Pumps Flashcards
List the components of a pump
- Casing (keeps liquid inside and protects internal parts)
- Inlet/suction (where the liquid enters the pump)
- Outlet/discharge (where liquid exits the pump)
- Component that physically moves the liquid through the pump (different for each type)
- Valves (installed in suction or discharge lines so the pump can be isolated/removed for maintenance)
- Driver or motor
What process conditions affect pump selection
- The rate at which the fluid needs to be pumped
- The viscosity
- Possible non-Newtonian characteristics
- How clear it is
- If it contains suspended particles
- Corrosiveness
What are the two main categories of pumps
- Positive displacement
- Centrifugal
How pumps are named
- By the liquid that it pumps
- By its specific function on the plant
- By its location
How does a positive displacement pump work
The volume of the liquid delivered is directly related to the displacement if the reciprocating pump. The flow rate of the liquid will be proportional to the pump speed and is not appreciably influenced by pressure
How does a centrifugal pump work
It depends on giving the liquid a high kinetic energy, which is partially converted into pressure energy. It doesn’t require precision engineering since there is a gap between the impeller and the casing. This means it can pump slurries
List some reciprocating pump types
- Piston pump
- Plunger or ram pump
- Diaphragm pump
- Metering pump
List some rotary pump types
- Gear pump
- Flow inducer or peristaltic pump
- Mono pump
- Screw pump
Describe the piston pump
The piston pump consists of a cylinder with a reciprocating piston connected to a rod which passes through a gland at the end of the cylinder. The liquid enters the suction line through a suction valve and is discharge through a delivery valve into the delivery line
The total pressure at the delivery of the piston pump is the sum of what components?
- The static pressure at the delivery point
- The pressure required to overcome frictional losses in the delivery pipe
- The pressure required for the acceleration of the fluid
Describe the plunger or ram pump
This pump is very similar to the piston type but it differs in that the gland is at the end of the cylinder rather than the middle, thus making replacement easier.
Describe a diaphragm pump
In one section a plunger or piston operates in a cylinder in which a non-corrosive fluid is displaced. The movement of the fluid is transmitted by means of a flexible diaphragm to the liquid to be pumped.
Why was the diaphragm pump developed
For handling corrosive liquids or liquids containing suspensions of abrasive solids
What are the two main types of diaphragm pumps
- Induced flow flexible discs
- Edge retained diaphragm
Describe a metering pump and when it is used
Positive displacement pumps driven by constant speed electric motors. They are used where a constant and accurately controlled rate of delivery of a liquid is required
Describe a gear pump
Two gear wheels operate inside a casing - one of the gears is driven and the other operates in mesh with it. The liquid is carried around in the space between the consecutive teeth and the casing and is then ejected as the teeth become meshed
What are the advantages of a gear pump
- Pump is self-priming
- There is uniform discharge with little pulsation
- The rotation is reversible
- Can handle gaseous fluids
- Due to small clearances there are only small flow variations with viscosity variations and pressure fluctuations
What are the disadvantages of a gear pump
- Can only pump clean fluids
- Cannot run dry
- They need close tolerances to operate, so limits and alignment is critical
- Needs pressure relief as with a piston pump
Describe the flow inducer or peristaltic pump
An elastic tube is compressed in stages by a rotor. The tubing is fitted to a curved track mounted around a rotor carrying three rollers. As the rollers rotate they flatten the tube against the track at the points of contact. The fluid is then squeezed forward by positive displacement.
What are the advantages of a peristaltic pump
- Self-priming
- It can run dry
- It is reversible
- Will pump gaseous fluids
- Can pump abrasive materials
- Has only one contact part
- There are no glands or mechanical seals
- Clinically clean
What are the disadvantages of a peristaltic pump
- Tube failure means loss of product and contamination
- Cannot run against a closed valve
- Pressure is limited to the tube strength
- At low rotational speeds pulsation can be a problem
- Relatively high cost or replacement tubes
Describe the mono pump
A specially shaped helical metal rotor revolves eccentrically within a double helix. A continuous forming cavity is created as the rotor turns. The cavity progresses towards the discharge
What are the advantages of a mono pump
- Gives a uniform flow
- Quiet in operation
- Will pump against high pressures
- Can handle corrosive and gritty liquids
Describe the screw pump
Fluid is captured in a channel between a screw and the cylinder wall of the pump casing and moved toward the discharge as the screw turns
Describe the centrifugal pump
The fluid is fed to the center of a rotating impeller and is thrown outward by centrifugal actio. Because of the high speed of rotation the liquid gains high kinetic energy and the conversion of kinetic energy into pressure energy gives increased pressure at the pump discharge
Explain how centrifugal pumps can be operated efficiently
The optimum conditions for operation is at the duty point, where a vertical line from the point of maximum efficiency crossed the head curve (graph on pg 151)
What is cavitation and how does it happen
Centrifugal pumps add pressure to a liquid, therefore the minimum pressure occurs on the suction side. If this pressure is less than the vapour pressure at the liquid temperature, boiling will occur. If there is sufficient vapour generated, the pump flow may stop. The pressure inside the pump causes the vapour bubbles to collapse. This is cavitation.
What can be done to prevent cavitation
Pumps should be arranged so that the inlet is under a suction head or the pump may be fed from a tank.
You can also increase the suction head or suction line diameter, by decreasing the fluid temperature, by reducing the pump speed or by throttling the flow in the delivery line.
What are the main advantages of a centrifugal pump
- Simple in construction and can be made from a wide range of materials
- Complete absence of valves
- Operates at high speed and can be coupled directly to an electric motor
- Gives a steady delivery
- Maintenance costs are lower
- No damage is done if the delivery line gets blocked
- Much smaller
- Liquids containing high proportions of suspended solids are readily handled
What are the main disadvantages of a centrifugal pump
- Will not develop a high pressure
- Operates at high efficiency over only a limited range of conditions
- Not self-priming
- If a non-return valve is not incorporated in the delivery or suction line, the liquid will run back into the suction tank as soon as the pump stops
- Very viscous liquids cannot be handled efficiently
