Module 4 : Centrifugal Pumps

Question 1

Centrifugal pumps : summary

Answer 1

Pumps that use a rotating impeller to impart energy to a fluid so that it can move through the piping system.
These pumps use physical action to move fluid, converting kinetic energy through the impeller into pressure energy in the casing.
Can be mounted horizontally or vertically (more space efficient for large pumps)

Question 2

Centrifugal pumps : parts

Answer 2

1. Pump casing
2. Cover
3. Impeller
4. Pump shaft

Question 3

1. Pump casing

Answer 3

Purpose:
Materials: Constructed of cast iron, cast steel, or bronze; sometimes brass, sometimes stainless (expensive). Small pumps may be of plastic or stainless steel with the type of material dependent on the media.
Construction: The casing is cast as one piece and machined to specifications. Feet are cast into the casing to allow it to be secured to a stool or suitable foundation. If the pump is to be mounted vertically, a large flange is cast into the top of the pump to support the drive motor. The casing has a cast flanged outlet (could be threaded but never welded) for connection to the piping system.
The shape is in the form of a volute (curved funnel with an increasing cross-sectional area from the impeller discharge to the pump discharge line).
Cast into the back of the casing is a raised cylinder that will house either a packed glad or a mechanical seal. A vent valve is often installed to bleed off trapped air.
Operation:

Question 4

2. Cover

Answer 4

Purpose: direct water onto the center of the impeller (axially)
Material: fabricated of the same material as the casing
Construction: Flanged with a central spigot (raised portion) that sits down in the casing to ensure correct alignment and an O-ring fitted around the spigot for sealing purposes
Operation: bolted to the front of the casing and sealed with a gasket or o-ring

Question 5

3. Impeller

Answer 5

Purpose:
Material: Cast of brass, bronze, or stainless (matertial must be resistant to corrosion and erosion), is circular in shape with a series of vanes that extend from the center hub (eye) to the outer rim. The number of vanes will depend on the manufacturer’s specifications, but generally there are 6-10.
Construction: The impeller can be shrouded (closed impellers, vanes are protected, the maximum amount of energy is transferred to the fluid and the pump discharge is sealed from the intake by close tolerances between impeller and case/cover) or unshrouded (open impellers, vanes are clearly visible on the impeller, less efficient due to energy lost to increased turbulence in the casing and increased slip)
Operation: located inside the pump and is situated between the casing and the top cover. The vanes are what transfers the kinetic energy to the fluid and causes the fluid to swirl to the outlet of the volute casing.

Question 6

4. Pump shaft

Answer 6

Purpose: connects the prime mover to the impeller
Material: high carbon or stainless steel for strength, ability to be polished and resistance to corrosion.
Construction: passes through the housing on the cover and is keyed and bolted to the impeller
Operation:

Question 7

Centrifugal pumps : operation

Answer 7

The impeller rotates in the casing. As fluid enters the pump axially through the eye (or center) of the impeller, and is acted upon by the impeller vanes, it receives kinetic energy.
Centrifugal force causes the fluid to be discharged radially around the entire circumference of the casing, and due to the shape, the kinetic energy is changed into pressure energy.
The liquid is discharged from the pump casing with full pressure.
As the fluid leaves the impeller, a vacuum is created at the eye of the impeller, drawing fluid in.
This type of pump is NOT self-priming and the casing must be flooded with fluid.

Question 8

Centrifugal pumps : slip

Answer 8

Due to the characteristics of the pump, suction is always located at the center of the impeller and there is a small amount of clearance between the impeller and the casing.
To reduce slip, close tolerances must be maintained between the impeller (suction) and the casing (discharge).
However, the clearance will increase due to erosion. One way to remedy this is to fit “wear rings” into the pump casing and on the impeller.
Wear rings are rings pressed into the casing and/or on the impeller that can be replaced when the clearance becomes excessive.
Note: wear rings can be located on the underside of the impeller as well. The rings are usually made from the same material as the impeller.
If the discharge is closed, it all slips back to the inlet (no possibility of infinite pressure)

Question 9

Centrifugal pumps : priming

Answer 9

Centrifugal pumps are not self-priming and require some means of removing air from the suction pipe and willing it with liquid.
There are three ways to prime a centrifugal pump:
1. The pump casing can be opened up and filled with water
2. When the liquid to be pumped is at a higher level than the pump, opening a valve or pipe plug installed on the casing will enable the air to be forced out of the pipe and fill the casing with liquid.
3. In some cases, a priming device may be fitted, essentially a small positive displacement pump

Question 10

Priming devices : operation / interface with the centrifugal pump

Answer 10

Air has to be removed from the suction of the pump. A float arrangement is fitted at the suction and when air is present, the float drops, allowing air to be removed by the device. When fluid is present, the float rises, shutting off the line to the device (to stop the priming device from sucking fluid)

Question 11

Water / liquid ring primer : construction

Answer 11

The most common priming device is a water ring primer consisting of an elliptical casing with contains a cast vaned rotor and a top cover which is fitted with suction and discharge ports.
The rotor is directly coupled to the centrifugal pump’s prime mover, usually by a belt, and the casing is partially filled with water.
A line connects the suction port of the primer to the suction side of the centrifugal pump through a float valve.

Question 12

Water / liquid ring primer : operation

Answer 12

When the prime mover is running, the rotor spins and the vanes force the water to the periphery of the elliptical casing. The water takes the shape of the casing and forms a ring around the inner surface. The tips of the vanes are sealed by the water ring and the volume between the vanes varies during rotation. Beneath the suction ports, the volume is increasing so air is drawn in. Beneath the discharge chamber, the volume decreases so air is forced out.
If air is present in the suction line of the pump, the float drops and opens the valve. Air is then drawn into the primer and discharged to the atmosphere through the exhaust. When the suction pipe is fully purged of air, the float will rise and close the suction to the primer.
The primer runs continuously and will only be activated when there is air in the suction.

Question 13

Water / liquid ring primer : drawback

Answer 13

The constant turning of the rotor in the casing causes the sealing water to heat up and if the sealing water overheats (due to friction), vapour (steam) will form, stopping the vacuum effect.
Passages are cast in a jacket around the casing for coolant, which will prevent the overheating. Coolant is supplied by the discharge side of the pump, goes through the passages, and then back to the suction side of the pump.
The natural flow (high pressure at discharge, low pressure at suction) will carry heat away from the casing.

Question 14

Priming devices for multistage centrifugal pumps

Answer 14

Multistage centrifugal pumps may be connected through a central priming system rather than having individual water ring primers fitted at each pumps.
Each pump will be connected to a vacuum tank through individual float valves, which operate similar to those of the water ring primer, and the tank will have dedicated electrically driven vacuum pumps.
When air is present in the suction of the pump, the valve operates and the vacuum from the tank removes it

Question 15

Multistage Centrifugal pumps : purpose

Answer 15

To raise the fluid pressure above what an individual pump is able to achieve
To increase the pressure of a centrifugal pump, you can connect impellers in series on the same shaft.

Question 16

Multistage Centrifugal pumps : construction

Answer 16

Multiple chambers (or stages) are connected in series.
The discharge from one impeller is directed to the suction of the other, which allows for pressure to be increased incrementally

Question 17

Multistage Centrifugal pumps : operation

Answer 17

Fluid enters the eye of the first impeller (first stage) at suction line pressure and leaves at some elevated pressure. Upon leaving the first stage, the fluid is directed so that it enters the eye of the impeller of the second stage, where pressure is increased further. This is repeated for as many stages as required.
The more stages the pump has, the higher the final discharge pressure.
These pumps produce higher and higher pressures with the addition of every stage, with flow rate being constant for a given RMP.

Question 18

Horizontally mounted multistage pumps

Answer 18

Consists of a casing made of individual segments which are similar in construction. When assembled, one end will have a suction casing fitted and the other end will have a discharge casing fitted.
When joined together, the assembly is held between bearing housings, maintaining alignment of the fixed and moving parts, and making the assembly more balanced for high pressures.
Typically handle higher flow rates than vertical multistage pumps
Best for applications like feed water supply and condensate pumps in boilers

Question 19

Vertically mounted multistage pumps

Answer 19

Have a vertical shaft and the stages are stacked one on top of the next.
Great for areas that don’t have much footprint to spare.

Question 20

Multistage pumps : cautions

Answer 20

• Maintenance intensive and it takes special skills and experience to overhaul them
• Can be expensive but this depends on the materials
• Does not tolerate debris or significant solids in the pumping medium as clearances are small
• Restricted use to clean water applications such as boiler feed systems or high pressure water systems
• Vulnerable to deadhead conditions (shutting off of the discharge valve)

Question 21

Vapour Pressure

Answer 21

At a specific combination of pressure and temperature, the molecules of a liquid will turn to vapor.
The pressure at which a fluid, at a set temperature, will turn into vapour is referred to as the Vapour Pressure

Question 22

Cavitation

Answer 22

A condition caused by a liquid flashing off to a vapour.
Cavitation occurs on a blade as it travels through a liquid. On one side of the blade, there is high pressure. On the other side, there is low pressure. If the pressure is low enough, the fluid will flash from liquid to vapour causing air bubbles. The bubbles travel down the blade to the trailing edge, where they will violently collapse when they hit the static fluic pressure. This creates a shockwave that hits the impeller and creates pump vibration and mechanical damage, possibly leading to eventual pump failure.
Cavitation is affected by two things:
1. Speed of rotation of the blade (faster blade, more chance of cavitation)
2. Static pressure of the fluid (less pressure, less depth of water, more chance of cavitation)
Cavitation can be reduced by decreasing the speed, but at a low enough speed, the pump will no longer function.

Question 23

Pump Head

Answer 23

Centrifugal pumps operate by drawing fluid in at the eye of the impeller. The impeller transfers kinetic energy to the fluid and the volute casing causes the kinetic energy to be transformed into pressure energy. In order to operate properly, the impeller must remain fully immersed in fluid (aka flooded).
There needs to be sufficient suction pressure at the eye of the impeller to draw in fluid yet not so low that cavitation will occur.
The higher a fluid is raised, the more negative pressure is applied to the pump casing and the more risk of cavitation.

Question 24

Total Suction Head

Answer 24

The maximum vertical distance that a centrifugal pump can raise a fluid before the pump air locks.
About 15 ft when pumping water from an open air tank

Question 25

Net Positive Suction Head

Answer 25

The maximum vertical distance that a centrifugal pump can raise a fluid without cavitation occurring.
The pump will still function but air will be in the casing.

Question 26

Discharge head

Answer 26

The maximum vertical distance that a pump can discharge water before the pressure at the outlet of the pump (due to height) becomes so high that it equals the pump discharge pressure and no flow occurs (maximum slip).
About 18 ft.
Can be varied by increasing the pressure at the suction of the pump. The higher the pressure at the impeller, the higher the pressure at the outlet and thus the higher the discharge head. However, a higher discharge head will lower the flow from the pump

Question 27

Total head

Answer 27

Sum of suction head and discharge head.
Constant

Question 28

Starting a centifugal pump

Answer 28

Commonly started with the discharge valve either fully shut or just cracked open for 2 reasons:
1. To ensure the impeller is fully immersed by allowing the priming system to remove the air
2. To decrease the starting current of the prime mover
Priming systems will only remove air in the system at a certain rate (dependent on the side and design of the primer). If the discharge is open, there may be excess air at both suction and discharge that will delay flooding of the impeller.
If the discharge valve is fully open and the discharge pipe is flooding, the prime mover will need to provide enough energy to get the impeller moving AND overcome the back pressure from the discharge piping. This can lead to high starting currents, which will either trip the circuit breaker of the primer mover OR eventually cause failure