Steam Boiler Pressure Reduction

[BronYrAur]

Anyone ever turned a 100 psig steam boiler plant down to 15 psig? I have a plant serving several buildings via steam tunnels. The intent is to take one of my boilers and convert it to low pressure for the smaller summer loads of reheat and domestic water heating. During the winter, the other boilers would be operated at full pressure.

Is there a checklist or other reference on what I need to do? I know my drip traps in the tunnel will probably need to be paralleled with low pressure traps. I may even run a separate low pressure main, depending on my load.

In the buildings, I plan to tie in downstream of PRVs, so I don't see any issues there. I have some pressure-powered pumps that I will switch to compressed air.

Anything else? What about the boiler? It is an old B&W water tube unit rated at 35,000Lbs/hr. The feedwater pumps maintain 225 psig at the control valves. Should i put in a PRV so that my inlet water pressure to this boiler is lower? What pressure should i set it to?

Any other issues?

 

[TBP]

I'd bet, by the time you factor-in reduced steam quality at lower pressures (this is uglier than most people realize), verifying that line-traps are suitable for the lower pressures, safety valve issues in the buildings with what amount to large bypass lines around your PRVs, feed pump issues, etc, the savings will be minimal, if any.

Does your plant have pressure compensated steam meters? Very few plants will spend much on steam meters - they typically border on "random". Most of the "savings" people sieze onto with projects like this are based on the steam meters reading WAY high on lower pressures, because they're not pressure compensated. They usually seem blissfully unaware that it's tough to have a boiler with 135% efficiency. They RAVE about how their boiler effiency has skyrocketed since the pressure drop. When asked about what they did with the steam meters, the typical response is "The what?"

 

[davefitz]

If the boiler was designed to operate at 100psig(114.7 psia) at full load, you would have to reduce the max permitted load to 25-30% full load in order to run at 15 psig (29.7 psia).

The likely issue will be the ability of the steam drum driers to ensure only dry steam leaves the drum- if you run at too low a pressure but retain high steam output, the steam velocity increases and this causes excess liquid carryover into the saturated steam outlet .

Another issue is one related to the water circulation in the waterwalls or firetube- at a low pressure , the steam specific volume is much greater than at fullpressure, so you could get steam voiding of metal sufaces that need to be wetted in order to prevent overheat.

And finally, the steam transfer pipe would have excessive noise and vibration if you operate at a low pressure but at full steam flow.

 

[rmw]

Did you do a search on this site? I remember this topic being discussed at length in another thread somewhere on this site. Used the "advanced search".

rmw

 

[Joesteam]

Aside from the boiler itself, you must look into the steam distribution system. The specific volume of 15 psig steam is about 3.5 times greater than that of 100 psig steam. So, you really need to calculate the maximum load and see if your steam velocities in the pipe are not outrageous.

 

[kiranne]

Hello all,

I would like to know if somebody have good results on reducing pressure of a steam boiler in order to reduce standing losses and thus running costs. Do you know anywhere where this could be documented ?

I am aware that reducing steam pressure is not a good idea as regards to :

- Higher water level in the boiler thus likely to have carry over, low quality steam and a lots of problems in the pipework because of that. (erosion etc...)
-the pipe work and trap sizes must be checked as regards to the new velocity due to reduced pressure.
- as was highlited above steam meters without density compensation would then show crazy figures.

It appears to me that savings could happen without causing problems in the steam system only if the boiler and pipe work have been oversized in the first place (which is often the case).

Thanks

 

[trashcanman]

I would strongly recommend you check with the boiler manufacturer before changing pressures. I have read about doing this and it leads to many unforseen problems - wet steam, undersized lines, condensate return problems, etc.

 

[BronYrAur]

Trashcanman,
I have done just that and decided it was a bad idea - for many of the reasons you cite. Instead we are scrapping one of the HP boilers and installing a series of LP ones in its place.

 

[kiranne]

Hello all

In the latest issue of Spirax newsletter there is an article giving the pros and cons (page 7). May be of interest...

"Higher boiler pressure for lower energy use"...
See :

http://www.spiraxsarco.com/uk/navigation/c306.asp

 

[rustbucketguy]

Don't forget, that when you reduce the operating pressure on a boiler, the water vaporizes easier so that you can convert more lbs/hr of water to steam and that the resulting steam will occupy more space. Consequently, the original relief valves will likely be undersized. The relief valves must be sized to remove all of the steam that can be generated. They are rated in terms of lbs/hr at the set point pressure conditions (110% if you really want to get technical). Several posts already talk about the possibility of vaporizing so much water that the tubes could run dry.