pump converts mechanical energy into pressure in a flowing liquid. A centrifugal pump does this by centrifugal action, in two steps. Refer to Figure.
(1) A centrifugal pump has two major components: the internal impeller and the outer casing. The liquid enters the
suction of the pump at A. It then flows to B and outward through the channels of the impeller marked C. As the liquid flows outward in the impeller, the impeller imparts a very high spinning or tangential velocity to the liquid.
(2) The liquid then enters the volute of the pump, area D. Here the velocity energy is converted to pressure.
Head is the term used to describe the energy imparted to the liquid. The units of head are foot-pounds (ft-lb) of force per pound of mass.
where:
V = Velocity of impeller tip, ft/sec
g = gravitational constant, 32.2 ft/sec2
Note that the important velocity is the tangential velocity at the tip of the impeller. This
velocity is proportional to the diameter of the impeller and the rotational speed.
The precise units of head are ft-lb (force) per lb (mass). However, it is conventional practice to cancel the lb units and to speak of head in terms of feet. Note that the pump vendor designs the impeller to produce the head required at the design point.
The pressure differential produced by a pump is equivalent to a column of the pumped liquid, where the height of the column is equal to the head produced by the pump.
Application of Centrifugal Pumps
Centrifugal pumps are the most commonly used in the process industries especially with Storage Tanks ,They are the first choice because they have very few moving parts, are simple to maintain, and are available for a wide range of flow rates and differential pressures.
There are a few exceptions where other types of pumps are more appropriate. These are services with a very high differential pressure, above about 2000 psi; very high viscosities, above 500 cSt; or very low flow rates, below 10 gpm. However, in most industries, more than 90% of the pumping applications will be covered by centrifugal pumps.
Read Also Pump Selection
Mechanical Components of Centrifugal Pumps
This is a diagram of a horizontal single-stage, overhung pump, the most common type. Horizontal refers to the orientation of the shaft; single-stage means there is one impeller. Overhung means that the impeller is outside of the two supporting bearings, not between the bearings.
The shaft runs through the center of the pump and holds the impeller at the left end. The drive motor is connected to the right end of the shaft through a flexible coupling. The liquid enters the suction nozzle, passes through the enclosed sections of the spinning impeller, and exits through the discharge nozzle at the top of the pump. The right end of the pump is the bearing housing. This housing contains two sets of ball bearings that support the weight of
the shaft. They also absorb the axial thrust on the shaft.
The casing contains the liquid under pressure. A seal is required where the rotating shaft enters the casing. This area is called the stuffing box and may actually contain a stuffing or packing. However, most modern pumps have mechanical seals at this point. Sealing the shaft is very important to prevent leakage of the pumped fluid, which is frequently hazardous, flammable, or toxic. Therefore, careful attention must be paid to the design, installation, and
maintenance of the seals. Many different types of seals are available for different process conditions.
Heat is generated by friction in seal area of the shaft, and sometimes cooling is required. A channel called the flushing connection is available for this purpose.
The amount of head that can be generated by a single impeller is limited to a maximum value. If more head is required, pump designs incorporate two or more impellers. These may be arranged in a horizontal multistage configuration or a vertical multistage configuration.
Impellers may be the open, semi-closed, or closed. These are shown in Figure. In the petroleum and gas process plants, most impellers are the closed type. Closed impellers can generate higher heads at greater efficiencies. Open and semi-closed impellers are used for liquids that contain solids. They will not clog as easily as closed impellers.
Centrifugal Pump Types:
Horizontal-Single Stage
The most common type
– Used for moderate head, <500 ft
– End suction top discharge
+ or top suction, top discharge
Vertical In-line
– Supported by piping or small foundation
– Motor is supported by pump; piping forces do not affect alignment
– Lower cost, simpler maintenance
– Slightly higher NPSHR than horizontal pump
Horizontal Multistage
– Up to 8 impellers for higher head
– Shaft supported between bearings
Vertical Can
– Used when low NPSHR is needed
Vertical-Submerged Suction
– Like vertical can type, without the can
– Used in sumps or shallow wells
– Used to pump water from the sea, or from reservoirs
Submersible
– Used in oil production wells.
System Resistance:
The discussion has centered on the head produced by an operating pump. Another important concept is system resistance. This is the head required to move liquid from one point in the process to another.
The total head (or differential pressure) required for a circuit can be divided into three components: (See Figure)
• Static pressure differential, the difference in pressure between the two vessels, P2 – P1.
• Elevation differential, the head required to lift the liquid from its initial to its final elevation.
• Friction resistance in the flowing system.
the figures show a typical pump circuit. This circuit contains all three components of system resistance.
The magnitudes of the three components are illustrated in the figure bellow . Notice that pressure differential and elevation are constant values, independent of the flow rate through the circuit. However, the dynamic friction resistance depends on the flow. The dynamic friction resistance is proportional to the square of the flow rate. Thus, at zero flow rate, the friction resistance is zero, but it rises exponentially as the flow rate increases.
To understand the dynamics of a pumped circuit, it is sometimes useful to plot the pump curve and the system curve together.
Read Also Bernoulli Equation
expressed either as feet of fluid or differential pressure (psi), as long as the units are consistent. At zero flow rate, the head produced by the pump is much greater than the head required to overcome the resistances of the system. However, as the flow rate increases, the head required increases. At the same time, the head produced by the pump decreases somewhat. At the design flow rate, the head produced by the pump is still larger than the
head required. The difference, or excess delta P, is taken up by a control valve.
The curve shows that if the flow rate is increased beyond the design value, the pressure drop available for the control valve becomes smaller and smaller. When the curves meet, the pressure drop available for control is zero, the control valve is wide open and the flow rate cannot increase further.
Conversely, if the flow rate is controlled at a value below design, the control valve will take a larger pressure drop.
Starting a Centrifugal Pump
The normal method for starting a centrifugal pump is as follows. Before startup, close both the discharge and suction block valves. Close the casing vent. Open the valve in the line to the seal.
1. Open the suction block valve to allow liquid to enter the pump.
2. Open the casing vent to release trapped gases or vapors.
3. Close the casing vent.
4. Start the pump motor; observe the pressure rise in the discharge line as indicated by the PI.
5. When the discharge pressure reaches the normal value, start to open the discharge block valve.
6. Gradually open the discharge block valve until it is fully open. If the discharge pressure starts to fall, close the block valve a small amount to reestablish discharge pressure.
Optional Features
Cooling water to stuffing box. Sometimes cooling water is provided to the seal housing to prevent vaporization of the liquid at the surface of the seal.
Steam quench. If the pump fluid is very hot and also flammable, steam is injected between the seal and the outside atmosphere. If there is leakage through the seal, the steam quench cools and dilutes the material. This prevents solidification of flammable pump fluid, such as oil, and reduces the risk of fire.
Casing vent line. The vapors will be vented to atmosphere through a connection at the pump discharge if the material is not toxic or hazardous. For toxic or hazardous materials, a pipe is installed to vent the material back to the suction drum. This is especially necessary if a pump is handling cold liquids. The vent line is left open for five or ten minutes before the pump is started. During this period, cold liquid circulates from the suction line through the pump and back to the suction vessel. This cools the pump to operating temperature before startup. If
this step is not carried out, vaporization can prevent successful starting of the pump.
Warm-up bypass. If the pump normally operates at high temperature, it must be heated before startup to avoid sudden heating and thermal shock. Gradual heating is done by circulating pumped liquid backwards through the idle pump. A small (1-in.) bypass around the check valve is used for this purpose.
References:
1. Centrifugal and Rotary Pump – Fundamentals and Applications.
2. Centrifugal Pumps – Saudi Aramco.
