Piston Pump: Working Principle, Types, and Features

Horizontal rotary piston pump and electric motor

Figure 1: Horizontal rotary piston pump and electric motor.



Piston pump is a kind of positive displacement pump. It relies on the reciprocating motion of the piston in the pump cylinder to make the working volume of the cylinder expand and reduce periodically to suck and discharge liquid. The reciprocating piston pump has self-priming ability, and can maintain almost constant flow under severe pressure changes.

It is especially suitable for conveying viscous liquids under the situation of small flow rate and high lift. The main use of the reciprocating piston pump is to transport special fuel oil and lubricating oil, but also can be a boiler feed pump, etc.

1. How Does a Piston Pump Work?

The piston pump usually consists of pump cylinder, piston, inlet and outlet valves, inlet and outlet pipes, connecting rod and transmission device. Its piston is driven by power (for example electric motor) to reciprocate in the pump cylinder.

The following figure is an example to introduce the working principle of the piston pump. When the piston moves downward, the water inlet port is closed, the water inlet valve above the piston opens, and the water outlet valve is closed. At the same time, the water in the lower chamber of the pump cylinder is pressed into the upper chamber.
When the piston moves upwards, the water inlet port is opened, and water enters the pump cylinder. At the same time, the water inlet valve above the piston is closed, the water outlet valve opens, and the water on the upper part of the piston is lifted up with the piston and into the water outlet pipe. Through such reciprocating motion of the piston, the water is continuously discharged from the water outlet.



The work process of single acting piston pump

Figure 2: The work process of the piston pump.

2. Classification of Piston Pumps

Piston pumps are divided into single and double-acting types; single, double and multi-cylinder types; horizontal, vertical and inclined types; motorized, manual, pedal, and animal power types. According to the structures, it can also be divided into axial piston pump, radial piston pump and inline (reciprocating) piston pump.

The flow rate of the piston pump is generally Q=0.71~6000m3/hour, and the discharge pressure is P2≤39.2MPa, or in most cases P2≤24.5MPa.

The flow of the piston pump is determined by the diameter of the pump cylinder, the stroke of the piston and the reciprocation number of the piston per minute. The lift depends on the piping characteristics of the unit. The flow rate of the same piston pump remains the same, but the lift can change with the characteristics of the device pipeline, that is, the lift is increased, but the flow rate is unchanged. Only in the high pressure area, the flow rate is slightly reduced.

2.1 Single-acting and Double-acting Piston Pumps

As shown in Figure 3, the main components of a reciprocating piston pump are a pump cylinder, a piston, a piston rod, a suction valve and a discharge valve. Both the suction and discharge valves are one-way valves (check valves). The electric reciprocating piston pump usually uses a crank connecting rod mechanism to convert the rotary motion of the electric motor into the reciprocating motion of the piston.



Working principle diagram of reciprocating piston pump

Figure 3: Working principle diagram of reciprocating piston pump.



1—Piston; 2—Pump cylinder; 3—Discharge pipe; 4—Discharge valve; 5—Working chamber; 6—Suction valve; 7—Suction pipe; 8—Container; Pn—Ambient pressure

When the piston moves to the right, the volume of the pump cylinder increases to create a low pressure. The discharge valve is closed by the liquid pressure in the discharge pipe; the suction valve is opened by the pressure difference between the liquid level of the storage tank and the pump cylinder, so that the liquid is sucked into the pump cylinder.
When the piston moves to the left, due to the push force of the piston, the liquid pressure in the cylinder increases, the suction valve is closed, and the discharge valve is opened, so that the liquid is discharged from the pump cylinder to complete a working cycle. The distance the piston moves between the two ends of the pump cylinder is called the stroke.

A pump in which the piston reciprocates once in the pump cylinder and the fluid is only discharged once is called a single-acting pump, as shown in Figure 3. When both sides of the piston act, that is, one side sucks fluid, and the other side discharges, at this time, two suction and discharge processes are completed in one reciprocating stroke, and the flow rate is about twice that of a single-acting pump. This kind of pump is called a double-acting pump, as shown in Figure 4.



Working principle diagram of double-acting reciprocating piston pump

Figure 4: Working principle diagram of double-acting reciprocating piston pump.

3. Features of Piston Pumps

Piston pump is also called an electric reciprocating pump, which is characterized by high lift. Due to the complex structure of the reciprocating piston pump, the many wearing parts, the pulsating flow, and the heavy machine when the flow is large, it is replaced by a centrifugal pump on many occasions.
However, various types of reciprocating piston pumps are still used in the case of high pressure, small flow, highly viscous liquids, and requiring accurate measurement and small changes in flow with pressure. It has good suction performance and can suck liquids of various media and different viscosities. It has the following advantages and disadvantages:

Advantages:
Wide pressure range - very high pressures can be reached.
Pressure can be controlled without affecting flow.
Changes in pressure and flow rate have little effect on performance.
Capable of moving viscous fluids, slurries, etc., with proper valve design.

Disadvantages:
Operation and maintenance costs are high.
Usually bulky, etc.

The reciprocating piston pump does not need to be primed before starting, and can automatically suck in liquid, that is, it has self-priming ability. However, in actual operation, it is still hoped that there is liquid in the pump cylinder when starting, so that not only can the liquid be sucked and discharged immediately, but also dry friction of the piston in the pump cylinder can be avoided to reduce wear.



Double-acting reciprocating piston pump diagram

Figure 5: Double-acting reciprocating piston pump diagram.



The speed of the reciprocating piston pump (i.e. the reciprocating frequency) has an impact on the self-priming ability of the pump. If the rotational speed is too high, the liquid flow resistance will increase, and when the pressure in the pump cylinder is lower than the liquid saturated vapor pressure, the pump will be evacuated and the liquid suction ability will be lost. Therefore, the reciprocating pump speed n should not be too high, generally between 80~200r/min, and the suction height (installation height) is 4~5m.

4. What is a Piston Pump Used For?

Piston pump is suitable for high pressure and small flow occasions. Especially when the flow rate is less than 100 m³/hour and the discharge pressure is greater than 9.8 MPa, it shows its higher efficiency and good operating performance.

Piston pumps are widely used in the petrochemical industry, machinery manufacturing industry, papermaking, food processing, pharmaceutical production, etc. The low and medium speed piston pump has a low speed, can be operated by manpower and dragged by animal power, and is suitable for rural water supply and small irrigation.

Piston pump is suitable for conveying oil emulsion without solid particles at room temperature, etc.
It can be used for oil fields, coal seam water injection, oil injection and oil recovery.
It can be used as the power pump of the chamber press and the hydraulic press, and can be used for hydraulic sand cleaning, ammonia transportation in chemical fertilizer plants, etc.
If the flow passage components are stainless steel, it can transport corrosive liquids.
In addition, according to the different structural materials, it can also transport high-temperature tar, mineral sludge, high-concentration mortar, high-viscosity liquid, etc.

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