CENTRIFUGAL PUMPS
A centrifugal pump is of very simple design. The only moving part is an impeller attached to a shaft that is driven by the motor.
The two main parts of the pump are the impeller and diffuser.
The impeller can be made of bronze, stainless steel, cast iron, polycarbonate, and a variety of other materials. A diffuser or volute houses the impeller and captures the water off the impeller.
Water enters the eye of the impeller and is thrown out by centrifugal force. As water leaves the eye of the impeller a low pressure area is created causing more liquid to flow toward the inlet because of atmospheric pressure and centrifugal force. Velocity is developed as the liquid flows through the impeller while it is turning at high speeds on the shaft. The liquid velocity is collected by the diffuser or volute and converted to pressure by specially designed passageways that direct the flow to discharge into the piping system; or, on to another impeller stage for further increasing of pressure.
Centrifugal Pump Classification
Centrifugal pumps can be classified based on the manner in which fluid flows through the pump. The manner in which fluid flows through the pump is determined by the design of the pump casing and the impeller. The three types of flow through a centrifugal pump are radial flow, axial flow, and mixed flow.
Radial Flow Pumps In a radial flow pump, the liquid enters at the center of the impeller and is directed out along the impeller blades in a direction at right angles to the pump shaft.
Axial Flow Pumps In an axial flow pump, the impeller pushes the liquid in a direction parallel to the pump shaft. Axial flow pumps are sometimes called propeller pumps because they operate essentially the same as the propeller of a boat.
Mixed Flow Pumps Mixed flow pumps borrow characteristics from both radial flow and axial flow pumps. As liquid flows through the impeller of a mixed flow pump, the impeller blades push the liquid out away from the pump shaft and to the pump suction at an angle greater than 90 Degree.
According to type of casting
Volute type pump
Vortex type pump
Diffuser type pump
According to number of stages
Single stage- One impeller in pump
Multi stage- more than one impeller in pump
Multi-Stage Centrifugal Pumps
A centrifugal pump with a single impeller that can develop a differential pressure of more than 10 Kg/cm2 between the suction and the discharge is difficult and costly to design and construct. A more economical approach to developing high pressures with a single centrifugal pump is to include multiple impellers on a common shaft within the same pump casing. Internal channels in the pump casing route the discharge of one impeller to the suction of another impeller. The water enters the pump from the top left and passes through each of the stage impellers in series, going from left to right. The water goes from the volute surrounding the discharge of one impeller to the suction of the next impeller. A pump stage is defined as that portion of a centrifugal pump consisting of one impeller and its associated components. Most centrifugal pumps are single-stage pumps, containing only one impeller. A pump containing seven impellers within a single casing would be referred to as a seven-stage pump or, or generally, as a multi-stage pump
SUBMERSIBLE PUMP
The submersible pump is a centrifugal pump. Because all stages of the pump end (wet end) and the motor are joined and submerged in the water, it has a great advantage over other centrifugal pumps. There is no need to recirculate or generate drive water as with jet pumps, therefore, most of its energy goes toward "pushing" the water rather than fighting gravity and atmospheric pressure to draw water.
Virtually all submersibles are "multi-stage" pumps. All of the impellers of the multi-stage submersible pump are mounted on a single shaft, and all rotate at the same speed. Each impeller passes the water to the eye of the next impeller through a diffuser. The diffuser is shaped to slow down the flow of water and convert velocity to pressure. Each impeller and matching diffuser is called a stage. As many stages are used as necessary to push the water out of the well at the required system pressure and capacity. Each time water is pumped from one impeller to the next, its pressure is increased.
The pump and motor assembly are lowered into the well by connecting piping to a position below the water level. In this way the pump is always filled with water (primed) and ready to pump. Because the motor and pump are under water they operate more quietly than above ground installations; and, pump freezing is not a concern.
We can stack as many impellers as we need; however, we are limited to the horsepower of the motor. We can have numerous pumps that have 1/2 HP ratings - pumps that are capable of pumping different flows at different pumping levels; they will, however, always be limited to 1/2 HP. Another way to look at it is that a pump will always operate somewhere along its design curve.
To get more flow, the exit width of the impeller is increased and there will then be less pressure (or head) that the pump will develop because there will be less impellers on a given HP size pump. Remember, the pump will always trade-off one for the other depending on the demand of the system. If the system demands more than a particular pump can produce, it will be necessary to go up in horsepower; thereby, allowing us to stack more impellers or go to different design pump with wider impellers.
IMPELLERS OF CENTRIFUGAL PUMPS
Pumps Impellers can be open, semi-open, or closed.
The open impeller consists only of blades attached to a hub.
The semi-open impeller is constructed with a circular plate (the web) attached to one side of the blades.
The closed impeller has circular plates attached to both sides of the blades. Enclosed impellers are also referred to as shrouded impellers.
There are three main categories of impeller due type of impeller’s vane, which are used in the centrifugal pumps as;
Radial vanes
Backward vanes
Forward vanes
Impellers of pumps are either Single-Suction and Double-Suction Impellers based on the number of points that the liquid can enter the impeller and also on the amount of webbing between the impeller blades. Impellers can be either single- suction or double-suction. A single-suction impeller allows liquid to enter the center of the blades from only one direction. A double-suction impeller allows liquid to enter the center of the impeller blades from both sides simultaneously.
The impeller sometimes contains balancing holes that connect the space around the hub to the suction side of the impeller. The balancing holes have a total cross-sectional area that is considerably greater than the cross-sectional area of the annular space between the wearing ring and the hub.
Wearing Rings
Centrifugal pumps contain rotating impellers within stationary pump casings. To allow the impeller to rotate freely within the pump casing, a small clearance is designed to be maintained between the impeller and the pump casing. To maximize the efficiency of a centrifugal pump, it is necessary to minimize the amount of liquid leaking through this clearance from the high pressure or discharge side of the pump back to the low pressure or suction side.Some wear or erosion will occur at the point where the impeller and the pump casing nearly come into contact. This wear is due to the erosion caused by liquid leaking through this tight clearance and other causes. As wear occurs, the clearances become larger and the rate of leakage increases. Eventually, the leakage could become unacceptably large and maintenance would be required on the pump. To minimize the cost of pump maintenance, many centrifugal pumps are designed with wearing rings.
Wearing rings are replaceable rings that are attached to the impeller and/or the pump casing to allow a small running clearance between the impeller and the pump casing without causing wear of the actual impeller or pump casing material. These wearing rings are designed to be replaced periodically during the life of a pump and prevent the more costly replacement of the impeller or the casing.
Cavitation
The flow area at the eye of the pump impeller is usually smaller than either the flow area of the pump suction piping or the flow area through the impeller vanes. When the liquid being pumped enters the eye of a centrifugal pump, the decrease in flow area results in an increase in flow velocity accompanied by a decrease in pressure.
The greater the pump flow rate, the greater the pressure drop between the pump suction and the eye of the impeller. If the pressure drop is large enough, or if the temperature is high enough, the pressure drop may be sufficient to cause the liquid to flash to vapor when the local pressure falls below the saturation pressure for the fluid being pumped. Any vapor bubbles formed by the pressure drop at the eye of the impeller are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region where local pressure is greater than saturation pressure farther out the impeller vane, the vapor bubbles abruptly collapse. This process of the formation and subsequent collapse of vapor bubbles in a pump is called cavitation.
Cavitation in a centrifugal pump has a significant effect on pump performance. Cavitation degrades the performance of a pump, resulting in a fluctuating flow rate and discharge pressure. Cavitation can also be destructive to pumps internal components. When a pump cavitates, vapor bubbles form in the low pressure region directly behind the rotating impeller vanes. These vapor bubbles then move toward the oncoming impeller vane, where they collapse and cause a physical shock to the leading edge of the impeller vane.
This physical shock creates small pits on the leading edge of the impeller vane. Each individual pit is microscopic in size, but the cumulative effect of millions of these pits formed over a period of hours or days can literally destroy a pump impeller. Cavitation can also cause excessive pump vibration, which could damage pump bearings, wearing rings, and seals. A small number of centrifugal pumps are designed to operate under conditions where cavitation is unavoidable. These pumps must be specially designed and maintained to withstand the small amount of cavitation that occurs during their operation. Most centrifugal pumps are not designed to withstand sustained cavitation. Noise is one of the indications that a centrifugal pump is cavitating.
Net Positive Suction Head
To avoid cavitation in centrifugal pumps, the pressure of the fluid at all points within the pump must remain above saturation pressure. The quantity used to determine if the pressure of the liquid being pumped is adequate to avoid cavitation is the net positive suction head (NPSH). The net positive suction head available (NPSHA) is the difference between the pressure at the suction of the pump and the saturation pressure for the liquid being pumped.The net positive suction head required (NPSHR) is the minimum net positive suction head necessary to avoid cavitation
The condition that must exist to avoid cavitation is that the net positive suction head available must be greater than or equal to the net positive suction head required.
A formula for NPSHA can be stated as the following equation.
NPSHA = Psuction - Psaturation
When a centrifugal pump is taking suction from a tank or other reservoir, the pressure at the suction of the pump is the sum of the absolute pressure at the surface of the liquid in the tank plus the pressure due to the elevation difference between the surface of liquid in the tank and the pump suction less the head losses due to friction in the suction line from the tank to the pump.
NPSHA = Pa + Pst - hf - Psat
Where:
NPSHA = net positive suction head available,
Pa = absolute pressure on the surface of the liquid,
Pst = pressure due to elevation between liquid surface and pump suction,
hf = head losses in the pump suction piping,
Psat = saturation pressure of the liquid being pumped
Preventing Cavitation
If a centrifugal pump is cavitating, several changes in the system design or operation may be necessary to increase the NPSHA above the NPSHR and stop the cavitation. One method for increasing the NPSHA is to increase the pressure at the suction of the pump. It is also possible to increase the NPSHA by decreasing the temperature of the liquid being pumped. Decreasing the temperature of the liquid decreases the saturation pressure, causing NPSHA to increase.
It may also be possible to stop cavitation by reducing the NPSHR for the pump. The NPSHR is not a constant for a given pump under all conditions, but depends on certain factors. Typically, the NPSHR of a pump increases significantly as flow rate through the pump increases. Therefore, reducing the flow rate through a pump by throttling a discharge valve decreases NPSHR.
NPSHR is also dependent upon pump speed. The faster the impeller of a pump rotates, the greater the NPSHR. Therefore, if the speed of a variable speed centrifugal pump is reduced, the NPSHR of the pump decreases. However, since a pump's flow rate is most often dictated by the needs of the system on which it is connected, only limited adjustments can be made without starting additional parallel pumps, if available.
The net positive suction head required to prevent cavitation is determined through testing by the pump manufacturer and depends upon factors including type of impeller inlet, impeller design, pump flow rate, impeller rotational speed, and the type of liquid being pumped. The manufacturer typically supplies curves of NPSHR as a function of pump flow rate for a particular liquid (usually water) in the vendor manual for the pump.
Specific speed of pump
Maximum efficiency lies in the range: 2000<NS<3000
High head, low capacity pumps: 500<NS<1000
Low head, large capacity pumps: NS>15000
Energy input
Most commonly, this is electricity used to power an electric motor. Alternative forms of energy used to power the driver include high-pressure steam used to drive a steam turbine. Fuel oil used to power a diesel engine. High-pressure hydraulic fluid used to power a hydraulic motor. Compressed air used to drive an air motor. Regardless of the driver type for a centrifugal pump, the input energy is converted in the driver to a rotating mechanical energy, consisting of the driver output shaft, operating at a certain speed, and transmitting a certain torque, or horsepower.
The head or pressure that a pump will develop is in direct relation to the impeller diameter, the number of impellers, the eye or inlet opening size, and how much velocity is developed from the speed of the shaft rotation. Capacity is determined by the exit width of the impeller. All of the these factors affect the horsepower size of the motor to be used; the more water to be pumped or pressure to be developed, the more energy is needed.
A centrifugal pump is not positive acting. As the depth to water increases, it pumps less and less water. Also, when it pumps against increasing pressure it pumps less water. For these reasons it is important to select a centrifugal pump that is designed to do a particular pumping job. For higher pressures or greater lifts, two or more impellers are commonly used; or, a jet ejector is added to assist the impellers in raising the pressure.
Centrifugal Pump Protection
A centrifugal pump is dead-headed when it is operated with no flow through it, for example, with a closed discharge valve or against a seated check valve. If the discharge valve is closed and there is no other flow path available to the pump, the impeller will churn the same volume of water as it rotates in the pump casing. This will increase the temperature of the liquid (due to friction) in the pump casing to the point that it will flash to vapor. The vapor can interrupt the cooling flow to the pump's packing and bearings, causing excessive wear and heat. If the pump is run in this condition for a significant amount of time, it will become damaged.
When a centrifugal pump is installed in a system such that it may be subjected to periodic shutoff head conditions, it is necessary to provide some means of pump protection. One method for protecting the pump from running dead-headed is to provide a recirculation line from the pump discharge line upstream of the discharge valve, back to the pump's supply source. The recirculation line should be sized to allow enough flow through the pump to prevent overheating and damage to the pump. Protection may also be accomplished by use of an automatic flow control device. Centrifugal pumps must also be protected from runout.
Runout can lead to cavitation and can also cause overheating of the pump's motor due to excessive currents. One method for ensuring that there is always adequate flow resistance at the pump discharge to prevent excessive flow through the pump is to place an orifice or a throttle valve immediately downstream of the pump discharge. Properly designed piping systems are very important to protect from runout.
Priming Centrifugal Pumps
Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled with liquid before the pump is started, or the pump will not be able to function. If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping. To ensure that a centrifugal pump remains primed and does not become gas-bound, most centrifugal pumps are located below the level of the source from which the pump is to take its suction.
The pressure increase created is proportional to the density of the fluid being pumped. A pump designed for water will be unable to produce much pressure increase when pumping air. Density of air at sea level is 1.225 Kg/m3. Change in pressure produced by pump is about 0.1% of design when pumping air rather than water!
Priming solution
Applications with water at less than atmospheric pressure on the suction side of the pump require a method to remove the air from the pump and the inlet piping. Solutions-foot valve, priming tank ,vacuum source, self priming
Benefits of Self priming centrifugal pump
Require a small volume of liquid in the pump.
Recirculate this liquid and entrain air from the suction side of the pump.
The entrained air is separated from the liquid and discharged in the pressure side of the pump
Capacity reduction of centrifugal pump
A damaged sealing ring
Leaking gland
Obstruction (valve partly closed/foreign body)
Incorrect rotational speed
Excessive vibration may be caused by
Loose coupling
Loose impeller
Bearing damaged
Impeller imbalance
Shaft sealing in pump
To connect the motor to the impeller, the shaft has to pass through an aperture in the casing. To allow the shaft to rotate freely in the casing aperture there needs to be a gap, but this gap needs to be closed off to stop air from being drawn in from atmosphere or liquid from leaking out during operation.
There are two common methods.
Gland packing
Mechanical seal
Gland Packing
A stuffing box with a soft packing material is the traditional seal for pumps. Normally made from soft impregnated cotton, which takes the form of a length of square cross-section wound spirally onto a tube. This enables the correct length, to suit the external diameter of the shaft, to be manually cut to the correct size.
The stuffing box is then repeatedly filled with sections until almost full, the gland can then be tightened down to provide the axial compressive force. This in turn provides the necessary radial compressive force required to seal the gap due to the sloping bottom face of the aperture.
If the force is insufficient the stuffing box will leak, if the force is too great, the additional friction, and consequently heat generated by the rotating shaft can damage the soft packing and/or shaft.
Mechanical seal
Mechanical Seals are becoming the Sealing Devices of choice for today’s environmentally conscious consumers.
Mechanical seals consist of two basic parts, a rotating element attached to the pump shaft and a stationary element attached to the pump casing. Each of these elements has a highly polished sealing surface. The polished faces of the rotating and stationary elements come into contact with each other to form a seal that prevents leakage along the shaft.
Mechanical Seal Classifications
Inside and Outside Mechanical Seal
Inside Mechanical Seal
Outside Mechanical Seal
Single and Multiple Seal Ring Mechanical Seal
Single Seal Ring Mechanical Seal
Multiple Seal Ring Mechanical Seal
Split and Non-split Mechanical Seal
Split Mechanical Seal
Non-split Mechanical Seal
Component and Cartridge Mechanical Seal
Component Mechanical Seal
Cartridge Mechanical Seal
Metallic and Non-metallic Mechanical Seal
Metallic Mechanical Seal
Non-metallic Mechanical Seal
Pusher and Non-pusher Mechanical Seal
Pusher Mechanical Seal
Non-pusher Mechanical Seal
Double Balanced Mechanical Seal Unbalanced
Double Balanced Mechanical
Unbalanced Mechanical Seal
Balanced Mechanical Seal
Gas and Non-gas Mechanical Seal
Gas Mechanical Seal
Non-gas Mechanical Seal
MOC of Mechanical seal
Metal Parts
Stainless Steel, Alloy 20, Bronze, Nickel, Inconel, Hastelloy-C, Titanium, Monel, Hastelloy-B
Mechanical Seal Face Materials
Carbon-graphite, Tungsten Carbide, Ceramic, Silicon Carbide
Elastomer Parts
Fluorocarbon, Ethylene Propylene Rubber (EPR), Kalrez, Chemraz, TFE Elastomer, Buna-N (Nitrile), Neoprene, Perfluoro Elastomer, Silicone Elastomer, PTFE