What is priming in centrifugal pumps?
Priming is a technique used to drive away the air entrapped in the suction line of a centrifugal pump. If the air present in the suction line are not removed the pump won’t be able to suck the liquid from the reservoir as air is lighter medium whereas liquid is heavier medium.
So the whole suction line and the part of casing is filled with water and the air is removed via the air vent. Also if air is allowed to enter to the impeller they will damage the impeller by flashing (cavitation).
Normally cavitation won’t occur in pump which is continuously in service as there is a li즈le chance for the air to get into the suction
line.
Unlike a positive displacement pump that can pump a liquid to any head as long as the pump body is strong enough, and there is enough horsepower available, the centrifugal pump can only pump a liquid to its rated head.
You’ll recall that this head was determined by, and limited to the diameter of the impeller and the impeller speed (rpm.) Since the weight of water is approximately 8000 times that of air (50 miles vs. 34 feet or 80 Km. vs. 10 meters) the centrifugal pump can produce only 1/8000 of its rated liquid pressure. In other words, for every one foot water has to be raised to prime the pump, the
centrifugal pump must produce a discharge head of approximately 8000 feet (each meter requires a head of 8000 meters) and that is impossible with conventional impeller diameters and speeds.
All of this means that if you intend to use a centrifugal pump you’re going to have to come up with some sensible method of priming it.
Your choices will include:
a) Install a foot valve in the suction piping to insure the liquid will not drain from the pump casing and suction piping when the pump stops. Keep in mind that these valves have a nasty habit of leaking.
b) Evacuate the air in the system with a positive displacement priming pump operating between the pump and a closed discharge valve.
c) Fill the pump with liquid prior to starting it.
d) Convert the application to a self priming pump that maintains a reservoir of liquid at its suction.
Priming is a technique used to drive away the air entrapped in the suction line of a centrifugal pump. If the air present in the suction line are not removed the pump won’t be able to suck the liquid from the reservoir as air is lighter medium whereas liquid is heavier medium.
So the whole suction line and the part of casing is filled with water and the air is removed via the air vent. Also if air is allowed to enter to the impeller they will damage the impeller by flashing (cavitation).
Normally cavitation won’t occur in pump which is continuously in service as there is a li즈le chance for the air to get into the suction
line.
Unlike a positive displacement pump that can pump a liquid to any head as long as the pump body is strong enough, and there is enough horsepower available, the centrifugal pump can only pump a liquid to its rated head.
You’ll recall that this head was determined by, and limited to the diameter of the impeller and the impeller speed (rpm.) Since the weight of water is approximately 8000 times that of air (50 miles vs. 34 feet or 80 Km. vs. 10 meters) the centrifugal pump can produce only 1/8000 of its rated liquid pressure. In other words, for every one foot water has to be raised to prime the pump, the
centrifugal pump must produce a discharge head of approximately 8000 feet (each meter requires a head of 8000 meters) and that is impossible with conventional impeller diameters and speeds.
All of this means that if you intend to use a centrifugal pump you’re going to have to come up with some sensible method of priming it.
Your choices will include:
a) Install a foot valve in the suction piping to insure the liquid will not drain from the pump casing and suction piping when the pump stops. Keep in mind that these valves have a nasty habit of leaking.
b) Evacuate the air in the system with a positive displacement priming pump operating between the pump and a closed discharge valve.
c) Fill the pump with liquid prior to starting it.
d) Convert the application to a self priming pump that maintains a reservoir of liquid at its suction.
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