Twin 250HP Steam Boilers

Frank_5

Just finished installing two ( I call 'em twins ) 250 HP steam boilers in a 260 unit apt. house. Set up in lead-lag, with all the do-dads and fidgets properly installed. When the IC burners go into high fire to produce steam, the water level drops ( as it should ) the MM 157 kicks the boiler feed pump rated at 35 gal./min. ( one pump for each boiler ) and the system shuts down on low water. The boiler is producing steam faster than the BFP can refill the boiler. Question: How much steam is produced in this boiler? According to the mfg of the tank/pumps, the feed pumps are the right size. The boiler is a Federal FST, w/a single 10" supply riser w/ 6" equalizer. There is no lift and about 30' of 3/4" feed pipe from the 3/4" pump to the boiler fitting.

Tony Conner

What Pressure...

...do the boilers operate? What is the discharge pressure on the feedpumps?

Where I've seen this problem before is where the feedpumps had a discharge pressure of just about what the boilers ran at. The pumps could just maintain the water level at low steam loads, but as soon as a high load showed up, they couldn't move enough water. If the boiler safety valves lift at 15 PSIG, the feedpumps should be able to maintain the water level with that pressure in the boiler. This means that the pumps would need a dishcarge pressure of at least 20-25 PSIG

John_2

Water required

About 10,500 LBS/HR, on high fire. One 250 HP steam boiler should require 17.5 gallons of water per minute, on high fire. ( 0.07 gallons per hp) This is with no blowdowns. The pump will usually be sized for more because you want the pump to still be able to "keep up" after several years when it is getting tired. The person selling the pump probably did not know how many restrictions you have in the pipe (90s, tees, etc.). One more reason for the pump to be capable of twice as many gallons is that it will not take a long time to fill an empty boiler. You may have to put an orifice plate in the outlet of each boiler to keep pressure on top of the water. You may have water leaving the boiler with the steam.

Floyd

17.5 gpm Hmmmmmmmmmmmmmmmmm

Would seem like that 3/4 pipe would be a bit small....................
Jump that 3/4 pipe up to about 1-1/2 or 2 " and I bet your
problem will go away!! I know that the pump is 3/4, but you have
too much restrction in the 3/4 pipe.
No way will you get 35 gpm through that.
Check the blowdown on the header ... you will soon know if
it's throw'in water.

Floyd

canuckDale

Been there sort of....

Clear what you are supplying.
But, what's the maximum load? What is the system asking from the boilers, and when?
Maybe a clue for you?

Earthfire

feeding

check your makeup tank temp. If over 180 deg.F. you could be having cavitation problems as the pumps can cause the water to flash to steam in the volute . That 3/4" pump discharge concerns me at least 1 1/2" pipe in and out. Make up pumps are designed to induce flow at low pressure . Need volumne at inlet (No cavitation) And Volumn at outlet (No restriction to flow)