Chapter 15 - Pumps Flashcards
(112 cards)
1
Q
What are the applications for pumps
A
- machine to convey fluids/semi-fluids
- feed water to boilers
- convey/meter fuel and chemicals
- circulate coolants and condensates
2
Q
What are the two basic classification of pumps
A
- dynamic pumps
- positive displacement pumps
3
Q
How does a dynamic pump move fluid?
A
- moved fluid by using centrifugal force to spin it outwards
4
Q
How do centrifugal pumps react when pressure builds beyond their ability to overcome it?
A
- they allow the fluid to slip after the required pressure is built up
5
Q
What are the 4 categories of centrifugal pumps?
A
- radial
- mixed
- axial
- peripheral
6
Q
Define - Pump head
A
- the pressure that a pump has to overcome to be able to move
7
Q
Define - Static suction lift
A
- the vertical distance from the liquid supply level to the pump center line when the liquid supply level is below the pump center line
8
Q
Define - Static suction head
A
- the vertical distance from the liquid supply level to the pump center line when the liquid supply level is above the pump center line
9
Q
Define - Static discharge head
A
- the vertical distance from the pump center line to either the surface of the liquid in the discharge tank or to the point of discharge
10
Q
Define - Total static head
A
- the vertical distance from the liquid supply level to the surface of the liquid in the discharge tank or to the point of discharge
11
Q
Define - Frictional loss
A
- the pressure needed by the fluid to overcome friction
12
Q
Define - Velocity head
A
- the force causing the fluid to flow through a chamber which is due to the velocity of the fluid
13
Q
Define - Dynamic suction lift
A
= static suction lift + velocity head - all the frictional losses in the suction pipes and fittings
14
Q
Define - Dynamic suction head
A
= static suction head - velocity head + all frictional losses in the suction pipes and fittings
15
Q
Define - Dynamic discharge head
A
= static discharge head + velocity head + all frictional losses in the suction pipes and fittings
16
Q
Define - Total dynamic head
A
- the term most used in industry to describe a pump’s head
- for pumping systems which have suction lift:
= dynamic suction lift + dynamic discharge head - for pumping systems which have suction head:
= dynamic discharge head - dynamic suction head
17
Q
Describe vapour binding and the reasons for it
A
- if pressure on the suction side of a pump drops below the vapour pressure of the liquid, vapour forms. This could partially or completely stop liquid flow into the pump. The pump is then said to be vapour-bound.
Causes:
- insufficient suction head
- high suction lift
- excessive friction head
- high liquid temperature
18
Q
Describe cavitation and its results
A
- If pressure in side a centrifugal pump drops below the vapour pressure, vapour bubbles form. The bubbles flow with the liquid until they reach an area of higher pressure. ( normally at the outer area of the impeller). They then collapse, producing a shockwave. This process is called cavitation.
- during cavitation, the bubbles collapse (implode). Liquid then suddenly fills the space, hitting and eroding the surface. This action causes vibration and noisy operation. if continued, this erosion causes the impeller to become imbalanced, which mechanically destroys the pump (bearing failure).
19
Q
How are vapour binding and cavitation prevented?
A
- to prevent cavitation and vapour binding, and to ensure maximum flow through the pump, suction pressure must be greater than the vapour pressure of the pumped liquid. This required pressure at pump suciton is called the “net positive suction head” (NPSH).
20
Q
How is a pump’s capacity determined?
A
- determined by the volume of liquid delivered per unit of time. ex. US gallons per minute
21
Q
How is the slip factor and the pump’s volumetric efficiency expressed?
A
- percentage of theoretical capacity
22
Q
How is the pump’s rating and size expressed?
A
- rated according to the capacity at a given head and speed (rpm).
- size is expressed as the diameter of the discharge nozzle, suction nozzle and impeller in inches or mm.
23
Q
Describe the operating principles of radial flow volute pumps
A
- Radial flow pumps move the liquid out radially from the shaft’s axis. In volute pumps, the rotating impeller discharges the fluid into a spiral shaped cavity called a volute.
Action:
- Fluid is fed into the centre (eye) of the impeller and flows into blades.
- the rotating impeller causes fluid to discharged at its circumference at an increased velocity.
- as the fluid leaves the impeller, it moves outward at right angles to the shaft’s axes. It is flung into the volute at high velocity.
- because the chamber widens, the velocity of the fluid decreases and part of the velocity head is transformed into pressure.
24
Q
Compare diffuser pumps to volute pumps.
A
- diffuser pumps; are used in pump casings to create multiple volutes. They are stationary curved vanes in the casing which redirect the flow of the liquid. The vanes direct the flow outwards towards the discharge nozzle.
- volute pumps; same principle without the stationary diffuser vanes, and therefore a single volute translating the decreased velocity into pressure.
25
Describe the operating principles of axial-flow and mixed-flow pumps.
axial-flow;
- the impeller moves liquid through the casing without changing its direction. The pump develops its head by the lifting action of the vanes on the liquid. The liquid moves through the casing parallel to the shaft.
- Pump is usually mounted vertically, usually below surface of liquid being pumped.
- large volume output flow, low discharge head.
mixed-flow;
- develops its discharge head by using both centrifugal force and lifting action of the vanes on the liquid.
- pump is mounted vertically or horizontally.
- low-head, high-capacity applications.
26
How is greater pressure developed through multiple stages?
- the discharge of one impeller is connected to the suction of the next impeller and so on.
- this action increases the velocity of the liquid each time it passes through a stage thus pressure is increased at the discharge outlet.
27
Describe the operating principles of a regenerative turbine pump.
Action
- the liquid enters at the outer edge of the impeller
- as the impeller rotates, the vanes travel in machined channel in the casing. This gives the liquid a forward motion.
- As the liquid speeds up, centrifugal force throws it into the channel.
- Because of the channel's shape, the liquid returns between the vanes.
- This process is repeated several times. As a result, the liquid follows a spiral path around the outer wall of the casing, to the discharge outlet
- each time the liquid re-enters a vane, it receives an impulse. This series of impulses increases the pressure gradually from suction to discharge.
28
Compare the discharge pressure of a regenerative turbine pump to a similar radial flow pump?
- The regenerative turbine pump can develop a discharge pressure several times larger than a radial flow pump can with the same diameter and speed
29
Describe the types of casings used to encase pumps.
Split Casings
- axially split casings are split along the axis of the shaft. The suction and discharge nozzles are usually in the lower half of the casing. The upper half is easily lifted for inspection.
- radially split casings are split at right angles to the shaft.
Barrel casings
- In a multi-stage centrifugal pump, the inner casing is fitted into an outer casing. The barrel casing has no axial joints. The space between the two casings is subjected to the high discharge pressures, which tends to hold the sections of the inner casing together.
30
What pump casing materials would you order to pump water?
- cast iron casing with bronze fittings
31
What pump casing materials would you order to pump mild acids?
- bronze
32
What pump casing materials would you order to pump moderate alkalis?
- Ni-resist
| - stainless steel
33
What pump casing materials would you order to pump concentrated solids in slurry?
- chrome steel
- Ni-hard
- manganese steel
34
What type of impeller has a partial shroud on one side?
- open impeller
35
What type of impeller has shrouds on both sides?
- closed, single-inlet impeller
36
What type of impeller has inlets on both sides?
- closed, double-inlet impeller
37
What type of impeller has a mechanical screw?
- screw-style impeller
38
How are impellers mounted and held on the pump shafts?
- mounted either on the end of the shaft or mid-shaft
When mounted on the end of the shaft the impeller
- is screwed on to a threaded shaft and butting against a shoulder. The hand of the thread must have a tightening action as the shaft rotates.
- has a parallel bore with a sliding fit on the shaft. It is held in position by a shoulder or sleeve on one side, and a keeper (retainer) plate and nut (with a locking device) on the inlet side. A key is needed for positive drive.
- Is installed on a tapered shaft. A key is also used for driving. A nut (with a locking device) and a keeper plate is used to hold the impeller on the taper.
When mounted mid-shaft the impeller is driven by a key and held in place by
- a sliding fit against a shoulder retained by sleeves and a nut.
- a sliding fit against sleeves and retained by nuts on both ends of the shaft.
39
What pump and impeller type would you use for water at moderate temperatures?
Pump
- Single or double suction centrifugal pump
Impeller
- closed except for very small capacities
40
What pump and impeller type would you use for hot hydrocarbons?
Pump
- single suction centrifugal pump; often special refinery pumps designed for high temperatures.
Impeller
- closed with large inlets
41
What pump and impeller type would you use for pulpy solids?
Pump
- single suction centrifugal pump; use double suction with special end-clearance wearing fits only on very light concentrations of solids.
Impeller
- closed (open was the standard until end-clearance wearing fits changed)
42
How is internal leakage prevented between the discharge and suction side of a pump?
- The clearance between the casing and the suction passage of the impeller must be kept to a minimum
43
What happens when the wear rings in a pump wear out?
- If wear exceeds 3x the initial clearance, the pump loses too much efficiency.
44
How are wear rings fastened in a pump?
- a shrink fit
- a spigot, a groove or pins
- screw threads
45
How are wear rings lubricated?
- lubricated only by the pumped liquid, so their life is extended if they are always run in liquid.
46
Describe wearing cheek plates.
- With open impellers in radially split casings, wear cheek plates are used instead of rings. Wear plates are easily replaced plates which protect the front and/or back faces of the casing.
- clearance between the plate and impeller can be adjusted by moving the shaft axially. This is usually done by using shims.
47
Describe how hydraulic axial thrust is controlled using opposing impellers, holes in impellers, and balancing drums and disks.
Opposing impellers
- half the impellers have inlets facing in the opposite direction to the other half. If the inlets all face in the same direction, the axial thrust must be compensated.
Holes in impeller
- the excess pressure that builds up in the back face of the impeller can pass through these holes into the suction side.
Balancing drum
- any excess pressure built up in the back face of the impeller is allowed to pass through the radial clearance between the balancing drum and the balancing drum head. This pressure then returns to suction side of the pump.
Balancing disk
- the excess pressure passes through the axial clearance. As the pressure fluctuates, the axial movement of the shaft changes the axial clearance which acts as a valve.
48
What methods are used to control leakage between the casing and the shaft?
- mechanical seal
| - compression packing
49
Describe a mechanical seal.
- Set screws are used to hold it in place. O-rings are used as a secondary seal to prevent leakage from the mechanical seal to the housing and shaft.
50
Describe sealing with compression packing and lantern rings.
- a stuffing box holding multiple rings of compression packing and a lantern ring are held in place by a gland. This gland is adjustable by tightening nuts to compress the rings for the desired fit.
- when a pump operates at a negative suction pressure, air can be drawn into the casing. To prevent this, the stuffing box is fitted with a lantern ring and a sealing liquid connection. This connection can be external piping or an internal passage consisting of passage holes drilled in the casing. When used to pump abrasive fluids, the clean sealing liquid keeps the abrasives out of the stuffing box.
51
How is shaft wear and corrosion at the pump's stuffing box controlled?
- shafts of smaller pumps are usually made from corrosion and wear resistant materials
- larger pump shafts are usually protected by renewable sleeves. These are made of stellite, stainless steel or chrome plated brass/bronze.
52
How are seized sleeves removed from the pump shaft?
1 - grind two grooves with a small hand grinder. The grooves should be ground 180° apart, axially along the sleeve.
2 - split the sleeve with a cold chisel, being careful not to damage the shaft.
53
What are the functions of bearings in centrifugal pumps?
- support the shaft carrying one or more impellers
- allow the shaft to rotate with minimum friction
- keep the rotating shaft and impellers in correct position within the stationary parts of the pump
54
What two basic kinds of bearings are used in centrifugal pumps?
- friction bearings
| - anti-friction (rolling element) bearings
55
How are friction bearings lubricated on horizontal and vertical pumps?
- drip feed oiling
| - if housing has oil reservoir, an endless chain or a ring riding on the shaft.
56
Where are axial and radial rolling element bearings located in axial split and radially split pumps?
Axially split pumps
- radial load bearing are used at both ends of the shaft. This is due to the axial thrust of the shaft being theoretically balanced.
Radially split pumps
- radial load bearings are used at the impeller end and combination (radial and axial load) bearings) are used at the drive end.
57
Describe a suitable base to mount a pump and drive source.
- cast iron or fabricated steel
| - base is bolted to a solid foundation to absorb vibration.
58
Compare base mounting pumps to centre-line pumps
Base mounting
- mounting lugs on the bottom of the pump casing.
- any change in temperature of the pump while operating can cause the pump to lift and become misaligned with the coupling.
Centre-line mounting
- the pump body is carried on lugs extending from the centre of the pump casing which rest on the base
59
Describe close-coupled pumps.
- the motor shaft extends through the pump casing. The impeller is then mounted directly onto this shaft. The pump casing is normally mounted directly onto the motor casing.
60
What checks should be made prior to starting a pump?
- proper alignment
- unit turns over freely by hand
- pump driven in direction shown on pump casing
- bearings properly lubricated
- pump is primed
- air from the seal housing is vented
- plenty of quenching fluid is flowing to the seals
61
What safety precautions should be followed when working on a pump?
- always lock out and tag all electrical controls
- shut off, lock and tag all main valves to and from the pump, including the seal-fluid supply
- drain the pumps. This is important with suction-head mounting.
- flush the pump if it has been pumping acids or any other substance that could be hazardous.
62
List the preventative maintenance procedures for maintaining centrifugal pumps.
- volume and temperature of liquid leaking past the packing
- oil level
- bearing temperature and noise (vibration analysis)
- joints and seals for leaks
- flow rates at operating temperatures
63
What are the routine maintenance procedures carried out on centrifugal pumps?
- replace the compression packing
- inspect and, if necessary, replace the shaft sleeve
- change the oil
64
Describe what a pump overhaul usually consists of and where the information needed for this is obtained.
Overhaul
- inspecting/replacing the impeller
- inspecting/replacing the wear rings or plates
- inspecting/replacing the shaft and shaft sleeve
- inspecting the stuffing box and replacing the compression packing or mechanical seal
- replacing all bearings and seals
Pump manufacturers supply
- a drawing showing position of all parts
- an information sheet showing details such as pump rating, model, packing seal, and bearing details
- an installation and service manual giving step-by-step routine for general overhaul
65
What symptoms would occur if the impeller were running in the wrong direction?
- failure to deliver liquid
66
What symptom would occur if air were leaking through the stuffing box?
- reduced capacity or pressure
67
What are the possible reasons for a pump driver to be overloaded?
- speed is too high
- viscosity of the liquid different then recommended for the pump
- mechanical resistance in the pump
68
What are the causes of pump vibration?
- misaligned coupling
- insecure foundation
- unbalanced impeller due to a chipped blade
- unbalanced impeller due to excessive cavitation
- bent shaft
- worn bearings
69
What corrections should be made if an unbalanced impeller is due to cavitation?
- repair and replace impeller
| - increase suction pressure
70
What corrections are required if the pump casing wears?
- repair casing by means of welding or installing an insert
| - replace the pump casing with a more suitable material for the pump liquid
71
What corrections are needed if there is excessive packing wear due to overheating?
1 - replace with proper grade
2 - reduce shaft diameter then use appropriate packing size
- replace shaft
3 - replace shaft sleeve
4 - check and correct the faulty sealing
5 - ensure cooling supply to the stuffing box is operational
- ensure gland nuts are not too tight
72
What are the usual causes of casing damage?
- impact
- uneven base
- freezing
73
Where are positive displacement pumps used?
- where a constant flow or a measured amount of flow is critical
74
What are the two classifications of displacement pumps?
- reciprocating
| - rotary
75
Define volumetric efficiency of a displacement pump.
- actual output / theoretical output
76
What are the two types of reciprocating pumps and what type of flow is produced?
- plunger pump
- piston pump
- produces a pulsating flow
77
What does the volume of liquid per stroke depend upon?
- cross-sectional area of the piston, plunger or diaphragm
| - length of stroke
78
What is the difference between plunger and piston pumps?
Plunger
- has its plunger sliding inside packing
Piston
- has its packing sliding with its piston
79
Describe the operating principles of a single-acting plunger/piston pump.
- has a piston which slides inside a cylinder
- the cylinder acts as a chamber and as the piston moves in the cylinder it displaces most all the fluid in the chamber
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Action
1 - plunger/piston extends:
- outlet valve opens
- inlet valve closes
- fluid is forced out of chamber
2 - plunger/piston retracts:
- outlet valve closes
- inlet valve opens
- fluid is sucked into chamber
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80
Describe the operating principles of a double-acting plunger/piston pump.
- both sides of the piston moves fluid
- piston discharges at each stroke
Action
1 - piston extends
- liquid drawn into left end of cylinder
- liquid pushed out right end of cylinder
2 - piston retracts
- liquid drawn into right end of cyclinder
- liquid pushed out left end of cylinder
81
What types of packing are used to promote efficient operation of plunger/piston pumps?
Plunger
- V-ring or U-ring packing is used
Piston
- series of cast iron rings which are fitted into grooves in the perimeter of the piston
- with cup or U-ring packing material held in position by retainers
82
What pumps are diaphragm pumps similar to in action?
- plunger/piston pumps
83
List the types of check valves used in conjunction with reciprocating pumps and state the application for each type.
Ball check valves
- used where free opening of suction and discharge is needed
Stem-guided check valves
- used for low pressures
Wing-guided check valves
- used for moderate or high pressures.
Flap check valves
- used for low pressures and free flow of semi-solids
84
What type of flow does a rotary pump produce?
- continuous and smooth discharge
85
What are some applications for rotary pumps?
- fuel
- lubricants
- hydraulic oils
- liquids of various viscosities
- gases and liquified gases
86
Define fixed delivery and variable delivery.
Fixed delivery
- always delivering the same volume at a given speed
- gear / vane / piston
Variable delivery
- delivering volumes varying from zero to a given maximum
- vane / piston
87
Describe the construction and action of external and internal gear pumps.
- pair of meshed gears enclosed in a casing
- one pair drives the other
- meshing teeth prevent liquid from flowing back to inlet
External
- two oppositely-rotated, externally-cut gears inside one casing
Internal
- one internally cut gear and one externally cut gear, separated on one side by a crescent-shaped partition
Action
- fluid enters the pump at the inlet port into the space between the gear teeth.
- fluid carried around the casing to the discharge port
- at the discharge port the meshing of the teeth restrict the oil from returning to the inlet port thus forcing it out.
88
Describe the construction and action of sliding vane-pumps.
- has a rotor set slightly off centre in its casing. Vanes in the rotor are free to slide in channels, pushed outwards by centrifugal force as the rotor rotates.
Action
- fluid is drawn into the chambers at the inlet and is carried around by the vanes
- as chamber passes over the outlet, the chamber reduces in size and the fluid is forced out
89
Compare unbalanced vane pumps to balanced vane pumps.
Unbalanced
- pumping action only on one side of pump
- high capacity bearings required
Balanced
- made by mounting the rotor in the centre of an elliptical case with two inlets and two outlets
- fluid is drawn in and discharged during each half-revolution
- light or medium-service bearings can be used
- larger volume delivery
90
Describe the operating principles of a variable-capacity vane pump.
- the rotor may be centred or off-centre by means of moving the pressure-chamber ring
- automatic controls with a spring-loaded governor
- varies flow to meet demand; levels pre-set
- if discharge rate is seldom changed; its adjusted manually
- rely on pump action rather than relief valves to limit pressure
91
Describe the operating principles of an external vane pump.
- has an oval rotor rotating centrally in the housing
- one sliding vane separates the inlet and discharge ports
- as rotor rotates, it carries fluid from the inlet port to the discharge port
- the vane maintains a seal between the rotor and the housing
92
Describe the operating principles and state the applications for flexible tube pumps.
- uses a rotor with two lugs mounted on the periphery, 180° apart
- lugs compress the tube against the housing. This forms a seal that prevents the liquid from returning to the inlet port.
- as the rotor rotates it forces the liquid around the housing to the discharge port
- slow RPM >200RPM
- discharges liquid every half rotation
Application
- food products (juice)
- paint
- chemicals
- slurry
- sludge
- pulp
93
Describe the operating principles and state the applications for flexible liner pumps.
- uses an eccentric rotor to force the internal walls of a flexible liner outwards against the housing
- discharges liquid once per revolution
- liner has a divider between the discharge and inlet ports, which prevents liquid from returning to the inlet port
Application
- food products (juice)
- paint
- chemicals
- slurry
- sludge
- pulp
94
Describe the operating principles and state the applications for flexible vane pumps.
- has a rotor with flexible vanes
- vanes carry liquid around the housing from the inlet to the discharge ports
- insert mounted between the discharge and inlet, which compresses the vanes and forces the liquid out
- moderate speeds >6000RPM
Application
- coolants
95
Describe the operating principles of lobe pumps.
- has two rotors, each with one, two, or three lobes.
- rotors placed in casing with a set of external timing gears which synchronize the lobes
- liquid is trapped in the pockets formed by the lobes and the casing
- motion of the lobes carries the liquid around the casing to the outlet
96
How is proper timing assured for re-assembly of lobe pumps?
- check the timing gears for mate marks
| - mate marks are often put on at the factory to aid re-assembly
97
Describe the operating principles of single screw pumps.
- has one rotor (screw) inside a stationary lobed casing called a stator
- a cavity is produced between the inside surface of the rotor with the stator
- outside surface of the rotor remains in contact with the stator to prevent the fluid from moving back to the inlet
98
Describe the operating principles of two-screw pumps.
- has two rotors each with two opposing screw threads (right and left hand)
- one rotor is the drive and stays in proper mesh by means of timing gears
99
Describe the operating principles of multiple-screw pumps.
- has multiple rotors
- the centre (drive) screw thread has one hand and the other screw threads have the opposite hand
- as the screw rotates, liquid is carried between the threads and the casing, axially towards the outlet
- threads are in constant mesh with each other, restricting the fluid from returning to the inlet
100
Describe the operating principles of axial piston pumps.
- has its pistons positioned axially in the pump
- pistons connect to a drive shaft which rotates
- a cylinder block houses the pistons and rotates against a stationary valve plate which houses the intake and outlet ports
Action
- the drive shaft rotates the cylinder block and pistons
- when the housing angle is other than zero degrees, the distance between the piston and valve surface continually changes
- each piston moves away from the valve surface during one half of the revolution, causing suction and flow
- each piston moves toward the valve surface during the second half of the revolution, causing compression and discharge
101
Describe how axial piston pumps can be of variable capacity.
- If the housing angle is zero, piston length is zero, and there is no pumping action
- If there is a small housing angle, there is small piston stroke length, and some fluid is pumped
- If maximum housing angle, stroke length is maxed, and full pumping capacity
102
Describe the operating principles of a constant displacement, radial piston pump.
- has a rotating eccentric shaft rather than a cylinder block
- the piston cylinders are contained in the fixed housing of the pump
- each piston has a check valve for fluid to enter and another for fluid to exit
Action
- as the eccentric shaft rotates (180°) from the high to low point, the piston cavity fills with fluid
- as the eccentric shaft rotates the other 180°, the piston cavity discharges the fluid
- This cycle acts on every piston in sequence
103
How is capacity regulated in a positive-displacement pump?
- regulated by varying their speed
| - output is only slightly affected by pressure variations, dropping gradually as pressure rises
104
Why is excessive pressure relieved with positive-displacement pumps?
- relieved by a pressure relief valve that redirects fluid after it reaches a set pressure limit
- at the preset pressure limit, the valve opens and dumps excess fluid back to the supply
- without a pressure relief valve, a blocked line may cause the motor to stall or the pump to break
105
What are two important factors in the installation of positive-displacement pumps?
- pump should be mounted on a strong and rigid base
| - alignment to power source
106
What does the allowable wear for a pump depend on?
- depends on the demands put on it
| - higher the capacity the less tolerance for wear
107
Are positive-displacement pumps repairable?
- yes, by complete overhaul or small repair
108
What correction is needed if the pump is leaking around the shaft seal?
- inspect the bearings and replace if necessary
| - replace the shaft seal as specified
109
What is the symptom and correction needed if the oil viscosity is too high for the pump to pick up and prime?
- use thinner oil and follow recommendations for temperature and service
- heat oil for cold-weather startup
110
What corrective action should be taken if oil is freely re-circulating to the tank through the system?
- check directional control valves (DCV)
| - the valve return may be open
111
How would you test for a small air leak on the pump suction line?
- test by pouring pumped fluid on the joints while listening for a change in the sound of operation
112
What are the symptoms of a partially clogged intake line or filter, or restricted intake pipe and what corrective measures should be taken to repair it?
Symptoms
- pump makes unusual noise
- pump doesn't deliver fluid
Correction
- clean the lines and filter to prevent cavitation
- clean or replace filters. Filter capacity should be at least twice the pump capacity
