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Triplex positive displacement pump – the best choice for high pressure liquid stream smooth flow rate

25 March 2014

In any application where a high pressure liquid stream is required the triplex positive displacement (PD) pump is arguably the most suitable choice. Such is the versatility of the technology, the triplex PD pump can be found providing high levels of accuracy and operating safety in many applications within the chemical processing industry, where both benign and corrosive liquids are encountered.

Triplex PD plunger and piston pumps should be the first consideration when high pressure and high operating efficiency is required (Fig.1). Where the application pressure exceeds 15 bar the efficiency for roto-dynamic pumps falls rapidly, unlike triplex PD pumps which actually increase in efficiency at higher pressures. Furthermore, many designs of rotary positive displacement pumps find it difficult to achieve even 10 bar unless the liquid is viscous. Conversely, the triplex PD pump is most widely used in applications where the pumped liquid is of low to medium viscosity and has little inherent lubricity, factors that contribute to a highly diverse range of applications, including process flows, dosing/metering, pressure testing and cleaning.

The triplex PD Pump

The global pump market is dominated by the roto-dynamic principle, mainly centrifugal pump, so it is not unusual for system designers and equipment specifiers to take this as their default choice when considering pumps for specific applications. However, roto-dynamic pumps have major limitations in respect of the pressure they can generate and their overall operating efficiency. It is issues such as these that make the triplex PD pump an attractive alternative. This type of pump uses a very different technology and as a result, it requires the specifier and user to change their mindset when considering a pumping system.

There are two significant advantages of reciprocating positive displacement pumps to be considered. Firstly, as they are almost 100% positive they will always deliver a predictable volumetric flow irrespective of pressure. This is 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. Secondly, the pump performance is independent of specific gravity so the input power and output flow are unaffected by the liquid density. Provided the pump chambers can be filled, the power required becomes just a function of the pump displacement multiplied by the pressure.

A Cat Pumps triplex PD pump employs three synchronised plungers or pistons operating 120° out of phase, which gives it the ability to deliver a predictable and constant flow. This flow is far smoother than that of simplex and duplex pumps. The peak flow in a simplex (single piston, plunger or diaphragm) pump is over three times the mean flow rate and for 50% of each cycle there is no flow at all.  Even a Duplex (twin chamber) pump has a peak-to-mean variation of 157% and twice per cycle the flow drops to zero.  In contrast, for a triplex PD pump (three pumping chambers) in which the pumping stokes overlap, the peak-to-mean variation is only 5%. This is a huge gain compared to the simplex and duplex pumps, and the benefit is vastly reduced flow and pressure variations in the discharge line. The much smoother flow greatly reduces the possibility of system failure caused by excessive pulsation.

Such is the versatility of Cat Pumps product range, applications range from hydrostatic pressure testing to fixed and mobile cleaning systems, misting and humidity control, machine tool cooling, chemical injection, desalination and fire suppression.

The advantages of triplex PD pumps lie in their high efficiency, their ability to generate high pressure and to produce constant volume irrespective of the pressure. Consequently, where an application demands controlled variable flow, they are eminently suited to variable speed drives. Using a triplex PD pump with a variable speed drive can provide very high efficiencies, up to 95% depending on the pump type design and how it has been installed, across a wide range of flow rates and pressures.

Industry Connections: CAT Pumps Ltd UK

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