THIS MONTH'S PUZZLER
Our liquid ring vacuum pump repeatedly tripped during high pressure, high flow conditions at startup. When it finally started, the pump ran noisily. On inspection, we noticed it was installed backwards. We reversed the direction of the pump but got only low vacuum and the motor became very hot — exceeding the temperature ratings on the motor insulation. We're operating at 300 torr with water. Why did the pump operate, though noisily, with the flow reversed? What is causing it to run so poorly now? Is the pump shot?
TAKE A NUMBER OF STEPS
Installing and operating a liquid ring vacuum pump (LRVP) backwards will severely impact its operation. On these machines, the vapor inlet port is several times larger than the outlet port. When operated backwards, the smaller discharge port (now acting as the inlet) will cause a restriction and impede the flow of vapors into the LRVP. The resulting turbulence will create unusual noises.
After the rotation was corrected you report the motor became very hot. Assuming there are no problems with the motor itself, high motor temperature would indicate high horsepower draw. Amperage readings can be taken off the motor leads to verify the approximate horsepower draw.
High draw is often the result of: 1) high discharge pressure; 2) mechanical problems such as a bad bearing, foreign object or product buildup inside the machine; or 3) high seal liquid flow rate.
First, verify the discharge pressure is within design by checking the pressure at the separator vessel. Be sure to check the pressure during startup, as the higher mass flow rates can result in high pressure downstream of the LRVP.
To check for internal mechanical problems without disassembly of the LRVP, remove the coupling and rotate the pump by hand. Check for severe binding or scraping, or other problems. Some scraping, especially if you can still rotate the pump by hand, is not unusual and can be ignored. However, a stainless pump can't tolerate any scraping. If stainless, have the pump repaired immediately.
If the above checks are okay, you may have too high a seal flow rate. Install a regulating valve, compound pressure gauge (or better yet a flow meter) and temperature gauge in the seal liquid piping near its entrance into the LRVP. With the pump operating in vacuum service, the seal liquid pressure reading should typically be close to 0 psig. Compare your readings against the manufacturer's recommendations.
If necessary, throttle back the seal liquid flow and you should see improvement in the power draw. As a final check, look at the temperature rise of the seal liquid. Compare the seal liquid inlet with the LRVP discharge. A temperature rise of 10–15°F is typical. Readings well outside these values should be further investigated.
"Is the pump shot?" Once the horsepower issue is solved, you can concentrate on performance. However, more information is needed. How does the operating level of 300 torr compare to design? How does the mass flow rate from your process compare to design? Is the process load constant or does it change during operation? How accurate is the pressure gauge that is being used? Very important and often overlooked: the pressure gauge must be an absolute pressure gauge, and must be recently calibrated.
If the mass flow rate from the process can't be measured, can the non-condensable flow rate be measured downstream of the LRVP, perhaps by adding a rotameter at the separator discharge? Do this in a manner that the rotameter is only used to check capacity, as hard piping it can lead to backpressure, especially during startup.
Finally, an off-line capacity test can be performed. Disconnect the LRVP suction from the process and run a dry air test using an appropriately sized set of orifices. Compare the test results to the book performance curve to evaluate the condition of the LRVP.