Metering Or Proportioning

Conventional reciprocating pumps can be adapted to function as metering or proportioning devices in the transfer of liquids. The principal adaptation is the addition of a means of varying the pumping rate and predicting what that rate will be, which makes the modified units suitable for use as final control elements in continuous-flow processes. Metering pumps are often employed where two or more liquids must be proportioned or where mixture ratios must be controlled. These effects are achieved by changing the displacement per stroke (by moving the crankpin by special linkages or by partial stroking) or by changing the stroking speed through the use of variable-speed transmissions or electric motors. Accurate cyclic volume compensation can also be performed using electronic calibration.

Four basic types of positive displacement reciprocating pumps, and several variations, are used for this service: packed plunger pumps, pumps with a mechanically actuated diaphragm, pumps with a hydraulically actuated diaphragm, and pumps with hydrauli-cally actuated pistons.

Packed Plunger The packed plunger pump is the most commonly used type because of its relatively simple design and wide range of pressure capability. It is an adaptation of the conventional reciprocating transfer pump (Figure 1). Its advantages are

1. Relatively low cost

2. Pressure capability to 50,000 lb/in2 (345 MPa) gage

3. Mechanical simplicity

4. Wide capacity range, from a few cubic centimeters per hour to 20 gpm (4.5 m3/h)

5. High accuracy, better than 1% over a 15:1 range

6. Only slightly affected by changes in discharge pressure

FIGURE 1 Packed plunger pump
FIGURE 2 Mechanically actuated diaphragm pump

Its disadvantages are

1. Packing leakage, making it unsuitable for corrosive or dangerous chemicals

2. Packing and plunger wear and the resulting need for gland adjustment

3. Inability to pump abrasive slurries or chemicals that crystallize

Mechanically Actuated Diaphragm The mechanically actuated diaphragm pump is commonly used for low-pressure service where freedom from leakage is important. This pump utilizes an unsupported diaphragm, which is moved in the discharge direction by a cam and returned by a spring (Figure 2). Its advantages are

1. Relatively low cost

2. Minimum maintenance at 6- to 12-month intervals

3. Zero chemical leakage

4. Ability to pump slurries and corrosive chemicals

Its disadvantages are

1. Discharge pressure limitation of 125 to 150 lb/in2 (860 to 1030 kPa) gage

2. Accuracy in 5% range and as much as 10% zero shift with change from minimum to maximum discharge pressure

3. Capacity limit of 12 to 15 gph (0.045 to 0.057 m3/h)

Hydraulically Actuated Diaphragm The hydraulically actuated diaphragm pump is a hybrid design that provides the principal advantages of the other types. A packed plunger is used to pulse hydraulic oil against the back side of the diaphragm. The reciprocating action thus imparted to the diaphragm causes it to pump in the normal manner without being subjected to high pressure differences. A flat diaphragm design is shown in Figure 3 and a tubular diaphragm design in Figure 4.

The advantages of the hydraulically actuated diaphragm pump are

1. Pressure capability to 5000 lb/in3 (34.5 MPa) gage

3. Minimum maintenance

4. Zero chemical leakage

5. Ability to pump slurries and corrosive chemicals

6. Accuracy around 1% over 10:1 range

Its disadvantages are

1. Subject to predictable zero shift of 3 to 5% per 1000 lb/in2 (6.9 MPa) gage

2. Higher cost

Hydraulically Actuated Pistons This type of design is used when high accuracy at relatively low pressure is required. Figure 5 shows the complete pumping and metering unit, whereas Figure 6 shows a sectioned view of the metering unit. The pumping unit shown in Figure 6 can be a gear or vane type, driven by an electric motor. This is integral to a dual-measuring unit (Figure 5), mounted on top of the pumping unit. Alternatively, a centrifugal pump located remotely in the product storage tank can provide the pumping function. The pumping unit is run at a constant speed of about 850 rpm. A bypass valve in the pumping unit varies the output from 0 to 100% in response to a control valve located downstream of the meter.

The metering unit shown in Figure 5 is a dual unit providing two separate measured outputs. Each meter unit has two pistons and three chambers. Scotch yokes that drive a

FIGURE 3 Hydraulically actuated diaphragm pump with flat, circular diaphragm

FIGURE 4 Hydraulically actuated diaphragm pump with tubular diaphragm

FIGURE 5 "Duplex" metering pump (Dresser-Wayne)

rotary valve that ports the fluid in and out of the measuring chambers connect the pistons. A shaft encoder and calibration unit is integral to the meter to provide accurate adjustment of the pumped volume together with batch and running volume totals. The advantages are

1. High accuracy (0.25%) across complete flow range to 25 gpm (6 m3/h)

2. Compact, totally integrated multi-function design.

3. Minimum maintenance every 1.3-1.5 million gallons (5000-6000 m3)

4. Low initial and ownership cost

Its disadvantages are

1. Pressure limitation of 100 lb/in2 (700 kPa).

2. Maximum flow rate of approximately 25 gpm (6 m3/h)

3. Only suitable for low viscosity noncorrosive products.

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Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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