Self Priming Pumps: Where to use them, why to use them, and the misconceptions
Self priming centrifugal pumps are often overlooked because they are not as commonly used as either standard centrifugal pumps or submersible pumps, and not understood by many who specify pumps. They are also misapplied or not properly applied to pumping applications by people not properly trained in their application.
What is a Self Priming Pump?
A self priming pump is like any other centrifugal pump, but it is designed with an external casing that “floods” the inner pump or volute. The casing is filled with liquid [usually the same as the liquid to be pumped] and the pump is ready to go. When the impeller rotates in the casing, a low pressure area is formed at the eye of the impeller. Because the pressure is lower than atmosphere, water is pushed up the suction pipe by atmospheric pressure, forcing all the air in the suction line into the pump. The self priming pump mixes the air with the recirculating water in the casing. The air separates from the liquid and is discharged from the casing. When all the air in the suction line has been displaced, the pump is dynamic and delivers flow like any other centrifugal pump.
Where to use them?
Self priming centrifugal pumps can be applied to any application that standard centrifugal pumps are applied, within their hydraulic limitations. Applications from water, fuels, effluent or “grey water”, right through to raw sewage, industrial wastewater and sewage sludge can be effectively handled by self priming centrifugal pumps. Provided they are within their hydraulic limits, and they are manufactured with the right features for their intended duty, self priming pumps can deliver the safest and most cost effective of pump solutions.
Why use a Self Priming Pump?
Self priming pumps are mounted above the liquid source [generally at “ground level”], so less infrastructure is required to set them up. And because they are located at ground level, they are easier to access and much easier to maintain and repair. As there is no mechanical equipment in the wet well, there is no need to open wet well lids, and there is no need for hoists or cranes [as in the case of submersible pumps], making them a much safer option for operators.
How to apply Self Priming Pumps
Firstly, the right type of pump needs to be selected for the duty [fuel pumps for fuel applications, solids handling pumps for wastewater applications etc].
The right size pump for head and flow then needs to be selected.
Then, an NPSH calculation needs to be done to ensure there is enough atmospheric pressure to support the intended flow, and the speed of the pump needs to be fast enough to prime the pump.
Selection of the suction line size is also important. It needs to be large enough to support the desired flow, and small enough to keep priming times to a minimum. It is best to avoid long suction lines, but if there is no choice, keeping part of it full during priming cycles will help [talk to Hydro Innovations about the “P Trap” design].
What are the limitations of Self Priming Pumps?
Self priming centrifugal pumps are limited in their suction lift [usually a maximum of 7.6m]. Some claims by some that 8.0 or even 8.5 metre suction lifts are possible, are misleading. Although some pumps in Australia may be able to generate a vacuum in excess of these figures [as can Gorman Rupp pumps], they cannot physically pump on these lifts because of limitations in NPSH of the pump, the temperature of the liquid, and friction losses in the suction line.
A single pump is also limited to a flow rate of about 300 litres per second. And heads above 90 metres are also difficult to achieve for true self priming pumps.
Misconceptions about the use of a Self Priming Pump
Some of the misconceptions about the self priming pump are as follows:
- Self priming pumps need to be re-primed with water after a pumping cycle. This is not the case with “guaranteed re-primers”, like Gorman-Rupp’s Super T, Super U, and Ultra V. These pumps only need to be charged with liquid once, and they retain enough liquid in their casings to guarantee re-priming each and every time they are called upon to pump.
- Self priming pumps are inefficient. In most cases the hydraulic efficiency of a self priming pump will not be as high as that of a standard centrifugal pump or submersible pump. If engineers looked no further than this, self priming pumps would never be used, and asset owners would pay a lot more for infrastructure costs and/or the maintenance of these pumps for the life of the asset. But as many engineers and asset owners have found, the cost savings in infrastructure and maintenance can far outweigh the savings one may achieve in selecting a slightly more hydraulically efficient pump.
- Self priming pumps are too expensive. It is true that self priming pumps are more expensive than standard centrifugal pumps. This is because there is a lot more material [because of the need for an outer casing], more components and more machining. This should to be weighed up against the capital cost savings [no dry well], the ease of access and the ease of maintenance offered by a self priming pump. Also, some self priming pumps are more expensive than others. Again the facts need to be weighed up. A pump purchased that has been engineered and produced by a company that has been manufacturing pumps for over 80 years [like Gorman-Rupp] can be considered as an investment in reliability. Pumps manufactured by companies attempting to copy market leaders will invariably not be able to meet efficiency, quality, repeatability and reliability standards of the market leaders, and purchasers can rightly consider these as costs.
Troubleshooting Self Priming Centrifugal Pumps
The self-priming pump is one of the easiest pumps to troubleshoot if you have all the right tools. You will need a good set of gauges and the manufacturer’s performance curve for the particular pump you are analysing.
Issues with the pump are often problems with the system. A good set of gauges will lead you to the problem:-
- A high discharge gauge reading points to a blockage in the discharge line
- A high suction reading points to blockage in the suction line.
- A low or no suction gauge reading points to a suction leak
- A fluctuating suction gauge points to vortexing or some other form of air entrainment.
- A low reading on both suction and discharge gauges points to a problem with the pump.
Some common troubleshooting issues include:-
- Noisy operation.
- Pump won’t prime or is slow to prime.
- Pump stops or fails to deliver rated flow.
- Pump clogs frequently.
The first thing to establish when confronted with a noisy pump is whether the noise is mechanical or hydraulic. To determine this, drain the liquid from the pump casing and run the pump for a short period of time. If the noise is still apparent, the problem is mechanical. Check for loose hardware and bearing noise [both pump and motor].
If the noise has gone, the problem with be hydraulic. It can be cavitation, vortexing or air entrainment.
Cavitation is the most damaging. A dictionary defines cavitation as “the formation of partial vacuums in a flowing liquid as a result of the separation of its parts. When these collapse, pitting or other damage is caused on metal surfaces in contact”. In pumping systems, negative pressure can and will be created in certain areas. If this negative pressure exceeds the vapor pressure of the liquid being pumped, vacuum bubbles form and remain until they migrate through the system and find enough local pressure to cause their collapse. When these vacuum bubbles collapse, the concentrated force of implosion can exceed 100,000 PSI. If this implosion occurs next to metal, it is likely to cause a tiny chip to flake off the surface.
Cavitation will sound like the pump has rocks or marbles going through it. The most accurate way to detect cavitation is to take gauge readings [both suction and discharge], obtain an accurate speed [rpm] of the pump shaft, then look at the pump performance curve to determine where the pump is operating. If the suction gauge reading is higher than it should be, the problem will be suction cavitation. The cause could be: blockage in suction line [clear blockage]; Suction line too small [increase size of line]; suction lift too high [raise the off level or get the pump closer to the water].
If the discharge gauge reading is too high, the pump will be suffering discharge or “tip” cavitation. The discharge line could be blocked [remove blockage], or the discharge line could be too small, or the static head could be too high. These conditions can often be overcome by increasing pump speed, but check gauge readings against manufacturer’s performance curve to determine the correct course of action.
An impeller showing signs of “tip” cavitation.
If the suction gauge is fluctuating and the pump is making a growling noise, the problem will be vortexing. This means that the pump is pulling air from the surface of the liquid. Often a whirlpool will form on the surface, but not always. This can be solved by lowering the suction line, increasing the diameter of the suction line inlet [suction “bell”], or lowering the off level.
Suction line shown with “Bell Mouth”
If the pump won’t prime or is slow to prime, there could be a suction leak. Check this by wrapping cling wrap around each joint. Repair the joint where the cling wrap is “sucked in”. If there is no leak, the suction line may be too long or its diameter too large. These things could possibly be solved by speeding the pump up, reducing the pipe diameter or construction a “P Trap”. Talk to Hydro innovations about this.
If the pump stops or fails to deliver the desired flow, check the gauges. A high suction reading will indicate a blocked or partially blocked suction line. A high discharge gauge reading will indicate a blocked or partially blocked discharge line. If both gauges are low, the pump may need a clearances adjustment or the pump may have a blocked or partially blocked impeller.
Gauges are your “window” to the pump system.
If the pump is blocking frequently, check where the pump is operating on its curve. If it is too far left, it may not have the velocity through the pump to avoid blockages. Speed pump up. If the pump is operating at its BEP [best efficiency point], the pump may not be suited to the application. Hard to fix without changing pumps [unless the pump can be upgraded with a solids handling impeller, cutting mechanism or other form of solids handling system] or fitting a strainer to the suction line [which may only move the problem].
Some well designed pumps will have easy access for blockage removal.