Displacement pumps create pressure imbalances to move liquids, with positive displacement pumps having expanding suction and decreasing discharge cavities. Safety valves are necessary to prevent bursts and spills. Reciprocating and helical root pumps are the two main types, with diaphragm pumps used for hazardous materials and helical root pumps for high viscosity liquids.
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 used only with liquids and is a simple method of 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 the liquid flows, the suction side expands. This changes the pressure inside the pump, which causes liquid to flow into the discharge cavity to balance the internal pressure. As the liquid moves, the suction cavity narrows, recreating the original pressure imbalance.
There is no locking head in this type of pump, so care must be taken to ensure that the safety or relief valve is operational. Correct use of this valve ensures that the proper pressure balance is maintained within the pumping system. If the discharge cavity is closed, but the pump is running, the line or tube in which the liquid moves may burst. This would be caused by the increasing amount of liquid being pushed towards the drain cavity. If the line breaks, all of the liquid that was trying to pass into the discharge cavity would spill out of the pump, creating a big mess.
There are two main classes of positive displacement pumps: reciprocating and helical root. A reciprocating pump is also known as a piston 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 fluid into the cylinder.
A plunger pump generates a lot of wasted energy at opposite ends of the pumping cycle. A diaphragm pump uses the same movement of the pressurized plunger to move the diaphragm within the pump. 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 root 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 helical rotors twisted at right angles to generate a low pulsation frequency. A triangular shaped sealing line is used at both the inlet of the suction and discharge cavities to ensure a continuous and even flow.
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