Basic Knowledge of Pumps Selection and Pump Alignment

by Newgram

Introduction

A pump is an equipment commonly used in industrial and commercial building to transfer fluid such as gas, liquid and/or slurries from one location to another by conversion of energy from electrical, fossil fuel (gasoline, diesel), air or other form to mechanical energy (kinetic and potential). The function of a pump is to elevate the fluid to a higher level (in most cases), provide pressure for some application (sprinkler, lifting device and washing device) and overcome frictional losses during conveyance.

The history and first invention of pump can be traced back to Egyptian in  2,000BC when water was raised using a long suspended rod with a bucket at one end and counter weight at the other. The pump we used today was improvise by Archimedes (ancient Greek physicist and mathematician) and is considered one of the greatest inventions of all time known as Archimedes screw that could raise water from river so that it can be used for irrigation.

Types of Pumps

Centrifugal Pumps:

Pumps can be classified as dynamic pumps and positive displacement pump. Most dynamic pumps are designated as centrifugal pump where the impeller enclosed within a stationary housing and it does not have a fixed closed volume. Fluid will be forced into center of the housing and set into rotation by rotating impellers. The resulting centrifugal force will accelerate the fluid to a certain momentum until it is thrown to a certain periphery of the impeller into the casing. The casing will collect the liquid and convert the high kinetic energy into potential energy before directing it out from the pump outlets to piping.

Centrifugal pumps can be single or multistage depending on requirement. A single stage pumps has only single impeller disc while multistage pumps have more than one impeller disc connected in series where the outgoing flow of fluid from first impeller will be directed into the center of second impeller disc and etc.

3 sub types of the centrifugal pumps are shown in table 1.1 below:-

For speciality pumps, a jet pumps are kinetic pumps attached with an ejector at discharge outlet using venture effect and motive fluid to generate potential energy. A turbine pump is another type of centrifugal pumps that uses rotary mechanism with pressure through several impellers and vanes to transfer energy to a fluid. Finally, a cantilever pump uses a long cantilever used in sump pump application.

Positive Displacement Pumps:

A positive displacement pumps trap a fixed volume fluid in a cavity through an inlet using mechanical force to direct the fluid through the system. This system is known as positive displacement as a direct force is applied to the confined space fluid, and when cavity closes, fluid will be squeezed through an outlet. The flow rate of positive displacement pump depends on the size of enclosed chamber and the rate of moving component in the pump. The flow rates are controlled by the drive speed of the pump and in each cycle, the fluid pump out is equivalent to the cavity.

The biggest advantage of positive displacement pumps allows high viscosity of fluid to be transferred compare to centrifugal pump. Most of the pump performance is independent from the fluid viscosity. A positive displacement pump is usually more expensive than a centrifugal pump for the same size in comparison as a positive displacement pump requires a sturdy design to withstand the pressure build up within the casing and a pressure relief valve is required in some cases to prevent excessive build up in pressure.

Table 1.2 below shows a comparison between positive displacement pump and centrifugal pumps

Sizing of Pumps

When sizing a pump, one should have understanding on the concept of cavitation.  Cavitation is the formation of vapor cavities in liquid due to rapid changes of pressure when the pressure is low. Upon increase of momentum within the pump confined space, pressure will build up and causes shock-wave. This will results in loss of production, equipment wear and tear or even casualty depending on situation. Depending on the type of fluid requires to be transferred, there is two type of pump mentioned above is commonly used in industry. During the sizing of a pump, pump manufacturer will post several questions such as flow rate requires, head requirement, material of construction (corrosive, hygienic design, abrasive). Bear in mind that most factories these days requires expansion and change in process which may change the sizing of the pump. A pump that works yesterday may not work perfectly in the current stage as the condition has alter which moves the pump graph away from its Best Efficiency Point (BEP). This will help to reduce in expansion cost in the future. Working capacity should always be designed 20% above the current requirement to cater for changes in process or expansion.

In selection of a pump, the parameter that may affect a pump capacity, Net positive suction head (NPSH), specific gravity, fluid viscosity, solid content and other parameter should be spelled out to a supplier to avoid disagreement prior purchase.

Pump capacity is the first and most important parameter in selection of pumps. Capacity is most commonly measured in gallon per minute (GPM) or cubic meter per hour (m3/hour) or litre per minute (l/min). In other word, it is the capability or flow rate of a pump to deliver the fluid to require destination within the time frame. A minimum and maximum flow rate is required to be specified to avoid pump overheating, mechanical damage or even pipe damage during sizing (if existing pipe already exist).

Another important parameter that requires consideration during sizing would be the Net positive suction head (NPSH) which is to prevent cavitation of liquid vaporization. This is the total head at the suction flange of the pump minus the vapour pressure converted to fluid column height of the liquid (liquids vapour head). The NPSH available should be calculated during the design and construction of system. NPSH available should always be higher than NPSH required which can be obtained from the pump manufacturer. Remember, pump can only pump only liquid in most cases and not vapour because the vaporization of a liquid increase the volume greatly (1m3 of liquid can be vaporize into thousands of meter cube of steam). As a consequence of rapid expansion of liquid into vapour, the temperature will increase while pressure drop in liquid causing pump to stop functioning as pump will have insufficient of pressure

Other parameter such as specific gravity determines pump heads require, pump type and pump power requirement. Maximum temperature will affect type of mechanical seal, coolant requirement. Solid content in a fluid should also be considered during the sizing stage as this impact erosion in pump wall, disintegration of particle, mechanical seal design, shaft sealing, impeller design and etc.

Considerations when sizing the pump system is available for download from the link below.

Consideration when sizing the pump

Disclaimer

Information provided in this article are provided “as is” without warranty of any kind, either express or implied, including without limitation warranties of merchantability, fitness for a particular purpose and non-infringement. The author provides, with reasonable effort, accurate and up-to-date information in this article. It does not, however, make any warranties or representations as to its accuracy or completeness. It periodically adds, changes, improves or updates the information and documents in this article without notice. The author assumes no liability or responsibility for any errors or omissions in the content of its article. You are using the article at your own risk. Under no circumstances shall the author or any other party involved in creating, producing or delivering the contents of this article be liable to you or any other person for any direct, indirect, special, incidental or consequential damages arising from your access to, or use of, this article.

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