Pump Operation Section ,centrifugal pump
Pump Operation Section
Pump Objectives. In this section we will examine…
Identify different types of pumps and related parts.
Identify the main purpose of a motor starter.
Describe the main use of AC and DC motors.
Describe the operations of level sensor controls.
Identify and describe the most commonly used pumps.
Identify the suction and discharge valving.
Distinguish between discharge head, total head, suction head, and suction lift.
Describe information to be obtained from pump performance graphs.
Identify types of couplings, bearings, seals and other pump components.
Describe the importance of the alignment of couplings.
Indicate when packing seals need to be replaced.
Describe cavitation.
Describe water hammer.
State the basic principles of positive displacement pumps.
Here are the important points to consider about suction piping when the liquid being pumped is below the level of the pump:
First, suction lift is when the level of water to be pumped is below the centerline of the pump. Sometimes suction lift is also referred to as ‘negative suction head’.
The ability of the pump to lift water is the result of a partial vacuum created at the center of the pump.
This works similar to sucking soda from a straw. As you gently suck on a straw, you are creating a vacuum or a pressure differential. Less pressure is exerted on the liquid inside the straw, so that the greater pressure is exerted on the liquid around the outside of the straw, causing the liquid in the straw to move up. By sucking on the straw, this allows atmospheric pressure to move the liquid.
Look at the diagram illustrated as “1”. The foot valve is located at the end of the suction pipe of a pump. It opens to allow water to enter the suction side, but closes to prevent water from passing back out of the bottom end.
The suction side of pipe should be one diameter larger than the pump inlet. The required eccentric reducer should be turned so that the top is flat and the bottom tapered.
Notice in illustration “2” that the liquid is above the level of the pump. Sometimes this is referred to as ‘flooded suction’ or ‘suction head’ situations. Points to Note are: If an elbow and bell are used, they should be at least one pipe diameter from the tank bottom and side. This type of suction piping must have a gate valve which can be used to prevent the reverse flow when the pump has to be removed. In the illustrations you can see in both cases the discharge head is from the centerline of the pump to the level of the discharge water. The total head is the difference between the two liquid levels.
Pump Definitions (Larger Glossary in the rear of this manual)
Fluid: Any substance that can be pumped such as oil, water, refrigerant, or even air.
Gasket: Flat material that is compressed between two flanges to form a seal.
Gland follower: A bushing used to compress the packing in the stuffing box and to control leakoff.
Gland sealing line: A line that directs sealing fluid to the stuffing box.
Horizontal pumps: Pumps in which the center line of the shaft is horizontal.
Impeller: The part of the pump that increases the speed of the fluid being handled.
Inboard: The end of the pump closest to the motor. Inter-stage diaphragm: A barrier that separates stages of a multi-stage pump.
Key: A rectangular piece of metal that prevents the impeller from rotating on the shaft.
Keyway: The area on the shaft that accepts the key.
Kinetic energy: Energy associated with motion.
Lantern ring: A metal ring located between rings of packing that distributes gland sealing fluid.
Leak-off: Fluid that leaks from the stuffing box.
Mechanical seal: A mechanical device that seals the pump stuffing box.
Mixed flow pump: A pump that uses both axial-flow and radial-flow components in one impeller.
Multi-stage pumps: Pumps with more than one impeller.
Outboard: The end of the pump farthest from the motor.
Packing: Soft, pliable material that seals the stuffing box.
Positive displacement pumps: Pumps that move fluids by physically displacing the fluid inside the pump.
Radial bearings: Bearings that prevent shaft movement in any direction outward from the center line of the pump.
Radial flow: Flow at 90° to the center line of the shaft.
Retaining nut: A nut that keeps the parts in place.
Rotor: The rotating parts, usually including the impeller, shaft, bearing housings, and all other parts included between the bearing housing and the impeller.
Score: To cause lines, grooves, or scratches.
Shaft: A cylindrical bar that transmits power from the driver to the pump impeller.
Shaft sleeve: A replaceable tubular covering on the shaft.
Shroud: The metal covering over the vanes of an impeller.
Slop drain: The drain from the area that collects leak-off from the stuffing box.
Slurry: A thick, viscous fluid, usually containing small particles.
Stages: Impellers in a multi-stage pump.
Stethoscope: A metal device that can amplify and pinpoint pump sounds.
Strainer: A device that retains solid pieces while letting liquids through.
Stuffing box: The area of the pump where the shaft penetrates the casing.
Suction: The place where fluid enters the pump.
Suction eye: The place where fluid enters the pump impeller.
Throat bushing: A bushing at the bottom of the stuffing box that prevents packing from being pushed out of the stuffing box into the suction eye of the impeller.
Thrust: Force, usually along the center line of the pump.
Thrust bearings: Bearings that prevent shaft movement back and forth in the same direction as the center line of the shaft.
Troubleshooting: Locating a problem.
Vanes: The parts of the impeller that push and increase the speed of the fluid in the pump.
Vertical pumps: Pumps in which the center line of the shaft runs vertically.
Volute: The part of the pump that changes the speed of the fluid into pressure.
Wearing rings: Replaceable rings on the impeller or the casing that wear as the pump operates.
What is a Pump?
Pumps are used to move or raise fluids. They are not only very useful, but are excellent examples of hydrostatics. Pumps are of two general types, hydrostatic or positive displacement pumps, and pumps depending on dynamic forces, such as centrifugal pumps. Here we will only consider positive displacement pumps, which can be understood purely by hydrostatic considerations. They have a piston (or equivalent) moving in a closely-fitting cylinder and forces are exerted on the fluid by motion of the piston.
We have already seen an important example of this in the hydraulic lever or hydraulic press, which we have called quasi-static. The simplest pump is the syringe, filled by withdrawing the piston and emptied by pressing it back in, as its port is immersed in the fluid or removed from it.
Pump Safety Regulations
It is a necessity that your safety department establishes a safety program based upon a thorough analysis of industrial hazards. Before installing and operating or performing maintenance on the pump and associated components described in this manual, it is important to ensure that it covers the hazards arising from high speed rotating machinery. It is also important that due consideration be given to those hazards which arise from the presence of electrical power, hot oil, high pressure and temperature liquids, toxic liquids and gases, and flammable liquids and gases. Proper installation and care of protective guards, shut-down devices and over pressure protection equipment must also be considered an essential part of any safety program.
Also essential are special precautionary measures to prevent the possibility of applying power to the equipment at any time when maintenance work is in progress. The prevention of rotation due to reverse flow should not be overlooked. In general, all personnel should be guided by all the basic rules of safety associated with the equipment and the process. It should be understood that the information contained in this manual does not relieve operating and maintenance personnel of the responsibility of exercising good judgment in operation and care of the pump and its components.
In the following safety procedures you will encounter the words DANGER, WARNING,
CAUTION, and NOTICE. These are intended to emphasize certain areas in the interest of personal safety and satisfactory pump operation and maintenance. The definitions of these words are as follows:
“DANGER” Danger is used to indicate the presence of a hazard which will cause severe personal injury, death, or substantial property damage if the warning is ignored.
“WARNING” Warning is used to indicate the presence of a hazard which can cause severe personal injury, death, or substantial property damage if the warning is ignored.
“CAUTION” Caution is used to indicate the presence of a hazard which will or can cause minor personal injury, death, or substantial property damage if the warning is ignored.
Pump Applications
Pumps are used throughout society for a variety of purposes. Early applications include the use of the windmill or watermill to pump water. Today, the pump is used for irrigation, water supply, gasoline supply, air conditioning systems, refrigeration (usually called a compressor), chemical movement, sewage movement, flood control, marine services, etc. Because of the wide variety of applications, pumps have a plethora of shapes and sizes: from very large to very small, from handling gas to handling liquid, from high pressure to low pressure, and from high volume to low volume.
Complicated Pumps
More complicated pumps have valves allowing them to work repetitively. These are usually check valves that open to allow passage in one direction, and close automatically to prevent reverse flow. There are many kinds of valves, and they are usually the most trouble-prone and complicated part of a pump. The force pump has two check valves in the cylinder, one for supply and the other for delivery. The supply valve opens when the cylinder volume increases, the delivery valve when the cylinder volume decreases.
The lift pump has a supply valve and a valve in the piston that allows the liquid to pass around it when the volume of the cylinder is reduced. The delivery in this case is from the upper part of the cylinder, which the piston does not enter.
Diaphragm pumps are force pumps in which the oscillating diaphragm takes the place of the piston. The diaphragm may be moved mechanically, or by the pressure of the fluid on one side of the diaphragm.
Some positive displacement pumps are shown below. The force and lift pumps are typically used for water. The force pump has two valves in the cylinder, while the lift pump has one valve in the cylinder and one in the piston. The maximum lift, or "suction," is determined by the atmospheric pressure, and either cylinder must be within this height of the free surface. The force pump, however, can give an arbitrarily large pressure to the discharged fluid, as in the case of a diesel engine injector. A nozzle can be used to convert the pressure to velocity, to produce a jet, as for firefighting. Fire fighting force pumps usually have two cylinders feeding one receiver alternately. The air space in the receiver helps to make the water pressure uniform.
The three pumps above are typically used for air, but would be equally applicable to liquids. The Roots blower has no valves, their place taken by the sliding contact between the rotors and the housing. The Roots blower can either exhaust a receiver or provide air under moderate pressure, in large volumes. The Bellows is a very old device, requiring no accurate machining. The single valve is in one or both sides of the expandable chamber. Another valve can be placed at the nozzle if required. The valve can be a piece of soft leather held close to holes in the chamber. The Bicycle pump uses the valve on the valve stem of the tire or inner tube to hold pressure in the tire. The piston, which is attached to the discharge tube, has a flexible seal that seals when the cylinder is moved to compress the air, but allows air to pass when the movement is reversed.
Diaphragm and vane pumps are not shown, but they act the same way by varying the volume of a chamber, and directing the flow with check valves.
Fluid Properties
The properties of the fluids being pumped can significantly affect the choice of pump. Key considerations include:
• Acidity/alkalinity (pH) and chemical composition. Corrosive and acidic fluids can degrade pumps, and should be considered when selecting pump materials.
• Operating temperature. Pump materials and expansion, mechanical seal components, and packing materials need to be considered with pumped fluids that are hotter than 200°F.
• Solids concentrations/particle sizes. When pumping abrasive liquids such as industrial slurries, selecting a pump that will not clog or fail prematurely depends on particle size, hardness, and the volumetric percentage of solids.
• Specific gravity. The fluid specific gravity is the ratio of the fluid density to that of water under specified conditions. Specific gravity affects the energy required to lift and move the fluid, and must be considered when determining pump power requirements.
• Vapor pressure. A fluid’s vapor pressure is the force per unit area that a fluid exerts in an effort to change phase from a liquid to a vapor, and depends on the fluid’s chemical and physical properties. Proper consideration of the fluid’s vapor pressure will help to minimize the risk of cavitation.
• Viscosity. The viscosity of a fluid is a measure of its resistance to motion. Since kinematic viscosity normally varies directly with temperature, the pumping system designer must know the viscosity of the fluid at the lowest anticipated pumping temperature. High viscosity fluids result in reduced centrifugal pump performance and increased power requirements. It is particularly important to consider pump suction-side line losses when pumping viscous fluids.
Environmental Considerations
Important environmental considerations include ambient temperature and humidity, elevation above sea level, and whether the pump is to be installed indoors or outdoors.
Software Tools
Most pump manufacturers have developed software or Web-based tools to assist in the pump selection process. Pump purchasers enter their fluid properties and system requirements to obtain a listing of suitable pumps. Software tools that allow you to evaluate and compare operating costs are available from private vendors.
Pumps as Public Water Supplies
One sort of pump once common worldwide was a hand-powered water pump, or 'pitcher pump'. It would be installed over a community water well that was used by people in the days before piped water supplies. Because water from pitcher pumps is drawn directly from the soil, it is more prone to contamination. If such water is not filtered and purified, consumption of it might lead to gastrointestinal or other water-borne diseases. A notorious case is the 1854 Broad Street cholera outbreak. At the time it was not known how cholera was transmitted, but physician John Snow suspected contaminated water and had the handle of the public pump he suspected removed; the outbreak then subsided.
Modern hand-operated community pumps are considered the most sustainable low-cost option for safe water supply in resource-poor settings, often in rural areas in developing countries. A hand pump opens access to deeper groundwater that is often not polluted and also improves the safety of a well by protecting the water source from contaminated buckets.
Pumps such as the Afridev pump are designed to be cheap to build and install, and easy to maintain with simple parts. However, scarcity of spare parts for these types of pumps in some regions of Africa has diminished their utility for these areas.
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