This chapter is from the book
This chapter is from the book
This chapter is from the book
1.4 Redundancy and Operability
While the focus of this book is on modeling and control system design, it should be understood that processes need to be designed to ensure safe operation even when components fail. For processes that operate continuously for long periods of time, it is important to have redundant pumps placed in parallel. Then, if one pump fails, the other can be immediately switched on. On the other hand, due to high capital costs, it is less common to have redundant compressors except in the case of instrument air systems. Usually, when a process compressor fails, the process must be shut down while the compressor is being repaired or replaced.
In Example 1.1 and in most of the examples given in this textbook, we use a simplified representation for the main components in a control loop. A classic piping and instrumentation diagram (P&ID) would also include pipe sizes and additional equipment for redundancy and safety. Figure 1–9 is a characteristic process and instrumentation diagram (P&ID without piping specifications) that shows redundant pumps, check valves to assure flow through pumps occurs in one direction, a pressure relief valve, and a bypass line around the control valve. When the control valve fails, the adjacent block valves can be closed; the control valve can then be removed and replaced. During the interim, the bypass valve can be adjusted manually to maintain the desired flow rate. Generally, these control valve “stations” are placed at ground level for easy access, even if the pipeline is in a pipe rack far above the ground. Pictures of redundant pumps and a control valve with bypass are shown in Figure 1–10.
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Figure 1–9 Process and instrumentation diagram for surge drum level control. There is a pressure relief valve on the drum, redundant pumps, check valves on the discharge of each pump, bypass around the control valve, and high (LAH) and low level (LAL) alarms as part of the controller interface (LIC, level indicating controller).
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Figure 1–10 Redundant pumps and control valve bypass