First of Two Parts
by Ray Hardee ( Engineered Software, Inc.)
July 9, 2015

Under this condition, it takes approximately 37 minutes to add 8,000 gallons to the tank. The pump turns off at the high working tank level. With 400 gpm leaving the system and the working capacity of the tank at 8,000 gallons, it takes 20 minutes to drain the working capacity from the tank.

The cycle time for the pump is 57 minutes, (20 minutes pump off and 37 minutes pump on). With the system in operation 8,000 hours per year and a 57 minute cycle time, 8,421 cycles occur each year. As a result, the pump operates 5,195 hours per year (8,421 cycles per year x 37 minutes per cycle x 1 hour/60 minutes). This results in $9,660 per year of operating cost.

Manual Control

A third way to handle the excess pump capacity is to install a throttle valve on the pump discharge and have an operator manually limit the flow rate to the destination tank. When an operator adjusts the valve hand wheel to the closed position, the head loss across the valve increases, causing the pump to run back further on its pump curve.

This results in a reduction in the flow rate to the tank. To increase the flow rate, the valve is opened, reducing the head loss across the valve and causing the flow rate of the pump to increase. The closer the flow rate through the throttle valve is to the system flow the slower the change in tank level.

This process is successful when the operator can maintain this tank level in the normal course of their duty. A tank overflow should be provided to handle unexpected rapid changes in tank levels or inattention by the operator.

The advantage of this approach is low initial cost, provided the operator can maintain the tank level without excessive attention. The disadvantage is the possible variability in the tank level and inadvertent loss of process fluid.

Next month's column will detail active controls options.

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