Graham Corporation Technical Article CASE STUDY: Liquid Ring Vacuum Pump Cavitation and Instability

CASE STUDY: Liquid Ring Vacuum Pump Cavitation and Instability

May 2023

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Liquid ring pumps are both rugged and reliable, but there are process conditions that if allowed to persist will damage them. Two of the more common issues that face liquid ring pumps are cavitation and instability issues at low loads. At lower loads, the water ring in the liquid ring pump will boil and damage the pump. Low loads can also cause the water ring to become unstable which can also be damaging to the pump. This article is going to cover an example of cavitation and instability, along with how it was resolved.

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CASE STUDY: Liquid Ring Pump Cavitation and Instability

Installed at a power plant was a turbine exhaust steam surface condenser and air venting ejector system, alongside a pair of air venting LRVP skids. The site had both LRVPs and air removal ejectors because the ejectors would not have steam to run until after the turbine was running as they used extracted steam. The liquid ring pumps were designed to act in place of the steam ejectors when needed and for system hogging at start up. Due to installation issues with the piping and motive steam, the air removal ejectors were not functional when the equipment was initially started, forcing the site to rely on the liquid ring pumps.

Upon starting the system, the liquid ring pump would run as expected for the first 20-25 minutes as the vacuum pulled in. Once the turbine was brought up to speed and the vacuum pulled down to design, the pump would become very noisy and had high vibrations. After only 3-4 weeks of operation, the first pump failed. It was removed from the installation and opened for an inspection.

The evaluation of the pump found that the mechanical seals had failed and the impeller showed signs of cavitation damage around the impeller hub and port plates. The pump needed to be rebuilt before it could be placed back into service. With a lead of 5-6 weeks for parts and shipping time, there was concern about operating the remaining liquid ring pump as the site no longer had a functional back up and its entire operation relied on the remaining pump.

Graham Corporation’s service team was dispatched to the site to help troubleshoot the issue. When the backup pump was started, it was stable for the first 20-25 minutes, but the pump became noisy and ran rough the same as the first pump had. Pressures and temperatures were taken at the pump’s suction, discharge, and seal liquid supply. The vacuum was measured to be 28.7inHgV (1.2 inHgA) and the seal water temperatures were 73 ºF in and, the outlet temperature was 85 ºF. The boiling point of water at 1.2 inHgA is 85 ºF. This means that the seal water temperature was limiting the vacuum as it was boiling off and preventing the pump from pulling down to a deeper vacuum. This is what had caused the cavitation damage to the impeller hub of the first pump. See our previous article that was written about cavitation.

Cavitation, Liquid Ring Vacuum Pumps Article Link

A measurement of the air leakage rate into the system showed a rate that was near 16 SCFM with the pump capacity sized for 194 SCFM. The main condenser was running at a loading of 130,000 lb/hr of steam, which is only 52% of its 252,000 lb/hr design. The low loading and low air leakage rate means that the vacuum could pull down below its design and below the vapor point of the seal water. This is a common issue for liquid ring pumps in this service.

The low air leakage rate was also causing instability issues. Without a noticeable source of air leakage the pumps seal ring became unstable. This was not present initially when the system was started as the large volume of air being evacuated meant substantial noncondensable flow to the pump’s suction. Once that air was fully evacuated and the flow rate into the liquid ring pump matched the air leakage rate into the system, the water ring inside the pump became unstable. Without the water ring being concentric, a hydraulic force was placed on the pump that caused it to vibrate and produce an audible groaning. This force over time is what caused the pump’s mechanical seals to fail. See pictures below of Graham’s glass faced liquid ring pump, highlighting this phenomena.

Stable Operation

Unstable Operation

The solution to this problem was very simple. The suction line of the pump had an isolation valve and test connection for local instrumentation alongside the transmitter. That valve was slowly opened allowing air to bleed into the pump’s suction. The pressure at the pump suction slowly degraded as this was done. The pressure was set to 1.7 inHgA by manually controlling this valve. This had a twofold effect. At the suction pressure of 1.7 inHgA, a seal water hotter than 96 ºF would be needed at the pump’s discharge in order for the pump to cavitate, meaning the pump was no longer cavitating at a seal water temperature of 85 ºF. The additional airflow also allowed the water ring inside of the pump to stabilize, which prevented the high vibrations and associated wear on the pumps bearings and mechanical seals.

While the problem was fixed in the field by opening the valve, it was understood that this was a manual process. If an operator was unware of why that valve was open, the valve could manually be closed. As a long-term fix, a low-pressure relief valve was added. This allowed air to automatically be pulled into the pump’s suction when the pump reaches the set pressure of 1.7 inHgA without manual adjustment by operations.