Pumps & Primers Flashcards
State the practical limitations of suction lift
It has been shown that water cannot rise to a vertical height greater than approximately 10m in a completely evacuated tube. However, the atmospheric pressures ability to lift water is reduced to below the theoretical capabilities of about 10m because:
Create flow I.e to give the water kinetic energy as it changes from its static state in the open supply, to its moving state in the suction hose
Overcome frictional resistance to flow in the suction hose couplings
Overcome pressure loss due to turbulence and shock as the water enters the pump impeller. It is known as entry loss and varies with the design of the pump
Overcome pressure loss as the water is forced through the strainer and changes its direction of flow after entering
Overcome any tendency for the water to vaporise as it nears the impeller. This tendency increases rapidly with temperature. In fact the vapour pressure of the water becomes equal to atmospheric pressure at its boiling point, so making it totally impossible to lift water at, or near this temperature. At normal outdoor water temperatures the effect, however, is small.
Whilst suction lifts of 8.5m or more sometimes obtained under very good working conditions, 8m can be considered the approximate practical maximum.
Describe the two main parts of a centrifugal pump
Volute - decreases velocity & turbulence & channels water to outlets
Impeller - water at the centre of the impeller is thrown outwards by centrifugal force as the impeller rotates. To increase pump efficiency some impellers have guide vanes to ensure that flow is as near as possible streamlined.
Describe the characteristics of a centrifugal pump
At any given speed, when there is no flow the pressure is at a maximum; it decreases as the delivery valves are gradually opened and the flow increases.
When pump speed increases, both pressure and flow increase
When suction lift increases both pressure and flow decrease
Identify the type of primer used on both a major and lightweight pump
Reciprocating primer - used on vehicle mounted pumps
Exhaust gas ejector - used on portable pumps
Rotary sliding vane - used in portable pump
Identify the two main types of gauges in use within the fire service
Pressure gauge - usually a bourdon tube design
Compound gauge - usually a diaphragm design
How does the cooling system on a portable pump work?
Smaller lightweight pumps use an air cooled engine, although the majority of lightweight pumps employ an indirect closed loop water cooling system. This system uses water from the pump on a closed loop to cool the water in the engines cylinder head and water tank
Explain the effect of atmospheric pressure in relation to suction lift
Atmospheric pressure (regarded as 1 bar) acts on the surface of any open body of water. If the atmospheric pressure is removed from above a portion of water (if a vacuum is formed) then the surface of the water is subjected to unequal pressures. The water being resistant to compression but not to change of shape, that part of the surface which is relatively free from pressure will tend to rise.
Effectively atmospheric pressure pushes down on a surface of water, which when subjected to unequal pressures, is then pushed up the suction lift pipe.
List the maximum practical and theoretical lift
Theoretical - 10.3m
Practical - 8m
Explain ‘cavitation’
As pressure decreases, so does the temperature at which water boils. Depending on the degree of suction lift and other factors, such as friction loss in the suction hose, conditions can occur inside the pump, such that the water can vaporise at the ambient temperatures. Vapour bubbles then form (known as cavitation) and the pump gives off a distinctive rattling sound.
Name the parts of the exhaust gas ejector primer (diagram in notes)
1 - primer rod 2 - butterfly valve 3 - ejector housing 4 - ejector nozzle 5 - priming pipe 6 - non-return valve 7 - non return valve spring 8 - diaphragm 9 - sealing washer 10 - diaphragm spring 11 - silencer
What could it mean if you get an increased delivery pressure reading?
A hand control branch has been closed down.
A heavy object or a vehicle is parked on the delivery hose.
A bad kink in the delievery hose.
A blockage in either the strainer, pump, hose or branch.
What are the 2 possible reasons for getting a decreased pressure reading?
A burst length of hose on the delivery side.
A hand control branch being opened.
What could a high vacuum reading mean?
Blockage in the metal or basket strainer
Faulty suction hose, hose may have collapsed
What could a no vacuum reading mean
Strainer at the end not properly submerged.
Loose or faulty suction joints.
A open drain cock or loose drain plug.
Air leaks in suction hose.
A defective seal.
Lack of water.
What could a decreased vacuum reading mean?
A rise in the level of static supply
Less water being taken by branches
