Why do I need a metering pump?

April 27, 2021 0 Comments

A metering pump, and any chemical injection pump, is a positive displacement type pump.

What is a positive displacement pump?

A positive displacement (PD) pump moves a fluid by repeatedly enclosing a fixed volume and moving it mechanically through the system. The pumping action is cyclic and can be driven by pistons, screws, gears, rollers, diaphragms or vanes.

How does a positive displacement pump work?

Although there are a wide variety of pump designs, the majority can be placed into two categories: reciprocating and rotary.

Reciprocating positive displacement pumps

A Reciprocating Positive Displacement pump works by the repeated back-and-forth movement (strokes) of either a piston, plunger or diaphragm (Figure 1). These cycles are called reciprocation.

In a piston pump, the first stroke of the piston creates a vacuum, opens an inlet valve, closes the outlet valve and draws fluid into the piston chamber (the suction phase). As the motion of the piston reverses, the inlet valve, now under pressure, is closed and the outlet valve opens allowing the fluid contained in the piston chamber to be discharged (the compression phase). The bicycle pump is a simple example. Piston pumps can also be double acting with inlet and outlet valves on both sides of the piston. While the piston is in suction on one side, it is in compression on the other. More complex, radial versions are often used in industrial applications.

Plunger pumps operate in a similar way. The volume of fluid moved by a piston pump depends on the cylinder volume; in a plunger pump it depends on the plunger size. The seal around the piston or plunger is important to maintain the pumping action and to avoid leaks. In general, a plunger pump seal is easier to maintain since it is stationary at the top of the pump cylinder whereas the seal around a piston is repeatedly moving up and down inside the pump chamber.

A diaphragm pump uses a flexible membrane instead of a piston or plunger to move fluid. By expanding the diaphragm, the volume of the pumping chamber is increased and fluid is drawn into the pump. Compressing the diaphragm decreases the volume and expels some fluid. Diaphragm pumps have the advantage of being hermetically sealed systems making them ideal for pumping hazardous fluids.

The cyclic action of reciprocating pumps creates pulses in the discharge with the fluid accelerating during the compression phase and slowing during the suction phase. This can cause damaging vibrations in the installation and often some form of damping or smoothing is employed. Pulsing can also be minimized by using two (or more) pistons, plungers or diaphragms with one in its compression phase whilst the other is in suction.

The repeatable and predictable action of reciprocating pumps makes them ideal for applications where accurate metering or dosing is required. By altering the stroke rate or length it is possible to provide measured quantities of the pumped fluid.

Rotary positive displacement pumps

Rotary positive displacement pumps use the actions of rotating cogs or gears to transfer fluids, rather than the backwards and forwards motion of reciprocating pumps. The rotating element develops a liquid seal with the pump casing and creates suction at the pump inlet. Fluid, drawn into the pump, is enclosed within the teeth of its rotating cogs or gears and transferred to the discharge. The simplest example of a rotary positive displacement pump is the gear pump. There are two basic designs of gear pump: external and internal (Figure 2).

An external gear pump consists of two interlocking gears supported by separate shafts (one or both of these shafts may be driven). Rotation of the gears traps the fluid between the teeth moving it from the inlet, to the discharge, around the casing. No fluid is transferred back through the centre, between the gears, because they are interlocked. Close tolerances between the gears and the casing allow the pump to develop suction at the inlet and prevent fluid from leaking back from the discharge side. Leakage or “slippage” is more likely with low viscosity liquids.

An internal gear pump operates on the same principle but the two interlocking gears are of different sizes with one rotating inside the other. The cavities between the two gears are filled with fluid at the inlet and transported around to the discharge port, where it is expelled by the action of the smaller gear.

Gear pumps need to be lubricated by the pumped fluid and are ideal for pumping oils and other high viscosity liquids. For this reason, a gear pump should not be run dry. The close tolerances between the gears and casing mean that these types of pump are susceptible to wear when used with abrasive fluids or feeds containing entrained solids.

Two other designs similar to the gear pump are the lobe pump and vane pump.

In the case of the lobe pump, the rotating elements are lobes instead of gears. The great advantage of this design is that the lobes do not come into contact with each other during the pumping action, reducing wear, contamination and fluid shear. Vane pumps use a set of moveable vanes (either spring-loaded, under hydraulic pressure, or flexible) mounted in an off-centre rotor. The vanes maintain a close seal against the casing wall and trapped fluid is transported to the discharge port.

A further class of rotary pumps uses one or several, meshed screws to transfer fluid along the screw axis. The basic principle of these pumps is that of the Archimedes screw, a design used for irrigation for thousands of years.

What are the main features and benefits of a positive displacement pump?

There are two main families of pumps: positive displacement and centrifugal. Centrifugal pumps are capable of higher flows and can work with lower viscosity liquids. In some chemical plants, 90% of the pumps in use will be centrifugal pumps. However, there are a number of applications for which positive displacement pumps are preferred. For example, they can handle higher viscosity fluids and can operate at high pressures and relatively low flows more efficiently. They are also more accurate when metering is an important consideration.

What are the limitations of a positive displacement pump?

In general, positive displacement pumps are more complex and difficult to maintain than centrifugal pumps. They are also not capable of generating the high flow rates characteristic of centrifugal pumps.

Positive displacement pumps are less able to handle low viscosity fluids than centrifugal pumps. To generate suction and reduce slippage and leaks, a rotary pump relies on the seal between its rotating elements and the pump housing. This is considerably reduced with low viscosity fluids. Similarly, it is more difficult to prevent slippage from the valves in a reciprocating pump with a low viscosity feed because of the high pressures generated during the pumping action.

A pulsing discharge is also a characteristic of positive displacement, and especially reciprocating, pump designs. Pulsation can cause noise and vibration in pipe systems and cavitation problems which can ultimately lead to damage or failure. Pulsing can be reduced by the use of multiple pump cylinders and pulsation dampeners but this requires careful system design. Centrifugal pumps, on the other hand, produce a smooth constant flow.

The back-and-forth motion of a reciprocating pump can also be a source of vibration and noise. It is therefore important to construct very strong foundations for this type of pump. As a consequence of the high pressures generated during the pumping cycle it is also vital that either the pump or discharge line has some form of pressure relief in case of a blockage. Centrifugal pumps do not need over-pressure protection: fluid is simply recirculated in this eventuality.

Feeds containing a high level of abrasive solids can cause excessive wear on the components of all types of pumps and especially valves and seals. Although the components of positive displacement pumps operate at considerably lower speeds than those of centrifugal pumps, they remain prone to these problems. This is particularly the case with piston and plunger style reciprocating pumps and gear rotary pumps. With this type of feed, a lobe, screw or diaphragm pump may be suitable for more demanding applications.

The following table summarises the capabilities of centrifugal and positive displacement pumps.

Pump Comparison: Centrifugal vs Positive Displacement

What are the main applications for positive displacement pumps?

Positive Displacement pumps are commonly used for pumping high viscosity fluids such as oil, paints, resins or foodstuffs. They are preferred in any application where accurate dosing or high pressure output is required. Unlike centrifugal pumps, the output of a positive displacement pump is not affected by pressure so they also tend to be preferred in any situation where the supply is irregular. Most are self priming.

Summary

A positive displacement pump moves a fluid by repeatedly enclosing a fixed volume, with the aid of seals or valves, and moving it mechanically through the system. The pumping action is cyclic and can be driven by pistons, screws, gears, lobes, diaphragms or vanes. There are two main types: reciprocating and rotary.

Positive displacement pumps are preferred for applications involving highly viscous fluids such as thick oils and slurries, especially at high pressures, for complex feeds such as emulsions, foodstuffs or biological fluids, and also when accurate dosing is required. 

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