A Simple Guide to Self-Priming Pumps

A Simple Guide to Self-Priming Pumps

Basically, a self priming pump draws liquid to the pump. So if there is air in your pipe then the pumps will be able to get rid of the air so you can begin to pump the liquid… Basically.

In this article we will cover the following:
Can Pumps Pump Air?

For systems where pumps are placed above liquid (like pumping from an Tank below ground), there will be air in the suction pipework when the pump starts. Now This air will need to be removed before the pumps can displace the liquid, since pumps cannot pump air. Or can they?

Well the answer to the above question is, no, pumps by definition cannot pump/displace air. To effectively displace air you will need a compressor (since air is compressible). Some pumps have compressible capabilities, or has a smart method of using a vacuum principle to draw air towards the pump. However it is done, ‘This methods of displacing the air before the liquid reaches the pump, is called self-priming. However, various methods can be used to get around this problem.

  1. A secondary vacuum pump (compressor) can be used to remove the air in the suction line.
  2. A foot valve will stop liquid from draining from the suction line when the pump is stopped. But you do need to fill the line with liquid first.
  3. A header tank can pull a vacuum to allow liquid to enter a pump.
  4. A pump itself can incorporate a header tank to draw a vacuum and allow liquid to enter the pump. – For Centrifugal Pumps.
  5. The pumps can compress air to allow liquid to be pulled by Vacuum to reach the pump, before displacing the liquid – For Positive Displacement Pumps.
Centrifugal Self Priming Pumps

A centrifugal pumps uses rotational energy, to pump liquid. These pumps have slip, which means when the they stop, liquid will drain back through the pumps if the liquid level is below the pump – like a syphon. In this situation, air can enters the system and break the pumps capability of transferring liquid i.e break its prime. Other than the fact that at this point the pump is deemed useless since it cannot achieve its purpose, the real danger is when the pump is unaware of its non-primed condition and when it is turned on, it begins to overheat and break.

Although it seems doom and gloom, this is not the end for some manufacturers. As mentioned above in bullet point 4 some pumps have the impeller and volute casing surrounded by an internal tank so that it can always be immersed in a liquid. This effectively allows the pumps to remain primed and create a vacuum in the suction pipe, which then displaces the air and begin to pump liquid. Consideration for priming time to get the pump started and provide the pump with lubrication and cooling must be taken to ensure it is not a long time.

Filling the volute casing is an integral part to commissioning the pumps as the pumps cannot run dry. However, since the pumps will be subjected to air there is a small element of dry running, therefore another important aspects to self priming centrifugal pumps is using the appropriate bearings and seals, to allow for these short period of dry running.

Positive Displacement Self-Priming Pumps

In principle, all positive displacement pumps are self-priming. This includes rotary gear pumps (internal and external), lobe pumps, vane pumps and diaphragm pumps. The reason for its self priming nature is because it has virtually no slip, meaning once it mover a column of Fluid (liquid or gas) then that fluid cannot return, and therefore resulting in pulling a vacuum resulting in self priming.

However virtually no slip is exactly that – Virtually. Meaning depending on the effectiveness of the seals created by these close-tolerance parts, a positive displacement pump may have a very small form of slip with will hinder its effectiveness of priming. Also, during priming, the pumps are effectively dry running, which leaves the pumps vulnerable to overheating and high wear which in turns hinders the performance to maintain its tight tolerance and good non slip capabilities.

Cavitation is a major concern for reciprocating pumps, when liquid starts to enter the pump and there is a liquid/air mixture. In which case, vapour bubbles form and expand on the suction side of the pump before violently imploding on the discharge side of the pump causing vibration and damage to the pumping elements.

A self priming arrangment

When selecting either form of self priming pumps, it is very important to refer to the manufacturer information to understand the limitation and design requirements for their pump.

Self-Priming Limitations

Physics dictates that in perfect conditions, the atmospheric pressure allows pumps can achieve a 10m suction lift. However due to NPSH requirements it is typical for pumps to at best achieve 7m f suction lift.

Common Problems with Self-Priming Pumps

The following list are typical issues with self priming pumps:

  • Blocked pipeline.
  • Suction line I not air-tight.
  • Volume of suction pipe is too large for the pump
  • Pumps are operating off Curve (inefficiently)
  • Pumps have warn.
  • Liquid is near freezing point

 

Summary

When liquid level is below the pump and needs to be drawn toward the pump Self-priming pumps are required. This can be done with a Positive displacement pumps with very little slip, or centrifugal pumps with self priming capabilities. In both cases, dry running will be a factor that needs to be considered to prevent overheating.

Submersible Pumps

As the name implies these can be fully submerged in water. The units are supplied as typically IP68 Rated and has a fully sealed motor directly connected to the volute. Submersible pumps don’t typically suffer from pump cavitation (Subject to NPSH). These efficient pump are used mostly for liquids within wells and sumps. Typically the liquid are in the form of dirty water (with solids), sewage, drainage, slurry, and general industrial pumping. In applications where a jet pump or a submersible pump can be used, the submersible pump is more efficient.

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Pool Pumps

A circulator pump specifically made for swimming pools. Usage of the pumps can vary from 24 hours a day for public pools, while residential swimming pool pumps run 4 hours per day when the pool is not in use, and between 6 and 12 hours a day on an electric timer during swimming season. They typically use 0.5kW to 2kW power. Some pool pumps have two speeds to cut down on power costs, while some now use smaller motors with heavier windings. They are usually self-priming, and most have a strainer or filtering unit to keep hair and other foreign objects from getting into the pump.

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Clean Water Pumps

Either self-priming or just standard centrifugal booster pumps. Clean water pumps must comply with water regulation and be WRAS approved within the united Kingdom. a built-in ejector system is a normal feature for self priming units. The pump are often made of stainless steel, and come in single or multi-stage models. As the name implies, they are appropriate for drinking water.

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Peristaltic Pumps

Interesting fact, the Peristaltic pumps mimic the gastro-intestinal tract’s method of moving food, called peristalsis… Ok not that interesting, but due to the nature and material range of peristaltic pumping they are very versatile. Product goes through a hose, which squeezes at one end to force water through to the other end, just like how you would squeeze tooth paste from its tube. However, as oppose to your fingers peristaltic pumps use rollers that move in the direction the fluid is being pushed. Some uses for peristaltic pumps: wineries, chlorine tanks, sewage treatment plants, slurry and the pumping of viscous products.

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Hopefully now you understand the differences between the pumps and can decide which pump may be best for you. But it doesn’t stop there. This is only an introduction to the top 7 pump range, however for further information, please feel free to visit other articles which goes into more details.Alternatively, pick up the phone and speak to one of our engineers who will be more than willing to help you assist you with your application.