Controlled-volume Metering Pumps

7.0 General description of controlled-volume metering pumps

Controlled-volume metering pumps (also known as metering pumps, proportioning pumps, chemical injection/ feed pumps, or dosing pumps) are reciprocating positive displacement pumps typically used for the injection of chemical additives, proportional blending of multiple components, or metered transfer of a single liquid. These types of pumps are used in applications requiring highly accurate, repeatable, and adjustable rate of flow.

The rate of flow of a controlled-volume pump is a function of the cross-sectional area of the plunger or piston, or displacement of the diaphragm; the stroke length; and the stroking speed. The pumping action is created by a reciprocating piston and controlled by suction and discharge check valves. The rate of flow is adjusted by changing the stroke length and/or the stroking speed.

Controlled-volume metering pumps are characterized by their ability to meet specific performance requirements concerning steady state accuracy, repeatability, and linearity.

7.1 Types and nomenclature

Because controlled-volume metering pumps are employed in a number of different environments and applications, several types have been developed with different liquid ends and drive and control mechanisms.

The basic elements of the pumps are the driver, the flow control, pumphead, and the gearbox.

These are shown in Figure 7.1, which illustrates the general arrangement of these elements and how they are combined to form a controlled-volume metering pump. Detailed descriptions of these elements follow below.

7.1.1 Construction characteristics of controlled-volume metering pumps

NOTE: The following definitions and illustrations represent typical construction characteristics of controlled-volume metering pumps but do not necessarily represent recommended designs. Variations in design may exist without violating the intent of this standard.

Liquid end

• Plunger
• Piston
• Mechanical coupled disc diaphragm
• Hydraulic coupled disc diaphragm
• Hydraulic coupled tubular diaphragm
• Hydraulic coupled conical diaphragm

Drive and control mechanisms

• Electromagnetic drive (solenoid)
• Reciprocating air/gas drive
• Motor driven,1 variable speed
• Motor driven,1 mechanical lost motion
• Motor driven,1 hydraulic lost motion
• Motor driven,1 variable eccentric (nonlost-motion)

7.1.2 Description of liquid ends

The pump liquid end assembly (also called the reagent head assembly, pumphead, or wet end) includes all parts that contain or are in direct contact with the liquid being pumped.

7.1.2.1 Plunger

A plunger liquid end (see Figure 7.2) contains a reciprocating plunger in direct contact with the liquid being displaced. It uses packing within a stuffing box or seals to restrict leakage. It is a simple design that is easily understood. This design is not inherently leak free.

The plunger design allows for lower required “minimum suction head,” limited only by the vapor pressure of the liquid, and this design can be used in high-pressure applications. In operation the process liquid is admitted through the suction check valve as the plunger moves away from the wet end. As the plunger moves towards the wet end, the suction check valve closes and the discharge check valve opens discharging liquid.

7.1.2.2 Piston

A piston liquid end (see Figure 7.3) contains a reciprocating piston in direct contact with the liquid being displaced.

It uses packing or seals located on the piston to restrict leakage. It is a simple design that is easily understood. This design is not inherently leak free.

The piston design allows for lower required “minimum suction head,” limited only by the vapor pressure of the liquid, and can be used in high-pressure applications.

In operation the process liquid is admitted through the suction check valve as the piston moves backwards.

As the piston moves to the front, the suction check valve closes and the discharge check valve opens discharging liquid.

7.1.2.3 Mechanical coupled disc diaphragm

A mechanically coupled disc diaphragm liquid end (see Figure 7.4) contains a flexible, round diaphragm, clamped at the periphery that is in direct contact with the process liquid being displaced. This type of design is inherently leak free. The diaphragm material is typically PTFE or a PTFE/elastomer composite. A connecting rod is connected directly to the diaphragm.

The diaphragm is not pressure balanced as the process pressure is acting on one side of the diaphragm and atmospheric pressure is acting on the other side.

This results in higher stress levels in the diaphragm and therefore these pumps are typically used for lower pressure applications. In operation the process liquid is admitted through the suction check valve as the diaphragm/ connecting rod assembly moves away from the wet end. As the diaphragm/connecting rod assembly moves towards the wet end, the suction check valve closes and the discharge check valve opens discharging liquid.

7.1.2.4 Hydraulic coupled disc diaphragm

A hydraulic coupled disc diaphragm liquid end (Figure 7.5A) contains a flexible, round diaphragm, clamped at the periphery, and is in direct contact with the process liquid being displaced. This type of liquid end design is inherently leak free. The diaphragm material is typically PTFE, elastomer, or a PTFE/elastomer composite. Liquid end designs featuring flexible metallic diaphragms are available and used in applications where severe operating conditions prohibit the use of PTFE or other elastomers.

In operation, the diaphragm is moved by a hydraulic fluid, which in turn is displaced by a reciprocating plunger or piston. The stresses in the diaphragm are minimal, as the process pressure acting on one side of the diaphragm is balanced by the hydraulic pressure acting on the opposite side. The process liquid is admitted through the suction check valves as the diaphragm moves rearward. As the diaphragm moves towards the wet end, the suction check valve closes and the discharge check valve opens discharging liquid.

Liquid end designs of this type may include provisions such as contour plates, springs, or diaphragm positioning hydraulic control valves (Figure 7.5B) to ensure the diaphragm does not move beyond its elastic limits.

7.1.2.5 Hydraulic coupled tubular diaphragm

A hydraulic coupled tubular diaphragm liquid end (see Figure 7.6) contains a flexible tube, clamped at both ends, that is in direct contact with the process liquid being displaced. This type of liquid end design is inherently leak free. The diaphragm material is either PFA or an elastomer. These liquid ends are typically used for viscous liquids or slurries. In operation, the plunger moves rearward and through hydraulic coupling expands the tube admitting liquid into the tube’s cavity through the suction check valve. As the plunger moves towards the wet end, the hydraulic fluid constricts the tube moving the process liquid through the discharge check valve. A disc diaphragm constrained by contour plates may be used in series hydraulically.

This disc diaphragm is used to ensure that the tube operates within its elastic limits.

7.1.2.6 Hydraulic coupled conical diaphragm

A hydraulic coupled conical diaphragm liquid end (see Figure 7.7) contains a flexible conical diaphragm, clamped at the periphery that is in direct contact with the process liquid being displaced. This type of liquid end design is inherently leak free. The diaphragm material is an elastomer. In operation, the plunger moves away from the wet end and process liquid is admitted through the suction check valve. As the plunger moves towards the wet end, the discharge check valve opens, allowing the process liquid to discharge and the conical diaphragm expands, storing elastomeric energy to return it to its original position.

7.1.3 Drive and control mechanisms

The drive and control mechanism is typically located between the driver and the liquid end. Its purpose is to create reciprocating motion of a piston, plunger, or push rod at a controlled frequency (stroke rate). As a controlled-volume metering pump, its secondary purpose is to allow controlled variation of the stroke length by means of its inherent stroke adjustment control mechanism (except in the case of the fixed strokelength design, see Paragraph 7.1.3.3).

7.1.3.1 Electromagnetic drive (solenoid)

The electromagnetic drive (see Figure 7.8) employs an electromagnet that, when pulsed, generates a linear motion transmitted to the liquid end. Each pulse results in one discharge stroke of the pump. The flow is controlled by changing the rate of pulses to the electromagnet and/or by varying the stroke length. These pumps can be run indefinitely in the stalled condition without damage to the pump or most systems. They are used in low-power applications.

7.1.3.2 Reciprocating air drive

Reciprocating air drives use an air cylinder to transmit linear motion to the liquid end. The flow is controlled by changing the pulse rate of the air entering and leaving the cylinder, changing the displacement by adjusting a physical stop, or by changing the rate of flow of air supplied to the air cylinder. These drives can be run indefinitely in the stalled condition without damage to the pump or pumping system. Most designs can be powered by air or other gases.

7.1.3.3 Fixed stroke-length drives

In a fixed stroke-length drive (see Figure 7.10) the stroke length is constant. The stroking speed is changed to vary the flow. As the velocity and acceleration of the mechanisms used are harmonic, the resulting velocity and acceleration of the process liquid is also harmonic. Unlike the lost-motion drives, amplitudes are maintained throughout the speed adjustment range.

7.1.3.4 Mechanical lost-motion drives

Mechanical lost-motion drives (Figure 7.11) have adjustable mechanical stops limiting the stroke length of the piston, plunger, or push rod to some portion of the total stroke. Rate of flow is controlled by the adjustment of the stop. These pumps are generally used at lower rates of flow. Figure 7.4 also illustrates the construction of a mechanical lost-motion drive in conjunction with a mechanically coupled diaphragm liquid end.

7.1.3.5 Hydraulic lost-motion drives

Hydraulic lost-motion drives (see Figure 7.12) use a constant mechanical stroke length to pump hydraulic fluid to actuate a diaphragm. To adjust the rate of flow, a portion of the hydraulic fluid is bypassed so it imparts less motion to the diaphragm. These pumps are generally used at lower rates of flow.

7.1.3.6 Variable stroke-length drive mechanisms (nonlost-motion drive)

For all design configurations, the effective radius of the crank arm is changed varying the stroke length of the piston. The velocity and acceleration of the crank arm is a harmonic function and the resulting velocity and acceleration of the process liquid is also harmonic.

Unlike the lost-motion drives, harmonic acceleration profiles are maintained throughout the stroke adjustment range, resulting in lower amplitude pressure pulsation.

Therefore, these non-lost motion drives (see Figure 7.13) tend to be used in higher flow applications or when long pipe runs are required.

7.1.4 Nomenclature

The nomenclature and definitions in these standards were prepared to provide a means for identifying the various pump components covered by these standards and also to serve as a common language for all who deal with this type of equipment.

7.1.4.1 Automatically controlled

The mechanism for varying the pump rate of flow is controlled by an external electronic or pneumatic signal. The external signal may control the stroke length and/or input speed to the pump.

7.1.4.2 Check valves

A controlled-volume pump uses suction and discharge check valves to produce directional flow of process liquid in the liquid head assembly. Various configurations of check valves are available for specific applications.

7.1.4.3 Diaphragm hydraulic system

Hydraulically actuated diaphragm pumps require hydraulic valves for proper operation. These valves are used to relieve, replenish, and/or bleed the hydraulic systems.

7.1.4.4 Diaphragm leak detection

Diaphragm leak detection systems are sensing systems that detect if a diaphragm has been compromised.

These systems may detect reactions in barrier fluid, mechanical degradation of a diaphragm, changes in pressure/vacuum between the diaphragms, or direct leakage of the process liquid by external means.

7.1.4.5 Diaphragms

Diaphragms provide isolation of the various liquids encountered in a diaphragm pump. To perform adequately, they should be of sufficient thickness and of appropriate material to minimize degradation or permeation in specified process liquid. Many diaphragm materials and configurations exist, but all perform the same basic function.

7.1.4.6 Liquid end assembly

The pump liquid end assembly (also called the reagent head assembly or wet end) includes all parts that contain the liquid being pumped.

7.1.4.7 Manually controlled

The mechanism for varying the pump rate of flow is manually adjusted.

7.1.4.8 Multiple diaphragm

A multiple diaphragm design uses two or more diaphragms operating in unison to provide redundant process liquid isolation. The diaphragms may be of similar or different design but must be synchronized to provide proper accuracy.

7.1.4.9 Multiplex pump

Contains two or more liquid ends. Multiplex pumps may be configured to produce specific flow profiles.

7.1.4.10 Remotely mounted liquid end

Most applications use liquid ends mounted directly to the drive mechanism. However, for extreme temperature services or a highly contaminating or explosive process, remotely mounted liquid heads may be specified to isolate the drive and/or control mechanisms.

7.1.4.11 Simplex pump

Contains one liquid end to provide motive force to a liquid.