The service liquid utilized in one of these pumps needs to be subjected to a certain set of conditions for the pump to cavitate. The most common service liquid used in these pumps is water, although there are pumps that have been specifically designed to utilize oils or other fluids. If a re-circulation service liquid system is used, the vapor pressure of the resulting mixture must be used when evaluating cavitation. We are going to refer to pumps that utilize water for the service liquid in the below example. In order for the water ring to vaporize inside of the pump, it needs to be exposed to the right conditions. Water will have a boiling point and that boiling point will vary with two things, pressure and temperature. The suction pressure that a liquid ring pump develops is controlled by the pump’s suction load. The temperature of the water ring is controlled by the supply temperature and flow rate of the pump’s service liquid. If the pump’s suction pressure is lower than the corresponding boiling point of water at the pump’s discharge temperature, then the service water would be flashing inside of the pump causing cavitation. In a liquid ring pump, the process and service liquid are both discharging at a common location, the pump’s discharge connection.  Measuring the temperature at the pump discharge connection will give you the hottest or worst case service liquid temperature.

Knowing the pump’s suction pressure and the service liquid inlet and outlet temperatures, one can then determine whether or not the pump is cavitating. By determining the vapor pressure at the pumps outlet temperature and applying a safety factor, the vacuum at which the pump will cavitate at can be calculated, see the example below.

Example: (Cavitation Calculation)

Measured Service Water Supply Temperature = 81 ºF

Taken at service liquid supply line

Measured Service Water Outlet Temperature = 89 ºF

Taken at liquid ring pump process/service liquid discharge connection

Calculated Service Liquid Temperature Rise = 8 ºF

Difference between supply temperature and discharge temperature

 

Vapor Pressure of Water at Outlet Temperature of 89 ºF = 35 mmHgA

 

Apply a Safety Factor:

35 mmHgA + 5 mmHgA = 40 mmHgA

 

At the current service liquid conditions, the pump is in danger of cavitation if the pump’s operating vacuum is deeper than 40 mmHgA.

A pump’s load can change over time, which impacts the pump’s suction pressure. The service liquid supply temperature can also change over time, which impacts the service liquid’s boiling point. If the calculation above is done at lowest load (deepest suction pressure) and hottest month of the year (warmest service liquid supply temperature), it should adequately protect the pump from cavitation.

If the pump pulls down below the vapor point of the service liquid and more or colder service liquid is not available, then you need to approach the issue from a different perspective. In this instance, the suction load to the pump can be manipulated via an air bleed, if the process allows, or a recycle line which would bring process from the discharge back to the pump’s suction. By manipulating the load you can control the pump’s suction pressure, raising the suction pressure to prevent cavitation.