Pump priming, Centrifugal or Self Priming

This article will cover what is pump-priming, what are self prime pump and how to do pump priming.

Pumps are essential equipment when it comes to chemical processes and product manufacturing. Pumps convert rotational kinetic energy into hydrodynamic energy and help to transfer fluid from one point to another.

The pump runs by motors, the impeller is connected to the shaft, and the shaft is connected to the motor. As the motor rotates, the impeller rotates which gives a moment to the fluid, and if got pressure and discharged from the pump. Apart from selecting the proper pump for the process, operating the pump also plays an important role.

The improper operating pump can result in damage to the pump and malfunction which will increase pump maintenance costs. Improper operation of the pump will also not allow you to have proper pump performance. Pump priming is a factor that has a direct effect on the pump impeller.

pump priming chemicaltweak
pump priming

Previously, we had learned what are the types of pumps used in industries in detail, and in this article, Let us understand what is pump-priming and why it is necessary for the pump. In the meantime, if you are interested in learning pumps then you must read Centrifugal Pump Curve In Detail.

What is pump priming?

Priming is the process in which the impeller of a centrifugal pump is submerged fully submerged in the fluid which is to be transferred with no air trap inside. This process is essential prior to the pump’s startup. It is recommended to prime the centrifugal pump and then only start the pump. This can be considered as the initial step to prepare the pump for operation and avoid the dry running of the pump.

Now as we have learned what priming a pump means let us understand why it is required.

Why pump priming is required | Is priming the pump is necessary?

When we start the pump, the impeller will rotate. Centrifugal pump impeller with developed pressure is proportional to the density of the fluid in the impeller. If the pump casing is not filled with water/fluid and the air is present, the impeller will run in air, producing negligible pressure which will not be sufficient to suck fluid from the source, and because of this, we won’t be able to transfer fluid. This is why pump-priming in the centrifugal pump is required.

Now you might be thinking is there any solution to this or do we have to do priming of every pump prior to starting?. The solution to this is a self-priming pump. These are available in the market that are not required to prime. Let’s move forward and study them in detail.

How To Prime A Centrifugal Pump

There are two case the centrifugal pump is installed,

  1. Suction above pump
  2. Suction below pump

In both, cases, priming centrifugal pump steps are a bit different. We will discuss both cases. Now that you had learned why priming is necessary, let us jump to the steps to prime a centrifugal pump.

Case 1 – Suction above the pump

priming the pump

As you can see in the above figure, suction is above the pump. Steps for priming the pump are described below

  1. Firstly open the vent line present in the pump or discharge valve on the pump.
  2. Now slowly open the suction valve of the pump. This will allows liquid from the tank to enter the pump. Keep it open until liquid comes out of the vent line.
  3. As the liquid comes out of the vent line, it indicates that the air present in the pump is replaced with the liquid. Close the vent valve.
  4. Now the priming procedure is completed, you can start the pump.

Note – Big pumps that are used in industries come with vent lines as it is not possible to check if liquid comes out from the discharge valve or not but the pump installed at home i.e. pump to transfer water from the underground tank to tank at top (water pump) as pump discharge is easily visible, doesn’t have vent valve.

Don’t forget to check Types Of Chemical Reactors used in chemical industries.

Case 2 – Suction below the pump

centrifugal pump priming

If the liquid from the pit or the tank is installed at the ground and the pump is installed above the tank surface, this falls under suction below the pump as the pump had to draw liquid below its level. In these types of pumps, a foot valve is installed in the bottom which will help in priming (Discussed in priming by installing foot valve section) and the external line is provided at the discharge of the pump by which we use it to prime it. Detailed steps of are described below.

  1. Firstly close the discharge valve. (It is obvious that if the pump is not running, the discharge valve will be closed)
  2. Now open the vent valve of the pump
  3. Now open the valve of the external supply line, this will fill liquid in the pump, and as the liquid comes out of the vent line close it.
  4. Liquid coming out of the vent valve ensures that air inside the pump is replaced with liquid.
  5. Now the pump is primed and one can start the pump.

Priming by installing Foot Valve

In this method, when the suction of the pump is below the pump and the pump had to suck liquid from below its level foot valve is installed. The foot valve is nothing but a non-return valve [NRV Valve] which will open in only one direction i.e. down to up. As the pump was in running condition, the foot valve would be open and it would allow the liquid to pass from the sump to the pump.

As the pump is stopped, the pump will stop the suction of liquid, and liquid from the pump will flow down to the sump. Here foot valve is an NRV valve and will be closed and won’t allow pump liquid to get drained in the sump, and because of this liquid will remain in the pump and it is not required to prime the pump.

These were the methods that we can use to properly prime industrial pumps. In order to avoid priming operation, self-priming pumps are also available. let us know what are self-priming pumps, and the working of self-priming centrifugal pumps are in detail.

What is self priming pump | Self Priming Centrifugal Pump

Not every pump are needed to be prime, there is a self-priming pump as well which doesn’t require manual priming. Priming mode and pumping mode are the two phases of operation in self-priming centrifugal pumps. In priming mode, the pump acts as a liquid-ring pump.

The rotation of the impeller generates a vacuum at the impeller’s ‘eye’ which pulls air from the suction into the pump and at that instance, it also creates a cylindrical ring of liquid on the inside of the pump casing. This gas-tight seal stops air from the discharge line to the suction line. Air pockets trapped in the liquid get expelled from the pump discharge by the rotation of the impeller.

Now as all the trapped air gets expells, the liquid will gradually start to get enter the pump. Now the pump is in pumping mode and started to pump liquid/discharge liquid.

Why is priming necessary for centrifugal pump and not in reciprocating pump?

The suction of liquid is important for the pump and to get low pressure needs to be created. In the Reciprocating pump, the piston is used as a part of pump operation. While the pump is in running condition, the piston moves backward and creates low pressure in the pump chamber. This creates suction because of the pressure difference between the pump chamber & tank and it will draw the liquid from tank to pump irrespective to air is present. So we don’t need to perform priming steps.

While, In a Centrifugal Pump, the working principle is the centrifugal force, and there no seal present in it which makes them incapable of replacing air present in the pump with the impeller. This makes the Centrifugal pump incapable to suck the liquid and only can push the liquid. In a centrifugal pump, the atmospheric pressure of the suction tank makes fluid enter in the suction of the centrifugal pump hence centrifugal pump requires priming.

FAQ (Self Priming Pump)

Que 1 – How does pump-priming work?

Pump priming will fill the pump casing with liquid which is needed to transfer and remove air pocket present in the pump.

Que 2 -Can self-priming pumps run dry?

Pumps are made to pump liquid, self-priming pumps are designed in such a way that they don’t require priming but it doesn’t mean that self-priming pumps can run dry. Even for a short period of time, pumps should not run dry else mechanical seal can be damaged.

Que 3 -Which pump does not need priming?

Self-priming pumps are pumps that are designed that they don’t require priming.

Que 4 -Are positive displacement pumps self-priming?

Yes, all positive displacement pumps are self-priming pumps, and no additional step of priming prior to starting a pump is required.

Wrapping up

This was a detailed article on pump priming and why it is required. if you haven’t checked the other articles then do read that where we discussed different types of pumps in brief. if you have any doubts or questions related to the article, do ask in the comment section, and we will try to answer them ASAP. comment down the topic on which you want us to post on the blog.

Me - Founder of Chemical Tweak - A Chemical Engineer by profession and a part-time self-trained technology blogger.

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