Positive Displacement Pumps – what they are & how they differ from centrifugal pumps
More engineering training people, this time on positive displacement pumps. There are basically two types of pumps, centrifugal and positive displacement.
In one of the first posts to the site, we began with centrifugal pumps and some of their characteristics. This post will deal with the other half of those pump classes – positive displacement.
It’s important that you know the differences between the two classes, mainly because the type of pump you choose, or that should be installed on that system, will be dictated by the fluid being pumped and the purpose of the system.
A building or stationary engineer should be able to look at a pump and be able to give out at least some rough classification characteristics like if it’s a positive displacement type or centrifugal. You should also be able to determine what type of pump SHOULD BE installed based upon the system and purpose. How do you tell? No idea, but the information below just may help.
What is a Positive Displacement Pump?
The primary characteristic that ALL positive displacement pumps share is that with every stroke of the pump – the exact same amount of fluid will be pumped each time if operated at a constant or the same speed, regardless of system pressure downstream.
This makes these pumps hot commodities when a high pressure and/or an exactly metered output is desired.
This is also the main difference between this class of pump and the centrifugal pump. A centrifugal pump varies its output based upon what is happening further down the system and piping. A positive displacement pump don’t care what system pressure is son. He’s putting out the same discharge pressure with every stroke or revolution.
In a rotary pump for example, which is positive displacement, for each revolution of the pump, a fixed volume of fluid is moved regardless of the resistance against which the pump is pushing.
A positive displacement pump has both good and dangerous characteristics because of this.
Positive because it will always try to pressurize more fluid and send it down line. Dangerous because it WILL ALWAYS try to do this while it is running.
Therefore, any blockage in the system could quickly cause damage to the pump or a rupture of the system components or piping. Whoever is running the pump must always ensure that the system is properly lined up so a complete flow path exists for fluid flow.
Because they always displace a positive amount of fluid, this class of pumps require a relief valve to protect the pump and piping system. The relief valve will lift at a preset pressure, thus preventing the fluid being pumped from actually “hitting” the system fully. Usually these relief valves return the fluid to either the suction side of the pump or back to the supply tank.
One plus of a positive displacement pump is that they are virtually self priming. This is another way they differ from centrifugal pumps. Centrifugal pumps typically are located BELOW the fluid they are pumping.
In this way, gravity is helping the centrifugal pump. The fluid is always falling down into the suction, like water into a sink drain.
A positive displacement pump like the rotary type, makes that fluid flow by creating a suction pressure. Picture a syringe. I don’t know if you ever played with them in the tub or a pool but they act like a positive displacement pump.
When you draw back the plunger on the syringe, the liquid gets drawn into the chamber. Same principle with these pumps. A rotary pump can even be located above the fluid being pumped. Usually the fluid is stored in a supply tank that is vented, or open, to the atmosphere.
This means that it can feel atmospheric pressure. The 14.7 psia we have talked about before is then pushing down on the surface of the liquid. If the pressure outside of the vented tank IS GREATER that the pressure on the suction side of a positive displacement pump, fluid gets sucked into the suction, or intake.
Types of Positive Displacement Pumps
A number of types, besides rotary, are included in this classification. To name a few: gear pump, screw pump, and moving vane pump. Some main features of common types of positive displacement pumps are described below.
Rotary pumps are designed with very small clearances and tolerances between the rotating/moving parts and the stationary parts to minimize leakage from the discharge side back to the suction side. Rotary pumps are designed to operate at relatively slow speeds to maintain these tight tolerances. Operation at faster speeds can cause erosion and lead to excessive wear. Erosion and excessive wear lead to increased clearances with a subsequent decrease in the pumping capacity.
Rotary pumps are useful for pumping oil and other heavy viscous liquids. Rotary pumps can be used to move lube and fuel oils and are good for liquids that cover a wide range of viscosities.
A simple gear pump can have two spurred gears that mesh together and revolve in opposite directions. One is the driving gear, and the other is the driven gear.
The clearances between the gear teeth and the casing and then between the end face and the casing are tiny, as in only a few thousandths of an inch. As the gears turn, the gears separate and fluid flows into the now vacated openings.
This creates the suction that draws the liquid into the pump, like the syringe example above. The liquid is then transported along in the openings formed by the gear teeth and the casing. On the discharge side, the liquid is sent out due to the meshing of the gears and essentially squeezed or forced out through the discharge side of the pump.
A gear pump sort of operates like when you roll up the end of a tube of toothpaste trying to get every last ounce of Crest.
As you roll it up and create those folds in the tube, you are forcing the paste up towards the opening, a lower pressure area. If you left the cap on would the toothpaste go anywhere?
Gear pumps can be used in petrochemical applications, as well as paint, resins, pulp, paper, and food.
There are a bunch of different types of screw pumps. A couple of the differences between the types of screw pumps are the quantity of inter meshing screws and the pitch of the screws.
In a screw pump, fluid is trapped and forced through the pump by the action of rotating screws, sort of like the gear pump. As the rotor rotates, the fluid flows in between the threads at the outer end of each pair of screws. The threads carry the liquid along within the housing to the center of the pump where it is discharged.
Screw pumps are often used to pump fuels or oils.
Sliding Vane Pumps
The sliding vane pump is perhaps the most intimidating to try and understand. A sliding vane pump has a cylindrical bored housing with a suction inlet on one side and a discharge outlet on the other side.
A rotor that is smaller in diameter than the cylinder it is housed in is driven about an orbit path that is arranged so that as it rotates, it provides a minimum clearance between the rotor and cylinder at the top and provides a maximum clearance at the bottom.
The rotor carries vanes, that move in and out as the rotor rotates, to maintain sealed spaces between the rotor and the cylinder wall. The vanes trap liquid on the suction side and carry it to the discharge side, where contraction of the space expels liquid through the discharge line. Again, almost squeezing it out.
Vane pumps are used for oil service and transfer, fueling and defueling and for handling lighter viscosity fluids.
Positive Displacement Pump Classifications
Of course these pumps also break down into classes, in this section we will look at some of those. Classification of rotary pumps is generally based on the types of rotating element. Generally they fall into either the rotary or reciprocating style.
Reciprocating positive displacement pumps usually have pistons and cylinders, like an engine or compressor.
A rotary type pumps in a more continuous manner. Like gears turning, squeezing fluid through.
If you operate positive displacement pumps, you must be aware of certain practices:
- Never run one with the discharge valve closed due to over pressurization.
- Test the proper operation of the pump relief valves every so often.
- Verify the pump has a positive suction head.
Now, we have covered the two main classes of pumps – centrifugal previously and positive displacement pumps here. You should pretty much be an expert now.
Positive displacement pumps discharge the same amount of fluid with each stroke or revolution. They do not care what system pressure is, this is not how centrifugal pumps operate.
Positive displacement pumps can be located above the liquid being pumped because they create a lower pressure on the suction side so the liquid is drawn in.
There are many different types of positive displacement pumps such as the gear and screw types, as well as the sliding vane style and even one called a lobe pump. Classifications of these pumps is usually done by the type of rotating element.
Not sure, if there is any more information out there about these pumps. If you happen to have more knowledge or would like to make a comment, please share. We are always looking for new voices.