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2. Forsthoffer's Rotating Equipment Handbooks - Pumps
of: William E Forsthoffer
Elsevier Trade Monographs, 2006
ISBN: 9780080949338 , 198 Pages
Format: PDF, ePUB, Read online
Copy protection: DRM
Pump types and applications
Definition of pump types
A pump is defined as a device that moves a liquid by increasing the energy level of the liquid. There are many ways to accomplish this objective. At the conclusion of this chapter one will be able to identify all of the different types of pumps on site and to state the function of each specific type. Refer to Figure 2.1 and observe all of the different types of pumps which will be discussed in this chapter. Note that the pumps are divided into two distinct groups. One group pumps the liquid by means of positive displacement – the other group pumps the liquid by means of dynamic action.
Figure 2.1 Types of pumps
Refer to Figure 2.2 for the definition of positive displacement and dynamic action and the characteristics of each type of pump.
Figure 2.2 Positive displacement – dynamic pump comparison
Regardless of whether the pumps moves the liquid by positive displacement or dynamic means, each pump is divided into a hydraulic and a mechanical end. Figure 2.3 identifies these features for a positive displacement pump.
Figure 2.3 Power pump (Courtesy of Union Pump Co)
One important fact to remember is that the liquid doesn’t care how it is moved; that is, the performance relationships of head, horsepower and efficiency will remain the same for all pumps regardless of whether they are positive displacement or dynamic. These relationships will be discussed.
As far as the mechanical end is concerned, the mechanical components of shafts, bearings, seals, couplings and casings perform the same function regardless of the pump type. Figure 2.4 is a chart that shows the similarities between the hydraulic and the mechanical ends of pumps regardless of their type.
Figure 2.4 Pump similarities – hydraulic and mechanical ends regardless of pump type
Again it can be seen from this chart that the relationships for pump performance are identical regardless of pump type. The only difference being the efficiency of one pump type relative to another. Secondly, it can be shown that the mechanical ends, housings, bearings, seals, etc. are very similar in each type of pump so that the function of a bearing or seal remains the same regardless of the type of pump.
Positive displacement pumps
As can be seen from Figure 2.2 a positive displacement pump is a constant flow variable head device. Refer to Figure 2.5 which shows a schematic of a double acting piston pump.
Figure 2.5 Double acting piston pump
As the piston moves from left to right the pressure of the liquid will be increased and the pump will displace the liquid regardless of its specific gravity and viscosity as long as sufficient power is available from the pump driver. The types of positive displacement pumps which can be found in any petrochemical plant, refinery or gas plant are noted below. Refer to Table 2.1 for the application limits of positive displacement pumps.
Table 2.1
Application envelope positive displacement pumps
*horsepower per cylinder, some applications use multiple cylinders
Reciprocating pumps
Reciprocating pumps are those types of positive displacement pumps that increase liquid energy by a pulsating action. The types are power pumps, direct acting steam pumps, diaphragm pumps and metering pumps. All reciprocating pumps produce pulsations that can cause damage to the pumps and/or process system if the system is not properly analyzed and designed. Anti pulsation devices (volume bottles, orifices or pulsation bottles) are usually required.
Power pumps
A picture of a power pump is shown in Figure 2.6. Power pumps are used normally for high pressure, low flow applications, typically carbonate, amine service or high pressure water or oil services. They can either be horizontal or vertical. The major parts of a power pump as shown in Figure 2.6 include the liquid cylinder with pistons and rods, the valves and power end. The power end consists of the crankshaft with bearings, connecting rod and crosshead assembly. It is termed, ‘Power Pump’ because it is driven by an external power source, such as an electric motor, or internal combustion engine, instead of steam cylinders as in direct-acting pumps.
Figure 2.6 Power pump (Courtesy of Union Pump Co)
Diaphragm pumps
A schematic of a diaphragm pump is shown in Figure 2.7. In this type of pump the power end and liquid end areas are approximately the same. This results in the pump being capable of pumping against pressures no greater than that of the motive fluid. This pump has limited use in the refining and petrochemical industry and is used primarily for metering services.
Figure 2.7 Diaphragm pump
Metering pumps
A diaphragm type metering pump or ‘proportioning’ pump is shown in Figure 2.8.
Figure 2.8 Diaphragm type of metering pump
This type of pump is most commonly used for chemical injection service when it is required to precisely control the amount of chemical or inhibitor being injected into a flowing process stream. Volume control is provided by varying the effective stroke length. There are two basic types of metering pumps:
1. Packed plunger pump – the process fluid is in direct contact with the plunger and is used for higher flow applications.
2. Diaphragm pump – process fluid is isolated from the plunger by means of a hydraulically actuated flat or shaped diaphragm and is used for lower flow applications or where escape of the pumped liquid to atmosphere is not acceptable.
Metering pumps can be furnished with either single or multiple pumping elements.
When the pumped liquid is toxic or flammable, diaphragm pumps can be provided with double diaphragms with a leak detector to alarm on failure of either diaphragm. The American Petroleum Institute has published standard 675 which covers the minimum requirements for controlled volume pumps for use in refinery service.
Rotary pumps
There are a number of different types of pumps which are classified as ‘rotaries’. Rotary pumps are positive displacement pumps that do not cause pulsation. The inherent high efficiency and versatility of the ‘rotary’ (screw, gear and others) makes this design very suitable for use in lube oil, seal oil and other high viscosity oil services. They can handle capacities from a fraction of a gallon to more than 5,000 GPM, with pressures ranging up through 5,000 PSI when pumping liquids with viscosities from les than one (1) centistoke to more than 1,000,000 SSU.
Screw pumps
Figure 2.9 illustrates the screw pump design. Fluid flow is carried axially between the threads of two or more close clearance rotors so that a fixed volume of fluid is displaced with each revolution. This design is frequently used for lube and seal service.
Figure 2.9 Screw pump (Courtesy of IMO Industries)
Gear pumps
A picture of a commonly used gear pump is shown in Figure 2.10. With this type of pump, fluid is carried between the teeth of two external gears and displaced as they mesh. Gear pumps are used for small volume lube oil services and liquids of very high viscosity (asphalt, polyethylene, etc).
Figure 2.10 Gear pump (Courtesy of IMO Industries)
Dynamic pumps
Centrifugal pumps can be referred to as ‘dynamic’ machines. That is to say they use centrifugal force for pumping liquids from one level of pressure to a higher level of pressure. Liquid enters the center of the rotating impeller, which imparts energy to the liquid. Centrifugal force then discharges the liquid through a volute as shown in Figure 2.11.
Figure 2.11 Dynamic pump principle
The centrifugal pump is one of the most widely used fluid handling devices in the refining and petrochemical industry. Every plant has a multitude of these types of pumps operating. A brief description of the various designs found in operating plants follows. Refer to Table 2.2 for the application limits of dynamic pumps.
Table 2.2
Application limits – Dynamic pumps
Single stage overhung pump
The single stage overhung pump design shown...
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