Pump Trips and Check Valve Closure
Systems running multiple pumps in parallel can undergo serious equipment and piping damage during a pump trip caused by a power outage or pump mechanical failure. Uncontrolled reverse flow in the system can occur and if improperly selected check valves are used it can result in pumps running backwards or transient pressure spikes in the system, i.e., water hammer. Water hammer will occur if reverse flow occurs prior closure of the check valve and the effect increases with higher reverse flow velocity. Becht has worked with clients on analysis of the design of their systems, e.g., a water treating facility running multiple 52,000 gpm pumps in parallel, a seawater pump station pumping cooling water through a several mile pipeline to an inland facility and a boiler feed water circulation system for a 3000 psig forced circulation boiler.
In a three pump system (two operating and one spare), one and two pump trips were analyzed to determine the response of the system, i.e., fluid deceleration, time at the occurrence of reverse flow vs. time of check valve closure, pressure transients and unbalanced forces that are imposed on the piping and pumps. The figure shows the forward fluid velocity vs. time as the speed of the pump(s) slows, the point of reverse flow, the time at check valve closure and pressure spikes in the system. Based on the analysis, the required performance of the check valve to minimize reverse flow can be determined and a valve selected. No valve will close precisely at "zero" fluid velocity; however, certain type valves perform significantly better than others.