Pump System Improvement
by Ray Hardee
June 21, 2017
Sample fluid piping systemFigure 1. When designing the drain collection system, a design flow rate of 400 gpm was used. In this column, we will be looking deeper into the system’s expected operating conditions and ways to improve the design. (Graphics courtesy of the author)

In the example system, the pipes and associated valves and fittings were sized to achieve an average design flow rate of 10 feet per second. Based on the system design flow rate of 400 gpm and the available pipe sizes for the specified pipe material, a 4-inch diameter pipe was selected. As Figure 1 shows, the pipe is sized for the design flow rate, but is oversized for the remainder of the year. As a result the pipe diameter could be reduced in size because of the lower flow rates expected during most of the year. This would result in an additional reduction in construction costs.

Control Elements

The control elements of the example system are based on level switches within the tank. The system starts with the pump off and the tank filling. When the level in the tank reaches and activates the high level switch, the transfer pump starts. The pump is sized so the flow rate through the pump is greater that the inlet flow rate into the tank, causing the tank level to decrease. When the level in the tank drops and activates the low level switch, the transfer pump stops.

With this simple control, the pump pushes a relatively constant flow rate out regardless of the flow rate into the tank. The pump and system are sized for the maximum flow rate even through the flow rate into the system varies and is less than 200 gpm 95 percent of the time.

Pump Elements

As Table 1 shows, the system was designed to achieve an inlet flow rate of 400 gpm. As a result, the pump design needs to account for the incoming flow plus some additional flow. This allows for a reduction in tank level at the most extreme operating condition. In the example, assume the pump was sized for a flow rate of 125 percent of the maximum flow rate, or 500 gpm. The pump will then operate at this flow rate regardless of the incoming flow to the system.

One way to achieve a more efficient design is to replace the single transfer pump with two transfer pumps sized for half the flow rate and an additional high-high level switch in the collection tank. With both pumps off and flow going into the system, the collection tank level increases.

When the level in the collection tank reaches the high level switch, one of the smaller pumps is started. If the single smaller pump is unable to keep up, the liquid level in the collection tank will continue to increase until the high-high level switch is activated, causing the second smaller pump to start.

With both smaller pumps operating, the level in the collection tank decreases until the low level switch is activated, causing both pumps to turn off. Using this approach, the system will operate more efficiently.

This approach will require the use of two pumps of a smaller size, which will typically cost more than one large pump. However, most plants have operation requirements that the loss of a single item will not affect the system operation. As a result, most plants would have a standby pump installed in the event the primary pump is lost. The system with two half-size pumps would only need a third half-size pump as a backup, which will more than likely even out the
pump costs.

Another option is to change the control system by incorporating a level controller on the collection tank and vary the pump operation using a variable speed drive. Using this approach, the flow rate through the transfer pump is controlled to equal the flow rate into the collection tank. As the flow rate into the tank increases, the level in the tank will increase. This will be sensed by the level control circuit, which will increase the pump speed by means of a variable speed drive controller until the outlet flow equals the inlet flow.

Similarly, if the flow rate into collection tank decreases, the pump’s outlet flow will cause the tank level to decrease. The reduction in tank level will cause the level controller to reduce the speed of the variable speed drive causing a reduction in the pump’s flow rate until the pump outlet flow equals the flow rate into the collection tank.

Conclusion

In this example, we saw how each item in the system was designed or sized based on the design flow rate. This method assures that the system can meet the worst case scenario.

As we saw though, most piping systems operate over a range of conditions, and as such many of the items are oversized for normal plant operation. The individual system elements could be improved, but without a clear understanding of how all the items in a system work together, the changes may not improve the system as a whole.

By completing a computer simulation of a fluid piping system, one can design the system to meet the design case and then look for ways to further improve the design. With this approach, users can reduce capitol cost, operating cost and maintenance cost while improving system uptime.

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