as Hazard and Operability method (HAZOP), may also be used to analyse failure modes and effects. Basically, these methods identify failures and deviations, which lead to incident and assess severity of potential consequences. Event likelihood may be derived by some empirical knowledge or by analytical methods such as Failure modes, effects, and criticality analysis (FMECA). Event probability and severity may be assessed in a qualitative way. Otherwise a quantitative approach may be adopted. Quantitative risk analysis methods use logic models, which consist of event trees and fault trees. Event trees delineate initiating events and combinations of system successes and failures, while fault trees depict ways in which the system failures represented in the event trees can occur [2].
3. Overall safety level in the plant lifetime
Along chemical process plant lifetime, the actual safety level could differ from the level evaluated in initial risk assessment procedures. In design phase components and equipment failure rate may also be inferred, as there are not yet data from operation. In operation phase, equipments breakages, maintenance, repairs, modifications and substitutions have to lead to reconsider failure rates. Values derived from operational data may be very different from values inferred at design time. For this reason, accidental
events probability may change along plant lifetime. Process hazards and consequences severity, as identified by initial hazard identification study, change only in the case of plant modifications, but failure probability may increase, or decrease, due to equipments conditions and also the risk, which combines event probability and severity, depends on plant operations. As time progress, more actions are made which will determine equipments conditions and affect overall safety level. As in a complex plant many types of equipment are custom made, failure rate is unknown before starting operation and may be deduced only from the failure rate of similar equipments or some other empirical argument. Operations may provide actual data on failures per time unit, useful to tune failure probability for these equipments.
Risk analysis methods are based on many hypotheses, due the lack of operational data at design time. In operation time a lot of information and data may be collected and used to improve the initial safety assessment. The understanding of actual deterioration mechanisms and the rate at which deterioration actually occur is essential to reconsider, in plant lifetime, risk assessment and inspection planning. Indeed the risk is increased when there is a lack of, or uncertainty in, key information required to assess the equipment integrity.
4. Pressure Equipment Management in Plant Lifetime
To face all these issues, a unified information system should be very useful. The establishment operator should use this system to follow the history of all pressure equipments, with a view on inspections, maintenance, repairs, modifications and substitution. The operator, by such a system, should keep all information and data about equipments history and use them to have a control “on line” of actual risk levels in order to manage in a better way, inspections and all risk based activities.
Fig. 1. Risk analysis on a heat exchanger: failure modes identification, consequences and likelihood qualitative evaluation
For each item of the plant, this system should handle the technical documentation, from design to construction, from installation to testing and approval, from inspection to fixing and modification, from casting off to substitution. Such a system should integrate the digital representations of equipments and plant, as they had been designed, with the representation of operations history, in order to have a sort of digital representation of the plant as it actually is, now and here. A few recent papers [3-5] demonstrated the feasibility of integrating digital plant representation and risk assessment. A first implementation of the system should consider only qualitative methods for consequences and probability evaluation. The qualitative approach requires data inputs based on descriptive information using engineering judgment and experience as the basis for the analysis of probability and consequence of failure. Inputs are often given in data ranges instead of discrete values. Results are typically given in qualitative terms such as high, medium and low, although numerical values may be associated with these categories. The value of this type of analysis is that it enables completion of a risk assessment in the absence of detailed quantitative data. The accuracy of results from a qualitative analysis is dependent on the background and expertise of the analysts.
5. PELM System
This paper presents a software prototype, which is aimed to demonstrate the feasibility of the integration of digital representations in plant lifetime management. The software focuses just pressure equipments, as in most process plants pressure equipments are critical for overall safety. Furthermore European directives and national laws rule pressure equipments approval and inspection and sound codes of technical rules, coming from decades of experiences in process industry, address pressure equipments design, construction, testing and operation.
Fig. 2. Verification of ISPESL-VSR rules on a pressure equipment
The software has been named PELM, acronym for “process plant equipment lifetime management”. It is a database, which handles plant and equipments 2D and 3D digital models, process hazard study representations, inspections results records. For the digital representation of plants and equipments, we used IBM CATIA©, an advanced tool for the computer aided design and management of components and plants. The prototype is sort of “toolbox”, organized in three main modules: DESIGN, STARTUP, and OPERATION. Each module has specific tools.
In the implementation we considered just ISPESL pressure equipment safety codes and namely ISPESL VSR code for stability verification of pressure vessels, ISPESL M code for pressure vessels material verification, ISPESL E code for pressure vessels operation. 5.1. Design
PELM-Design module is aimed to collect all initial information about plant and equipment. At this stage the system requires, for each equipment, digital models and data about:
position and function in the plant,
operation parameters (temperature, pressure, levels and so on), potential failures with consequences,
potential deviations with causes, consequences and safeguards, drawings and 3D model.
A specific tool of PELM-D allows reviewing plant in a systematic way in order to identify equipment failures, which could lead to undesirable consequences. The methodology is applied to each component of the plant, or to each component of the selected logic unit. It uses P&ID digital representation to find out equipment position in plant. Analysis, following a hazop like method, provides failure identification, consequences identification and evaluation. Token words are used to describe failures and consequences. Figure 3 shows the PELM-D user interface.
A qualitative consequences evaluation method is embedded in this module. It involves identification of the equipment, and the hazards present as a result of operating conditions and process fluids. On the basis of expert knowledge and experience, the consequences of failure can be estimated separately for each equipment group or individual equipment item. Consequences severity categories are “very severe”, “severe”, “light” and “ very light”.
A qualitative probability evaluation method is embedded in this module. It involves identification of the units, systems or equipment, the materials of construction and the corrosive components of the processes. On the basis of knowledge of the operating history, future inspection and maintenance plans and possible materials deterioration, probability of failure can be assessed separately for each equipment grouping or
individual equipment item. Engineering judgment is the basis for this assessment. A probability category is assigned for each failure mode. The categories are described with words such as “very high”, “high”, “low” or “ very low”.
5.2. Startup Module
At this stage the system requires, for every equipment, digital models and data about:
construction materials,
fluid physical and chemical characteristic, compliance with coded design rules, supplier - builder,
construction, and installation time, testing and approval records,
inspections scheduling,
maintenance scheduling.
A specific tool of STARTUP module allows verifying the compliance of pressure vessels CAD drawings with VSR rules [6]. In order to by pass the problem of different digital representations, as supplied by manufacturers, into a coherent representation schema this tool rebuild a synthetic digital representation of the equipment, using just the data supplied by manufacturer in the certification request form. In such a way boring problems of format conversion have been avoided. Furthermore details useless for safety purpose are omitted. This module builds a synthetic model, check VSR rules and transfer it to a CAD system to have a visual representation on the computer screen (Figure 2).
The core of this module is a tool, which allows defining inspection scheduling, based on equipment risk priority classification (Figure 3). Priority is derived combining severity and probability levels, as defined in PELM-Design module.
5.3. Operation
PELM-Operation Modules requires, for each pressure component, digital documents and data about:
safety component inspection records,
integrity inspections evidences such as radiographic images, eco-graphic images, maintenance records,
failures records,
if any measure instrument is present on the equipment, averaged values, preventive maintenance reports,
approval of mechanical modifications with related 2d and 3d models, fluids physical and chemical characteristics change,
Fig. 3. Inspection scheduling for a heat exchanger
PELM allows all access functions, namely insertion and extraction of graphical and textual document and numerical data. Document may be searched by key words, by date, by equipment, by operation, by supplier. Furthermore, for customised equipments, failure probability may be tuned by means of operation data. Value of risk-based priority may be changed in PELM-S module.
PELM keep records at each stage in equipment lifetime and is able to produce reports as well warnings and suggestions. As nowadays industry is featuring a fast personnel turnover, technical experiences on equipment operation may be dissipated. Systems like that, if used for a quite long time, may become the vault of all operation experiences on plant equipments. This experience vault could be used to build a base of “frequently asked questions and answers”. Such a tool may be very useful to transfer technical experience to unskilled operators.
Competent authorities that are in charge of the surveillance for major accident hazard in process industries have to manage large databases of equipments, in order to harmonize and coordinate inspection activities. Using PELM they should integrate in such databases also 3D digital representations.
PELM Software architecture is very flexible, as rules are not embedded in the main code. For this reason, changes in rules definition are quite easy and do not affect software main code. The developed prototype works just with VSR the Italian standard for pressure vessels, but the extension to the harmonised European code, EN 13445, may be
considered quite easy, as they have the same logical internal structure. Also vapour generators, low-pressure tanks, piping, and pumps could enter in application range of PELM. At the end, any body of technical rules, leading to formulas, could be considered for a potential extension of PELM.
References
1. API 580: Risk Based Inspection. American Petroleum Institute Washington DC 2003 2. API 510: Pressure Vessel Inspection Code—Inspection, Repair, Alteration, and
Rerating. American Petroleum Institute Washington DC 2000.
3. Giannini, F. Monti, M. Ansaldi, S. Bragatto, P.: Hazard identification in process plant through CAD, CAE and PDM systems. Advances in Safety and Reliability Proceeding of ESREL, Balkema, Leiden.
4. Ansaldi, S. Giannini,F. Monti, M. Bragatto, P.: PDM-based tool for hazard identification in plant design. Proceeding of International Conference on Product Lifecycle Management Inderscience, Geneva, 2005.
5. V. Venkatasubramanian, Prognostic and diagnostic monitoring of complex systems for product lifecycle management: Challenges and opportunities. Computers and Chemical Engineering 29 1253–1263, 2005.
6. Bragatto,P. Pittiglio,P. Ansaldi,S.: Knowledge Based CAD for Pressure Vessel Stability Verification. Proceeding of SRA Europe Conference, 2005.
