Design intention

The inherent contribution to reliability should be compressor assemblies that truly reflect dimensions intended by the designer. This in turn should make the compressor perform to the level intended. It should oi t -set the desire on the part of maintenance people to improvise when parts don t fit. This improvising can certainly become a problem as the original design intent is not normally known or understood at the field maintenance level. 

The procedure for a HAZOP study is to apply a numher of guide words to various sections of the process design intention. The design intention informs what the process is expected to caiTy out. Table 12-6 shows these guide words, and Figure 12-42 summarizes the entire procedure. Common property words are ... 

No Complete negation of the design intention. Application to flow, concentration, react, heat transfer, separate and similar functions. No level means an empty vessel or a two-phase interface is lost. 

As well as Design intention achieved together with something else. Flow as well as describes contamination m a pipeline. React as well as covers side reactions. 

Part of Design intentions only partly achieved. It IS more precise to use the guide word less" wherever possible. However, fluctuations m a property word are covered by part of. 

HAZSEC generates two types of records. The first page is the log sheet with the time, date, rc% ision number, team leader, and team members. This page also contains a section that describes the part of the plant design under investigation, and a statement of the design intent, i.e., the expected equipment performance under normal and accident conditions. The pages that follow repeat this... 

System (node) Selection Design Intent Guide Words Cause Identification Consequence Identificatidn Resolution Review Record... 

Potential failure mode (the manner in which the item or process could potentially fail to meet the design intent)... 

Solution We suppose that the mass transfer and diffusion steps are fast compared with bulk transport by convection. This is the design intent for ion-exchange columns. The reaction front moves through the bed at a speed dependent only on the supply of fluid-phase reactants. Assuming piston... 

As well as something in addition to the design intention such as, impurities, side-reactions, ingress of air, extra phases present. 

Reverse the reverse of, or opposite to, the design intention. This could mean reverse flow if the intention was to transfer material. For a reaction, it could mean the reverse reaction. In heat transfer, it could mean the transfer of heat in the opposite direction to what was intended. 

The design intent was mainly to produce transportation fuels, yet some chemicals were co-produced, rather than being converted into fuels. For example, the refinery design of the aqueous product refinery could have been simplified,43 if the design intent had been to produce only fuels. 

By approaching the refinery design from a crude oil perspective, the advantage of preseparation by stepwise condensation after HTFT synthesis was reduced. The refinery design included primary separation steps typically found in crude oil refineries, namely, an atmospheric distillation unit (ADU) that is followed by a vacuum distillation unit (VDU). Despite the design intent, the operation of these units, out of necessity, had to be different. The reboiler temperature of the ADU was... 

The design intent was to produce transportation fuels, and the design did not specifically make provision for chemicals co-production. It is in principle possible to extract chemicals from the HTFT syncrude, such as the alcohols that are being recovered from the Fischer-Tropsch aqueous product. Extraction of linear a-olefins may also be considered, which has indeed been investigated,57 and many other opportunities exist. However, it should be noted that the Mossgas facility is much smaller than the Sasol Synfuels facility, and recovery of valuable products in HTFT syncrude may not have economy of scale. 

The HAZOP team examines each study node for potentially hazardous process deviations. First, the design intent is defined to delineate the purpose of the equipment and the process parameters. Process deviations are determined by combining guide words with the important process parameters. The established set of guide words is shown in Table 4.14. 

In addition, it provides a list of potential actions that should be evaluated. Table 4.16 is an example of a HAZOP study worksheet. A typical HAZOP study report should include a brief system description, a list of drawings or equipment analyzed, the design intents, the HAZOP study tables, and a list of actions items. 

Describe the design intent of the study node. For example, vessel V-l is designed to store the benzene feedstock and provide it on demand to the reactor. 

NO, NOT, NONE The complete negation of the intention No part of the design intention is achieved, but nothing else happens. 

PART OF Qualitative decrease Only some of the design intentions are achieved, some are not. 

No, Not, None More Less As well as Part of Reverse Other than Complete negation of design intentions Quantitative increases Quantitative decreases of any relevant physical parameters Qualitative increase A qualitative decrease Logical opposite of intention Complete substitution No part of intention is achieved and nothing else occurs Quantities and relevant physical properties such as flow rates, heat, pressure Same as above All design and operating intentions are achieved as well as some additional activity Some parts of the intention are achieved, others are not Activities such as reverse flow or chemical reaction, or poison instead of antidote No part of intention is achieved something quite different happens... 

HAZOP - A formal systematic critical safety study where deviations of design intent of each component are formulated and analyzed from a standardized list. Risks are typically expressed in a qualitative numerical series (e g., 1 to 5) relative to one another. 

For the majority of projects, it is sufficient to use pre- and postconditions as the basis of test harnesses and to document just enough of a refinement that other developers and maintainers clearly see the design intent. We ll see how to do that later in this chapter. 

It is essential that the design engineer recognize the job is not complete until the structural system has been detailed in a manner that assures the response will be consistent with the design intent. The development of details should also consider cost and constructability. 

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