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Explain Hydrant Pumps | Working principle, Construction & Characteristics

Introduction

Centrifugal pumps a machine which raises water or any liquid from a layer level to a higher level and from one place to another by the action of centrifugal force. For controlling major fires large quantities of water at higher-pressure is needed. Centrifugal pumps are employed generally for the purpose.

Centrifugal Force:

It is that force which acts upon a body moving in a circular path, tending to force it farther away from the centre of the circle in which it is moving.

Working Principle:

When the rotating member of the pump Impeller gives fast rotary motion to mass of the liquid contained in the casing surrounding it, the centrifugal force forces it out of the casing through the discharge outlet, the vacuum thus created makes atmosphere pressure to pushing more water into the casing through the centre, called the “eye” of the impeller.

Construction:

The Centrifugal pump has two main components:

  • Impeller 
  • Casing 

Impeller:

Centrifugal Pimp

It is the only moving part in the Centrifugal pump. The vanes of the impeller extend from the centre of rotation to the periphery. These vanes closed by means of two discs, one on each side. These discs called as shrouds. The vanes are curved slightly forward (in the same direction as that of rotation). It has a round opening in the centre which is called ‘eye’ of the, impeller.

The main function of the impellers to provide high velocity to the mass of water entering through its “eye“.

The Casing of Centrifugal Pump:

the casing is the enclosure in which impeller rotates. Designed as to reduce the turbulence created by the water as it is discharged in the volute which steadily increases in diameter till it reaches the delivery outlet.. The main function of the casing is to convert velocity energy into pressure energy. The high pressure liquid in the volute recalculates behind the shroud of the impeller cause in ginned thrust tending to force the impeller off the shaft, A thrust ball bearing installed to counteract this force.

Motor:

The pump connected to a motor (Electric or diesel engine) by means of a shaft with the help of a coupling joint. To prevent leakage of water a mechanical seal is provided between the casing and the shaft. It is also done by providing a packing of fire resistant material such as asbestos called “gland packing”. A small leakage is retained in this case to prevent its damage by the frictional heat.

Vital Parameters:

  • Capacity: The quantity of fluid discharged per unit lime. E. g. Litre/Minute or Gallons/Minute,
  • Head: the increase in pressure. g. Kg/cm2.
  • Power: the energy consumed by the machine. g. KWH
  • Efficiency: the ratio of the energy supplied to the fluid by the machine and the energy supplied to the machine

Characteristics of Centrifugal  Pump:

  • At any given speed, when there is no flow the discharge pressure is at the maximum.
  • The pressure decreases progressively with an increase in the discharge rate.
  • Both pressure and flow increase with the increase in speed and vice versa.
  • The power an absorbed by the machine increases as the flow increases and vice versa.
  • Keeping the revolutions per minute constant the discharge flow rate falls as the suction lift increases.

Installation of Centrifugal pump :

  • Centrifugal Pump commonly called “Fire Pump” is installed.
  • To cater fixed Fire Hydrant System.
  • On the Fire Engine (Movable).

Water Supply:

  • The pump takes suction from water reservoirs constructed overhead (Positive Head) from underground (Negative Head) or static reservoirs.
  • When connected from Positive Read Source the pump is provided with isolation valve called “Suction Valve”.
  • When connected from Negative Head Source the suction is provided with a’ foot valve. All pumps have an isolation valve in the discharge side called “Discharge Valve”.
  • All pumps are provided with priming facility designed suitably for different applications.

Operating Procedure (Positive Head):

Start up of pump:

Step – I:

Keep Discharge Valve closed and check free movement of shaft by hand rotation.

Step – II:

Open suction valve.

Step – III:

Open the priming valve and let the air come out. Keep the valve. Open till a smooth flow of water is established through the waste pipe and then close the value.

Step – IV:

Now press the power connection to the motor.

Step – V:

Open the discharge valve slowly.

Shut Down of pump:

Step – 1:

Close the discharge valve fully.

Step – II:

Switch off power supply.

Step – IlI:

Close suction valve if the pump is to be kept stopped for longer periods. Otherwise octane left open.

OPERATING PROCEDURE (For Negative Pressure Suction) of Centrifugal  Pump:

Start up of pump:

Step – 1:

Make sure that discharge valve is close and shaft is free to move with hand rotation.

Step – II:

Prime the pump by filling the suction line from its foot valve to the priming point of the pump discharge. This can be done by drawing water from a small capacity water drum kept above the pump level or it can be done manually. Priming can also be done with the help of small capacity pumps of either reciprocating type or an ejector pump.

Step -IlI:

Start the motor.

Step – IV:

Open the discharge valve gradually.

Shut down of pump:

Step – I:

pose the discharge valve slowly.

Step – Il:

Stop the motor.

Operating Procedure (Fire Engines/Tenders):

Start Up of fire engines:

Step – I:

Make sure discharge valves are kept closed and the shaft is free to move within hand.

Step – II:

Open the suction valve.

Step – III:

Open the priming valve allow all air to come out till a smooth flow of water is established.

Step – IV:

Engage the lever to take power off from wheel and connect it to the pump.

Step -V:

Connect water make up from sources with positive pressure such as overhead tanks/Tankers or hydrants if available.

If the source of waters underground storage or well or ponds etc. Then connect suction hose or hoses according to the requirement of length. The last hose shall have a foot value. At the end. Lower the suction hose well into the water source.

Step – VI:

Slowly close the suction valve watching the discharge valve.

Step – VII:

Open the discharge valves one by one maintaining the discharge pressure by adjusting the accelerator to maintain required speed of shaft.

Shut Down of fire engines:

Step – I:

Close discharge valves.

Step – II:

Depress the’ Power Take Off lever to take power off the pump and give to back to wheel.

Step -IlI:

Stop make up when the water tank on the vehicle is full. Disconnect suction hoses.

Step -IV:

Disconnect suction hoses.

Cavitation:

It is the formation of vapour bobbies in the pump casing and is accompanied by a distinctive rattling noise. This is the indication of water being drawn more than normal, This is the remedy is to reduce the discharge rate.

Water boils at 1000 C at atmospheric pressure (760 mm Hg). If the pressures reduced it will boil at lesser temperature. If the pressure reduced sufficiently, water can start boiling at atmospheric temperature. When more and more of waters drawn from the pump, it keeps on reducing pressure at the suction till water starts boiling at the room temperature.

This creates water vapors and bubbles which enter. The impeller creating “Cavitation”. This can also happen if there is obstruction in the suction line. G blockage in the foot value or suction strainer.

Advantages of Centrifugal Pump Over Other Pump:

  • Uniform Flow.
  • Less Cost.
  • Occupies less space.
  • Less maintenance.

Related Topic:-

Fire Hydrant System | Operation, appearance, inspection of Hydrants

Hose and types of hose | Maintenance of hose | Grouping and Coupling