Vacuum System Fix Fizzles

Use of scavenged equipment leads to problems.

By Andrew Sloley, Contributing Editor

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At 55 gpm of inlet fluid, the LRPs would work more like washing machines than vacuum pumps. They would exhibit excessive power demand and poor vacuum, and experience vane damage in short order.

Figure 1 also shows the pressures gathered when checking the vacuum system. The seal-oil supply pressure, upstream of a manual globe valve controlling seal oil flow to LRP1, is 13 psig. The pressure in the outlet drum is 4.5 psig. The valve V1, a regulating valve in the water system but now a gate valve, was open. The line originally for recirculation was now for seal oil bypass. Roughly 10 gpm of seal fluid was going to LRP1 and 45 gpm was bypassing it.

Attempting to close the bypass line revealed problems with the seal-oil return pump P2, a scavenged unit with a 60-gpm BEP and 660 ft of dynamic head. It had suction recirculation problems if its feed rate was too low. An astute operator had addressed this by opening valve V1 to create a bypass flow of 45 gpm. P2 now worked correctly.

P2 was a poor choice. Its capacity is too high for efficient use in the service. P2 also has a second problem: its 660 ft of head vastly exceeds what's needed. The return requires less than 50 ft of head to get back through the system to the supply tank at 10 gpm.

Using a variable speed motor on P2 provided a solution, albeit not a perfect one. Lower speed reduced the suction recirculation problem, allowing closing of the bypass, and also decreased pump head. P2 still isn't a great choice but at least now suffices. Both the rate and head reductions cut energy costs. These energy savings of 13 hp alone paid for the new variable speed control system.

This example provides a general lesson: higher installation and excessive energy costs over many years can outweigh any capital savings from re-using idled equipment. Before re-use, always consider whether the unit will be operating far from its normal performance envelope.

ANDREW SLOLEY, Contributing Editor

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