Water Supplies, Appliances and Equipment Flashcards
Describe the purpose and operating principles of a reciprocating primer.
Purpose - To remove air from the pump casing of a centrifugal pump (allowing atmospheric pressure to force water up into the pump from open water).
(i) Induction stroke - small piston driven by eccentric cam creates a vacuum in the primary valve body .
(ii) AP causes primary valve to open drawing in air through the inlet flap valves of the suction tube.
(iii) on exhaust stroke, air forced out through exhaust flaps.
(iv) once pump pressure generated the drive to the primer is disengaged by using water pressure to lift the pistons.
Describe the purpose and operating principles of a water ring primer.
Purpose - To remove air from the pump casing of a centrifugal pump.
A vaned impellar with a hollow centre rotates in an elliptical housing around a stationary hollow boss.
(i) Impellar rotates
(ii) liquid rotates outwards and back inwards following contour of the housing form forming a hollow elliptical vortex.
(iii) on outward rotation air is drawn into the impellar through centre ports.
(iv) on inward rotation impellar vanes dip into the liquid ring causeing air cavities to be formed which are forced through the impellar and discharged at the central boss.
(v) when air feed cease i.e. all air has been expelled on the suction side, the primer begins pumping water at which point the device is automatically stopped and the central pump takes over.
Describe the opertion of a Centrifugal Pump
- No valves, pistons or plunger
- 2 main parts Impellar ans Casing
- Makes use of centrifugal force (tendancy of a revolving body to fly out from the centre of rotation).
Operation:
(i) Impellar rotates at speed to produce neccessary pressure, inducing flow through the pump.
(ii) Water from the inlet is picked up by the impellar vanes.
(iii) Water passes through the periphery, increasing in speed as it nears the impellar rim, where it is thrown off.
(iv) water has high kinetic energy here as it enters the pump casing where the velocity is greatly reduced converting the energy into pressure energy.
Impellar job - to impart high velocity to the water.
Pump casing job - transforms the velocity energy into pressure energy.
Describe the methods of increasing delivery pressure used in multi-stage pumps with a single impellar.
(i) Increase the diameter of the impeller - not efficient as it would make the pump too bulky.
(ii) increase the rotational speed of the impeller - needs an increased engine speed which may not be practicable.
Describe how increased delivery pressure is achieved using a multi-stage pump.
Using a pump with more than one impeller:
(i) the impellers are driven by the same **rotating shaft **with water fed from the periphery of the first to the entry of the second impeller.
(ii) neglecting frictional loss, the pressure increasing ability of the pump is applied a number of times.
(iii) most fire appliances consist of only 2 stages using a Regenerative impeller for the second stage (due to several stages being required with a simple impeller).
- Practical multi-stage pumps have the advantage that water may be discharged at relatively low pressure after passing the first stage only.
- Or at high pressure (upto ~55 bar) after passing the subsequent stages.
- Also possible to deliver water at both high and low pressure simultaneously.
- Driven through PTO of a stationary appliance.
Describe the operation of a Regenerative Impeller
(aka peripheral impeller/peripheral pump).
(i) Regenerative impeller has a ring of guide vanes at its periphery, enclosed in a casing, with a channel around the part containing the vanes.
(ii) water enters the channel via the inlet and drops to the base of the guide vanes.
(iii) water is then thrown outwards between the vanes by centrifugal force.
(iv) water then moves to the base of the vanes and again the process is repeated with the water following a spiral path around the channel.
(v) cumulative effect is to develop sufficiently high pressure for hose reel operation and wat fog production (but at a lower pump speed ~3000rpm).
