Safety Shutdown Systems

These controls are normally provided as backup pump protection for the continuous bypass and automatic bypass control systems. They should only operate if the normal system fails due to mechanical or operator causes. In each case, the protection system trips the pump off.

FIGURE 9 Operation of a typical pilot-operated ARC bypass valve actuated through a lever mechanism

Head This requires only an accurate pressure switch, set for a discharge pressure corresponding to the total pump head when the pump is close to the shutoff capacity. For conservative results, calculate based on minimum suction pressure. To use this method, the pump characteristic must rise continuously to shutoff. Control circuitry may require delays to reject spurious pressure surges.

Temperature A temperature sensor mounted on the pump casing or in the discharge and close to the pump can signal for a shutdown of all or some of the operating pumps. The trip can be on either a high temperature sensing or a high differential temperature sensing between inlet and discharge. This method can protect against large temperature rises (occurring at perhaps 5% or less of capacity) but is less sensitive to smaller rises at higher percentages of capacity where adverse hydraulic effects begin for large and highpower pumps.

Low-Flow Systems These protection systems can be configured in various ways. A low-flow switch can trip the pump off at a predetermined setpoint. A flow meter can signal a controller to shut down the pump. If this protection circuit is installed, the pump start circuit must temporarily bypass this trip in order to start the pump. This is normally accomplished by a spring-loaded switch that bypasses the low-flow trip when held in the "Start" position, but activates the trip protection in the "Run" position.

FIGURE 10 Pilot operated multistage bypass trim actuated by an in-line pilot valve

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