What is the Difference Between Piston and Plunger Pumps

Figure 1: Comparison diagram of piston pump (left) and plunger pump (right) from Wikipedia.

Plunger and piston pumps are very similar in construction and both are reciprocating positive displacement pumps. They suck water, oil or other media into the chamber through the inlet valve, and discharge it from the pump body through the outlet valve. These valves are designed as one-way valves, which means that the inlet valve opens under negative pressure and the outflow valve opens under positive pressure.

What is a Piston Pump?

The piston pump relies on the reciprocating motion of the piston in the cylinder, so that the working volume of the pump chamber changes periodically to achieve suction and discharge of liquid. It consists of a pump cylinder, a piston, inlet and outlet valves, inlet and outlet pipes, a connecting rod and transmission device, as shown in the following picture.

During the reciprocating motion of the piston, when the piston moves outward, the outlet check valve is closed under the action of its own weight and the pressure difference, and the inlet check valve is opened under the action of the pressure difference, and the liquid is sucked into the pump chamber.
When the piston moves inward, the pressure in the pump chamber rises, so that the inlet check valve is closed, the outlet check valve is opened, and the liquid is pressed into the outlet pipe.

Figure 2: Piston pump diagram.

What is a Plunger Pump?

The plunger pump is an important device in the hydraulic system. It relies on the reciprocating motion of the plunger in the cylinder body to change the volume of the sealed working chamber to achieve oil suction and oil compression.

The plunger is connected to the rotor by a connecting rod. As the ceramic plunger reciprocates, it creates suction pressure that draws fluid through the inlet. As the rotor continues to turn, the plunger is pushed back down, delivering fluid through the discharge valve. The high pressure seal of the plunger pump remains stationary, allowing the plunger pump to be used at much higher pressures than the piston pump.

The plunger pump has the advantages of high rated pressure, compact structure, high efficiency and convenient flow adjustment. Plunger pumps are widely used in occasions where high pressure, large flow and adjustable flow are required, such as hydraulic presses, construction machinery and ships.

Figure 3: 3H-8/450 plunger pump diagram.

1. Pump casing assembly 2. Crankshaft assembly 3. Connecting rod assembly 4. Crosshead assembly 5. Plunger connection assembly 6. Plunger assembly 7. Fluid end assembly

The difference Between Piston Pump and Plunger Pump

One of the problems that often confuses beginners in hydraulics is that they cannot tell which is a piston pump and which is a plunger pump. Many times it seems that the piston pump has already referred to the piston pump and the plunger pump.

From the appearance point of view, generally people think that the one that has long diameter is generally a plunger pump, and the one that has small diameter is a piston pump, but this is not a fundamental difference between them. In addition, some differences between piston pump and plunger pump are as follows:

1. The piston of the piston pump will not extend out of the cylinder, while the plunger will do that.

2. The piston generates suction and discharge force by changing the volume in the cylinder, and the plunger generates suction and discharge force by changing the volume outside the cylinder, as shown in the picture 2, 3.

3. The main difference between plunger and piston is sealing. The plunger pump adopts a static seal, and the seal is on the cylinder body, while the piston pump adopts a dynamic seal, and the seal is on the piston. Depending on the machining accuracy, the clearance between the plunger and the cylinder is very small, so as to achieve the sealing effect. The clearance between the piston and the cylinder is larger, and the seal is achieved by the close contact between the piston ring on the piston and the cylinder.

Because of the different sealing methods, their shapes are also different. In order to seal the plunger well, the length of the plunger is often lengthened, while the length of the piston is shorter. Similarly, the manufacturing cost is also high due to the high precision required for plunger machining.

4. Piston pump seals wear faster and cannot withstand high pressures compared to plunger pumps. As the seals wear, the power washers will be affected by the accumulation of decompression, resulting in weakened flow and inefficient operation.

5. The plunger and the piston are both reciprocating components, but the plunger can only discharge the medium on one side (such as a needle for injection), and the piston can discharge the medium on both sides (such as the cylinder piston on the pneumatic valve).

Figure 4: Double acting reciprocating piston pump.

What is the Advantage of the Plunger Pump over the Piston Pump?

The biggest advantage of plunger pumps compared to piston pumps is that they can be used under higher pressure conditions. The plunger pump is a hydraulic pump that relies on the reciprocating motion of the plunger in the cylinder to cause the change of the sealing volume to achieve oil suction and oil compression. Compared with the piston pump, this pump has many advantages.

Figure 5: High pressure plunger pump.

First of all, the parts constituting the sealing volume are cylindrical plungers and cylinder holes, which are easy to process, can obtain high matching accuracy, have good sealing performance, and still have high volumetric efficiency in high-pressure work.
Second, simply changing the working stroke of the plunger can change the flow rate, which is easy to implement.
Third, the main parts in the plunger pump are all subject to compressive stress, and the material strength properties can be fully utilized.

Because the plunger pump has high pressure, compact structure, high efficiency and convenient flow adjustment, it is widely used in systems that require high pressure, large flow and high power and in occasions where the flow needs to be adjusted, such as gantry planers, broaching machines, hydraulic presses, construction machinery, mining and metallurgical machinery, ships, etc.

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