Vacuum control in a liquid ring pump system is best achieved by recycle control. In this arrangement, control is achieved by taking the noncondensable load from the discharge separator’s vapor outlet and recycling it back to the LRVP's suction. Recycling the noncondensable load back to the pump’s suction will allow the pump’s suction pressure to be varied in accordance with the pump’s performance curve. With feedback from an absolute pressure transmitter, the control can be automated to maintain a specific set pressure at the pump’s suction. Because this control arrangement does not introduce any external load, it’s often used when the process load cannot be mixed with air or when the off‐gas flow needs to be treated or processed. This is the most common arrangement in most refining and chemical applications.

Bleed control is similar to recycle control with the exception that recycle gas is not used. By bleeding in a noncondensable load to the pump’s suction, the pump’s suction pressure will move accordingly on its performance curve. With feedback from an absolute pressure transmitter, the control can be automated to maintain a specific suction pressure. This control arrangement often utilizes atmospheric air as the bleed’s load source, which has no cost. Some processes will not allow air to be mixed with the process load, which include most refinery and chemical applications. In these instances, nitrogen, some other inert gas, or recycle control is best. If nitrogen or other gases are used, the bleed control arrangement has an operating cost which should be taken into consideration. Also, if the off‐gas is being sent to an off‐gas processing system, the additional load from the bleed control will result in an increase in the off‐gas flow rate which could have treatment and processing costs.

Unlike other commonly used pumps, like a centrifugal pump for example, the use of a VFD is not recommended for use with liquid ring pumps. While some level of vacuum control can be achieved by using a variable frequency drive or VFD, the inherent properties of the liquid ring pump limit its effectiveness. There are upper and lower limits to a liquid ring pumps RPM range. Running the pump above its design speed will result in increased horsepower that can overload the pump's motor, or exceed the mechanical limits of the pump's design, potentially resulting in damage to the pump.

Liquid ring vacuum pumps also have a minimum speed that must be maintained. At lower speeds the pump will not have enough energy to maintain its internal liquid ring. The result is that the ring will collapse and become unstable, causing a complete loss of performance and high vibrations. Pump turndown is often limited to 50-60% of design. While VFD's may be effective within this narrow operating range, they are the most expensive option with regards to vacuum control and offer the most limited range of control and are therefore not recommended for use in most liquid ring pump applications. The other control options are more effective and come with a lower cost.

Vacuum control is important when protecting a liquid ring vacuum pump from cavitation. By introducing a load either via a bleed connection or a recycle line, the suction pressure of the pump can be prevented from pulling down into the cavitation range. The cavitation pressure of a liquid ring vacuum pump is determined by the vapor pressure of the service fluid at the service liquid outlet temperature. That outlet temperature is a product of the service liquid supply temperature, service liquid flow rate, and heat load on the pump due to compression and any condensable loading. Because of these variables, the cavitation point will be different for each installation and should be calculated on a case by case basis. A previous article was written showing how that can be achieved and a copy of that article is available via Graham’s website. Please contact Graham if you have any questions on cavitation.

In order to prevent cavitation, feedback from the pump’s pressure transmitter can be used to limit how deep a vacuum the pump pulls. Pressure limits can be included in the pump’s control logic, preventing the pump’s suction pressure from being set at a point lower than the calculated cavitation point. As long as that value is selected correctly and the transmitter is calibrated on a regular basis, the pump will then be protected from cavitation damage.

Graham also offers a lower cost analog solution for installations that do not utilize a pressure transmitter with feedback. In these instances, Graham can provide a low pressure relief valve. At a suction vacuum set by the valve’s adjustment, the valve will open allowing air to be pulled into the system. This sets a limit as to how deep of a vacuum the pump can pull, which in turn protects the pump from cavitation damage. This arrangement does not offer precise vacuum control, and should be used in applications where atmospheric air can be safely introduced into the process stream.