Introduction to Priming in Pumps
First of all lets try to get a brief of What is a Pump?
Pump is a machine or mechanical equipment which is required to lift a fluid (liquid, semi-solid, gas, steam etc) from low level to high level or to flow fluid from low pressure area to high pressure area or as a booster in a piping network system. Principally, pump converts mechanical energy of motor into fluid flow energy.
- For introduction to basics on pumps, check out Introduction to Pumps.
Now assuming that you have briefed yourselves about basic of pumps lets get to our present topic – Pump Priming.
Pump Priming is the process of removing air from the pump and suction line. In this process the pump is been filled with the liquid being pumped and this liquid forces all the air, gas, or vapor contained in the passage ways of pump to escape out. Priming maybe done manually or automatically. Not all pumps require priming but mostly do. There are Self Priming Pumps and also some layout situations where priming is not required. Same will be discussed in this article as it progresses.
Priming a pump is probably the first and one of the most important thing one should do before operating it. Not priming a pump or not doing it properly makes majority of pump problems. Any problem in pump due to lack of priming may cause financial impact due to pump maintenance and the downtime of piping system due to a malfunctioning pump.
Why Priming is Required?
Priming reduces the risk of pump damage during start-up as it prevents the the pump impeller to becomes gas-bound and thus incapable of pumping the desired liquid.
For reliable operation, pumps must first be primed; that is, air or gases to be expelled from the suction and impeller eye area and replaced with liquid to be pumped. The pump would not function properly when not completely filled with liquid. Along with compromised performance, not priming the pump and allowed to run without fluid, it will overheat the pump system and there will be a danger of damage to critical internal pump components.
Priming – Centrifugal Pump vs Positive Displacement Pump
In principle, all Positive Displacement Pumps are self-priming. In particular, this includes different type of rotary and reciprocating pumps. The priming of Positive Displacement Pump is required only at the time of first starting as under dry running conditions the pump may overheat. But in a Centrifugal Pump (except Self Priming Pump) priming is required in starting after every shutdown.
Centrifugal Pumps are designed to pump liquids not gases. Centrifugal Pump can not suck the liquid, but it pushes the liquid from suction to discharge. Due to pressure difference created by the liquid pushed to the discharge with an additional push on liquid from the atmospheric pressure in the storage tank connected to pump suction piping, more liquid enter in the suction side of pump provided suction line is completely filled with liquid (primed). Its sort of that pushes the liquid out and pulling effect is not so prominent. During the start up of the pump if any air pocket is present at the suction side, then pump will push the air. As a result air present in the suction side will try to expand and it will block the liquid from entering into the centrifugal pump.
Also explained in other words, in Centrifugal Pump the head developed (in meters of liquid that is pumped) depends on the velocities determined by diameter of the impeller and the impeller speed (rpm.). As the pressure developed is related to the head by the equation head = pr / sp. weight, the pressure available will be proportional to the specific weight of the liquid. This means that the pressure (or pressure difference) created with air will be only around 1/800 times that with water (density of water = 1000 kg/ m3 and dry air at S.T.P has a density of 1.2 kg/m3 ). Therefore, if the pump is not primed, the suction pressure created will not be sufficient to lift water.
Whereas in Positive Displacement Pump, during suction phase, piston moves backward and form a low pressure zone in the pump. This pressure difference between suction & storage tank is large enough to pull the liquid, even if air pocket is present in the suction line. In short, it creates a high initial vacuum during the start of suction stroke. Positive displacement pumps can evacuate all the air in its cylinder by virtue of its motion and therefore a better pressure (vacuum) is also generated. So we need not have priming operation in positive displacement pumps.
Also a common feature of all Positive Displacement Pumps is the use of close tolerance parts to prevent fluid returning from the discharge to the suction side. Depending on the effectiveness of these seals created by these close-tolerance parts, a positive displacement pump is capable of venting air from its suction to discharge and prevent the vented air from returning back. Whereas in Centrifugal Pumps, the pumping action is generated by the transfer of rotational energy from the impeller to the liquid. There are no seals between the suction and discharge sides of the pump making it ineffective with gases.
With Positive Displacement Pumps, there is a danger of cavitation occurring at the point when liquid starts to enter the pump and there is a liquid/air mixture. Under these conditions, vapor bubbles form and expand on the suction side of the pump. Upon reaching the high pressure, discharge side of the pump, the bubbles collapse violently causing vibration and damage to the pumping elements. For these reasons, it is important to refer to the manufacturer standard and operating procedure before using a positive displacement pump in an application where it must self-prime and, of necessity, be run dry for any period.
However, with a few modifications to the basic design, a centrifugal pump can be made Self Priming. The details of Self Priming Pumps will be discussed in this article later on.
When Priming is Not Required?
Priming is only not required when the pump is either capable of removing air and gases from itself (also known as Self Priming Pumps) or the layout conditions are so much favorable that the pump will be always completely filled with the liquid to be pumped. Few such conditions are detailed out below.
- Priming is not required when pump is submerged (Submersible or Vertical Sump Pumps).
- Priming is not required when the pump is at a lower elevation than the supply and this ensures that pump suction will be completely filled with liquid at all times (known as “Flooded Suction Condition”).
- Self Priming Pumps.
Methods of Priming
Priming of a pump can be achieved by either layout consideration, or by means of some external arrangements that ensures priming or by use of Self Priming Pumps. Few of the external arrangements that ensures priming of a pump are detailed out below.
- With Vacuum Pump
- With Jet Pump
- With Separator
- Installing Foot Valve
In this method of pump priming, liquid is poured in the pump suction. This can be achieved by pouring liquid directly in suction or with the help of other devices like a funnel and the pump will be manually primed with a gravity feed. While priming is being done, all the air escapes through air vent valve.
Priming with Vacuum Pump
In this method of pump priming, a small size vacuum pump or self priming pump or a positive displacement pump is being used for priming the main centrifugal pump. The suction line of positive displacement pump is connected to the discharge line of main centrifugal pump. This positive displacement priming pump evacuate all the air in the primary pump and suction piping.
Priming with Jet Pump
In this method of pump priming, water available at high head is allowed to flow through a nozzle. The nozzle is so designed that at the jet outside the nozzle the pressure is less than the atmospheric pressure so it is possible to suck water from the sump.
Priming with Separator
In this method of pump priming, air-water separation chamber is provided on the delivery side of pump and a bent suction pipe portion is provided at the inlet of the pump. Bent suction pipe portion always maintain some liquid in the pump. Air is separated and expelled through pump discharge or air vent and liquid, being heavier than air, falls back into separation chamber.
This design is made part of some self priming centrifugal pumps too. In self-priming centrifugal pumps with a separation chamber the fluid pumped and the entrained air bubbles are pumped into the separation chamber by the impeller action. The air escapes through the pump discharge nozzle whilst the fluid drops back down and is once more entrained by the impeller. The suction line is thus continuously evacuated. This design has two major drawbacks. Firstly reduced pump efficiency and secondly large dimensions due to incorporation of separation chamber.
Priming with Ejector
In this method of pump priming, ejector is provided on the suction side of pump. Ejectors operate by creating a vacuum inside the suction line of the pump. The vacuum draws the liquids from sump up to the pump elevation. Ejectors require a Compressed Air Supply as an energy input.
Priming by Installing Foot Valve
In this method of pump priming, a foot valve (functioning as a NRV) is installed in the suction piping to insure that the liquid will not drain from the pump casing and suction piping once the pump stops operating. A foot valve is a form of check valve installed at the bottom, or foot, of a suction line. When the pump stops and the ports of the foot valve close, the liquid cannot drain back from pump suction if the valve seats tightly. Keep in mind that these foot valves have a nasty habit of leaking.
Prevention of Pump Operation without Priming
For prevention of a pump, where priming is required, operation without being primed various methods are being used. The basic of these methods are to trigger some form of alarm or auto shutdown of pump if the pump is not filled with liquid completely. One such scenario is discussed below.
In some pumps, a form of float switch in a chamber connected with the suction line is being used. If the level in the chamber is above the impeller eye of the pump, the float switch control allows the pump to operate. If the liquid falls below a safe level, the float switch acts through the control to stop the pump, to prevent its being started, to sound an alarm, or to light a warning lamp.
Self Priming Pumps
Self Priming Pumps are designed to have the ability to prime themselves automatically, when operating under a suction lift, to free themselves of entrained air or gases, and to continue normal pumping without external priming. They can be broken down into three basic types:
- Liquid Primed
- Compressed Air Primed
- Vacuum Primed.
Liquid Primed Self Priming Pumps
Liquid Primed Self Priming Pumps have their own in-built or separate liquid reservoir (known as “Priming Chamber”) that must be filled with liquid in order to “self prime” the pump. Without this initial liquid charge filled in priming chamber, a liquid primed self priming pump will not prime or pump. Liquid primed self priming pump generally operate in an air-liquid mixture by transforming this mixture into a fluid that can be pumped without help of any external auxiliary devices. Priming chamber allows liquid primed self priming pumps to recirculate liquid within the pump at will, ridding the pump of the air that prevents it from operating whenever necessary.
A Liquid Primed Self Priming Pump has two phases of operation: “Priming Mode” and “Pumping Mode”. During priming mode, the pump essentially acts as a liquid-ring pump. The rotating impeller generates a vacuum at the impeller’s ‘eye’ which draws air into the pump from the suction line. At the same time, it also creates a cylindrical ring of liquid on the inside of the pump casing. This effectively forms a gas-tight seal, stopping air returning from the discharge line to the suction line. Air bubbles are trapped in the liquid within the impeller’s vanes and transported to the discharge port. There, the air is expelled and the liquid returns under gravity to the reservoir (“Priming Chamber”) in the pump housing. Gradually, liquid rises up the suction line as the air is evacuated. This process continues until liquid replaces all the air in the suction piping and the pump. At this stage, the normal pumping mode commences, and liquid is discharged. If the attached discharge piping does not allow this separated air to escape out to the downstream discharge piping system, a bypass line may be required to evacuate it.
When the pump is shut off, the design of the priming chamber ensures that enough liquid is retained so that the pump can self prime on the next time it is operated. Liquid primed self priming pumps ability to operate in a mixture of air and liquid makes them far more versatile than their non self priming counterparts, which allows them to work in a broader range of environments and industries.
Compressed Air Primed Self Priming Pumps
In Compressed Air Primed Self Priming Pumps, compressed air is blown through a jet into a tapered tube to create a vacuum, so air from the pump casing and suction line is drawn in with the compressed air and exhausted to the atmosphere. A non-return ball check valve seals out air from the discharge, allowing fluid to enter the pump body. Water then replaces the air which allows the pump to begin pumping. This pump type also avoids the potential build up of solids, since it has no priming chamber, so it can be used for sewage applications, plus it has dry running capability.
Vacuum Primed Self Priming Pumps
Vacuum Primed Self Priming Pumps typically has a vacuum pump and positive sealing float box installed at the pump discharge, close to the discharge valve. This allows it to pull a vacuum on the pump until it is full of water. Note that the maximum height that water can be lifted with a vacuum is 34 feet (at sea level), and that is with a perfect vacuum, and no liquid flowing. This pump type can have dry run capability, and is also capable of handling sewage.
Precautions for Self Priming Pumps
- Even a self priming pump has to be primed prior to its first operation. No matter the design, there is a priming chamber (integral or external) or some portion of the volute that will require filling prior to startup.
- The discharge line must not be pressurized or blocked. The air in the suction side of the system being displaced by the liquid has to have somewhere to go, otherwise the pump will air bind.
- The suction line must be air-tight. If air continues to be drawn into the pump, the pressure will never be reduced and fluid will not be drawn up the suction line.
- Volume of the suction side piping to be minimized to reduce the priming time. With excessive priming times, there is a danger that the liquid charge will evaporate before the pump is primed.
- If the liquid contains any solids, debris may collect in the re-circulation port, impeding the circulation of fluid and the generation of the liquid ring. For this a strainer may be required to keep solids from accumulating in the priming chamber and displacing the priming liquid.
- When pumping liquid in cold conditions, the fluid in the priming chamber of the pump, usually water, will solidify if the ambient temperature drops below freezing for a sufficient period of time. When water freezes it expands and the casing will crack. Either drain the fluid out of the pump or supply a heat source when the ambient temperature is predicted to be below freezing.
- The pump suction piping should be designed such that no high points are created, where air can be trapped/accumulated, thus prevent priming.
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