Startup Control

A pressure switch located so as to sense falling pressure at the earliest moment should be used to activate the standby pump. The switch should be so connected to the system as to permit testing the startup circuit without shutting down the compressor. Figure 8-8 shows four piping arrangements. The figure at b would be the recommended method.

SERVICE LINE

ALARM(S)

SERVICE LINE

ALARM(S)

I . RESERVOIR

SERVICE LINE

I . RESERVOIR

SERVICE LINE

C ALARM(S)

TO RHSERVOIR OR DRAIN LINE

SERVICE LINE

Figure 8-8. Pressure gauge and pressure switch piping arrangements [1].

It should be emphasized at this point that the speed of response is critical. The pressure transient pressure should not fall to less than 50% of the difference in pressure between the standby pump start pressure and the low oil pressure trip pressure. This is normally achievable with good design practice and the use of a switch and direct wiring. There is some tendency to use a transmitter and control through a remote computer. The latter arrangement is difficult to check on a shop test and normally is too slow to meet the requirement. An accumulator can be added and must be used if the requirement cannot be met. This additional hardware contributes to higher initial cost and possible reliability problems in the future. The direct switch method is therefore highly recommended.

Provision must be made in the wiring to maintain power to the standby pump driver. This will prevent power interruption due to the pressure switch contacts opening when lubricant pressure is restored. Cycling of the standby pump will occur if the circuit is not maintained. A reset will have to be furnished to permit shutting down the standby pump when the main pump is back on stream.

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