*3.3.2. Moderating the transient initiation event*

A reduction of the rate of velocity change will reduce the transient pressure amplitude. A variable speed drive or soft start/stop functionality may be effective measures for normal operations, but their effect is negligible in case of a power failure. A flywheel increases the polar moment of inertia and thereby slows down the pump trip response. It should be veri‐ fied that the pump motor is capable of handling the large inertia of the flywheel during pump start scenarios. Experience shows that a flywheel is not a cost-effective option for pumps that need to start and stop frequently.

If inadmissible pressures are caused by valve manipulations, the valve closure time must be increased. The velocity reduction by a closing valve is not only influenced by the valve char‐ acteristic, but also by the system. The valve resistance must dominate the total system resist‐ ance before the discharge is significantly reduced. Therefore, the effective valve closure time is typically 20% to 30% of the total closure time. A two-stage closure, or the utilization of a smaller valve in parallel, may permit a rapid initial stage and very slow final stage as an ef‐ fective strategy for an emergency shut down scenario. The effective valve closure must be spread over multiple pipe periods to obtain a significant reduction of the peak pressure. Ex‐ isting books on fluid transient provide more detail on efficient valve stroking (Tullis 1989; Streeter and Wylie 1993; Thorley 2004).
