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Pump Clinic Issue 36

PC36 Metering Pumps Page 1 of 14 12-Aug-09

METERING PUMPS

The metering pump is a positive displacement chemical dosing device with the ability to vary capacitymanually or automatically as process conditions require. It features a high level of repetitive accuracyand is capable of pumping a wide range of chemicals including acids, bases, corrosive or viscous liquidsand slurries.

The pumping action is developed by a reciprocating piston, plunger or diaphragm which is either indirect contact with the process fluid, or is shielded from the fluid by a diaphragm. Diaphragms may beactivated by direct mechanical link or by hydraulic fluid.

Metering pumps are generally used in applications where one or more of the following conditions exist:

Low flow rates are required High system pressure exists

High accuracy feed rate is demanded

Dosing is controlled by computer, microprocessor, DCS, PLC, or flow proportioning

Corrosive, hazardous, or high temperature fluids are handled

Viscous fluids or slurries need to be pumped

METERING PUMP CHARACTERISTICS1. The pumping action is developed by the reciprocating action. This reciprocating motion develops a

flow sine wave. Actual flow rate is determined by the following formula:

Figure 1


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2. Unlike centrifugal pumps, flow rate is not greatly affected by changes in discharge pressure.

Figure 2

3. The metering pump flow vs. stroke characteristic curve is linear. It is not however, necessarilyproportional in that 50% stroke setting may not equal 50% flow. This is due to the fact that the calibrationline may not pass through 0 on both axes simultaneously. By measuring flow at 2 stroke settings, plottingboth points and drawing a straight line through them, other flow rates vs. stroke can be accuratelypredicted.

The steady state accuracy of a correctly installed industrial grade metering pump is generally +/- 1.0% orbetter. Although a metering pump can generally be adjusted to pump at any flow rate between 0 and itsmaximum capacity, its accuracy is measured over a range determined by the pump's turndown ratio.Most metering pumps have a turndown ratio of 10:1 which simply means that the pump is within itsaccuracy rating anywhere between 10% and 100% of capacity. Some newer designs of metering pumpsfeature higher accuracy, and a greater turndown ratio of 100:1. Therefore, this design will accurately doseanywhere between 1% and 100% of capacity.

Figure 3


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METERING PUMP DESIGN

LIQUID END The liquid end design and materials of construction are determined by the service conditions, andthe nature of the fluid to be handled. Temperature, flow rate, fluid viscosity, corrosiveness andother factors are considered.

DRIVE MECHANISM The drive mechanism translates the rotary motion of the driver into reciprocating movement.

FLOW ADJUSTMENT Pump flow rate is adjustable by varying stroke length, effective stroke length or stroking speed. Mostmetering pumps are supplied with a micrometer screw adjustment similar to the one shown here.The micrometer can also be replaced by an electronic or pneumatic actuator to adjust pump flow rate inresponse to a process signal

DRIVERS The pump is usually driven by an AC constant speed motor. Electromagnetic drive is available in smallflow pumps.

Figure 4

DRIVE MECHANISMS

Metering pumps can be powered by a variety of drivers however, the almost universal driver is anelectric motor. The motor speed is normally reduced to pump design speed by the use of gearing builtinto the pump power end. This rotary power is converted to a linear motion through one of threemethods:

- a crank mechanism with either fixed (Figure 5) or variable (Figure 6) stroke length

- an eccentric or cam arrangement (Figure 7)

Depending on the type of adjustable output flow mechanism used, the power can be utilised on both theforward thrust of the crank and the back thrust of the crank. The eccentric or cam arrangement,however, can provide power in only one direction.

Metering pumps with solenoid power ends (Figure 8) are another type of drive and create linear,reciprocating motion using electromagnets. The solenoid type could be considered the ideal power end

as it does not require any type of transmission to convert motion from rotary to linear. Anotheradvantage is that it has in built overload protection as the pump simply stops at excessive load. Adisadvantage is that availability is limited to very small powers.


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Figure 5

Figure 6


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Figure 7

ADJUSTMENT SCREW

Figure 8


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pumped fluid, and the actuation of the diaphragm is by hydraulic power instead of mechanical power

(Figure6).The measuring piston or plunger reciprocates within a precisely sized cylinder at an established strokelength, displacing a volume of hydraulic liquid, not the product liquid. The hydraulic liquid is stable andhas excellent lubricating qualities. The piston uses the hydraulic oil to move the diaphragm forward andbackward, causing a displacement that expels the product liquid through the discharge check valve and,on the suction stroke, takes in an equal amount through the suction check valve. The diaphragm isolatesthe liquid product being contained within the liquid chamber and check valves. These are the only partsthat must be made of chemically compatible material.

The diaphragm's only job is to separate two liquids. It normally does no work, carries no load, and pumpsno liquid; rather it serves as a moving barrier between liquids during periods of pressure imbalance. It issimply a moving partition with pressure hydraulically balanced on both its sides; on one side is the liquid

product and on the other side is the hydraulic oil. At full deflection, the diaphragm undergoes totalcombined stresses well within the endurance limit of the diaphragm material. Contoured support platesare provided on either side of the diaphragm to ensure that stresses are kept within limits. When properlyinstalled and working within the recommended temperature range and not affected by corrosion orabrasion, the diaphragm has an unlimited life.

As previously stated, the piston or plunger handles only hydraulic oil. Conventional seals are used on thepiston or plunger, which does not require power flushing and complicated drain systems as are found onconventional piston or plunger pumps handling corrosive or hazardous liquids.

Even the slightest leakage past the piston is replaced on the suction stroke through the automaticfunctioning of a compensation system, which draws in replacement oil from the oil reservoir (Figure 11).

Figure 11 Function of oil make-up valve

Any excess pressure within the hydraulic system or the liquid product chamber is relieved through theautomatic action of a pressure relief valve. This valve blows off oil, under excess pressure ahead of the

piston, back into the oil reservoir. This valve blows off oil, under excess pressure ahead of the piston, backinto the oil reservoir (Figure 12).


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Figure 12 Function of pressure relief valve

The vacuum and pressure compensator systems actually perform three important functions that the otherdescribed types of metering pumps cannot do unless auxiliary equipment is added to their piping systems.

As described previously, they compensate for any leakage occurring within the hydraulic system of thepump, ensuring a balanced diaphragm movement. In addition, they serve to protect the process systemfrom an over-pressure condition produced by the pump. For instance, the positive displacement pump,because of its design, must over pressure the system to the point of damaging the pump, bursting pipes,or damaging other downstream equipment should an operator mistakenly close a shut-off valvedownstream from the pump.

The hydraulic diaphragm pump will, however, relieve any pump-produced pressure beyond the setpressure of the pressure relief valve, thus avoiding the dangerous build up of pressure. Thecompensation system also serves to protect the pump from a closed suction line or a partially cloggedstrainer in the suction line. Should this occur, the backward movement of the diaphragm is prevented andthe vacuum relief system would automatically open to relieve the starved suction condition within thepump.

In doing so, however, a surplus of hydraulic oil enters into the system between the diaphragm and piston. As the piston starts forward on its discharge stroke, the diaphragm is displaced forward and will come intocontact with the contoured dish support plate in the process liquid chamber, because of the surplus oildrawn into the hydraulic chamber.

At the moment of diaphragm contact with its support plate, an over-pressure condition starts to developwithin the hydraulic system. The pressure relief valve now opens to relieve the surplus oil back into thehydraulic reservoir, preventing a dangerous build up of pressure. The interaction of the two compensationsystems continue stroke after stroke to activate a fluid-clutch-type action to prevent overloading of thepump's power end until the condition plugging the suction or discharge lines is found and corrected.


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FLOW A DJUSTMENTFlow control can be achieved in a number of ways, however the most common are as follows.

1. Changing the stroke rate within the pump by changing internal gearing in the drive mechanism. Thisis done at time of pump manufacture and further changes cannot be made after pump installation.

2. Changing the stroke rate by changing the driver speed. When the driver is an electric motor, using afrequency inverter will achieve this result. With solenoid operated pumps, this can be done by settingthe switching on the solenoid.

3. Changing the stroke length. This can be achieved in a number of ways and is dependent on thepump construction. Details are given below.

4. Adjustable cranks and hydraulic bypass have also been used in the past but are rarely seen these

days.

STROKE LENGTH ADJUSTMENTThere are two main categories of stroke length adjustment mechanisms - lost motion and full motion.Each of these designs changes the way in which the internal piston travels within the piston cylinder.

Adjustments can be made manually as shown in all the diagrams below or adjustments can be automaticwith the use of electric or pneumatic actuators. A 4-20 mA electric or 3-15 psig pneumatic process signalwould be required for the actuators.

LOST MOTION (Figures 13, 14 and 15)Smaller pumps (low flow) are typically of the lost motion stroke length mechanism design. The pumpmotor turns the worm shaft which, in turn, rotates the eccentric gear within the pump gearbox. The cam

rotates with the eccentric gear and actuates the piston via the cam follower. On each discharge stroke ofthe pump the cam follower pushes the piston towards the pump reagent head displacing the pumpdiaphragm. After the piston reaches its full forward position the piston is retracted via spring force.

Displacement per stroke is controlled through limiting the rearward travel of the piston. Adjusting thestroke length mechanism extends and / or retracts the internal adjustment screw. When a stroke length ofless than 100% is desired the internal adjustment screw is rotated and the rearward piston travel is limitedbased on the stroke length setting. As a result there is no contact with the cam for a portion of the camrotation and the piston stops moving until the cam rotates to a position in which contact is re-establishedwith the cam follower.

Figure 13


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Figure 14

Figure 15

The flow characteristics produced from a lost motion style of pump are shown below. As indicated, theflow at 100% stroke length can be represented as a sine wave. When the stroke length is decreased themaximum amplitude of each stroke is maintained while the full potential volume per stroke is decreased.


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LOST MOTION FLOW CURVE

FULL MOTION (Figures 16,17 and 18)

Pumps with larger flow requirements are typically handled by full motion pumps. Full motion pumps rely oninternal linkages for the adjustment of stroke length. The pump motor turns the worm shaft which, in turn,rotates the eccentric gear within the pump gearbox. The eccentric gear transmits motion to a connectingrod which is attached to an oscillating housing. The oscillating housing is stationary at its top theresulting motion is similar to that of a pendulum. Within the oscillating housing exists a housing blockwhich is, in turn, connected to a connecting rod.

The connecting rod is attached to the piston. It can be seen from Figure 16 that when the housing block isat its full bottom position (100% stroke length) the piston will maximize its horizontal movement. As aresult the pump will produce its greatest potential displacement per stroke. When the housing block isadjusted to its full top position (0% stroke length) the piston will be stationary and, as a result, the pumpwill produce no flow. Adjustment of stroke length is actually an adjustment of the housing block positionwithin the oscillating housing this adjustment will determine how far back and forth the piston can traveland, as a result, the volume per stroke that the pump can produce.

DISCHARGE

270

60% STROKE LENGTH

100% STROKE LENGTH

360

SUCTION

- - (


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PC36 Metering Pumps August 12, 2009 Page 13 of 14

Figure 16

Figure 17

Figure 18


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PC36 M i P A 12 2009 P 14 f 14

FULL MOTION FLOW CURVEFull motion flow characteristics are detailed below. As with lost motion pumps the flow characteristic at100% st roke length can be characte rized as a sine wave . Adjus tment of stroke length decreases thesine wave amplitude (displacement per stroke).

With the use of variable speed motors and drives many metering pumps are controlled through variablespeed in lieu of stroke length. The flow characteristics of full motion and lost motion pumps are identical ifstroke length is maintained at 100% and motor speed is used to adjust flow. Instead of changing volume(amplitude) per stroke the adjustable motor speed will modify strokes per minute (frequency). Theresulting output will be identical regardless of stroke length type.

Finally, a discharge pulsation dampener is a typical recommendation for all styles of metering pumps. Thedischarge pulsation dampener transforms a diaphragm metering pump's reciprocating flow to laminar flow.

As a result, the flow characteristics downstream of a metering pump, regardless if it is lost motion or fullmotion, will be identical when a discharge pulsation dampener is installed.

Acknowledgements:Metering Pump Handbook (Pulsafeeder Inc)www.pulsa.comwww.miltonroy.com

100% STROKE LENGTH

60% STROKE LENGTH

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