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Positive displacement pumps move liquid by creating a pressure imbalance within a sealed environment. This imbalance causes the liquid to move from one location to another in an attempt to balance the pressure. This type of pump is only used with liquids and is a simple method for creating a constant flow.
In positive displacement pumps, there are two cavities: an expanding suction cavity, and a decreasing discharge cavity. At the beginning of the cycle, the suction cavity is smaller than the discharge cavity. As liquid flows in, the suction side expands. This changes the pressure within the pump, which causes the liquid to flow to the discharge cavity in order to balance the internal pressure. As the liquid moves, the suction cavity shrinks, recreating the original pressure imbalance.
No shutoff head exists in this type of pump, so care must be taken to ensure that the relief or safety valve is operational. The proper use of this valve ensures that the appropriate pressure balance is maintained within the pumping system. If the discharge cavity is closed, but the pump is operating, the line or pipe that the liquid is moving through may burst. This would be caused by the increasing amount of liquid being pushed toward the discharge cavity. If the line bursts, all the liquid that was attempting to move into the discharge cavity would pour out the pump, creating a large mess.
There are two main classes of positive displacement pumps: reciprocating and helical roots. A reciprocating pump is also known as a plunger or diaphragm pump. The plunger pump has a cylinder head, where both the suction and discharge cavities are located. With the forward stroke, the plunger pushes the liquid out. The suction stroke retracts the plunger and opens the suction cavity to pull the fluid into the cylinder.
A plunger pump generates a lot of wasted energy at the opposite ends of the pumping cycle. A diaphragm pump uses the same pressurized plunger motion to move the diaphragm within the pump itself. This type of pump is used for hazardous materials, as the diaphragm material is used to contain the liquid away from the operator.
The helical roots pump is also known as a Wendelkolben pump. It is used in situations where the liquid has a high viscosity and a smoother pumping action is required. These positive displacement pumps use two right angled helical twisted rotors to generate a low pulsation rate. A triangular-shaped sealing line is used at both the suction and discharge cavity entrances to ensure a continuous, even flow.