Selection of Boiler Feed Water Pump

[SAK9]

Guys,

I need to design a steam generator system with two electric boilers rated at 250 kg/gr @ 8 bar pressure to deliver a total output of 500kg/hr @8 bar.I need some guidance to select the boiler feed water pump and the condensate return pump( ie flow rates,pressure,type of pump etc).What should be the margin between pump outlet pressure and boiler rated pressure? What should be the flow rate for sizing the condensate line?I was once advised it should be twice the steam rate.Any specific reasons for this? What should be the capacity for the feed water and condesate tanks?

Thanks for any help

 

[davefitz]

For the BFP , the margin may depend on wheher you use a constant speed pump or variable speed pump.

For large boilers which also have some excess steam drum volume to limit the upset in drum water level , one usually oversizes the BFP by 15-20% in MCR flowrate, ie, the pump design point would be flow = 120% MCR flow at a discharge pressure equal to drum MAWP plus gravity head to the drum plus feddwater and economizer piping and valve pressure losses at a flowrate of 120% MCR.

The pump actually delivered to the site usually has an additional 5% factory margin in developed head. The design pressure of piping at the outlet of the pump would be based on pump dead head pressure ( full speed at min permitted flow incuding 5% margin) divided by 1.10 .

 

[SAK9]

Dave,
Thanks for your post.What should be the flow rate for the Condensate return pump?

On the boiler feed water bypass line to the tank,should I use an orice plate or a pressure control valve?

 

[TBP]

SAK9 - If you're selecting a feedpump for a small boiler (as you are), then typically they have off-on control. When the water in the boiler drops to a predetermined level, the feedpump starts. Once the upper set point is reach, the pump shuts off. Feedpumps for this kind of application are normally twice the capacity of the boiler. If you have a larger boiler, like davefitz is referring to, then you don't need to go that much oversized.

Remember not to pick a feedpump that has a discharge pressure that is the same as the boiler operating pressure. No pressure drop, no flow. I've seen a number of smaller boiler installation suffer from that mistake. You should be able to get water into the boiler at the safety valve setting.

 

[TBP]

I should have said "a larger boiler with level control valve". In this arrangment, the feedpump runs all the time, and is sized marginally larger than the steam rate of the boiler to allow for blowdown, etc.

 

[KeItSiSt]

condensate return pump should be twice the load as a minimum making sure it has enough head to pump back to wherever its going.

You would only need a bypass line in your boiler feed pump if the pump is vastly oversized or running continuous with a modulating feedwater valve on your boiler.

 

[pablo02]

SAK9 -- I assume the boiler feedwater bypass line mentioned is used for minimum flow conditions where the pump needs to discharge more water than the boiler demand (minimzes cavitation and allows the pump to run cooler) -- we have used an orifice in a start-up BFP application and found that as the boiler came up on pressure, our flow to the boiler dropped off accordingly -- depending upon your particular application instead of either the orifice or a pressure control valve, you may want to use a flow-control valve -- maintains minimum flow through the pump -- if enough water is flowing to the boiler, the bypass is closed, but opens as boiler (makeup water) demand drops off.. [again, look at the application -- small packaged type boilers usually use an on-off scheme rather than a continously operated pump -- as commented on in some of the previous posts]...

 

[Heula]

We should also see that any flow orifice measurement is given . Becasue they also contribute a major pressure drop. Also , we need to consider the pressure loss on eco coils and LP heaters etc. if they are in series after the pump discharge. Hence , calculation of design head for pump depends on water circuit equipments , lay out of piping etc. All these things are to be considered on arraiving head.

 

[yogibear1]

Davefitz, Huela and others above have a good point about actual and theoretical flow: The aim is get water into the steam drum. In a utility power plant the BFP may be located on the ground floor or turbine room floor, so consider not only differential elevations, and max unit pressure, but also all of the circuitry the water normally flow thru, such as the several feedwater heaters and economizer; there is a considerable friction loss due to the mass flow at MCR, plus allowance for oversizing. Start with your P&ID, convert that to a hydraulic model and run the iteration which meets your maximum theoretical flow.

 

[mashbob]

dear sak9,

for condensate recovery schemes, you may look the site of GESTRA.

http://www.gestra.de

and also you must keep in mind that the condensta flow must be calculated in max. conditions - during starting up and stop of the boilers.