(Newswire.net — July 30, 2021) —
What is a Positive Displacement Pump?
A positive displacement pump is commonly used for professional applications such as pumping viscous liquids or liquids containing suspended or brittle solids. These pumps generally may not have the same flow rate as a centrifugal pump, but they can create more pressure.
Working principle of the positive displacement pump
The positive displacement pump displaces fluid by trapping a fixed volume of liquid and forcing (replacing) the trapped volume in the drainpipe or drain system. Some positive displacement pumps use an expansion chamber on the suction side and a reduction chamber on the discharge side. As the suction cavity expands, fluids flow into the pump, and when the cavity collapses, fluids exit the discharge port. During each pump run, the volume remains constant.
Positive displacement pumps do not use impellers but instead pressurize rotating components or equivalent to push the fluid directly into the closed cavity until there is sufficient pressure to transfer the fluid to the drainage system. This pump does not rely on the passage of liquids through the impeller to increase the speed of the liquid like a centrifugal pump. Therefore, the fluid velocity in the positively displaced pump is less than that of the centrifugal pump. This is often a desirable feature for some applications, such as when pumping environments containing brittle solids.
Types of Positive displacement Pumps
There are several types of positive displacement pumps on the market, and they are generally classified according to the mechanism used to displace fluids. For example:
1) Rotary Positive Displacement Pump
This includes gears, screws, blades, peristalsis, cams and screws, all of which use rotating parts to transfer fluid in and out of the pump reservoir. Some rotary pumps, such as gear pumps, must have a compressed distance between the rotating element and the chamber wall and between the rotating parts, which means that they cannot be used in general. Other types, such as lobe pumps and screw pumps, are designed to transport liquids containing solids.
2) Reciprocating Pump
The reciprocating pump has one or more sets of check valves at the inlet and outlet of the pump to help guide fluid through the pump and prevent backflow.
These pumps use different reciprocating components such pistons, diaphragms, pistons, hydraulic systems and others to periodically expand and expand the chamber.
3) Linear Pump
A chain pump is a water pump in which many discs are in an endless chain. Part of the chain is submerged in water, and the chain passes through a tube slightly larger in diameter than the disc. When the chain is pulled into the tube, water is trapped between the discs, rising and draining from the top.
4) Diaphragm Pump
The diaphragm pump is a special type of positive displacement pump that uses the corresponding movement of a flexible diaphragm to transfer fluid in and out of the pump housing.
The flexible diaphragm creates a vacuum in the chamber, which enters the chamber and sucks in liquids. When the diaphragm moves in the opposite direction, it reduces the volume of the pump chamber and removes fluid from the pump drain port.
Diaphragm pumps use pump casing inlet and outlet control valves to ensure liquid flows in one direction and the other with no return leakage.
Positive Displacement Pump Features
Theoretically, it can produce the same flow at a given speed (RPM) regardless of the discharge pressure, unlike centrifugal pumps or dynamic rotor pumps, positive displacement pumps. Therefore, a positively displaced pump can be considered a constant flow device. However, as the pressure increases, a slight increase in internal leakage prevents a constant true flow rate.
In use, a positive displacement pump cannot operate with the valve closed on the outlet side of the pump because it does not shut off like a centrifugal pump. When the pump is operating under a closed drain valve, it continues to generate flow and the pressure in the drain line increases until the hose bursts or the pump is badly damaged, or both. Therefore, to avoid this, it is usually necessary to install a pressure relief valve or a safety valve on the discharge side of the displacement pump. The safety valve can be installed inside or outside the pump.
Pump manufacturers can usually supply internal auxiliary valves or safety valves. The internal valve is usually used only as a safety precaution. Still, the external pressure relief valve provides added safety on the discharge line and the return line of the suction or supply tank return line. For a better understanding, you also need to understand what is a water velocity meter.
Advantage of Positive Displacement Pump
In general, positive displacement pumps are suitable for applications that require constant flow. They produce moderate to high pressures and are often a great way to pump oil and other liquids.
Positive displacement pumps are also useful for applications that require a combination of low flow and high pressure.