Suppose the pressure switch used to control the "lead" potable water pump is set at 45 psig cut-in for pump start and 55 psig cut-out for pump stop. What would be an appropriate setting for the "lag" potable water pump?
• Lead vs. lag pump operation in a pressure-controlled potable water system • How cut-in (start) and cut-out (stop) pressures relate to system pressure dropping and rising • Why the lag pump should only start when the lead pump cannot maintain pressure
• As system pressure drops from 55 psig downward, which pump should start first, lead or lag? What does that say about their relative cut-in settings? • Once both pumps are running and pressure is restored, should they stop at different pressures or the same pressure? What would happen if one stopped earlier or tried to pump to a higher pressure than the other? • Which choice gives the lag pump a setting that keeps it in standby most of the time, but brings it in to assist before the pressure gets too low for the users?
• Make sure the lag pump cut-in is not higher than the lead pump cut-in, or it might start first when pressure falls. • Check that the cut-out pressure for the lag pump will not fight the lead pump (for example, trying to pump to a much higher or lower shutoff pressure). • Confirm that the lag pump will only start after the lead pump is already running and still cannot maintain the desired pressure range.
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