In this article, we will see Liquid Ring Vacuum Pump Working Principle and Pumping system. The Liquid Ring Vacuum Pumps are most widely used vacuum-producing devices in the industry. (The other vacuum producing device mostly used in industires is Ejectors).
Liquid Ring Vacuum Pump Working Principle
The two important things in liquid ring vacuum pumps are used to create vacuum are Liquid Sealent and Impeller.
The vacuum is created in the vacuum pump by using a liquid seal ,the most commonly used liquid sealant is water. The other liquid sealants used in the vacuum pumps are Oil and mixture of Methanol and Water.Before starting the vacuum pump, as a minimum volume of ¼ of the cylinder is filled with liquid sealent.
The impeller is located eccentric to the casing center line of the vacuum pump(refer below picture). Also the impeller is placed between two port plates. The port plates are act as suction and discharge valve of vacuum pump.
When the impeller starts to rotate, the liquid starts to move from the centre line of vaccum pump due to centrifugal force. Because of this an area of void space is created without liquid and the liquid form a ring shape(refer above picture).
The portion between the impeller vane and the liquid is called “impeller cell”. Let us consider an impeller cell at the top of the vacuum pump and the impeller rotates in the clockwise direction (Refer above figure). At the top of an impeller, the cell is filled with seal liquid. Due to the impeller rotation, the liquid recedes from the impeller centre will create a vacuum in the impeller cell. Due to this air or gas/ vapour is drawn from the vessel (the system to be maintained in vacuum) through the Inlet port of Vacuum pump. The inlet ports are located at the side of the impeller.
The impeller cell passes the inlet port and travels toward the discharge port, the sealant liquid is forced back toward the centre hub of the impeller, thereby the trapped gasses are compressed.
When the impeller cell near to the discharge port, the compression is at its highest, and the gases, along with some of the liquid sealant are exhausted through the discharge port to atmosphere. Although the diagrams show a very smooth ring of liquid, but in actual, the liquid sealant is highly turbulent. Due to turbulent the liquid sealants are discharged along with gas. This cycle will repeat again.
Vacuum Pump System
Vacuum pump system refers the arrangement which includes the vacuum pump, separator vessel, necessary pipes and fittings. The vacuum pump systems are classified into three types. They are
- Once-Through System
- Partial Recirculation System
- Total Recirculation System
The selection of Vacuum pump system depends upon the requirement and application of the pump to be used.
Refer below picture of once through system. The sealing liquid enters into the vacuum pump is discharge from the pump and sends to a separator vessel. In the separator vessel due to density difference the gas and the liquid are separated. Usually the gases are vented to atmosphere and the liquid is drained.
In this system no recirculation or recovery takes place. This is a common arrangement where conservation or contamination of the seal liquid is not a concern.
- This system is a simple and low initial cost
- Used for less critical service
- Sealing Liquid consumption is very high
Partial Recirculation System
In this system, the seal liquid enters and leaves the vacuum pump in the same manner as the once-through system arrangement (Refer below figure). But in this system, a circulation line is used to connect the separator tank and the vacuum pump.
An additional makeup seal liquid is added in to the system, in order to add an equal quantity of the liquid (plus any condensate) is discharged from the separator tank via an overflow connection. This is to maintain an constant working level of seal liquid in the system.
Fresh makeup sealing liquid is also used to maintain the proper temperature of sealing liquid. It is essential for better pump performance and avoid cavitation. (To read Cavitation in Vacuum pump click here).
This type of system is used where seal-liquid conservation is important (up to 50% reduction in sealing liquid consumption is possible, and if other than water is utilized, the consumption can be reduced more than 50%, depending upon the fluid vapour pressure and temperature).
- This system is a simple and low initial cost when compared with a total recirculation system
- Consumption of sealing liquid is low.
Total Recirculation System
In this system the sealing liquid is totally re-circulated within the system. The heat generated in the vacuum system is removed by a heat exchanger and the seal liquid will be reintroduced into the vacuum pump.
In this system, the sealing liquid level in the recirculation tank should be at, or slightly below, the centerline of the pump shaft. Provisions may also be made for high-level overflow and low-level makeup on total recovery systems. These level controls help prevent the starting of the pump with the casing full of water since this could overload the motor and damage the pump. If the pressure drop across the system is very high then a circulation pump is included in the system.
- This system is used for high critical gas/ vapour services.
- Consumption of sealing liquid is very minimum.
- Initial cost and maintains of the system is used high.