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Reciprocating positive displacement pumps for the OEM

01 May 2010

Unless one is an expert in fluid dynamics or pump technology, selecting the most appropriate pump for a specific application is not always straightforward. There are many factors that will influence the pump selection process, such as the pressure required and the nature of the fluid being handled, that will determine the type of pumping principle that is most appropriate. Brian Hubbard reports

Whereas an end user customer may buy one pump then adapt and adjust it to suit the process, OEMs expect the pump manufacturer to configure exactly the right pump to the application in order to maximise effective performance. They are also seeking a realistic original purchase cost that is in line with their budgets and justifies the time and effort made to get it right. The reality is that OEMs are looking for the best offering and not just the lowest initial purchase price that might see the product lasting 12 months and one day!

The global pump market is dominated by the roto-dynamic pumping principle, so it is not unusual for designers and OEMs to go down this route when looking for a suitable pump. However, roto-dynamic pumps have limitations in respect of the pressure they can generate and their overall operating efficiency. It is issues such as these that make the reciprocating positive displacement pump an attractive alternative. This type of pump uses a very different technology and as a result, it requires the OEM and potential user to change their mindset when considering a pumping system. Having said that, reciprocating positive displacement pumps are no more complicated to install and operate than many rotary pumps.

With reciprocating positive displacement pumps, there are two advantages to be considered. Firstly, as they are positive they will always deliver a predictable volumetric flow because on the inlet stroke the pump takes in a fixed volume of liquid and physically moves it through and out of the pump with virtually no losses or inefficiencies. This is not the case with the roto-dynamic type pump, which uses inertia to impart the flow. Secondly, the pump output is dependent on volume only, so it doesn’t necessarily matter what the density of the liquid is. Provided the cylinder can be filled, the power input is just a function of the flow multiplied by the pressure. Conversely, with a roto-dynamic pump, the performance output of the pump is proportional to the fluid density.

Triplex reciprocating positive displacement pumps contain three synchronised pistons or plungers, which produce a low-pulsation flow that is sufficiently smooth for most applications. The flow is highly stable and predictable because the flow rate is determined by the running speed of the pump and is virtually unaffected by back pressure, liquid viscosity or specific gravity.

These types should be a first consideration when high pressure and high operating efficiency is required. Where the application pressure exceeds 50bar the loss of efficiency for other types of pumps falls rapidly, which is not the case with triplex types (such as the Cat Pumps CP3 model pictured on this page). In fact, these pumps are actually increasingly efficient at higher pressures. Furthermore, many designs of rotary positive displacement pumps will find it difficult to achieve 50bar unless the liquid is viscous. Conversely, the triplex pump is most widely used in applications where the pumped liquid is of low to medium viscosity and has no inherent lubricity, factors that contribute to a highly diverse range of OEM applications.

There can be no doubt that if the most appropriate type of pump is selected for a specific application and duty, then there will be a benefit to overall performance of the machine or system into which it is installed. Whereas at one time it was not unusual to find OEMs selecting pumps on the basis of the cheapest pump that would work, today there is far more concern about reliability, efficiency, availability of spares, back-up and energy consumption as customers increasingly demand lowest life cycle costs.

Brian Hubbard is general manager, Cat Pumps

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