P and I diagrams

This chapter covers the preparation of the preliminary P and I diagrams at the process design stage of the project. 

The P and I diagram shows the arrangement of the process equipment, piping, pumps, instruments, valves and other fittings. It should include ... 

For simple processes, the utility (service) lines can be shown on the P and I diagram. For complex processes, separate diagrams should be used to show the service lines, so... 

The P and I diagram will resemble the process flow-sheet, but the process information is not shown. The same equipment identification numbers should be used on both diagrams. 

The symbols used to show the equipment, valves, instruments and control loops will depend on the practice of the particular design office. The equipment symbols are usually more detailed than those used for the process flow-sheet. A typical example of a P and I diagram is shown in Figure 5.25. 

Computer aided drafting programs are available for the preparation of P and I diagrams, see the reference to the PROCEDE package in Chapter 4. 

The following procedure can be used when drawing up preliminary P and I diagrams ... 

A brief outline of the technique is given in this section to illustrate its use in process design. It can be used to make a preliminary examination of the design at the flow-sheet stage and for a detailed study at a later stage, when a full process description, final flow-sheets, P and I diagrams, and equipment details are available. 

The P and I diagram shows all the components that make up a control loop. For example. Figure 5.8 shows a field-located pressure transmitter connected to a shared display pressure indicator-controller with operator access to adjustments and high and low alarms. The pressure controller sends an electric signal to a fail-closed diaphragm-actuated pressure control valve. 

The most significant hazard of this process is the probability of an explosion if the concentration of ammonia in the reactor is inadvertently allowed to reach the explosive range, >14%. Note that this is a simplified flow diagram, and a HAZOP based on the full P and I diagram would go into considerably more detail. 

A distillation column separates benzene from toluene using a control scheme similar to that shown in Figure 5.27c. Make a practice HAZOP study of the plant section and add any instrumentation that is needed to develop the full P and I diagram. 

In the analysis, each process step is looked at for deviations in temperature, pressure, level, etc., and the possible effects of physical problems such as pipe blockage, valve failure, corrosion, etc. All the details of this analysis should be recorded. On the findings of this analysis, the chemical engineer can refine his design and produce detailed plans including pipework layouts and instrumentation (P and I) diagrams. 

Although such diagrams relate to the operation of plant within a perimeter fence, mal-operation may result in aqueous or atmospheric discharges capable of producing an environmental impact beyond the fence. Thus safety teams who scrutinise P and I diagrams in laborious... 

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