Figure 11-14 shows a typical mechanical flowsheet for two centrifugal pumps installed in parallel. It is presented to illustrate some of the considerations necessary for a good pump installation. Every installation has different objectives and very few will be exactly like this one.
The suction piping is sized for about 2-3 ft/sec and the discharge piping for about 5-6 ft/sec. Although suction and discharge velocities are not as critical for centrifugal pumps as for reciprocating pumps, field
experience indicates lower maintenance when the velocities are kept below this range.
Each pump has isolation valves to enable it to be maintained while the other is running. Because of the possibility that a discharge valve could be left open while a suction valve is closed, the suction line up to and including the block valve should be rated for discharge pressure. As an alternative, a relief valve could be installed in each pump’s suction piping.
A check valve is installed in each pump’s discharge to prevent reverse rotation of the pump when it is not operating and its isolation valves are open.
A throttling valve (FCV) is installed to control flow without having to start and stop the pumps. It is also possible to start and stop one or both pumps based on level in the feed tank or to use a variable speed driver to control pump speed on level. Since it is possible for the throttling valve to close or for a pump to be started with a blocked discharge, a minimum flow bypass with an orifice (RO) sized to provide sufficient flow to avoid overheating the pump is installed. The bypass is piped back to the tank for further cooling, but this requires a valve on the bypass line that could be left closed, and it does not protect the pump from a closed suction valve. It would be possible to protect against these problems by piping the recycle directly back to the pump suction. The drawback of this type of bypass is that with the short loop from discharge to suction, the liquid will eventually overheat. The continuous bypass effectively reduces theefficiency of the pump. On large installations it could be attractive to install a pressure control valve on the pump discharge that would only bypass liquid when the pump discharge pressure approached the shut-in pressure. There is much debate on which scheme to use.
It is common to install low- and high-pressure switches in the discharge line (PSHL). The low-pressure switch usually has a delay for start-up. If the pump starts against a closed valve, the pressure will reach
the high setting, and the switch will shut the pump down and trigger an alarm that indicates the reason for the shutdown. If the suction valve is closed, the pump pressure will remain low until the low-pressure switch delay expires, and the pump will be shut down. If a large leak develops in the piping downstream of the pump and the pressure drops below the normal operating range, the low-pressure switch will sense the abnormal pressure and shut-in the pump.
The pump is provided with vent and drain connections and a cone-type strainer (SP-1) for startup. Piping must be arranged to allow removal and replacement of the strainer and to allow easy access to the pump for maintenance.
