What is Positive Displacement Pump

metering pumps are reciprocating positive displacement pumps for precise metering

Figure 1: Working principle diagram of the metering pump.

Positive displacement pump is also known as a volumetric pump, which uses the periodic volume changes of the working chamber (pump casing or cylinder) in the pump to transport the liquid. The mechanical energy of the power machine is directly converted into the pressure energy of the conveyed liquid through the pump. The pump flow only depends on the volume change value of the pump working chamber and and the frequency (pump speed) of the volume change in the working chamber, and has nothing to do with the characteristics of the pipeline. The pressure it generates only depends on the characteristics of the pipeline.

1. Classification of Positive Displacement Pumps

Positive displacement pumps can be divided into reciprocating pumps and rotary pumps. The reciprocating pump relies on the reciprocating piston or plunger to push the liquid; the rotary pump, also known as the rotor pump, relies on the rotating parts to push the liquid.

Reciprocating pumps include: piston pumps, plunger pumps, and diaphragm pumps.
Rotary pumps include: gear pump, screw pump, sliding vane pump (rotary vane pump), and peristaltic pump.

types of positive displacement pumps mainly including reciprocating pumps and rotary pumps

Figure 2: Positive displacement pump types.

2. How Does a Positive Displacement Pump Work

2.1 How a Reciprocating Pump Works

The reciprocating pump is a pump that uses the reciprocating motion of the piston to transport the liquid. Relying on the reciprocating motion of the piston, the energy is converted into static pressure energy to transfer the liquid. Since the liquid is incompressible, when the piston pushes the liquid, it can make the liquid withstand a high pressure, thereby obtaining a high lift.

2.2 How a Rotary Pump Works

The rotary pump consists of a stationary pump casing and a rotating rotor, and it has no suction valve and discharge valve. The pump sucks and discharges liquid by the rotation of one or more rotors in the pump. When the rotor rotates, the volume changes between it and the pump casing. In the process of the volume increasing, the liquid is sucked into the pump; in the process of the volume decreasing, the liquid is discharged.

The suction and discharge processes of the reciprocating pump are carried out alternately in the same pump cylinder and are controlled by the suction valve and the discharge valve. The rotary pump is through the rotation of working elements such as gears, screws, vanes or sliding vanes to force the liquid to flow from the suction side to the discharge side.

Items	Gear pump	Double acting vane pump	Single acting vane pump	Axial piston pump	Radial piston pump
Working pressure/MPa	＜17.5	6.3~21	＜6.3	10~14	10~20
Flow rate change	Unadjustable	Unadjustable	Adjustable	Adjustable	Adjustable
Volume efficiency	0.75~0.95	0.80~0.95	0.80~0.90	0.90~0.98	0.85~0.95
total efficiency	0.60~0.85	0.75~0.85	0.70~0.85	0.85~0.95	0.75~0.92
Oil sensitivity	Not sensitive	Sensitive	Sensitive	Sensitive	Sensitive
Self-priming performance	Good	Relatively poor	Relatively poor	Relatively poor	Poor
Noise	Big	Small	Relatively big	Big	Relatively big
Scope of application	Machine tool, construction machinery, agricultural machinery, aviation	Machine tool, construction machinery, hoisting machinery, aviation	Machine tool, injection machine	Construction machinery, mining machine, hoisting machinery, ship, aviation	Machine tool, hydraulic press, ship

Table 1: Main performance and selection range of hydraulic positive displacement pumps.

3. Working Characteristics of Positive Displacement Pump

1. The theoretical flow has nothing to do with the characteristics of the pipeline, but only depends on the pump itself. The flow rate of the positive displacement pump is constant at a certain speed, and it hardly changes with the pressure.
2. The pressure provided by the positive displacement pump only depends on the characteristics of the pipeline and has nothing to do with the pump itself.

3. The shaft power of the pump increases with the increase of the discharge pressure, and the efficiency of the pump increases accordingly. But if the pressure exceeds the rated value, the efficiency will decrease due to the increase of the internal leakage.
4. As the viscosity and the air content of the liquid increase, the flow rate and the efficiency of the pump decrease.

5. Positive displacement pumps generally need to be equipped with safety valves.
6. The flow rate of the positive displacement pump cannot be adjusted by the outlet control valve, and general methods used are bypass adjustment, speed adjustment and stroke adjustment.

7. Be sure to open the outlet valve before starting the positive displacement pump.
8. It is suitable for wide pressure, high viscosity and narrow flow range.
9. The pump speed is low, the conveying liquid has pulse, the average flow is constant, and the suction capacity is good.
10. It has self-priming ability when transporting high viscosity fluids.
11. There is relative movement of friction parts, such as the rotor and stator.
12. The pump features high efficiency, less power and energy saving.

4. Advantages and Disadvantages of Various Positive Displacement Pumps

4.1 Gear Pump (Internal ＆External)

Gear Pump

Advantages

Disadvantages

1. Low pulse, stable flow

2. High pressure up to 20 bar

3. Maximum flow up to 250 m³/hr

4. Maximum temperature up to 300 ℃

5. Able to be used for precise metering

6. Able to be used to transport high viscosity fluids

7. Wide range of applicable materials

8. Suitable for continuous operation

9.Simple structure

10.Able to run in small steps

1.The tooth surface is easy to wear

2.Dynamic seal required

3.Unable to run dry

4.The higher the accuracy, the more expensive the price

5.Limited chemical suitability

6.Not suitable for conveying fluids containing solid particles

7.Not suitable for transporting shear sensitive liquids

Table 2: Advantages and disadvantages of gear pumps.

4.2 Lobe Pump

Lobe Pump

Advantages

Disadvantages

1.Stable flow and small pulse

2.Suitable for conveying high viscosity fluids (up to 200,000 cps)

3.Able to operate at relatively high temperatures

4.Metal lobe pump able to perform dry operation

5.Compact pump head

6.Available in different types and directions of entrance and exit

7.Easy to install

8.CIP available, SIP optional

9.Reverse operation is possible (but the one-way valve does not work at this time)

10.Sanitary level complying with 3A, BISSC and ASME standards

1.Rotors are vulnerable to damage

2.Not easy to install

3.Shaft seal required

4.Complex bearing structure

5.2 drive shafts required - so the wear is doubled

6.Requiring 2 sets of mechanical seal structures

7.Inefficient when conveying low viscosity materials (slipping)

8.Seal failure can lead to leaks

9.Tendency to two-way pollution because of the contact between the pump body and the fluid

10.Not suitable for conveying abrasive fluids

11.Expensive spare parts

12.Rotors may to rub against each other caused by the vibration of the reducer

Table 3: Advantages and disadvantages of lobe pumps.

4.3 Screw Pump

Screw Pump

Advantages

Disadvantages

1.High outlet pressure

2.Able to transport high viscosity materials

3.Able to run at relatively high speed

4.With self-priming ability

5.Low pulse

6.It can be cascaded in multiple stages to increase the output pressure

7.Flow up to 150m³/hr

8.Flexible design

9.Good fluid adaptability, good compatibility, easy installation

10.Self-priming and low noise

11.Reversible operation and low price

12.Good compatibility

13.Capable of conveying higher temperature fluids

1.High processing costs due to high accuracy requirements between the screw and the pump body

2.Sensitive to changes in viscosity

3.High pressure delivery requiring increased pump length

4.Can not run dry (no fluid in the pump body)

5.Dry running protection device is recommended (requiring higher cost)

6.High maintenance cost

7.Many parts in contact with fluid

8.Complex assembly

9.Long maintenance time

10.Serious wear at high operating speed

Table 4: Advantages and disadvantages of screw pumps.

4.4 Diaphragm Pump

Diaphragm Pump

Advantages

Disadvantages

1.Able to run dry (but may cause damage to pump head or diaphragm)

2.Applicable to relatively high temperature fluids

3.Can be used in liquid submersion

4.Self-priming and low price

5.Small size, light weight, easy to transport

6.Can run in small steps

7.No sealing ring or sealing mechanism

8.Easy to install

9.Wide variety of materials and good chemical adaptability

10.Speed control

11.Zero leakage

12.Capable of closed loop control

1.Tendency to stop the delivery when the flow is very low

2.Small suction lift

3.Not suitable for conveying fluids with high solids content

4.Abrasive fluids can damage the spool and seat of the check valve, resulting in loss of flow.

5.Inconvenient maintenance due to numerous components

6.High maintenance costs

7.The valve may be blocked, affecting efficiency and accuracy

8.Loud noise (especially high pressure type), up to 70dBA

9.Big pulse

10.Unable to run in both directions

Table 5: Advantages and disadvantages of diaphragm pumps.

4.5 Piston Pump

Piston Pump

Advantages

Disadvantages

1.Compact

2.Capable of transporting high viscosity materials

3.Low noise

4.Able to be used for filling with an accuracy up to 98%

5.High pressure up to 100-400Bar

1.With many parts and expensive price

2.The check valve of a piston pump is very fragile and easily contaminated or clogged, leading to failure

3.Very high pulse

4.High maintenance cost

5.The pump body is in direct contact with the pumped fluid.

6.Not suitable for abrasive fluids

Table 6: Advantages and disadvantages of piston pumps.

4.6 Peristaltic Pump / Hose Pump

Peristaltic Pump

Advantages

Disadvantages

1.No pollution. The fluid does not contact the pump body

2.High precision

3.Low shear stress, ideal for conveying shear-sensitive, aggressive fluids

4.Good sealing

5.Good self-priming ability, able to idle and to prevent backflow

6.Easy maintenance

7.Capable of pumping liquids, gases, solids or multiphase mixtures. The diameter of the solids contained in the fluid is allowed to reach 40% of the inner diameter of the tubular element.

8.Able to transport all kinds of materials with abrasive, corrosive, oxygen-sensitive characteristics and various foods.

1.Pulse can be generated

2.The pump tube is easily damaged

3.Unable to reach excessive pressure

Table 7: Advantages and disadvantages of peristaltic pumps.

Related Info

What is Positive Displacement Pump?

1. Classification of Positive Displacement Pumps

2. How Does a Positive Displacement Pump Work

2.1 How a Reciprocating Pump Works

2.2 How a Rotary Pump Works

3. Working Characteristics of Positive Displacement Pump

4. Advantages and Disadvantages of Various Positive Displacement Pumps

4.1 Gear Pump (Internal ＆External)

4.2 Lobe Pump

4.3 Screw Pump

4.4 Diaphragm Pump

4.4 Diaphragm Pump

4.5 Piston Pump

4.6 Peristaltic Pump / Hose Pump

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