Positive displacement pumps are constant flow machines that force a fixed fluid volume into the discharge pipe, producing the same flow regardless of pressure. They require safety valves and small clearances to minimize leakage, and operate at slow speeds to prevent erosion and wear.

When would you choose a PD pump? Typically, PD pumps are selected for the following scenarios:

High Viscosity Products
(up to 1,000,000+ cps)

• As viscosity increases, flow actually increases. This is
  because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency.

Variable Viscosities

• Many liquids vary in viscosity depending on temperature or due to chemical reaction. A rise in viscosity will independently alter the flow rate and efficiency. Add to that the rise in pressure due to the increase in frictional line losses and PD pumps become the clear choice for variable viscosity applications.

Metered Flow

• A fixed volume of liquid is moved per revolution of a PD pump. Flow quantity can easily be metered by adjusting the speed of the pump.

High Pressure Conditions
(up to 500 psi)

• Pressure limits will depend on the design of each pump, but pressures of 250 psi (580 feet) are not unusual for a PD pump, with some models going over 3,000 psi (7,000 feet).

Materials with Particulates 

• Due to its gentle pumping action, PD pumps are able to handle particulates with minimal damage to the product.

Shear Sensitive Products

• Generally speaking, pumps tend to shear liquids more as speed is increased and centrifugals are high speed pumps. This makes PD pumps better able to handle shear sensitive liquids.

A diaphragm pump is a positive displacement pump using a rubber, thermoplastic, or Teflon® diaphragm and valves to pump fluid. It has two air chambers, air valves, fluid housing, inlet and discharge manifolds, diaphragms, shaft, mufflers, and exhaust port, delivering specific flow per stroke or cycle.

Air Chambers: The pump has two chambers, one on the left side and the other on the right side of it. These
chambers let the compressed air flow in and out of it.

Air Valve: The compressed air is directed to air chambers with the help of air valves. These have a valve cup and a valve plate. Air valves make sure that the compressed air enters the air chambers and leave from it through the exhaust port.

Check Valve: There are four fluid check valves in a double diaphragm pumps. Two of them are inlet check valves while the other two are outlet check valves. The flow of liquid in the fluid housing and manifolds is controlled by these check valves.

Fluid Housing: Each pump has fluid housing, one at each side of the pump. As the name implies, fluid housing is that part which holds the fluid and makes it flow through the pumping mechanism.

Inlet Manifold: Fluid enters the pumping container via the inlet manifold and flows evenly to the left and right fluid housing. This mechanism makes the distribution of fluid equal so that both fluid housings remain in operation.

Outlet or Discharge Manifold: When the fluid is coming out of the container, it passes through a couple of components. First, the fluid passes through one of the exit check valves and then this check valve directs the fluid to the outlet manifold to finally exit the container altogether.

Diaphragms: The air operated double diaphragm pump obviously has two diaphragms in it. The diaphragm is actually a kind of a separation sheet in between the air chambers and fluid housings. The diaphragms are good enough to adjust themselves according to the rise or fall of the air pressure, as the condition may be. Besides, the two diaphragms are allied with a shaft.

Muffler: The objective of muffler is to control noise of the exhaust air. There are multiple mufflers available that offer several levels of noise reduction to ensure effective and efficient pumping operation.

Exhaust Port: The exhaust port is the final exit point in the pump.

A centrifugal pump is a rotodynamic pump with a rotating impeller, used for liquid movement. It comprises a casing, impeller, shaft, bearings, and seals, with the impeller providing acceleration and the shaft rotating at motor speed.Casing – also known as the volute, is the outside visible part of the pump. For sanitary processing, the casing is typically a heavy-walled 316L stainless configured in a spiral design to even out flow and minimize turbulence. The end cover is clamped on and can be easily removed for access to the impeller.

Impeller – The impeller is the main rotating part that provides the centrifugal acceleration of the product. The impeller can have an open or closed vane. Generally closed vane impellers develop higher pressures but have a lower capacity. Open vane
impellers develop lower pressure but have a higher capacity. It is attached to the shaft and rotates inside the casing at the speed of the shaft. The design is balanced to prevent vibration.

Shaft – The shaft rotates insides the casing at the speed of the motor and transfers the torque from the motor to the impeller. The shaft is typically made of 316L stainless.

Bearings – The bearings support the shaft and keep it in alignment so that it does not wobble inside the casing and prevents it from touching the casing.

Seals and/or Packing – The seals are the essential area in terms of hygiene as they prevent the product from leaking back inside the pump or outside of the pump when it is under pressure. Pumps can have either single-seal or double-seal arrangements.