Pump Priming is a manual or automatic process by which air present in a pump and its suction line is removed by filling liquid. In the pump-priming process, the pump is filled with the liquid to be pumped and that liquid forces to remove the air, gas, or vapor present. With the exception of a few self-primed pumps, mostly all pumps are primed. Before starting a pump, Pump Priming is the most important first step and it avoids the majority of the pump problems.
Reason for Pump Priming
During starting a pump, if air, gas or vapor exists inside the pump casing, the pump will not be able to function properly. Pump will be subjected to the risk of damage. The air or gas present inside the pump will make it gas bound and the pump won’t be pumping the desired liquid. Pump will get overheated and it will damage pump internals.
To reduce the risk of pump damage and reliable operation, the gas present in the pump must be removed. So, the pump must be fully primed.
When is Pump Priming Required?
The main objective of priming a pump is to remove the gas present. So, if the air or other gases present inside the pump casing and suction line, it must be primed before starting. But if the pump suction line and the casing are already filled with liquid during start-up, priming is not required. The main reason behind locating most centrifugal pumps below the liquid source level is that the pump remains primed automatically. Refer to Fig. 1 below that clearly explains the requirement of priming a pump.
In general, Centrifugal Pumps need priming. Submersible Pumps or vertical sump pumps do not require priming. Positive displacement pumps are considered self-primed pumps.
What is Self-Priming?
A self priming pump is a specially designed end suction centrifugal pump with an external casing that always “floods” the inner pump or volute. Self priming pump has the ability to evacuate air from the suction side at startup and then it operates similar to a normal pump. The external casing is filled with liquid and the pump is always ready to start. When the impeller of a self priming pump rotates inside the casing, a low pressure area (below atmospheric pressure) is formed at the eye of the impeller. As a result, the liquid is pushed up the suction pipe by atmospheric pressure along with the the air present in the suction pipe.
This air is mixed with the recirculating fluid inside the casing. The air then separates out from the liquid and is discharged from the casing. Once, all the air from the suction pipe is removed, the pump operates dynamically like any other centrifugal pump. So, A self-priming pump can lift fluid from a level below the pump and evacuates the air of the suction line without using other external auxiliary devices.
Pump Priming: Centrifugal vs Positive Displacement Pumps
Normally, Centrifugal pumps need priming and Positive Displacement Pumps (Rotary Pumps, Reciprocating Pumps) does not require priming. However, for the first time operation all pumps need priming to avoid overheating and failure by dry running condition.
In a centrifugal pump, liquid is pushed from suction to discharge. The pump works by the transfer of rotational energy from the impeller to the liquid. In between the suction and discharge sides of the pump, there are no seals. For this reason, when the liquid level is below that of the impeller, centrifugal pumps are ineffective with gases and incapable of evacuating air from a suction line. So centrifugal pumps must be primed for proper working.
On the other hand, all positive displacement pumps use close-tolerance parts to prevent fluid from returning from the discharge to the suction side. Hence, a positive displacement pump is capable of venting air from its suction line to some extent.
Pump Priming Methods
Pump can be primed by layout considerations or using external arrangements. Few of the external pump priming methods are:
- Natural Priming
- Manual Priming
- Priming a pump with Vacuum Pump
- Pump Priming with Jet Pump
- Pump Priming using a Separator
- Priming a pump by Installing a Foot Valve
- Pump Priming with Ejector
Natural Pump Priming:
Natural pump-priming can be achieved by maintaining the impeller eye below the surface of the water. So, naturally, water will flow into the suction pipe and casing removing all the air present by gravitational force as shown in Fig. 2
Manual Pump Priming Method:
In the manual method (Fig. 3) of pump priming, pumping liquid is filled in the pump suction by manually pouring liquid directly in suction using a funnel. The pump is manually primed due to gravity feed and the air present escapes through the air vent valve.
Priming with a Vacuum Pump:
To prime, the main centrifugal pump, An additional small size vacuum pump or a self-priming pump, or a positive displacement pump is used. The discharge line of the main pump is connected to the suction line of the positive displacement priming pump that evacuates all the air in the primary pump and suction piping.
Priming with Jet Pump:
Water available at high head is allowed to flow through a nozzle in this method of pump priming. The nozzle is designed in such a way that at the jet outside the nozzle the pressure is less than the atmospheric pressure which causes water to be sucked from the sump.
Priming with Separator:
On the discharge side of the pump, an air-water separation chamber or separator is provided for pump priming with separator. A bent suction pipe portion is provided at the inlet of the pump that always maintain some liquid in the pump. Through pump discharge or air vent, Air is separated and expelled and the liquid falls back into separation chamber due to higher density.
Priming by Installing Foot Valve:
A foot valve that acts like a non-return valve is installed in the suction piping in this pump-priming method. The foot valve does not allow the liquid to drain from the pump casing and suction line once the pump operation is stopped. So, while starting the pump for the next operation, the pump is already primed and can work.
Priming with Ejector:
In this pump-priming method, an ejector (Fig. 7) is provided on the pump suction (or discharge) side. These Ejectors create a vacuum inside the pump suction line forcing the liquids to draw from the sump up to the pump elevation. However, Ejectors require energy input for their work.