Does lower pressure mean lower steam output?

[ione]

I was wondering whether firetube boilers designed to work with a given pressure, would produce the same amount of steam if operated at lower pressures, or would suffer a derating in steam output.

 

[21121956]

Hello everybody:

Steam density change is important at low pressures. By lowering the operating pressure, a larger volume of steam is generated; the pattern of circulation and heat distribution in the boiler is changed.

A down rating is expected to occur and, among other things, the resize of safety valves and other support equipment will be necessary.

 

[GenB]

same amount of steam, almost the same as is the pph generated and
related not much to the pressure but by the capcity or ''fire'' input you put
in it. ifr the boiler is designed to higher pressure and you then decide to runn iot at considerable lower pressure, you may not have the header/outlet sized
for the amount of steam at the lower pressure, means too small a nozzle.

 

[TBP]

Lots of carry-over when running a boiler at significantly under design pressure.

 

[ione]

I'm not surprised about incresed carryover as consequence of increased velocity related to decreased pressure. This phenomenon should in any case be more emphasized in watertube boilers than in firetube boilers. Anyway carryover affects steam quality, while my question was about the effect a reduced working pressure would produce on steam output.

 

[TBP]

Spirax Sarco used to have a video that was shot from the steam space of a running firetube boiler. When the steam pressure was dropped from - I believe - 115 PSIG to 75, it looked like a tornado of fog/water droplets being sucked up the steam outlet.

 

[Sharik]

Dealing with pph, if the pressure is reduced it takes more volume to have a pound of steam. So things are dependent upon the steam outlet connection and piping from the boiler. If they are of sufficient size to allow a larger volume for the same pph, then you are okay. If the connection/piping are restrictive, you will get fewer pph.

 

[dougarthur42]

Some times when a boiler is down rated the steam flow appears to change. We have a boiler that was down rated. I was told this about it, when the steam flow transmitter was initially calibrated it was calibrated for 1200 # steam pressure. It was accurate from 40000 pph to 420,000 pph. But when the boiler was operated @ less than 1000 # steam pressure the flow would appear skewed. Probably doesnt answer your question but it may help.