Periodically I am asked if there are self-priming slurry pumps. To answer this question accurately we must address the 3 main types of pumps separately.
Rotary positive displacement pumps utilize close-tolerance parts to prevent fluid from returning from the discharge to the suction side. This close tolerance is generally tight enough to also be effective on air, making it quite capable of evacuating air from the suction line. Although in principle pumps such as rotary gear pumps, lobe pumps, and vane pumps are self-primers, they are not typically designed to run dry for extended periods. As such the priming time must be minimized to avoid overheating and pump damage.
Reciprocating positive displacement pumps that have appropriate valves will displace air just as easily as a liquid however there may be a problem. PD pumps that are classified as contact pumps (the reciprocating component is in direct contact with the product) often rely on the product for lubrication. This makes them susceptible to heat and wears problems when running dry, so like the rotating PD pumps priming time must be minimized to avoid overheating and pump damage.
On the other hand, if you have a non-contact PD pumps, such as diaphragm pumps, these can usually run dry for quite some time but be sure that the valves are designed for high suction lift self-priming applications or there will be trouble. I know this from personal experience. Years ago I installed an electric shut-off valve between the tank and the fuel pump in my old car as an anti-theft device. One day I forgot to switch it to open. As I started the car and drove up my driveway one of the valves on the fuel pump quietly came to pieces. No fixing it, a completely new pump had to be installed.
Centrifugal pumps are the most common pumps and are used worldwide for millions of applications. Not surprisingly quite a few are in applications that are deemed as self-priming, but is the pump itself really self-priming? To answer this question we must look closely at how a centrifugal pump works.
The operational principle of this type of pump, as the name indicates, uses centrifugal force to accelerate a mass and create pressure. Liquids generally weigh over 50 lbs per cubic foot (800kg/m³) and as such, they have enough mass that when accelerated by centrifugal force they do develop a pressure, hopefully, enough to move the liquid out of the impeller and into the discharge line. As the liquid leaves the impeller a low-pressure zone is created inside the pump and atmospheric pressure pushes the liquid up the already flooded inlet pipe into the low-pressure zone, hence keeping the pump flooded with liquid.
When a pump sits void of liquid the impeller only has air to accelerate with centrifugal force. With a weight of 0.0765 lb/cu ft (1.225 kg/m³), there is close to no pressure available to effectively create a low-pressure zone and therefore no differential pressure to flood the pump.
A person could attempt to overcome this by filling the pump with water and thereby giving the impeller a mass to accelerate, but after a few rotations, the impeller would again be trying to accelerate air. Unless the suction line was almost already flooded the pump would have no chance of pumping. Although a standard centrifugal pump clearly can not be self-priming, engineers have worked around this issue by adding auxiliary items to the standard pump.
Priming tanks are one such item. When these tanks are added to the pump they can in effect create a self-priming unit. The priming tank when charged with liquid prior to start up provides sufficient liquid to flood some of the impeller allowing it to create a low-pressure zone in the suction eye. As the liquid leaves the impeller it carries with it some air as it enters a second priming tank better known as the separation tank. There, through gravity, the liquid leaves the expelled air and returns to the main priming tank. This allows the cycle to continue using the same initial charge of liquid as the suction line is progressively emptied of air. When manufacturers incorporated the priming and separation tanks into the pump they created a self-priming unit but simply called it a self-priming centrifugal pump. See the illustration below.
Compressed Air Self-Priming Pumps like the pumps described above rely on an auxiliary piece of equipment to create a pumping unit that is self-priming. In this case, however, it is a compressed air-powered venturi system that evacuates the air from the suction line and pump casing. Manufacturers that use this method of self-priming have an auxiliary air compressor installed on the pump that is driven off the main pump shaft. It continually supplies a jet of air into a tapered tube that is attached to a high spot in the pump suction. This, in conjunction with a special air-tight valve on the discharge side, creates a low-pressure zone inside the pump head. This in turn allows atmospheric pressure to push the liquid up the suction piping, thereby flooding the pump.
So in answer to the title question… When it comes to PD pumps there is clearly no myth, self priming pumps do exist. The standard centrifugal pump however cannot prime without the addition of auxiliary equipment. As a result, do we then say standard centrifugal pumps are not self-priming pumps? Or do we include the priming and separation tanks as part of the pump and say self-priming pumps do exist?
I’ll let you decide.
Bye for now!