A DETAILED LOOK AT CENTRIFUGAL SUMP PUMPS
What is a sump pump?
As a mechanical engineer, as well a pump and IC engine specialist, I thought it wise to do something to reflect on my academic profession, and to assist many online users planning to purchase a sump pump or those looking for a way to maintain their existing sump pump. I decided to restrict this hub on reciprocating sump pumps alone, to ensure I dealt with it in detail, as I continue to carry out more researches on compressors, turbines and other class of pumps.
What is a pump? A pump is a mechanical device which provides energy to a fluid in a fluid system. In a simple definition, it is a machine that raises either or both the kinetic or pressure energy of a flowing fluid by converting them to a more useful mechanical energy. A pump and a turbine are just the same with a slight variation; pump admits fluid flow from a lower pressure to a high pressure while turbine does it in reverse order.
Sump pumps have various classes, based on their characteristic features and mode of transfer of mechanical energy, which I am not going to discuss here.
The component parts of a centrifugal sump pump
The centrifugal pump has a very simple construction, which adds up to its numerous advantages. Listed below are the component parts that make up a centrifugal pump:
- The impeller: a rotor (wheel) with series of backward curved blades, usually mounted on a shaft, and coupled to an electric motor
- The casing: this is an airtight chamber covering the pump impeller, and also carries the suction and discharge connections, bearing supports and housing for rotor.
- The suction pipe: the pipe that that connects the eye of the impeller to sump from which liquid is lifted
- Delivery pipe: the pipe which is connected at the lower end or outlet of the pump through which the liquid is delivered to the required height.
The working principle of a centrifugal sump pump
Every centrifugal pump works on the principle that when a certain amount or mass of fluid is rotated by an external source (force), it is discharged from the central axis of rotation and centrifugal head is impressed which allows it to rise to a higher level.
WHY YOU SHOULD GO FOR A CENTRIFUGAL PUMP INSTEAD OF A RECIPROCATING (DISPLACEMENT) TYPE
If you have ever went to price a pump, you would discover that certain pumps cost more than others with slightly no much difference, why? If you go into the detail of its construction, you will discover that the costlier one is a reciprocating type pump. Therefore, one of the merits of a centrifugal pump over the reciprocating pump is the cost. For the fact that centrifugal pumps come with fewer working parts, they cost less than the reciprocating type.
Easy, cheaper installation & maintenance- centrifugal pumps are less complex in construction, as such, they are easily and cheaply installed and maintained. With a centrifugal pump, you don’t need an expert to install it, and even after installation, the cost of maintenance is variably low compared to a reciprocating pump. If you have a good psychomotor or technical ability, you should be able to install and maintain the pump with the help of the manufacturer’s installation and maintenance manual, but if you are not good at this, get a professional to assist you.
High discharging capacity- if you are expecting to discharge a high volume of fluid per time, then go for a centrifugal pump instead of a reciprocating pump.
Low space requirement- everyone is looking for a way to conserve space in his or her environment, gone are the days of free space. Centrifugal pumps are much more compact than reciprocating pumps of same capacity and energy transfer.
Better for lifting highly viscous liquid- I don’t know your reason of wanting to buy a pump, but if for any reason, you want to lift liquids of high viscosity (let say, thickness, e.g. sewage, paper pulp, oil, muddy, sugar molasses, etc), then go for a centrifugal type pump. Mechanically and technically, I would have loved to explain this in detail, but spce wouldn’t permit me to do so, just go for a centrifugal pump.
Operates at very high speed- cavitation and separation, are unallowable limitations in pumps, therefore, if you desire a pump that can operate at a very high speed without risk of cavitation and separation, go for a centrifugal pump.
Easy synchronization with electric motor- a centrifugal pump can easily or directly be coupled to an electric motor or even an oil engine for better performance.
All said and done, the reciprocating pump has a higher efficiency, and is better employed for high heads and small discharges, for example, in lifting oils from very deep oil wells.
HOW TO MAINTAIN YOUR CENTRIFUGAL PUMP
One of the greatest challenges with all pumps, is losses and efficiencies. If you are able to prevent losses, the pump’s efficiency will be normal and okay to serve you. Remember, this pump has a hydraulic system, therefore, there must be a hydraulic loss. Some these losses are uncontrollable though, little can be done to assist.
- Shock losses at entrance to the impeller and at the exit: ensure they are airtight, by ensuring that the rubber washer is always in good shape and prevention from tampering
- Losses due to friction in the impeller: you have nothing to contribute here, but you can assist by making sure the pump is not overloaded to avoid the impeller working at forced condition
- Friction and eddy losses in the guide blades, diffuser and casings: regular checks ups at intervals can help detect early damage
A pump like I mentioned earlier, is a mechanical device, therefore, there must be losses due to mechanical components.
- Frictional losses arising from the bearing and glands: greasing during maintenance will assist
- Losses due to disc friction between the impeller and the liquid which fills the clearance spaces between the impeller and casing: you have nothing to contribute here
Finally, we have losses due to leakage of liquid: ensure all fittings are airtight always. Regular checkups would assist
Note: always ensure that the centrifugal pump runs and discharges at its ‘designed speed’, to ensure maximum efficiency. If the discharge is either increased or decreased, its efficiency will also drop. This is as a result of head loss due to shock at entry to impeller.
CAUSES OF CAVITATION IN CENTRIFUGAL PUMPS
Cavitation is an undesirable effect in hydraulic machines. Cavitation begins to show up in centrifugal pumps when the pressure at the suction falls below the vapor pressure of the liquid. Its intensity increases with the decrease in the value of NPSH (the net head I meters of liquid that is required to make the liquid flow through the suction pipe from the sump to the impeller).
Effects of cavitation
- Erosion of surface
- Noise and vibration
- Sudden drop in head, efficiency and power delivered to the fluid
To control the onset of cavitation
- Ensure that the suction is not restricted
- Avoid running the pump at too high specific speed
- Ensure the flowing liquid is not at very high temperature
- Avoid high runner speed
HOW DO YOU PRIME A CENTRIFUGAL PUMP?
Priming for those who do not know already, is the process of filling the suction pipe, casing of the pump and a portion of the delivery pipe completely from outside source with the liquid to be raised, before starting the pump, to remove any air, gas or vapor from these parts of the centrifugal pump.
If a centrifugal pump is not primed before it is started, air pockets inside the impeller may give rise to vortices (whirlpools) and cause discontinuity of flow. Also running the pump dry, may result in rubbing and seizing of the wearing rings and cause serious damage.
Procedure for priming
- Small pumps: continue to pour liquid into the funnel provided for this purpose. While priming, open the air-vent valve provided in the pump casing, to allow for air escape. Continue priming until all air from the suction pipe, impeller and casing has been removed.
- Large pumps: evacuate the casing and suction pipe by a vacuum pump or by an ejector; the liquid is thus drawn up the suction pipe from the sump and the pump is filled with liquid.
COMMON PROBLEMS ENCOUNTERED WITH CENTRIFUGAL PUMPS AND THEIR REMEDIES
- Pump fails to start pumping:
o Pump may not be properly primed. (Reprime the pump)
o Total head against which pump is operating may be much higher than that for which it is designed. (Check the head with accurate gauges, reduce the head or change pump entirely)
o Impeller may be clogged. (clean the impeller)
o The rotation of the impeller may be in the wrong direction. (change the direction of rotation)
o Too high suction lift. (reduce the suction lift)
o Low speed. (increase the speed)
- Pump is not working up to capacity and pressure:
o Leakage of air into the pump. (plug the leakage)
o Some of the parts are damaged due to excessive wear and tear. (replace the worn out parts)
- Pump stops working:
o Presence of air in suction line. (remove the air by priming and plug the entry of air)
o High suction lift. (reduce the suction lift)
- Pump has very low efficiency:
o Speed may be too high. (reduce the speed)
o Head may be too low and the pump delivers the liquid in large quantity. (reduce the discharge or change pump)
o Pump may be operating in wrong direction. (correct the direction of rotation of impeller)
o Shaft may be bent, the impeller may be touching the casing, stuffing boxes may be too tight, wearing rings may be worn. (repair the affected parts)
HOW TO SELECT A PUMP
If you are considering buying a pump, below are the major things you must consider before concluding on the pump to purchase:
- Discharge value
- Head and speed. From the values, you can work out the specific speed of the pump and its type
- Type of impeller:
o Shrounded type- best for pumping fresh clean water
o Unshrounded or propeller type- for pumping solid-liquid mixture or near plastic material, molasses, etc
o Mixed flow impellers with diffuser blades- used for deep well or submersible pumps
- Go for axial flow pumps if you need a very low head of about 6 meters and for large discharges
- When you need a large head, go for radial flow pumps
If you encounter any difficulty or need further clarification on anything concerning centrifugal pumps, get back to me and I will be at your service. Thank you.