Positive Displacement Pump versus Centrifugal Pump.

When pumping fluids from one casing to another, there are several variables to be taken into account when choosing what pump should be used. Sometimes it is better to use a centrifugal pump and at other times a positive displacement pump is preferred. In this blog, we take a short look at the way the pumps operate. Then we go further into the differences between the two pumps and give advice on when to use which pump.

What is a centrifugal pump?
Centrifugal pumps are used to transport fluids by converting rotational energy driven by some sort of motor or engine into energy moving the fluid. The fluid enters the impeller of the pump by the rotating axis, where it is accelerated by the impellor towards a diffuser or scroll. The fluids gain both pressure and velocity while being pushed passed the impeller.

What is a positive displacement pump?
A positive displacement pump transports fluids by trapping a fixed amount of the liquid and forcing it into the discharge pipe. The movement is started by two or three spindles that move in opposite directions of each other; pumping and thus displacing the liquid.

What are the differences?
Both pumps displace fluids, however, there are quite some differences between the two.

Mechanics.
The obvious difference between the two is in the way they operate. As displayed above, centrifugal pumps impart velocity to the liquid, resulting in pressure at the outlet. Positive displacement pumps capture confined amounts of liquid and transfers it from the suction to the discharge port. In short, with centrifugal pumps pressure is created and flow results, whereas with positive displacement pumps flow is created and pressure results.

Performance.
Due to the fact that the flow is the result of pressure, with centrifugal pumps the flow varies with changing pressure. Since positive displacement pumps work the other way round, their flow is consistent with changing pressure. Both pump types are able to regulate the flow by changing the speed. The flow rate of a rotating positive displacement pump is proportional to the speed (turndown ratio often more than 10:1) and almost unaffected by the differential pressure. In contrast to centrifugal pumps (turndown ratio 3:1), both multiple applications and product-specific pump trains are needless. Operation can be controlled without liquid treatment or pump throttling. With centrifugal pumps, this is necessary despite the adjustable speed.

Viscosity.
For centrifugal pumps the efficiency decreases when viscosity increases. This is due to frictional losses inside the pump. For this reason, these pumps are typically not used on viscosities over 850 cSt. The performance of a centrifugal pump decreases already from a viscosity of 100 cSt. In contrast, the efficiency of positive displacement pumps increases with increasing viscosity.

Efficiency.
Centrifugal pumps peak at BEP (best efficiency point). At higher or lower pressure levels, the efficiency decreases. Within a window of 80-110% of its BEP, this pump is adequate. For the positive displacement pumps, it applies that efficiency increases with increasing pressure.

Inlet Conditions.
With centrifugal pumps it is indispensable for liquids to be in the pump to create a pressure differential. In addition, a centrifugal pump is not capable of delivering a GVF (Gas Volume Fraction) higher than 15 %. Therefore the suction piping and the pump have to be filled up with liquid. A dry pump will not prime on its own. With positive displacement pumps, a negative pressure is created at the inlet port (suction). The pump must also be filled once with liquid. However, unlike the centrifugal pump, it can also handle large quantities of gas and primes on its own with an NPSHr (Net Positive Suction Head Required) value of down to 1.8 m. This very good suction-lift capabilities can maximize linestripping operations while being able to tolerate the intake of entrained air and other gases without vapor locking.

So, which pump to use?
This question is not as simple to answer as it seems. In general, with increasing viscosity, positive displacement pumps become a necessity. The same goes for working with varying pressure or when working with a dry pump. However, to be a 100% sure on what type of pump to use, get in touch with our expert team at KRAL. Tell us your application and together we can decide on the best pump for your business.

28.10.2019