When it comes to selecting the correct transfer pump for your application, there are many things to take into consideration. The two main types of pumps that will be a major part of your selection process are the positive displacement pump and centrifugal pump.
There are some very major differences between these two types of transfer pumps, and choosing the correct pump between these two types can make or break your application. To understand the differences, let's take a look at the differences between the two designs.
A centrifugal pump, sometimes called a Kinetic pump, is a time tested design that is fairly simple.
Basically, a centrifugal pump uses a rotor spinning at high RPMs to create a pressure differential between the inlet and the outlet of the pump head. This pressure difference draws product into the pump, and creates enough centrifugal force to "throw" the product out the outlet.
While centrifugal pumps do have some advantageous qualities in terms of extremely high potential flow rates* and the ability to pump fluids with a certain amount of particulate matter, there are also some serious limitations on the centrifugal pump design.
A centrifugal pump loses the ability to pump very quickly as pressure increases. The high flow rate of an average 2" centrifugal pump can be stopped completely at as low as 20 PSI. The centrifugal pump is also limited in the range of viscosities it is able to pump due friction loss, and is not well suited to pump products above 850 CST.
Positive displacement pumps
A positive displacement pump operates based on a slightly more complicated design, but features an expanded range of operation as compared to centrifugal pumps.
There are several design variations of positive displacement pumps, including:
- Reciprocating pumps (piston, plunger, and diaphragm)
- Power pumps
- Steam pumps
- Rotary pumps (gear, lobe, screw, vane, regenerative, and progressive cavity)
Simply put, positive displacement pumps use forces applied to the boundaries of a product to create the force that moves that product through the pump. This means that by expanding the area on the inlet side and collapsing that area on the outlet side, a cavity is constantly being created and filled by the pump.
A positive displacement pump does have a more complicated design, and cannot generate the sheer volume a centrifugal pump can produce, but there are also some major advantages to using a PD pump.
Positive displacement pumps are capable of maintaining a consistent GPM output at varying pressure, and are able to operate at higher pressures than a centrifugal can handle. Positive displacement pumps are also able to self-prime and handle high viscosity products very well.
So which is better?
The answer is:
Each transfer pump is made for a different purpose, and each type of pump has a unique set of strengths and weaknesses.
If you want to pump water at 1000 GPM with no back pressure for as cheap as possible, consider a centrifugal pump. If you want to pump motor oil at over 100 GPM and pump it up and out of a tank, you are going to need a positive displacement pump.