Pumps (centrifugal) Flashcards
What are the two types of pumps?
Dynamic pump
Positive displacement pump
What are the different types of dynamic pumps?
Radial flow
Axial flow
Mixed flow
Peripheral flow
How is flow created in a dynamic pump?
By using centrifugal force to spin the fluid outwards.
What is pump head?
The pressure the pump has to overcome to be able to move liquid through the system.
Psi
kPa
Bars
What is head?
Pressure converted to a height of a column in which the pumped liquid can maintain.
What is vapour binding?
When pressure on the suction side of the pump drops below the pressure of the liquid, vapour forms.
Can be caused by:
Insufficient suction head
High suction lift
Excessive friction head
High liquid temperature
What is cavitation?
If pressure inside a centrifugal pump drops below the vapour pressure, vapour bubbles form. The bubbles flow with liquid to the high pressure area and collapse, producing a shock wave.
What is net positive suction head?
The required pressure at pump section.
How is the pumps capacity determined?
By the volume of liquid it can deliver per unit of time.
USgpm
L/min
What is the most common radial flow pump?
Volute pump
What’s the difference between radial and axial flow pumps?
Radial pump moves the liquid out radially from the shafts axis.
Axial-flow pump the liquid moves through the casing parallel to the shaft.
Mixed flow pumps use?
A combination of radial and axial pumps
Where do you find regenerative turbine pumps?
Used in compact areas and are suited for high pressure low capacity service
Where are mixed flow pumps used?
Low head, high capacity applications
What are the two types of pump casings?
Split casings:
Horizontally
Diagonally
Radial
Barrel casings
What casing material is best suiting for water, gasoline and other chemically neutral liquids?
Cast iron with bronze fittings
What are the different impeller designs? 7 of them
Open impeller
Semi open
Closed single inlet
Closed double inlet
Screw style
Axial flow
Open mixed flow
How is internal leakage prevented between the discharge and suction side of the pump?
Seals
What happens when the wear rings in a pump wear out?
Re surfaced or replaced when wear exceeds three times the initial clearance.
How are wear rings held into position?
Shrink fit
Spigot, groove, or pins
Screw threads
How are wear rings lubricated?
The pumped liquid
What is a wearing cheek plate?
Easily replaced plates which front and back of the casings
How is hydraulic axial thrust controlled
Using opposing impellers
Holes in impellers
Balancing drum/discs
How do you control leaking through the casing and the shaft?
Mechanical seals
Or
Compression packing
What is a mechanical seal?
Two lapped faces contacting each other to make a seal.
Best suited for high pressure
Describe sealing with compression packing and lanterns?
Usually rings of packing separated by a lantern (lantern ring separates layers of packing and allows a passage for lubrication.) the glad is either tightened or loosened to properly hold rings in place.
How is shaft wear and corrosion at the pumps stuffing box controlled?
Shafts are made of corrosion and wear resistant material.
Larger shaft usually have replaceable sleeves
What is the purpose of the centrifugal pump bearings?
Supports the shaft carrying one or more impellers
Allows shaft to move with limited friction
Keep the rotating shaft and impeller into position.
What are the two basic types of bearing used in pumps?
Friction
Anti friction
Where are axial and radial rolling elements located in axial split and radially split pumps?
On axial split pumps bearings are located at both ends of the shaft.
On radially split pumps radial load bearings are placed at the impeller end and combination bearings are placed at the drive end.
What is a suitable base to mount a pump and drive source?
Must be mounted to a secure base of cast iron or steel. And bolted to a solid foundation.
Compare base mounted pumps to centreline mounting pumps?
Base mounted pumps have mounting lugs on the bottom of the pump casting. Any Change in temp can cause pump to lift and become misaligned.
Centreline mounting pumps are carried on lugs from the centre of the casting which rest on the base. Casting can move in both directions with out changing alignment.
What is a close coupled pump?
Pump casing is directly mounted on the motor casing.
Motor shaft extends through the casing and the impeller is mounted onto it.
What checks should be made prior to starting the pump?
The unit will turn over freely by hand
The pump is driven the the direction shown on the casing.
The bearings are properly lubricated
The pump is primed
All air from seal housing is vented
Plenty of quenching liquid is flowing to the seals
What safety precautions should be followed when working on a pump?
Lock out
Shut off, lock and tag all main valves to and from the pump. Including seal fluid supply
Drain the pump
Flush the pump if it had been pumping acids or anything else that could harm workers
List the preventative maintenance procedures for centrifugal pumps?
Inspect:
Volume and temperature of liquid leaking past the packing.
Oil level
Bearing temperature and noise
Joints and seals for leaks
Flow rates at operating conditions.
What does a pump overhaul consist of?
Inspecting/replacing the impeller
Inspecting/replacing wear rings or plates
Inspecting/replacing shaft or sleeve
Inspecting stuffing box and replacing compression packing or mechanical seal
Replacing all bearings and seals
What symptoms would occur if the impeller where running in the wrong direction?
Failure to deliver liquid
What symptoms would occur if air were leaking past the stuffing box?
Reduced capacity or pressure
What are the possible reasons for the pump driver to be overloaded?
Speed is to high
Viscosity of liquid different then what is recommended for the pump.
Mechanical resistance in the pump.
What are the causes of pump vibration?
Misaligned coupling
Insecure foundation
Unbalanced impeller due to excessive cavitation.
Bent shaft
Worn bearings
What corrections are required if the pump casing wears?
Repair casing by welding or installing an insert.
Or
Replace the pump casing with a more suitable material for the pumped liquid.
What corrections are needed if there is excessive packing wear due to overheating?
Ensure cooling supply to the stuffing box is operational
Ensure gland nuts are not to tight.
What are the usual causes of casing damage?
Impact
Uneven base
Freezing
What are some other names for dynamic pumps?
Continuous flow, kinetic, centrifugal, or non displacement pump.
What is the equation to get Psi from the pump?
Psi = ft x .433 x sg
Specific gravity (SG) of water is 1
One foot head of water has a head pressure of .433
What is the pump equation to convert psi to feet?
Ft = psi / .433 x SG
Specific Gravity (SG) of water is 1
What is the most common dynamic pump in the industry?
Radial flow centrifugal pump
What are the two main parts of a centrifugal pump?
Rotating element
Stationary element
Where is the radial flow pump most typically used?
Where a reasonably high discharge is needed.
Where should axial flow pumps be mounted and why?
Below the surface of the liquid to be pumped because it had very low suction.
Where can you find mixed flow pumps?
Wells, processing, circulation, refrigeration, and air conditioning.
How deep Can a deep well mixed flow pump go?
1500ft
What are the two types of peripheral pump?
High speed pump up to 25 000 rpm
Regenerative turbine pumps (high head low flow)
What is an advantage of a vertical single stage end suction pump?
Can be mounted vertically with a c faced motor where there is limited space.
What does NPSHR stand for?
Net positive suction head required.
Must be higher the vapour pressure
What does NPSHA stand for?
How much greater should this be than NSPHR?
net Pressure suction head available
10% more
What are the stages of pumps
Single
Multi
Where do you find multi stage barrel pumps?
Exclusively in boilers
When do you replace wear rings on dynamic pump?
Replace when clearances are three time the original
What is the difference between suction head and suction lift?
Suction head the level of liquid is above the centreline of the pump. This has weight of liquid plus atmospheric pressure can push the fluid into the pump.
Suction lift the supply reservoir is below the pump. It can only use atmospheric pressure to help push fluid into the pump
What is the best configuration of imposed impellers for hydraulics, and abrasive solids would be?
What devices are used to reduce axial thrust on the bearings?
Balancing devices:
Wearing rings and balancing holes
Back pump out vanes
Double suction impellers
Opposed impellers
Balancing drum
Balancing disc
Where is the check valve normally placed on a pumping system?
Discharge side
If you are experiencing inadequate pump discharge what should you be checking?
Blockage: check suction and discharge valves are open, gate valves are all fully open, check and relief valves are working properly, check that strainers are not clogged.
Why do you have to prime a displacement pump before starting?
Damage to the pump parts may occur
What are some rules of thumb for suction piping in the displacement system?
Pipe must be large enough diameter to carry the flow required by the pump to minimize friction loss. One to two sizes larger than the diameter at the pump suction flange
There should be a minimum of 10 diameters between the elbow and the pump inlet
Pump should be as close as possible to the liquid
Use y fittings instead of T fittings
How is water hammer created?
By opening/closing valves or stopping the pump.
Can be controlled by changing the rate valves are opened/closed or installing a pulsation dampener
What is hydraulic slap and how is it resolved?
Similar to water hammer closing the discharge valve slowly before stopping the pump will help the check valve close normally
What could cause mechanically induced noise and vibration?
Bent shaft
Bound rotor
Worn bearings
Misalignment
How big should the foundation be for you pump?
2 1/2 times the total mass of the pump
What could be some reasons for excessive power consumption?
Tight packing
Tight mechanical seal
High viscosity
Bent shaft or bound rotor
Wrong rotation
Faulty motor windings
Excessive flow
Before servicing a pump you need to……
Drain, vent, purge, blind
When a shaft runout exceeds ______” it needs to be replaced.
.003”
If shaft radial runout exceeds .003” what needs to be checked/replaced?
Bearings and shaft size
If shaft axial movement exceeds .003” what needs to be checked/replaced?
Bearings
What are the high maintenance areas in displacement pumps?
Packing
Mechanical seals
Bearings
Wear rings
Shafts, sleeves, and impellers
Valves
Axial pumps should be mounted where? why?
Axial pumps should be mounted below the surface of the liquid because they have very little suction power
Axial pumps have relativly low discharge head but can produce _______?
large flow
Axial pumps use Volute or diffuser casings
Diffuser
Single stage end suction pumps make up about _______% of the centrifugal pumps used?
75
How is balance achieved with a single stage double suction pump?
The casing includes a single suction inlet that spits to allow equal flow to each side of the impeller and a volute that continues around the top and bottom halves of the casing. The double suction design results in opposing forces which balance the out axial loads.
How can pressure be increased with a single stage pump>
You can mount several impellers in series are mounted onto the same shaft (multi stage) each impeller is surrounded by its own volute or diffuser.
Multi stage horizontal pumps can handle heads up to 5000 feet and flows up to _______GPM
2500
What are some disadvantages of single casing radial split pumps?
- Casings all face the same direction so there is a build up of axial thrust. A balancing drum counters this thrust and an external line returns the drum leakage back to the suction.
- After rotor has been balanced, it must be dismantled and re-assembled with inner casing pieces
- Suction and discharge connections must be broken each time the pump is opened
Volute casing is used for lower pressure while diffuser casing can be used for _______ pressure
higher
Multi stage barrel pumps are used exclusively for?
Boiler feed water, water injection, oil refinery services
one disadvantage to single casing pumps is the potential for hazardous leaks through the casing joints to the atmosphere. how is this overcome?
installing a multistage pump inside a one piece outer casing that has radial split ends.
mutli stage pumps can handle pressures up to ________PSI and capacities up to _________gpm
6500
20 000
What are the different things that influence the pump efficiency and the net positive suction head(NSPH)?
- impeller eye
- Hub and outside diameters
- Width
- Suction area and specific speed
- Shape
- number and pitch of vanes
- Area between vanes and shape of suction passages
Most radial flow impellers use a back swept vane design, what does that mean?
vane tip bends away from the direction of rotation
What is more efficient, semi open or open impeller?
semi open impeller
Semi open impellers can handle fibrous slurries, and abrasives in chemical, paper and severe industrial services. its drawback is?
Cannot handle solids as well as an open impeller. also requires care in clearance adjustments to achieve high performance.
The impellers are more expensive to build but are more efficient and handle higher heads?
Closed exclusively used for clean liquids
What type of impeller has a rear cover that acts as a wear surface instead of the volute casing?
Reverse vane impeller. Can reduce maintenance cost.
What type of impeller results in a hydraulically balanced impeller with minimal axial thrust?
double suction impeller. clear and low viscosity fluids only
The double suction Eye impeller increases the suction area capacity, as well as _______ the NPSH requirements?
reduces
What are francis impellers suited for and what makes them different than other radial flow impellers?
Has double curvature vanes and and a chunkier profile. most efficient and suited for relatively high flow rates and moderate head rises.
Mixed flow impellers generate flow in which directions? and which way are they typically mounted?
Radial and axially
Normally mount on a vertical shaft
High speed impellers vane angle is close to 90 degrees and developes ______% more head than a backswept blade of the same diameter running at the same speed.
50
What are the different types of pump casings?
- Volute
- Double volute
- concentric
- Diffuser
what is a Volute casing?
cross sectional collecting area that gradually increases from the cutwater (or tounge) to the conical diffuser leading to the discharge flange.
what is more common in volute casings, centerline or Tangential volute?
Although not as efficient as the Tangential volute the centerline volute is the most common.
Volute casings are bulky and expensive. what is their main advantage?
the flexibility they allow in variation of pump capacity.
Volute casing impeller diameters can decrease up to _____% before hydraulic losses affect pump efficiency?
20%
Large volute casings can use the _______ design to balance out the radial forces on the impeller and strengthen the casing?
double Volute
Concentric casings are used when?
When the job requires impellers of different diameters when pumping waste water containing solids or when the job requires a pump larger than the flow.
what does a diffuser casing consist of?
a series of stationary, curved vanes, each space between the vanes gradually expanding to slow the liquids down converting the velocity head into pressure head.
Diffusers can either surround the impeller or adjacent to it. because of the concentric design _______ forces on the impeller are equalized and occupy less space of an equally performing volute.
Radial
Diffuser casings are found in most __________Pumps?
Horizontal multi stage pumps
A volute pump impeller can be reduced 20%, A diffuser impeller can only be reduced by?
5 to 10 % before pump efficiency is affected.
What do wear rings do?
Protect expensive casings and impellers. they should be replaced when clearance exceeds three times the initial clearance.
To reduce axial thrust in centrifugal pumps balancing devises may be used in conjunction with thrust bearings. These include:
- Wearing rings
- Balancing holes
- Back pump out vanes
- Double suction impellers
- Opposed Impellers
- Balancing drums
- Balancing discs
When would back pump out vanes be used?
generally found in pumps that handle solids, in order to keep the area between the back shroud and casing free from foreign material.
Front and back wear rings reduce axial and radial thrust in _______ pumps?
single suction
What is vacuum pressure?
Pressure below atmospheric pressure. Measure in inches of mercury.
Atmospheric pressure at sea level is 29.92 inches of mercury
