In this article, we will learn what are self priming pumps along with definition, meaning, working, types, advantages, disadvantages, etc.
Let’s explore!
What are Self Priming Pumps? Definition & Meaning
Self Priming Pumps Basics
Let’s try to understand the basics of self-priming pumps. A pump is a hydraulic machine that converts mechanical energy into hydraulic energy. This hydraulic energy is in the form of pressure energy. Now let’s define a self-priming pump.
Self-priming pumps are liquid pumps constituting a pump body/cavity filled with a liquid that aids in carrying out the process of pumping. They are useful if a pump needs to be located above the liquid level that must be pumped.
These pumps are completely or partially placed above the liquid’s level and direct the air into the suction line, which must be evacuated before the pump moves the fluid. Self-priming pumps remove the air initially before starting the pumping functions.
What are Self Priming Pumps?
Self-priming pumps are those which remove air by their own pumping action. These are pumps that generally have an air separator.
- Self priming pump has a dedicated external casing and it is always filled with liquid.
- In this type of pumps, no need additional priming even at the starting of the pump, only it is required to fill this casing with water at intial start.
- It removes air or gases with out any extra priming efforts.
Normally self-priming can be done in three ways,
- Priming used by Liquid
- Priming used by Compressed Air
- Priming used by Vacuum
Historical Overview of Self Priming Pump
Fluid pumping applications require pumps above the liquid level to allow air in the line, but this phenomenon offers challenges such as pump failure and overheating. Hence, self-priming pumps are the best go-to devices to evacuate air before the liquid is in motion to avoid such scenarios. Pumps are not new.
They are dated back to 2000 BC when Egyptians invented “shadoof”. It was used to raise water and the construction was based on a long-suspended rod having a bucket at one end and weight at the other. The basic working principle and design of a reciprocating pump are traced back to 200 BC when Ctesibius invented a device known as a “water organ”.
Also, the first centrifugal pump was a mud lifting device invented by Renaissance-Italian engineer Francesco di Giorgio Martini. From 2000 BC up till now several inventions have been made and many different designs of self-priming pumps are created while many old designs are modified for better results.
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How Does Self Priming Pump Work?
Working Basics of Self Priming Pumps
Fluid recirculation inside the pump head is the working principle of the self-priming pump. Priming is a significant phenomenon taking place inside the self-priming pump. A priming arrangement exists inside the pump, constituting components such as a discharge pipe, suction line, an impeller inside the casing, and a priming port.
Now, let’s discuss the working of the self-priming pump.
- The arrangement consists of a casing in which an impeller is present. A discharge pipe and priming port are also there in the casing.
- Initially, when the pump is switched on, the casing is filled with air, and when the impeller rotates, enough centrifugal force isn’t developed. As a result, water cannot be pulled from the suction line. In this scenario, the casing is first filled with water, known as priming.
- Water is added into the casing with the help of a priming port. The pump is switched on, and the impeller rotates, due to which the water inside the casing also rotates.
- It develops a centrifugal force on the water, which pushes the water in a radial direction against the axis of rotation. Hence, water is pushed out towards the discharge pipe.
- The pushing of water through the discharge tube creates a vacuum inside the suction line, and freshwater is pulled upwards.
So, we say a steady flow of water occurs due to the pushing and pulling. The pull developed due to the push is directly analogous to the push applied to the water due to centrifugal force.
On the contrary, if initially the casing isn’t filled with water and the pump is started, water flow is not possible. When the pump is started without performing priming, the impeller rotates, and there is air inside the casing.
Air is less dense than water hence the centrifugal force created in this scenario is not sufficient to carry out the process of pumping.
The rotating impeller will move the air towards the discharge pipe, but the centrifugal force created is almost 1000 times lesser than that created due to water inside the casing. If the push is insufficient, the pull will also be smaller and won’t make the water come up through the suction line.
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Automatically Self-Priming Pump
Some pumps are designed to be automatically self-priming. This feature enables the pump to trap fluid/liquid inside the pump’s body/casing, preventing liquid flow from discharge towards the suction side while the pump is not operating.
Such pumps are constructed so that the constant presence of fluid in the casing/pump’s body allows the device to handle the air pockets perfectly. Air pockets are particles of air that can interrupt the normal functioning of the pump.
Working Process of Self-Priming Pump
The working of the self-priming pump is based on two phases;
- the priming phase and
- the pumping phase.
The steps of working are, as follows:
- As discussed earlier in the above paragraph, the priming phase creates the vacuum in the casing due to the rotating impeller.
- It draws air from the suction line into the pump and, at the same time, generates a cylindrical ring of liquid inside the pump’s body/casing.
- It forms a gas-tight seal preventing the air from moving towards the suction line from the discharge pipe.
- Inside the impeller’s vanes, the air pockets are trapped and transported towards the discharge port.
- This way, the air is expelled out, and fluid/liquid under the action of gravity comes back to the reservoir in the pump housing.
- After the evacuation, the liquid is pulled up the suction line, as discussed earlier.
- The phenomenon continues until liquid/fluid replaces all the air in the suction line and pump.
- At this point, the pumping mode starts, and the discharge of liquid occurs.
Euler’s Turbomachine/Pump Equation
Euler’s turbomachine/pump equation is based on energy conservation and conservation of angular momentum. The equations play a crucial role in turbomachinery because it forms a relationship between specific work Y, the geometry, and velocities in the impeller.
Euler’s turbomachine/pump equations apply to both pumps and turbines. The equations are hereunder:
- Shaft torque: Tshaft = ρQ(r2Vt2 – r1Vt1)
- Water horsepower: Pw = ω. Tshaft = ρQ(u2Vt2 – u1Vt1)
- Pump head: H = Pw / ρgQ = (u2Vt2 – u1Vt1)/g
r1 and r2 demonstrate the impeller’s diameters at the inlet and outlet points, respectively.
u1 and u2 represent the impeller’s absolute velocities at inlet and outlet, respectively.
Vt1 and Vt2 depict the flow’s tangential velocities at the inlet and outlet, respectively.
Types of Self-Priming Pumps
The major types of self-priming pumps are mentioned below, along with an explanation.
Positive Displacement Pumps
All positive displacement pumps are self-priming, constituting lobe pumps, diaphragm pumps, vane pumps, external and internal rotary pumps, etc. These pumps displace a certain quantity of liquid with each revolution of the pumping components, which are screws, rotary pistons, vanes, gears, and lobes.
- This displacement occurs when the fluid is trapped between the stationary casing and pumping components.
- The pump consists of an expanding cavity on the suction side, and the discharge side has a decreasing cavity.
- When the cavity on the suction side expands and liquid flows out of discharge with the collapse of the cavity, the fluid flows into the pump.
- In each cycle of the pump’s operation, the volume is constant.
- No shut-off head is present in these pumps. Therefore, they should not be operated against a closed valve as it may damage the pump.
- Positive displacement pumps are further categorized as reciprocating and rotary positive displacement pumps.
Self-priming Centrifugal Pumps
Self-priming centrifugal pump is simple, inexpensive, and sturdy. They are perfect for low viscosity liquids, for instance, CIP solutions when suction conditions are challenging.
However, typical centrifugal pumps are not self-priming, but certain modifications can change a simple centrifugal pump into a self-priming centrifugal pump. This type of pump operates when the impeller rotates the liquid present inside the casing, producing centrifugal force and forcing the liquid towards the discharge pipe.
- Air is considered an enemy in this pump.
- Due to air pockets, the pump becomes air-bound and cannot function normally.
- Hence, self-priming pumps the binding of air because initially, in the priming process, it combines any residual air with the fluid in the self-priming pump’s mechanism.
- Compared to a typical centrifugal pump, the self-priming one features a fluid reservoir at the front or above the impeller inside the pump’s body/casing.
Self-priming Chemical Pump
The self-priming chemical pump is similar in operation to the centrifugal one. Both kinds allow the transport/flow of slurry, oils, dyes, and water.
Self-Priming Water Pump
Self-priming water pumps are widely used in commercial and agricultural applications for pumping clean water. They are mostly centrifugal pumps capable of pumping almost 40L/min of water.
Seal-less self-priming pumps
A seal less self-priming pump is ideal for heavy-duty work used for transporting acids, solvents, salts, liquids, slurries, etc. There are two sections of a seal-less self-priming pump; an outer wall that is blend-able and a thermoplastic interior (rigid). The material/media that is to be transported moves between these two walls.
Apart from the above-mentioned few types, there are various other pumps categorized as self-priming. They include gear, side channel, peripheral, progressing cavity, peristaltic, and diaphragm pumps.
Factors Affecting the Operation of Self-Priming Pumps
Several parameters affect the functioning of self-priming pump. Ensure that the discharge line is not blocked or pressurized while the suction line should be air-tight. Additionally, the priming time must be lesser since greater priming times will cause the evaporation of liquid before the pump is primed.
In case the priming times are greater, dry-running will surely cause damage to the pump. In centrifugal pumps, the impeller is an important component.
When debris or any solid gets stuck into the impeller, it won’t function accordingly affecting the efficiency. Additionally, time machines wear and so is the case with self-priming pumps.
Also, after maintenance, the improper assembly can affect internal clearances which in turn can affect the operation. In cold weather, the pump must be provided with some sort of heating to avoid freezing fluid inside the pipework.
Applications of Self-priming pump
Below are a few applications of self-priming pump.
- The pumps can pump fuels, greywater, clear water, raw sewage, and industrial water.
- They have applications in irrigation and sewage treatment processes.
- Liquid transfer systems also use self-priming pumps.
- The pumps can be used for increasing the pressure.
- Boats can also be bailed out using these pumps.
- In many areas, basement flood water pumps are quite successfully used.
- Sanitary processing industries use self-priming centrifugal pumps very often.
- Industrial and commercial applications require self-priming pumps because they involve the liquid transfer of high volume.
- The pumps include built-in check valves which ensure consistent self-priming.
- They are designed in such a way that radial load on the motor is minimized.
- Self-priming water pumps are used for agricultural purposes commonly.
- Self-priming water pumps are often used in Hydrology and marine applications.
- Agricultural and horticulture applications often require sampling pumps.
- Self-priming pumps are also used in oil production and exploration.
Advantages of Self Priming Pump
The advantages of self-priming pumps are hereunder:
- Self-priming pumps are easier to access, repair, and maintain.
- Self-priming centrifugal pumps are best for intermittent and frequent pumping operations.
- These pumps are sturdy and inexpensive.
- They are very safe to operate since there is no need to open wet good lids.
- The operation of pumps doesn’t require cranes or hoists.
- These pumps are designed to handle entrained gases and are also able to prime for about 30 minutes without drawing the fluid.
- They are available in various sizes.
- Due to their versatility, they are used in many applications.
Disadvantages of Self Priming Pump
The disadvantages of these pumps are mentioned below:
- These pumps are limited in case of suction life; i-e, they come with a limited height (7.6 m maximum).
- If there is no priming liquid initially, self-priming centrifugal pumps won’t work.
- Air pockets can disrupt the whole operation of self-priming pumps.
- A single pump is limited to about 300 liters per second flow rate.
- For true self-priming pumps, heads above 90 m are hard to get.
- Size is big due to additional casing.
- More space required.
- More cost.
- Less efficiency.
- Sometimes, re-priming is required due to evaporation effects.
- Any leakage in suctin pipe, will incur air and pump will not work.
- Although it is self-priming pump, however, at the first time, priming is required, that means pump should be filled with water.
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Conclusion
Pumps are hydraulic devices. While self-priming pumps are designed to remove air before starting the pumping operation. Fluid recirculation inside the pump head is the primary working principle of self-priming pumps. The blog covers the working of self-priming pumps in detail.
There are several types of self-priming pumps; a few of them are discussed in the blog. These pumps are versatile and hence offer many applications in various fields. Additionally, the blog also sheds light on the advantages, applications and disadvantages of these pumps.
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