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Centrifugal Pumps

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pump converts mechanical energy into pressure in a flowing liquid. A centrifugal pump does this by centrifugal action, in two steps. Refer to Figure below.

(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 type of pump most commonly used in the process industries. 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.

Pump Selection

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Type of Pump

The selection of type and construction of a pump is very important to meet the process specification and proper application. Knowledge of the variety of pumps in the market should be review. It is mentioned before, that there are two general types of pump in today’s industry: positive displacement and centrifugal (dynamic) pumps.

Positive Displacement Pumps
Positive displacement (PD) pumps work by allowing a fluid to flow into some enclosed cavity from a low-pressure source, trapping the fluid, and then forcing it out into a high-pressure receiver by decreasing the volume of the cavity. Some examples of PD pumps are: fuel and oil pump in most automobiles, the pumps on most hydraulic
systems, and the heart of most animals. see our Pump Video Course CD
Some general types of the positive displacement pumps are as below:

a) Reciprocating Pump
Reciprocating pumps create and displace a volume of liquid, their “displacement volumes”, by action of a reciprocating element. Liquid discharge pressure is limited only by strength of structural parts. A pressure relief valve and a discharge check valve are normally required for reciprocating pumps
Reciprocating pumps can be further classified into three types of pump as below:

i) Piston Pumps
ii) Packed Plunger Pumps
iii) Diaphragm Pumps

see also our Pump Books section

b) Rotary Pump
Rotary pumps function with close clearances such that a fixed volume of liquid is displaced with each revolution of the internal element. Rotary pumps include:

i) Gear Pump
ii) Lobe Pump
iii) Vane Pump
iv) Screw Pump

All those pumps above have the similar working principles: pumping the liquid with the help of rotating elements. The difference lies on the rotating elements; they could be gear, lobe, vane, or screw.

Centrifugal Pumps

Centrifugal pumps are dynamic pumps. A centrifugal pump raises the pressure of the liquid by giving it a high kinetic energy and then converts it into pressure energy before the fluid exits the pump. It normally consists of an impeller (a wheel with blades), and some form of housing with a central inlet and a peripheral outlet. The impeller is mounted on a rotating shaft and enclosed in a stationary casing. Casings are generally of two types: volute and circular. The impeller design and the shape of the casing determine how liquid is accelerated though the pump.Some general types of the centrifugal pumps are as below:

a) Overhung pump
A pump with the impeller(s) cantilevered from its bearing assemblies is classified as an overhung pump.

 

b) Between bearings pump
A pump with the impeller(s) located between the bearings is classified as a between bearings pump. The pump may be single-stage (one impeller), twostage, or multistage. It can be axially (horizontally) split or radially split.

c) Vertically suspended pump
A pump with the impeller(s) cantilevered vertically and the suction nozzle typically submerged is classified as a vertically suspended pump.

d) Seal-less pump
Seal-less pumps are special pumps which do not require shaft seals. Construction for seal-less pumps is driven by canned motors or magnetic couplings. It is normally used in process involve extremely hazardous fluid, where leakage cannot be tolerated.

e) Submersible pump
Submersible pumps are designed to prevent pump cavitation .The driver components inside are completely surrounded by the pumped fluid.

f) Horizontal self-priming pump
Horizontal self-priming pumps are designed to create a vacuum at the pump inlet. This enables the pump to “suck” fluid into its casing. The suction nozzle of the pump can therefore be located above the level of liquid being pumped.
Centrifugal pumps are used in more industrial applications than any other kind of pump. This is primarily because these pumps offer low initial and upkeep costs.
Traditionally these pumps have been limited to low-pressure-head applications, but modern pump designs have overcome this problem unless very high pressures are required. The single-stage, horizontal, overhung, centrifugal pump is by far the most commonly type used in the chemical process industry.
Basically, pump selection is made on the flow rate and head requirement and with other process considerations, such as material of the construction pumps for the corrosive chemical service or for the fluid with presence solids in the stream.

Process Requirements Parameters

In designing the pump, the knowledge of the effect of parameters; such as pump capacity, NPSH, pumping maximum temperature, specific gravity, fluid viscosity, fluid solid content, and the other process requirements are very important. All of these parameters will affect the selection and design of the pump which will affect the
performance of the pump in the process.
Pump capacity is a parameter plays an important role when selecting the pump. Capacity means the flow rate with which liquid is moved or pushed by the pump to the desired point in the process. It is commonly measured in either gallons per minute (gal/min) or cubic meters per hour (m3/hr). The capacity usually changes with the changes in operation of the process. A minimum required flow rate need to be specified, this is important to determining if a minimum flow bypass is required for the selected pump to avoid pump overheating and mechanical damage.
NPSH as a measure to prevent liquid vaporization or called cavitation of pump. Net Positive Suction Head (NPSH) is the total head at the suction flange of the pump less the vapor pressure converted to fluid column height of the liquid. The design engineer should always remember that pumps can pump only liquids, not vapors because when
a liquid vaporizes its volume increases greatly. For example: 1ft3 of water it will vaporize to produce 1700ft3 of steam. This will cause the rise in temperature and pressure drop in the fluid and pump will stop functioning because it has not sufficient suction pressure present.
Pumping maximum temperatures is important in deciding pump construction style and pump cooling and mechanical seal requirements. The minimum operating temperature is to ensure that the material has adequate impact strength.
Specific gravity is parameter determines the pump head required to produce a desired pressure increase. For pumps with limited head capability such as centrifugal pumps, it affects pressure rise capability. Pump power requirements are also affected by specific gravity.
Viscosity is important in the selection of pump type and has a significant effect on centrifugal pump performance. Minimum values of viscosity are important in determining rotary pump (positive displacement pump) performance, while maximum viscosity is important in determining debits to centrifugal pump performance.

Fluid solid content will affect the pump design. It affected the aspects of the design for the flow characteristic, consideration design of erosion resistance, flow passage size, impeller style, peripheral speed, design features to disintegrate large particles, and shaft sealing design. This parameter has to be added in the data sheet for design.
Other process requirement such as flexibility for expansion should be consider as well.
This is important for future capacity expansion; it helps to minimize the cost of expansion because to replace the pump will be a large sum of money. Working capacity of pump should always be design for more than 20% extra design capacity.

DEFINITIONS

Bearing Housing -The bearing housing encloses the bearings mounted on the shaft. The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. The bearing house also includes an oil reservoir for lubrication, constant level of oil, jacket for cooling by circulating cooling
water.

Capacity - Is the water handling capability of a pump commonly expressed as either gallon per minute (gal/min) or cubic meter per minute (m3/min).

Cavitation - Is the result of vapor bubbles imploding. This occurs when the amount of fluid flowing into the pump is restricted or blocked.

Discharge Port —Point where the discharge hose or pipe is connected to the pump.

Datum Elevation – It use as reference of the horizontal plane for which all the elevations and head are measured. The pumps standards normally specify the datum position relative to a pump part, eg. Centrifugal horizontal pump datum position is at the impeller shaft centerline.

Dynamic Discharge Head- The static discharge head plus the friction in the discharge line also referred to as Total Discharge Head.

Dynamic Suction Head - The static suction lift plus the friction in the suction line also referred to as Total Suction Head.

Endurance limit – Is the stress below which the shaft will withstand an infinite number of stress reversals without failure. Since one stress reversal occurs for each revolution of the shaft, this means that ideally the shaft will never fail if the maximum bending stress in the shaft is less than the endurance limit of the shaft material.

Friction Head-The head required to overcome the resistance to flow in the pipe and fittings. It is dependent upon the size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid.

Friction Loss - Refers to reductions in flow due to turbulence as water passes through hoses, pipes, fittings and elbows.

Impeller — A disk with multiple vanes. It is attached to the pump engine or motor and is used to create the centrifugal force necessary for moving water through the pump casing.

Mechanical Seal — A common wear part that forms a seal between the pump and the engine or motor. Also prevents liquid from seeping into the engine or motor.

Net Positive Suction Head (NPSHa) - Is the total head at the suction flange of the pump less the vapor pressure converted to fluid column height of the liquid.

Net Positive Suction Head Required (NPSHr) - NPSH in meters (feet) determined by Supplier testing, usually with water. NPSHR is measured at the suction flange and corrected to the datum elevation. NPSHR is the minimum NPSH at rated capacity required to prevent a head drop of more than 3% (first stage head in multistage pumps) due to cavitation within pump.

Pressure Head - Pressure Head must be considered when a pumping system either begins or terminates in a tank which is under some pressure other than atmospheric.
The pressure in such a tank must first be converted to feet of liquid. Denoted as hp, pressure head refers to absolute pressure on the surface of the liquid reservoir supplying the pump suction, converted to feet of head. If the system is open, hp equals atmospheric pressure head.

Static Suction Head -Head resulting from elevation of the liquid relative to the pump center line (datum). If the liquid level is above pump centerline (datum), hS is positive. If the liquid level is below pump centerline (datum), hS is negative. Negative hS condition is commonly denoted as a “suction lift” condition Static Discharge Head - It is the vertical distance in feet between the pump centerline and the point of free discharge or the surface of the liquid in the discharge tank.

Suction Port — Point where the suction hose or pipe is connected to the pump.

Vapor Pressure Head - Vapor pressure is the absolute pressure at which a liquid and its vapor co-exist in equilibrium at a given temperature. The vapor pressure of liquid can be obtained from vapor pressure tables. When the vapor pressure is converted to head, it is referred to as vapor pressure head, hvp. The value of hvp of a liquid increases with the rising temperature and in effect, opposes the pressure on the liquid surface, the positive force that tends to cause liquid flow into the pump suction i.e. it reduces the suction pressure head. (Vapor pressure can be said as the external pressure require to prevent fluid from evaporate become vapor).

Velocity Head - Refers to the energy of a liquid as a result of its motion at some velocity ‘v’. It is the equivalent head in feet through which the water would have to fall to acquire the same velocity, or in other words, the head necessary to accelerate the water. The velocity head is usually insignificant and can be ignored in most high head
systems. However, it can be a large factor and must be considered in low head systems.

Volute — A stationary housing inside the pump housing in which the impeller rotates. It is used to separate air and water.

Total Head - Pressure required in feet (meter) of head that the pump must produce. The head at the discharge pump flange minus the head at suction flange.