Pumps

Question 1

What are the two classifications of pumps?

Answer 1

kinetic (dynamic) and positive displacement

Question 2

Define pump capacity.

Answer 2

The rated volumetric flow rate for a specific set of system parameters.

Question 3

Define pump head terms.

Answer 3

Head refers to fluid levels above the eye of a pump.

Question 4

Define pump lift terms.

Answer 4

Lift refers to fluid levels below the eye of a pump

Question 5

Define pump Net positive suction head (NPSH).

Answer 5

The difference between the pump inlet, or suction, pressure and the saturation pressure of the fluid being pumped is called available net positive suction head.

Question 6

Define Pump efficiency.

Answer 6

The ratio of work required by an ideal pump to the work required by a real pump. The difference between the input pump power and the useful pump power actually converted to head is the power lost to friction in the pump. This energy is converted to heat and is carriedaway by the fluid.

Question 7

What is suction lift?

Answer 7

The suction lift of a pump is the work performed when a pump “lifts” a fluid the maximum distance possible. As pump suction lift increases (becomes more negative), the suction pressure approaches the vaporization point of the water. Water vaporizes when pump suction pressure reaches saturation pressure.

Question 8

What is velocity head?

Answer 8

A pump moves a fluid; therefore, this fluid obtains a certain velocity. Velocity head is the work required to maintain this velocity. This head is equal to the height the fluid would reach due to its kinetic energy.

Question 9

What is static suction head?

Answer 9

The energy available due to the actual vertical distancebetween the fluid level and the eye of the pump, as well as pressure exerted on the fluid surface.

Question 10

What is the relationship between psi and feet of head for pure water at room temperature?

Answer 10

1 psi = 2.3 feet of pure water at room temperature.

Question 11

What is total static head?

Answer 11

The vertical distance, normally in feet, between the free level of the liquid on the pump suction side and the free level of the liquid on the pump discharge side.

Question 12

What is the relationship between inches of Mercury vacuum and head?

Answer 12

For every inch of mercury (Hg) vacuum, approximately one additional foot of suction lift on the pump is required.

Question 13

Describe cavitation, including symptoms, effects on centrifugal pump operation and methods of prevention.

Answer 13

Formation and collapse of vapor in a pump due to suction pressure approaching saturation pressure. Can cause reduced capacity, noise, vibration, and pump damage. Decrease fluid temperature, increase static suction head, decrease fluid flow, decrease pump speed to correct.

Question 14

State the purpose of the pump casing.

Answer 14

It houses the majority of the major pump components. The inlet (suction) and outlet (discharge) connections are integral to the pump casing.

Question 15

State the purpose of the pump gaskets.

Answer 15

Semi-soft flexible materials used to seal mating surfaces or flanges

Question 16

State the purpose of the pump Impeller.

Answer 16

The rotating component of the pump that converts the mechanical energy of the prime mover (the pump motor) into kinetic energy (speed) in the fluid.

Question 17

State the purpose of the pump volute.

Answer 17

A gradually expanding spiral that is integral to the casing. It reduces fluid velocity and increases fluid pressure.

Question 18

State the purpose of the pump shaft.

Answer 18

The shaft connects the prime mover to the impeller

Question 19

State the purpose of the pump bearings.

Answer 19

Bearings provide two types of support, radial support forside to side motion and axial support for movement along the axis. The radial support is provided by anti-friction journal bearings. The bearings that provide the axial support are called thrust bearings.

Question 20

State the purpose of the pump seal rings.

Answer 20

They provide the seal needed between the impeller and the casing. This seal prevents high-pressure discharge water from leaking back to the low-pressure suctionside of the impeller. The wearing rings also prevent excessive wearing of the pump casing. The casing ring fits into the casing and the wearing ring fits onto the impeller and rotates with it. There is a very close clearance between the rings. The wearing ring is replaceable, as it is softer than the casing ring and is designed to wear.

Question 21

State the purpose of the pump shaft seals.

Answer 21

Seals where the shaft penetrates the casing to prevent the pumped fluid from leaking out around the rotating shaft

Question 22

Describe the operation of Volute and diffuser centrifugal pumps.

Answer 22

As fluid fills the volute, it slows down, and its energy of motion (kinetic energy) is converted into pressure (flowenergy). Diffuser pumps have a volute enhanced with stationary vanes. The diffuser vane helps balance the radial thrust loads on the impeller, shaft, and journal bearings for varying flow conditions.

Question 23

Describe the operation of Radial-flow, axial-flow and mixed-flow centrifugal pumps.

Answer 23

By radically changing the direction of flow and directing the fluid through a volute, radial-flow pumps are able to create a high discharge pressure. In contrast, axial-flow pumps can move tremendous volumes of fluid but at relatively low discharge pressures. Axial flow impellers provide flow where the main flow through the pump is along the axis of the shaft. Mixed-flow pumps combine the functions of radial-flow pumps and axial-flow pumps. Mixed-flow impellers are designed to discharge fluid at an angle greater than 0° and less than 90°.

Question 24

What are the three centrifugal pump affinity laws?

Answer 24

These laws apply only to centrifugal pumps in closed systems
Change in RPM directly proportional to Volumetric Flow. Change in RPM results in a Square of the pressure. Change in RPM results in a Cube of the Power

Question 25

State the purpose of the packing gland in centrifugal pumps.

Answer 25

Holds the packing (soft compressible material) in place. Can be housed in a lantern ring to receive gland sealing water.

Question 26

Describe the operation of a regenerative centrifugal pump.

Answer 26

The impeller in the regenerative pump resembles the turbine wheel of a steam turbine. The fluid is introduced on both sides of the impeller, helps balance axial thrust. As the impeller rotates, the water is circulated both in the
direction of rotation and in a circular motion on the face of the impeller and discharge passage in the pump casing. This inner rotation of the fluid within the discharge passage and its interaction with the impeller gives the pump its name, “regenerative.”

Question 27

Describe the operation of a centrifugal pump, including requirements for Net Positive Suction Head.

Answer 27

NPSH impacted by temperature of fluid pumped, speed of pump, system pressure and flow rate.

Question 28

Describe the operation of a centrifugal pump, including requirements for Starting of the pump.

Answer 28

Ensure proper directional rotation for driver. The arrow on the pump casing indicates the proper direction. Bearings should be supplied with lubricant. Cooling water should be introduced carefully to pump bearings and lubricating oil. Use only enough to keep the lubricant at a safe working temperature. Before starting the pump, bearing temperatures should be near ambient, the main concern is that the bearings are too cold due to cooling water being supplied with the pump off. Final inspection of all parts should be made carefully before starting the pump. It should be possible to rotate the shaft by hand. This starts the flow of oil to the bearing surfaces.

Question 29

Describe the operation of a centrifugal pump, including requirements for Operation with pump dead headed.

Answer 29

Overheating will occur if a pump is operated dead headed for more than a few minutes. This is called operating at shutoff head. Shutoff head is generally defined as the maximum value of head that a pump can produce. Indication that a pump is “deadheaded” would
include minimal flow and motor amps.

Question 30

Describe the operation of a centrifugal pump, including requirements for minimum flow.

Answer 30

The smallest amount of flow that prevents overheating is called the pump minimum flow requirement. A bypass or minimum flow line is utilized to prevent the pump from operating at shutoff head.

Question 31

Describe the operation of a centrifugal pump, including requirements for Venting of the pump.

Answer 31

If the pump is run without fluid, there is danger of damage to liquid lubricated internal parts. Gas binding can also result.

Question 32

What is gas binding of a centrifugal pump?

Answer 32

Gas binding is a condition in which a pump casing is filled or partially filled with air, gas, or vapor instead of
the liquid being pumped. It can occur as a result of non-condensable gases coming out of solution, or due to incomplete filing and venting of a system. Gas binding reduces the liquid capacity in the pump because gas surrounds the impeller and prevents the impeller from contacting the liquid to be pumped.

Question 33

What are symptoms of gas binding?

Answer 33

Gas binding would be indicated by low flow and low discharge pressure readings. Additionally, the motor would be drawing minimum current.

Question 34

What is pump run out?

Answer 34

It is an abnormally high flow rate from a centrifugal pump that can lead to mechanical stress on the pump and
excessive motor current. Results from of a loss of downstream pressure which speeds up the pump and causes excessive motor current, which may result in
damage to the windings. Indications would include very high flow, high pump vibration, and high motor current and low discharge pressure.

Question 35

Why is a centrifugal pump started with its discharge valve partially closed?

Answer 35

Because the pump operates at only 35-50% of full load when the discharge valve is closed. Pumps started with the discharge valve open require more starting torque and have a greater chance of water hammer. But if the liquid on the upper side of the discharge check valve is under sufficient head, the pump can be started with the
discharge valve open.

Question 36

How do jet pumps operate?

Answer 36

They convert high pressure developed by an external source into a high velocity jet flow at low pressure. The fluid to be moved surrounds this high velocity jet via
the low pressure at the outlet of the jet nozzle. The jet draws the surrounding fluid into the throat or mixing section. The mixed fluid flows into a divergent diffuser, where the expanding area converts the velocity back to high pressure. This results in a high pressure, high volumetric output pump.

Question 37

Explain safety precautions for centrifugal pumps.

Answer 37

1. Ensure the pump is filled and vented prior to pump start.
2. Cooling water and lubricating systems should be verified as operable before start.
3. The suction valve open before pump start. The
   discharge valve should be closed when a centrifugal pump is started, to prevent excessive starting torque and load on the motor, and to prevent water hammer.
   During pump start, observe pump motor current to ensure it drops to the normal operating range after the starting surge.
4. The discharge valve should not be closed for an extended period unless minimum recirculating flow is verified adequate. Some recirculating flow is necessary to prevent overheating the pump.
5. A centrifugal pump should be shut down immediately if it becomes excessively noisy.

Question 38

Describe the operation of reciprocating positive displacement pumps.

Answer 38

The pump consists of a cylinder, a piston, piston rings, a suction valve, a discharge valve, and the pump casing that contains the parts. Basically, the piston draws fluid into the cylinder, and then positively displaces the fluid
in the cylinder. The fluid gains the pressure needed to move through the discharge piping from the force of the piston pressing against it.

Question 39

Describe the operation of a rotary positive displacement pumps.

Answer 39

A rotary pump is a positive displacement pump employing rotary motion. Rotary type pumps include gear, screw, and vane pumps.

Question 40

What is the net positive suction head requirement for positive displacement pumps?

Answer 40

In varying flow rates with positive displacement pumps, care must be taken to ensure available NPSH is maintained above required NPSH. An increase in positive
displacement pump speed causes available NPSH for the pump to decrease due to the decrease in the suction pressure.

Question 41

What are the requirements for starting a positive displacement pump?

Answer 41

Started with both the suction and discharge valves open.

Question 42

What happens if a positive displacement pump is operated dead headed?

Answer 42

Starting or operating a positive displacement pump with the discharge valve closed can result in severe damage, including potential rupture of the pump casing and
discharge piping.

Question 43

Describe the operation of centrifugal pumps in series.

Answer 43

Raises the pump head at a given capacity.

Question 44

Describe the operation of centrifugal pumps in parallel.

Answer 44

Raises the system flow

Question 45

Describe the operation of positive displacement pumps in series.

Answer 45

The effect will be a doubling of the discharge head with a negligible change in flow.

Question 46

Describe the operation of positive displacement pumps in parallel.

Answer 46

Results in a doubling of flow rate with a negligible change in pressure

Question 47

Draw, label, and explain the characteristic curves for centrifugal pumps, including the effects of system changes on pump operation.

Answer 47

Draw

Question 48

Draw, label, and explain characteristic curves for positive displacement pumps, including the effects of system changes on pump operation.

Answer 48

Draw