Centrifugal pumps which have the capability to pump mixtures of liquids with entrained gas, as well as liquids, which have the ability to vent the suction line of air, are deemed to be self-priming pumps.
Self-priming pumps operate by fluid recirculating within the pump head, which creates a vacuum. This causes air to be evacuated from suction pipework, until fluid enters the impeller of the pump.
The design of self-priming centrifugal pumps reduces pump efficiency to some extent, due to the separation chamber within the pump head which works when fluid is retained within the pump head to effectively create a vacuum.
There are some designs of pumps which are classed as self-priming which include progressing cavity, gear, peristaltic, side channel, peripheral and diaphragm pumps due to their positive displacement design, which are designed for fluids more or less viscous than water such as oils, foodstuffs, solids, some types of fuel, and chemicals.
Standard centrifugal pumps can also be made self-priming with the addition of a check valve being placed at the bottom of the suction pipework providing the casing is filled with water on startup. This prevents the water from draining out of the suction pipework. A pump operating in this fashion is known to create a manometric suction lift.
Units which are unable to self-prime are incapable of venting suction lines containing air meaning if pumps are located above tanks, pits or have long suction lines they cannot draw fluid to their inlet.
Non-self-priming pumps should have their suction flooded either:
Via the use of a tank designed to supply the pump with sufficient fluid pressure at the inlet
Suction pipework contains valves to ensure that the pump head and suction line remains filled with liquid
A booster pump to ensure fluid at sufficient suction pressure enters the unit.
Self-Priming pumps can be used in a variety of applications, here are a few examples along with some of our previous projects:
ATEX Fluid Transfer – Rather than having an ATEX submersible pump immersed in a flammable liquid, having a surface mounted pump which primes from pits can be far safer, and lead to fewer issues as the motor and cable glands are not immersed within the flammable liquid. One ATEX self-priming pump we provided in the past was for a distillery to bulk transfer Methanol at 95% concentration.
Pit Emptying – Units are great at pit emptying eliminating the need for confined space entry.
Solid Handling – Such designs are typically engineered to handle solids and can often transfer these comfortably. One of our previous projects that required solid handling was for a wheel wash system at a quarry.
Wastewater Handling & Dewatering – Units can be kept in situ with hoses ported around site for dewatering various areas rather than having to relocate pumps each time an area needs dewatering saving time, labour, and resources.
Tanker Offloading – Self priming pumps ensure that tankers, and storage tanks are stripped of fluid, maximising fluid transfer as well as eliminating vapour locking in suction lines.
Varying inlet / Tidal Conditions – For transferring from tidal rivers or the sea to on land, having a pump which can handle fluctuating intake conditions can help eliminate any possible issues caused by changes in fluid suction height.
Before selecting the an application there are a few key factors you should consider:
1. Size of Solids – To ensure the impeller can sufficiently handle any particles without clogging.
2. Fluid Viscosity – This often determines pump speed, possible flow rate and if other devices are required such as a follower plate to help feed the pump.
3. Priming Distance – Not just the horizontal, and Vertical runs of pipework but also any accessories such as bends, strainers or valves which can add pressure losses and determine possible priming distance.
4. Fluctuating Fluid Levels – If water is being drawn from the sea, tidal rivers, or tanks where water levels change, care must be taken to ensure that the designed flow is possible at various heights without encountering issues due to a lack of suction pressure such as vortexing or cavitation due to insufficient NPSHA.
5. Priming Time – That the time taken to evacuate the suction pipe is checked and that the unit does not overheat or take too long to start up. During priming, fluid recirculates within the casing, creating a vacuum. As it recirculates it begins to heat up, and if left too long, it can heat above the temperature rating of gaskets and seals within the pump head.
6. Power Options – How to power the pump whether it be by single, or 3 phase, hydraulically driven or by engine.
7. Close Or Long Coupled – Whether a cost-effective unit is required or a robust long coupled unit.
8. Trailer Mounted, Baseplate or Pontoon Mounted – Whether your pump will stay in situ, require a trailer for portability across your land / domestic roads or installed on a pontoon for dewatering there are many options to consider.
9. Controls – How the pump will start, stop, and operate as part of your system, and whether certain events may be triggered.
There are many advantages to self-priming pumps, but also some drawbacks:
Entrained Gas Handling – Due to their ability to vent suction lines they can handle fluids with entrained gasses, priming for up to 30 mins without drawing liquid.
Easy to Maintain – They are far easier to maintain than a submersible pump which is immersed in the liquid and may need confined space training to maintain.
Reliable – Their designs are not based on use with other accessories meaning they do not rely on additional valves to function which can sometimes fail
Robust – As the motor and cable are not immersed in fluids, they are longer lasting and less exposed to pumping mediums when handling corrosive or aggressive fluids
Efficiency – Self-priming pumps tend to be less efficient than standard centrifugal pumps due to their internal volute and solid handling, meaning internal clearances are larger with a corresponding increase in energy consumption.
Priming – Units can need to be reprimed or have the casing filled with fluid prior to startup in particular if stood idle for long periods of time. Suction lines may also need to be checked to ensure the pump can empty suction lines of air in the time required.
Clogging – Pumps may still require suction strainers to prevent clogging, and not all designs can handle solids.
As with any pump it is always essential to keep a set of spare parts locally on site.
As a way of pre-empting any issues conduct the following:
Visual Inspection – Visually inspect the pump for any leaks, damage or unusual sounds indicating there is an issue present. Open the inspection port to check the impeller rotates freely and that there are no foreign particles present.
Lubrication – Ensure any bearings are adequately lubricated, or constant level oilers have sufficient lubricant.
Vibration / Alignment – Check pump is aligned, and that excessive vibration is not occurring.
Maintenance Log – Always keep a maintenance log local to the unit to detail what maintenance was done when.
If you're experiencing issues with your self-priming pump read our troubleshooting guide for some of the most common issues, alternatively we are always available to discuss any current issues.
Self-priming pumps should first have the casing filled with liquid via the priming port prior to startup.
All suction lines should be thoroughly checked for air leaks, and the discharge line should not only be free from obstruction but also not contain any non-return valves directly above the pump on the discharge which can prevent the pump from evacuating any air.
The unit should be started and observed to ensure it does not vibrate excessively, that all noises are normal and that that the unit is not cavitating.
The pump should be run to check it successfully primes, operates without issue, and is performing as per its curve. The pump should be started and stopped several times to ensure it reprimes without issue.
Our self-priming centrifugal pumps are unlike other designs as they are built with an armoured volute, which protects the main casing from abrasive wear, and high velocity solid impact.
Constructed in ductile iron, along with the impeller, it ensures that units are durable, handle solids up to 50 x 24mm in diameter without loss of flow and last for many years to come.
For more information contact our experts today, they can help resolve any issues you might be having or begin your journey on getting a new application installed.
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