PRACTICAL GUIDE TO DEVELOPING PROCESS FLOW DIAGRAMS AND PRELIMINARY ENGINEERING LINE DIAGRAMS...

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Process Safety Guide: GBHE-PSG-021

PRACTICAL GUIDE TO DEVELOPING PROCESS FLOW DIAGRAMS AND PRELIMINARY ENGINEERING LINE DIAGRAMS PROCESS

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Process Safety Guide: PFDs and ELDs CONTENTS 1 INTRODUCTION 2 DESCRIPTION OF METHODOLOGY 2.1 Philosophy of the Design Process 2.2 Outline of Methodology 2.3 Additional Aspects of Methodology

3 WORKBOOK 4 EXAMPLE: BATCH NEUTRALIZATION AND STRIPPING PROCESS 4.1 Start of Stage 1 4.2 Stage 1 4.3 Stage 2 4.3 Stage 3 5 REFERENCES TABLES 1 DESIGN OBJECTIVES 2 OUTPUT FROM EACH STAGE 3 STEP BY STEP APPROACH 4 FUNCTION 5 OPERATION 6 FAILURES 7 STAGE 3 PROMPTS (DATA SHEET) 8 PRELIMINARY MASS BALANCE 9 PRELIMINARY EQUIPMENT LIST 10 POTENTIAL PROBLEMS 11 STAGE 2: DEVELOPMENT OF OPERATING STATES AND TRANSMISSIONS 12 POTENTIAL PROBLEMS



FIGURES 1 DESIGN PROCESS 2 DESIGN PHILOSOPHY. 3 SUMMARY OF METHODOLOGY 4 POSSIBLE OUTCOMES FROM A KEYWORD PROMPT 5 OPERATING STATES 6 OPERATING STATES/TRANSITIONS 7 EXAMPLE: OVERALL BLOCK DIAGRAM 8 EXAMPLE: “PRIMITIVE PFD” 9 EXAMPLE: STAGE 1 “WORKING DIAGRAM”. 10 EXAMPLE: MAIN OPERATING STATES AND TRANSITIONS 11 EXAMPLE: DETAILED PFD (END OF STAGE 1) 12 EXAMPLE: STAGE 2 “WORKING DIAGRAM” 13 EXAMPLE: STAGE 3 “REV 0” ELD



1 INTRODUCTION This document presents a methodology to guide the development of projects from preliminary process design into Process Flow Diagrams (PFDs) and subsequently into Engineering Line Diagrams (ELDs). Key features of this approach are an early understanding of design issues and in particular a systematic way of designing out potential hazards. The guide provides a structure and set of tools to improve the effectiveness of the design process and especially to minimize design changes. The methodology is applicable to both batch and continuous processes at any scale of operation. It can be applied equally well to a plant complex or an individual section of a plant. This "synthesis" tool may also be used for analysis to develop a systematic understanding of an existing process or plant. It should therefore also be useful to newly appointed plant personnel, to those involved in purchasing licensed technology or to teams involved in acquisitions. The development of this methodology arose out of project OSPREY, a collaborative research project between GBH ENTERPRISES and RENALT ENERGY. The original remit of OSPREY was to produce a formal methodology to aid in the development of ELDs from PFDs, and thus to support and promote consideration of safety, health and environmental concerns during this stage of design and in particular to minimize changes identified during Hazard Studies. During the project it became apparent that there was no formal methodology for the development of PFDs. In order to bridge this gap, work on an extended remit was carried out in collaboration with RENALTENERGY and subsequently completed within GBH ENTERPRISES. Figure 1 shows a schematic of the design process from the initial "idea" through to a completed set of ELDs. There is a well-established methodology for the development of an initial "idea" through to preliminary process design ("Professor Douglas" Methodology, Ref.1). In contrast there appears to be no published approach to guide the development of the output from preliminary process design into PFDs and subsequently into ELDs. During this latter development successive layers of detail are added, the design team increases in size and it becomes more expensive and it takes more time to do any rework associated with design changes. In addition, the increase in detail makes it difficult to maintain a "global view" of the whole process as a single PFD is replaced by a set of detailed PFDs and ELDs. There is thus a need for a methodology to drive the development of PFDs and ELDs. This process SHE Guide represents a considered and documented approach for this step.



FIGURE 1 DESIGN PROCESS



2 DESCRIPTION OF METHODOLOGY

2.1 Philosophy of the Design Process This methodology is based on the fundamental approach illustrated in Figure 2. Here it is argued that any design process should start with an agreed intent, continue by steps to determine possible solutions, screen the possibilities against criteria such as cost, and work up the preferred solution into the final design. The original intent should always be checked to ensure that this is really what is wanted. This checking should occur both at the start and throughout the design process. The list of possible solutions should not be and cannot be exhaustive so some recycling back to the solution stage will occur if the criteria are not met. If no acceptable solution can be found, then there is a need to restate the intent. Every design project will be different and it is important that the design process takes a fresh view to meeting objectives and implementing solutions. It should however be recognized that successful projects will make use of existing knowledge and understanding and will integrate into the local culture. There is no point in re-inventing wheels but the skill will be in applying the right set of wheels. This fundamental approach is scale independent; thus it could be applied to the design of an entire chemical plant or to a minor modification. In the context of this guide, the approach is applied in stages to provide a structure and some ’milestones’ for the PFD and ELD development. 2.2 Outline of Methodology The methodology comprises three stages as follows: Stage 1: Design Strategy This stage takes the preliminary process design information and uses a structured approach to identify all the issues relevant to PFD development. This allows development of an overall PFD, together with a process description and operating strategy.



Stage 2: Design Intent This stage uses the overall PFD as a basis for notional ELDs, and uses a structured approach to add the next layer of detail with the emphasis on what is required, rather than how requirements are to be implemented in specific hardware. Stage 3: Design Implementation This stage turns intent into hardware proposals, which are displayed on "Revision 0" ELDs. These ELDs can then be refined further in an ongoing process of ELD development and review. Any viable process design should operate efficiently at steady state or throughout a batch cycle, should be capable of operating under different conditions and should be capable of minimizing losses and shutting down safely in the face of failures. Accordingly, the considerations at each stage are divided into three sub-stages: (a) Function: here the focus is on those design objectives that are key to the normal operation of the plant. (b) Operation: here the focus is on the various operating states and transitions (e.g. start-up, shutdown) that the plant must be designed to handle. (c) Failures: here the focus is on identification of likely failures, minimization of the risk of failure and limiting the effects of failures. A risk assessment can then be made, and where necessary safeguard actions proposed and implemented.



FIGURE 2 DESIGN PHILOSOPHY



Table 1 shows the various design objectives grouped into each substage. TABLE 1 DESIGN OBJECTIVES

These design objectives are expanded and used as thought prompts or guidewords in stages 1 and 2. The same prompts are used in both stages; this has the advantage of maintaining some linkage between the stages and makes for ease of cross-referencing. The distinction between function, operation and failures is retained in Stage 3, although a separate set of guidewords, based on defining equipment data, is used. Figure 3 summarizes the outline methodology and indicates the method of working. It will always be a matter of judgment as to the level of detail required at each stage during the development. A key to efficient design is timing: providing the right level of detail at the right time. Experience is vital to achieve that efficiency. Developing confidence that problems can be solved later should lead to a more effective approach. The possible outcomes from any guideword are shown in Figure 4. For each guideword the outcomes are recorded and any actions are identified and noted. There is no need for elaborate records; notes on the notional PFD or the notional ELD, together with a simple table of Guideword/Comments/Actions should be sufficient for subsequent reference and to demonstrate that a systematic method was used. The methodology can be used by an individual; however it is probably most powerful if used by a small group of individuals with complementary skills. An ideal core group would comprise the lead process engineer, the project engineer and a representative from operations. Such a team would require support from functional specialists from other disciplines (e.g. R&T, Control, Vessels, Machinery, etc).



2.3 Additional Aspects of Methodology 2.3.1 Operating States and Transitions A key part of "Operation" in Stage 1 is the definition of the operating states that the plant must be able to operate at and the transitions between these operating states that should be possible. Figure 5 shows an example of possible operating states for a plant together with transitions between them. Some transitions, such as between regeneration and ready to start-up, will be carried out in a controlled way; others, such as between normal operation and shutdown, may be initiated by automatic detection of a fault by the plant shutdown system. For each operating state it is possible to define the status of each process stream, service or item of equipment and thus define the actions required during any given transition (Figure 6). This provides a basis for the systematic development of operating instructions.



FIGURE 3 SUMMARY OF METHODOLOGY



FIGURE 4 POSSIBLE OUTCOMES FROM A KEYWORD PROMPT



FIGURE 5 OPERATING STATES



FIGURE 6 OPERATING STATES/TRANSITIONS

2.3.2 Interactions There are a number of global design issues, such as control, measurement and analysis, that span the whole plant. Furthermore some of these issues, such as isolation and relief, interact with each other. It is difficult to handle these issues once the level of detail is such that the design is presented on more than one PFD or ELD. It is also recognized that much of the detailed development, particularly on large projects, may be carried out by different teams at different times. It is therefore proposed to retain and use a single overall PFD in addition to the more detailed sectional PFDs that might be developed. This overall PFD should show the main plant items (MPIs) and lines, together with "global" details such as relief, isolation, control and instrumentation relevant to control of the whole plant (inventory control in particular). This PFD should suppress as much "local" detail as possible, such as local instrumentation and control (except control valves that might isolate vessels from relief devices), and local kick back lines. This global PFD should be updated as the project progresses.



3 WORKBOOK This clause describes how to develop a concept into a line diagram with its associated supporting design information. The description is based around a whole plant design and the development of a set of line diagrams but is equally applicable to a single diagram, e.g. in a modification to an existing plant. The methodology considers the development in a series of defined stages and uses a set of guidewords to define the purpose of the design and find acceptable solutions. Figure 3 shows the stages in the proposed method. This guide starts with a preliminary proposal and ends with a set of Rev "0" ELDs and supporting data sheets. Further details of the output and the typical support information, required at each stage, are given in Table 2. The approach to be adopted is described in outline in Table 3. It will be noted that at each stage, the proposed design is critically examined with respect to function, operation and failure, using a set of guidewords. These guidewords are described in Tables 4 to 6. A specific set, loosely based on the information needed for datasheets, is also available for stage 3 (Table 7). It is difficult to be specific in this guide as to the level of critical examination and detail necessary at each stage. Such detail will depend upon the complexity of the project. The detail should be adequate to identify and address the important issues and allow efficient progress. The detail should not be so great as to divert attention or to complete design work that may either change or be better carried out later. An experienced team is necessary. For a new plant, the overall PFD and section ELDs develop together. For a modification to an existing plant, these drawings and supporting design data should already be available. It is however critical to review the overall PFD and ELDs to ensure that all new potential interactions are fully considered.



4 EXAMPLE: BATCH NEUTRALISATION AND STRIPPING PROCESS This example illustrates the application of the methodology to a fairly simple batch process: the batch neutralization and steam stripping of an effluent stream. The notes below are not intended to be exhaustive, but to give some illustration of the use of the methodology and in particular to illustrate the level of detail that is appropriate at each stage. 4.1 Start of Stage 1 At this point the information available is as follows: An upstream plant produces an acidic effluent contaminated with acid soluble organics which must be neutralized and stripped before discharging to the site effluent system. Laboratory experiments show that the organics are easily stripped from the neutralized effluent by steam sparging, and that the resulting aqueous and organic layers in the condensate are immiscible and easily separated. Figure 7 shows a block diagram of the batch neutralization and stripping process in the overall context of the main plant producing the effluent. The effluent flow is continuous and averages 0.2m3/hr at 10% acid with about 1% organics, although there is considerable variability in both flow and composition and on occasions there will be a need to process decontamination washings from the main plant. The stripped organics can be returned to the main plant for reuse provided they are substantially free of free phase water. The organics have boiling points in the range 100- 120°C, have a low OEL and are flammable. The stripped alkaline effluent can be fed to the Works Treatment Plant (TP) provided the level of organics is below 10ppm. The Works TP can handle variations in pH, although the temperature of the stream must be below 50°C. Figure 8 shows a "primitive" PFD. The process concept is to neutralize the effluent with 10% caustic, strip with live steam to remove the volatile organics, condense the vapors produced and phase separate the condensate. The organic phase is returned to the upstream plant, the organic-saturated aqueous phase is recycled to D1 for stripping with the next batch. Table 8 is a preliminary mass balance, based on a 6 hr batch cycle. Table 9 is a preliminary equipment list:



TABLE 8 PRELIMINARY MASS BALANCE amounts in kg, 6 hr batch

TABLE 9 PRELIMINARY EQUIPMENT LIST



FIGURE 7 EXAMPLE: OVERALL BLOCK DIAGRAM



FIGURE 8 EXAMPLE: “PRIMITIVE PFD”

4.2 Stage 1 The overall objective of this stage is to convert the primitive PFD to a more detailed PFD using the guidewords under each of the three headings of function, operation and failure. A copy of the primitive PFD can be used as a working diagram (e.g. Figure 9) to record decision and comments. (a) Function The guidewords are used to check carefully the basis of design. A number of issues surface such as: (1) destination of recovered organics if main plant not able to accept? (2) destination for the vent: treat locally or tie into main plant vent system? (3) N2 padding on vent required to maintain an inert atmosphere; (4) etc.



(b) Operation One of the key activities at this stage is to make explicit the main operating states and the transitions between them. Figure 10 has been developed for this example. This in turn allows the main instrumentation and control requirements to be defined and added to the working diagram. Measurement of pH and organics, together with observation of the interface are flagged as areas that will need additional consideration. The need for process water for commissioning and for washout before maintenance is noted. (c) Failures At this stage, the potential problems and their possible solutions are listed in Table 10: TABLE 10 POTENTIAL PROBLEMS

A detailed PFD (Figure 11) can be produced from the working diagram. This diagram becomes the starting point for the Stage 2 work.



FIGURE 9 EXAMPLE: STAGE 1 “WORKING DIAGRAM”

FIGURE 10 EXAMPLE: MAIN OPERATING STATES AND TRANSITIONS



FIGURE 11 EXAMPLE: DETAILED PFD (END OF STAGE 1)

4.3 Stage 2 The overall objective of this stage is to convert the detailed PFD to a "concept" ELD that captures design intent, using the guidewords under each of the three headings of function, operation and failure. A copy of the detailed PFD can be used as a working diagram (e.g. Figure 11) to record decision and comments. (a) Function The guidewords should be revisited to check that: (1) problems and issues raised in Stage 1 have been resolved; (2) any new information (e.g. from an experimental program) is taken on board; (3) any changes in project scope are addressed. It is important to check that any "interface" issues raised by the guideword "input/output" are resolved at this stage; issues for the example include: (i) availability of caustic batches when required; (ii) are feed pumps required for caustic and the acid effluent? (iii) etc.



(b) Operation The operating states and transitions that were defined in Stage 1 are developed in more detail; this can be done most effectively in a tabular format like Table 11. This table can be used to write outline operating instructions: by way of example the outline instructions for the transitions "add caustic" and "add acid effluent" are as follows: (1) "add caustic"; (2) check exit valves shut, cooling water on, caustic available; (3) analyze acid strength in buffer storage; (4) calculate batch recipe; (5) charge caustic; (6) return aqueous "heel" from phase separator to D1; (7) start agitator; (8) check level and temperature; (9) check caustic valve closed; (10) "add acid effluent"; (11) add 90% of acid in recipe; (12) agitate for 5 mins, check level and temperature; (13) check pH (should be alkaline); (14) recalculate "trim" addition (should be acid effluent); (15) add "trim" addition; (16) agitate, check pH. Table 11, together with these outline operating instructions should enable detailed consideration of the remaining guidewords and allow development of the working diagram Figure 12.



TABLE 11 STAGE 2: DEVELOPMENT OF OPERATING STATES AND TRANSITIONS



(c) Failures A more detailed consideration is probably best carried out in a tabular form such as Table 12 below: TABLE 12 POTENTIAL PROBLEMS

Additions to the working diagram (Figure 12) are made as required. This diagram becomes the starting point for the Stage 3 work. 4.4 Stage 3 The overall objective of this stage is to convert the working diagram (Figure 12) which shows ELD intent into a "Revision 0" ELD which shows hardware proposals. The guidewords in Tables 4 to 6 should be applied to each line and/or vessel in turn. Figure 13 is the completed "Revision 0" ELD. 5 REFERENCES 1 IC02290: Process Evaluation & Preliminary Design Procedure. D C Woodcock et al. 1985



FIGURE 12 EXAMPLE: STAGE 2 WORKING DIAGRAM



FIGURE 13 EXAMPLE: STAGE 3 “REV 0” ELD

PRACTICAL GUIDE TO DEVELOPING PROCESS FLOW DIAGRAMS AND PRELIMINARY ENGINEERING LINE DIAGRAMS...

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