In this article, we see the Liquid Ring Vacuum Pump Working Principle and Pumping system. The Liquid Ring Vacuum Pumps are the most widely used vacuum-producing devices in the industry. (To read about the Ejector system click Here)
Liquid Ring Vacuum Pump Working Principle
The vacuum pump consisting an impeller which is located eccentric to the cylinder body(Vacuum pump housing). The vacuum is created in the vacuum pump by using a liquid seal. The most commonly used liquid sealant is water. The other liquids sealants used in the vacuum pump are Oil and water-methanol mixture.
Before starting the vacuum pump, the liquid(sealant) is filled the minimum of ¼ of the cylinder volume. In the vacuum pump, the impeller is placed between two port plates. The port plates act as suction and discharge of vacuum pump, which has shaped holes cut into them called ports
When the impeller starts to rotate, the liquid starts to move outward by centrifugal force. Due to this an area of void space is created without liquid and the liquid form a ring shape(refer below figure).
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 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 which is to be maintained at vacuum through the Inlet port at the sides of the impeller. After 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, creating the compression step.
As 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 thru the discharge port to atmosphere. Although the diagrams show a very smooth ring of liquid, in actuality, the liquid sealant is highly turbulent, which is why some of the liquid sealants are discharged with the gases. Again the cycle is repeated.
Vacuum Pump System
Vacuum pump system refers to the total arrangement of 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 /application the pump is used.
Once-Through System
In the once-through system, the sealing liquid enters into the vacuum pump. The liquid discharge from the pump sends to a separator vessel for removing the gas from the liquid and then 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.
Advantages
- This system is a simple and low initial cost
- Used for less critical service
Disadvantages
- 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 separator tank is used for recirculating the seal liquid with additional makeup water is introduced in the separator tank. Based on the quantity of the makeup water added an equal amount of liquid (plus any condensate) is discharged from the separator tank via an overflow connection to maintain the working level in the same horizontal plane as the pump shaft centre line.
Fresh makeup sealing liquid is introduced in sufficient quantity to maintain the proper temperature that is essential to good pump performance and avoid cavitation.
This type of system is used where seal-liquid conservation is important (up to 50% reduction in freshwater 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).
Advantages
- This system is the 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 in the system. Due to re-circulation, the sealing liquid picks up more heat, as a result, Cavitation is occurred in Vacuum Pump. To cool the seal liquid, a heat exchanger is used to cool down the Sealing liquid and 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.
Advantages
- This system is used for high critical gas/ vapour services.
- Consumption of sealing liquid is very minimum.
Disadvantages
- Initial cost and maintains of the system is used high.