Pumps Flashcards
Name the two primary classifications of pumps
Positive and non positive displacement pumps
Define Cavitation
The sudden formation and collapse of low pressure bubbles in water by mechanical forces
What is the minimum discharge pressure for a boiler feed pump
Each feedwater supply source must be capable of supplying feedwater to the boiler at a pressure 3% above the highest set safety valve
What are the affinity laws
The affinity laws state that, for centrifugal devices,
- the power varies as the cube of the speed,
- the head varies as the square of the speed
- the capacity varies directly with the speed
How to set the valves on a duplex pump
Close steam and exhaust valves, drain pump and loto Center the pistons Plumb the rocker arms Square the valves over the ports Equalize the lost motion Move one piston off center
Explain the operation of a centrifugal pump
As the impeller of a centrifugal pump rotates water enters the center of the impeller and is thrown outward by centrifugal force. The water leaves the impeller at high velocity and is converted to pressure in the volute section of the pump
What is the purpose of the diffusion ring on a centrifugal pump
The rings are held stationary in the casing and contain slots or openings that receive the water from the impellers at a high velocity. The velocity is converted to pressure
Provide Examples of pumps that are found in a power plant and are of the centrifugal pump design
Centrifugal pumps can be used in a variety of applications but can be found in the feed water system, chemical pumping, condensate pumps and circulating pump
If a liquid has a specific gravity of less than 1.0, what does this mean? What if it is more than 1.0
Specific gravities of less than 1.0 indicate liquids that are lighter than water and will float. More than one it will sink
Describe the effect of cavitation on the operation of a pump. How can it be avoided
Sudden formation and collapse of low pressure bubbles in the pump. When this occurs the pump will vibrate and make noise. This can be avoided by making sure the system has more than enough suction pressure than is required by the pump. Also occurs when the temperature of the water becomes too hot
Describe the NPSH for both a centrifugal and a rotary pump
For a centrifugal pump the NPSH is the amount of energy to overcome friction losses from the suction opening to the impeller vanes and to create the desired velocity of flow into the vanes. A rotary pump NPSH is the amount of energy to overcome friction losses from the suction opening into the gears or vanes and to create the desired velocity of flow into the gears or vanes.
How can the output of a centrifugal pump be controlled without causing the pump to overheat
The pump can be controlled by throttling the discharge valve or by changing the speed of the pump regulator
Why must a pump be primed
Pump must be primed if its suction line is below the pump centerline. The air must be evacuated from the pump suction before water will enter the pump and fill the pipe and casing
Would you prefer that a water supply tank be located above or below a pump? Why
Above a pump to give a good NPSH to the pump so it won’t have to be primed or have to lift a liquid from below
What is theoretical suction lift? What is practical suction lift
Theoretical suction lift is 33.9 feet.
Practical suction lift is 15-20 feet
Which pump has no moving parts
An injector/ejector is a pump with no moving parts
When do we prime a pump? When do we vent a pump
We prime a pump when the source of fluid is below (lower) the pump. We vent a pump when the source of liquid is above (higher) then the pump
The boiler feed pump just tripped. What will you do
Start stand-by pump if available and trouble shoot the cause of the pump trip
How many feedwater pumps are required for boilers
At least one (1) boiler feed-pump is required. Good engineering practice recommends two (2) 100% capacity pumps
How are centrifugal pumps classified
Centrifugal pumps are classified as single stage or multi-stage pumps
What limits the capacity of a centrifugal pump
Discharge head, Speed, Efficiency, horsepower. (Reference pump curve)
how does a multi stage centrifugal pump work
Discharge from first impeller becomes the suction for the next impeller, this is how they stage up pressure to reach the desired pressure
What is a diffuser pump
It is a centrifugal pump that uses internal vanes that act just like the volute and perform the same function as the volute. ( Change velocity to pressure). These vanes are not externally visible
What are wearing rings
Wearing rings usually sit on both sides of the impeller and act like a seal to prevent water leakage. Wearing rings are made of soft material that wears easily so if you have a rub (impeller hits against it) the damage is incurred to the wearing rings and not incurred on the impeller
What are lantern rings
Lantern rings are used with pumps that have packing. Sealing water is directed over the lantern rings and is dispersed over the packing to keep the packing cool
How do you obtain information about a centrifugal pump
The pump manufacturer supplies a pump curve which shows you information about that pumps
What is head pressure
The distance in feet from the level of the water to the pump datum (centerline of the pump suction)
What is NPSH
Its the minimum suction conditions to prevent cavitation of the pump
What is required NPSH (NPSHR)
Required NPSH, (NPSHR) is set at by the manufacturer and is the distance required (head) to prevent cavitation
Available NPSH (NPSH3)
Available NPSH (NPSH3), takes into account valves, fittings, strainers, ETC. (friction) that will reduce NPSH
