Piston Pumps

There are two ordinary types of piston pumps, simplex double-acting pumps and duplex double-acting pumps.

Simplex Double-Acting Pumps These pumps may be direct acting (i.e., direct-connected to a steam cylinder) or power-driven (through a crank and flywheel from the cross-head of a steam engine).

Duplex Double-Acting Pumps These pumps differ primarily from those of the simplex type in having two cylinders whose operation is coordinated. They may be direct-acting, steam-driven, or power-driven with crank and flywheel.

Plunger pumps differ from piston pumps in that they have one or more constant-diameter plungers reciprocating through packing glands and displacing liquid from cylinders in which there is considerable radial clearance. They are always single-acting, in the sense that only one end of the plunger is used in pumping the liquid.

Plunger pumps are available with one, two, three, four, five, or even more cylinders. Simplex and duplex units are often built in a horizontal design. Those with three or more cylinders are usually of vertical design. The driver may be an electric motor, a steam or gas engine, or a steam turbine. This is the common type of power pump.

Occasionally plunger pumps are constructed with opposed cylinders and plungers connected by yokes and tie rods; this arrangement, in effect, constitutes a double-acting unit. Simplex plunger pumps mounted singly or in gangs with a common drive are quite commonly used as metering or proportioning pumps. Frequently a variable-speed drive or a stroke adjusting mechanism is provided to vary the flow as desired. These pumps are designed to measure or control the flow of liquid within a deviation of 62 percent with capacities up to 11.35 m3/h (50 gal/min) and pressures as high as 68.9 MPa (10,000 lbf/in2).

Reciprocating Pumps

There are three classes of reciprocating pumps: piston pumps, plunger pumps, and diaphragm pumps. Basically, the action of the liquid-transferring parts of these pumps is the same, a cylindrical piston, plunger, or bucket or a round diaphragm being caused to pass or flex back and forth in a chamber.

The device is equipped with valves for the inlet and discharge of the liquid being pumped, and the operation of these valves is related in a definite manner to the motions of the piston. In all modern-design reciprocating pumps, the suction and discharge valves are operated by pressure difference. That is, when the pump is on its suction stroke and the pump cavity is increasing in volume, the pressure is lowered within the pump cavity, permitting the higher suction pressure to open the suction valve and allowing liquid to flow into the pump. At the same time, the higher discharge-line pressure holds the discharge valve closed. Likewise on the discharge stroke, as the pump cavity is decreasing in volume, the higher pressure developed in the pump cavity holds the suction valve closed and opens the discharge valve to expel liquid from the pump into the discharge line.

The overall efficiency of these pumps varies from about 50 percent or the small pumps to about 90 percent or more for the larger sizes. Reciprocating pumps, except when used for metering service, are frequently provided on the discharge side with gas-charged chambers, the purpose of which is to limit pressure pulsation and to provide a more uniform flow in the discharge line. In many installations, surge chambers are required on the suction side as well. Piping layouts should be studied to determine the most effective size and location. If surge chambers are used, provision should be made to keep the chamber charged with gas. A surge chamber filled with liquid is of no value. A liquid-level gauge is desirable to permit checking the amount of gas in the chamber.

Reciprocating pumps may be of single-cylinder or multicylinder design. Multicylinder pumps have all cylinders in parallel for increased capacity. Piston-type pumps may be single-acting or double acting; i.e., pumping may be accomplished from one or both ends of the piston. Plunger pumps are always single-acting.

Positive Displacement Pump

Whereas the total dynamic head developed by centrifugal, mixed flow or axial flow pump is usually determined for any given flow by the speed at which it rotates, positive displacement pumps and those which approach positive displacement will ideally produce whatever head is impressed upon them by the system restriction to flow. Actually, with slippage neglected, the maximum head attainable is determined by the power available in the drive and the strength of the pump parts. An automatic relief valve set to open at safe pressure higher than normal or maximum discharge pressure is generally required on the discharge side of all positive displacement pumps.

In general overall efficiencies of positive displacement pumps are higher than those of centrifugal equipment because of internal losses are minimized. On the other hand, the flexibility of each piece of equipment in handling a wide range of capacities is somewhat limited.

Positive displacement pumps may be of either the reciprocating or the rotary type. In all positive displacement pumps, a cavity or cavities are alternately filled and emptied of the pumped fluid by the action of the pump.

Propeller and Turbine Pumps

Axial-Flow Pumps
These pumps are essentially very-high-capacity low-head units. Normally they are design for flows in excess of 450 m/h3 (2000 gal/min) against heads of 15 m (50 ft) or loss. They are use to great advantage in closed loop circulation systems in which the pump casing becomes merely an elbow in the line. A common installation is for calandria circulation.

Turbine Pumps
The terms ‘Turbine Pump’ is applied to unit with mixed flow (part axial and par centrifugal) impellers. Such unit are available in capacities from 20 m3/h (100 gal/min) upward for heads up to about 30 m (100 h) per stage. Turbine pumps are usually vertical.

A common form of turbine pump has the pump elements mounted of the bottom of a column that serve as the discharge pipe. Such units are immersed in the liquid to be pumped and are commonly used for well condenser circulating water, large volume drainage, etc. Another form of the pump has a shell surrounding the pumping element which is connected to the inlet pipe. In the form, pumping is used on condensate service in power plant and for process work in oil refineries and elsewhere.

Regenerative Pumps
Also referred to turbine pumps because of the shape of the impeller, regenerative pumps employ a combination of mechanical impulse and centrifugal forced to produce head of several hundred meters (feet) at low volumes, usually less than 20 m3/h (100 gal/min). The impeller, which rotate at high speed with small clearances, has many short radial passages milled on each side at the periphery, passing alternatively from the impeller to the casing and receiving successive impulses at it does so.

These pumps are particularly useful when low volume of low viscosity liquids must be handled at higher pressure than are normally available with centrifugal pumps. Close clearances limit their use to clean liquids. For very high heads, multistage units are available.

Sump Pumps and Multistage

These are a small single-stage vertical pumps used to drain shallow pits or sumps. They are of the same general construction as vertical process pumps but are not designed for severe operating conditions.

Multistage Centrifugal Pumps
These pumps are used for services requiring heads (pressures) higher than can be generated by a single impeller. All impeller are in series, the liquid passing from one impeller to the next and finally to the pump discharge. The total then is the summation of the heads of the individual impellers. Deep-well pumps, high pressure water supply pumps, boiler feed pumps, fire pumps, and charge pumps for refinery processes are example of multistage pumps required by various services.

Multistage pumps may be of the volute type, with single, or double suction impellers or of the diffuser type. They are have horizontally spilt casing or, for extremely high pressure, 20 to 40 MPa (3000 to 6000 lbf/in2), vertically split barrel type exterior casting with inner casing containing diffusers, interstage passage, ets.

Vertical Pumps

In the chemical Industry, the term vertical process pump generally applies to a pump with a vertical shaft having a length from drive and to impeller of approximately 1 m (3.1 ft) minimum to 20 m (66 ft) or more. Vertical pumps are used either wet-pit pumps (immersed) or dry-pit pumps (externally mounted) in conjunctive with stationary or mobile tanks containing difficult to handle liquid.

They have the following advantages:

• The liquid level is above the impeller
• The pump is thus self-priming
• The shaft seal is above the liquid level and is not wetted by the pumped liquid which simplifies the sealing task (a safety consideration for highly corrosive or toxic liquid)
• The vertical wet-pit pump may be the only logical choice.

These pumps have the following disadvantages:

• Intermediate or line bearing are generally required when the shaft length exceeds about 3 m (10 ft). In order to avoid shaft resonance problem, these bearing must be lubricated whenever the shaft is rotating.
• Since all wetted parts must be corrosion resistant, low-cost materials may not be suitable for the shaft, column, etc.
• Maintenance is more costly since the pump are larger and more difficult to handle.

For abrasive service, vertical cantilever designs requiring no line or foot bearings are available. Generally, these pumps are limited to about 1 m (3.1 ft) maximum shaft length.