Calculation for a Boiler Feed Pump

http://cr4.globalspec.com/thread/30464

Dear subhranshshekhar:

you have to take care when you deal with boiler water feeding pump as follow:

1- Consider the surface pressure in the boiler (the max.pressure of the steam) when you calculate the total discharege head,you have to added this value to both static pressure an the friction losses.

2- This pump is always a multistage one to overcome the surface pressure inside the boiler.

Thanks

Aly

BOILER FEED WATER PUMP SELECTION:

FEED WATER PUMP

Every steam boiler needs one or two feed water pumps

Example:

Fire tube steam boiler. Rated pressure 8 bar . Maximum steam production 1000 Kg/h.

Piping pressure drop 1,3 bar Pump head 0,5 bar

Pump dimensioning:

Head = ( 8+1,3+0,5) + 5% = 10,3 bar (103 m.)

We may so assume that the head is within 1,25 and 1,3 the rated pressure, while the capacity have to be the 200 % of the steam production.

Feed water pump capacity

Dear Mr. subhranshekhar,

The net positive suction head requirement mainly depends upon the boiler feed water temperature.

You know any centrifugal pump creates a vaccuum at the pump inlet and water is sucked into the pump for which atmospheric pressure is aiding the suction. As the feed water temp.increases, if the suction is not positive head, the difference in the temp. of feed water and the saturation temp. relating to the vaccuum, will make a small portion of the water to flash in to vapour and this vapour will block the entry of feed water in to the pump.

The higher the feed water temp., the higher will be the flashing effect of water, and create more problem.

To suppress this flashing effect, we need more pressure as positive suction, the higher the feed water temp., higher will be the required positive head. For ordinary water pumping from a well the water temp. will be very near to ambient temp., and 26 feet or 7.92 Metres below the pump shaft level, water will be pumped.

If the temp.rises, this height of 26 feet or 7.92 metres will reduce and at 70 Deg.C of water temp., the negative suction effect will cease and we need positive suction head.

If the feed water temp. is 115 Deg. C, pl. refer the steam table for the pressure corresponding to 115 Deg.C plus frictional head plus some marginal head is to be provided for trouble free pumpimg of feed water. For 115 Deg.C we may need 9.5 metres as positive suction head, further higher temp. means we need further more head.

DHAYANANDHAN.S