How to Control Liquid Ring Vacuum Pumps
Liquid ring vacuum pumps are versatile machines because they can handle “wet” loads and can operate down to around 1” HgA. They require proper operation and monitoring, however. This article will address several monitoring and troubleshooting situations involving liquid ring vacuum pumps (Figure 1).
A check valve will prevent a backflow of gases at start-up or shutdown of the system. When choosing a check valve, a low pressure drop type should be considered to minimize pressure loss in the system. A butterfly style check valve normally has the lowest pressure drop.
Typically for pressures of 100 mmHgA and less, some sort of anti-cavitation or suction pressure control is needed. The simplest type is a manual bleed valve such as a globe valve. If automatic opening and closing is required, then an inexpensive spring operated diaphragm valve can be used. In this design, the suction pressure opposes the spring. When the vacuum in the sensing line pulls the diaphragm down, air will be bled in to restore the pressure. Barometric pressure will affect the setpoint range, so periodic adjustment may be required. By bleeding air in on reduced load conditions, this device will prevent the pump from cavitating, but it will not accurately control suction pressure.
For accurate control of suction pressure, an absolute pressure transmitter, a pressure controller and a recycle valve (typically a
pneumatic diaphragm operated globe valve) should be considered. This combination will maintain a constant suction pressure by recycling non-condensables from the separator discharge to the vacuum pump suction.