Vacuum Pumps and Seal Oil System
Figure 1. Revamp led to far more flow than the liquid ring pumps could properly handle.
When water served as the seal fluid, LRP1 required nine gpm of seal water. Adding make-up water to the system prevented the seal water from getting too hot. (The process gas entering the LRP directly contacts the seal fluid and heats it.) At low make-up rates, the seal water over-heated and the pump couldn't always provide desired process vacuum. To get a -14-psig suction pressure, seal water temperature had to remain below 80°F — this wasn't possible during the summer.
To help maintain vacuum, the plant switched to a once-through system using water taken directly from its cooling tower. Mixing the process gas with the water put impurities into the water, so it now had to go to treatment. The 18 gpm of water (from both LRPs) taxed water treatment capacity. Additionally, on the hottest summer days, water temperatures still could exceed 80°F.
So, the plant decided to switch to oil as the seal fluid. It used an existing out-of-service pump to supply the seal oil and added a second scavenged pump to return the oil. This change and some minor tweaks seemed to solve the vacuum system problems.
As part of other work, the plant had to check the capacity of the seal-oil supply pump (P1). The total seal oil rate needed was 33 gpm (9 gpm for each LRP and 15 gpm for other users). However, P1 had a best efficiency point (BEP) of 160 gpm. At 33 gpm, it should suffer severe maintenance problems. Instead, it ran fine. Investigation showed P1 was pumping 125 gpm — other users received 15 gpm while each LRP got 55 gpm.
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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