Vacuum System Fix Fizzles

Use of scavenged equipment leads to problems.

By Andrew Sloley, Contributing Editor

A liquid ring pump (LRP) actually is a type of compressor. The pump name comes from its common historical use as a vacuum pump. The LRP has an eccentric-mounted rotor with integral vanes that reach into a liquid ring. The vanes create a series of spaces between the rotor and liquid. As the rotor spins, the larger spaces between the vanes are compressed as they move from an area where the rotor is far from the liquid to where it's close. Ports allow vapor to be drawn in and expelled in the correct locations.

The units would work more like washing machines than vacuum pumps.

Some seal fluid also leaves the LRP through the outlet ports. So, maintaining the liquid ring in the pump requires continuous addition of seal fluid. Downstream of the LRP, a separator drum recovers the seal fluid. Systems can use once-through seal fluid flow, partial recycle or full recycle with partial flush. Recycling systems normally include a heat exchanger to cool the seal fluid.

Water most commonly serves as the seal fluid, with hydrocarbon oils probably second in popularity. However, you can use nearly any fluid compatible with the process vapor and system materials.

Seal-fluid vapor pressure is the most common restriction on a LRP vacuum. When pump suction pressure drops to the seal-fluid vapor pressure, the seal fluid starts to vaporize. The lower vapor pressure of many oils permits drawing a deeper vacuum.

One plant using water as the seal fluid required a deeper vacuum due to process changes. Moving to oil allowed for better performance. However, staff didn't understand all the consequences of the change. Oil use was exceptionally high, resulting in significant costs. Fixing the problem demanded some modifications.

Figure 1 depicts the system after modifications and gives some operating data. The seal-oil supply pump provides oil for two identical liquid ring pumps (LRP1 and LRP2) plus some other users. LRP1 and LRP2 have identical configurations, and each gets roughly 55 gpm of seal oil. The other users consume 15 gpm.

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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