Check valves on steam supply risers?

The Steam Whisperer (Formerly Boilerpro)

I was wondering if anyone has ever done this and/or what the problems/limitantions may be.  I am looking closer at setting up piping for multi-stage atmospheric gas steam boilers and one of the problems is steam from "on" boilers flowing into "off" boilers, heating them up and causing large standby losses.  A condensate pump or boiler feed pump with zone valves would be needed on the return side to control flow on this side.

I suspect that the vavles would be need to be sized small enough to not cause chatter, but how small?

Brad White

The principles are the same

as on commercial boilers.



I only use stop-check valves on high pressure steam piped in parallel, sure. Not on low pressure.



There is not enough "lift" to be reliable especially if one boiler is already "at pressure",  the second boiler has to overcome the system pressure PLUS the check valve lift pressure.



To do this, one has to over-fire the second boiler and once the check valve lifts, the boiler pressure would drop of course and the system pressure would get a sudden boost. If the pressure controls are in a common line, they may shut down the just-needed boiler. You can see the various situations of control at low pressure.



Similarly, automatic valves could be used but the same problem happens, how fast the valve opens, versus system pressure response. On high pressure boilers, this is less of a problem because the range of pressure and control is wider than on a boiler running on ounces. There is enough range for response.



A couple of years ago we designed a replacement plant using two (2) 103 BHP low pressure steam boilers (Burnham V-1117s), piped in tandem.  No check valves in the supply.  Instead we specified waterline equalizing F&T traps to "skim" any overflow and keep the waterlines equal. Burnham has this covered in their piping diagrams and their Hydronic Helper booklet.



As you noted, yes, with the system under pressure both boilers will have full steam chests. The only solution I can see is to have our US made boilers be fitted with 3 inches or 4 inches of blanket insulation versus the one inch insulation which is standard. Or you just admit defeat on that issue.



There are more details of course and more defined diagrams out there. The above is just the general provisions I have specified and which work.



My $0.02



Brad

The Steam Whisperer (Formerly Boilerpro)

Brad, I was hoping you'd respond....

The standby loss up the stack is huge with atmospheric, and if a damper is installed then it just dumps into the boiler room. With power burners, it is probably quite small, which is the case I think you're describing.   I wonder how it would work if 2 unequally weighted check valves were used, so one would open first to begin supplying steam and then the second could open as boiler pressure built some more.  This would probably limit the pressure spiking you described.

However, since I am usually  looking at space heating using  a multistage, multi boiler heating plant feeding a single zone multiple unit building I probably would not be staging the boilers on pressure.  Instead I would only use either outdoor stats or indoor temp feedback (a multi-stage thermostat, for instance). 

The application I am speaking of would probably use staged Peerless Series 64 boilers, but just about any atmospheric would be the same, I suspect.



 So with the control issue out of the way, what other problems might I encounter and how would I size and spec. the check valves?

Jamie Hall

I can only respond...

from the standpoint of engineering thermodynamics -- not being a heating pro.  Very low pressure saturated steam, however, is strange stuff (we'll all agree on that!) and one of the strangenesses is that if you have a significant pressure drop -- such as might be across a valve or even a venturi -- you will get some condensation.  If your steam is superheated -- even to a relatively small degree -- this won't happen.  But in the case of low pressure saturated, I'd want to be a little wary of check valves, as it is almost inevitable that you will get a pressure drop.



If you need valves, how about a powered gate or plug valve (full port), operated off the burner controls?

The Steam Whisperer (Formerly Boilerpro)

Thanks.....

I forgot about that.   I suppose a little condensation wouldn't be much of an issue since it would just blow through the valve into the common header of the boilers.

Brad White

You did say atmospheric

so you are entirely correct- the radiation dumping would be larger in an atmospheric boiler than a power-burner arrangement. Duh, would that make a difference. My bad.



I take it that this application is an add-on to an existing boiler? In other words, if you were going in "from scratch", you would have other options and not be asking this.  A single boiler with modulating fire is, well, ok, we can dream.



I see the staging going on pressure as with any boiler, the thermostat only furnishing the "demand call".  Regardless, if the thermostat (say 2-stage), demanded more fire, your pressure condition between fired and unfired boiler under a check valve, would still exist. And pressure rules.



Sorry, I do not see a way to make that work right now. NOT saying that it cannot be done, just that I have no idea how to make that work right now. Others may! We can only hope. So I will sit back and watch.  Sorry- best I can do right now.

Dave in QCA

Check Shut-off Combination Valves

Back in my hospital complex days, we had a couple of Kewanee, 338 HP boilers running at 55-75 psi.  The outlets of each boiler had a shutoff/check valve.  Even though the leakage was insignificant, there was some very minor spillage on the 3 pop off valves.  On a very cold day, there were tiny whisps of steam that would appear where they discharged throug the roof of the boiler room.  Also, there was some very minor back leakage on the checks.  I assumed that the minor leakge there provided enough venting of air to allow steam from the active boiler to leak into the inactive boiler, and to condense and raise the water line over time, although this was a very VERY slow process.



I note from the WM drawings for the LGB, they too show an F&T trap to prevent boiler flooding on multiple boiler setups.  I note that the F&T trap gaurantees that the inactive boiler's steam chest will be filled with steam from the active boiler, because the F&T trap will be acting as a vent.  These drawings, of course, relate only to boilers that are filled by a feed pump.



On a dual set up, running with gravity return, the boilers will be self equalizing.  (won't they?)  (hey... I am the novice... just trying to sound smart!) 



And, on the inactive boiler, how is the steam going to get into the steam chest unless there is a way for the air to get vented out?  Steam can't go where air is in the way, right?    I know that if you have both boiler running and making steam, then shut one off, since there is already steam there, the active boiler will continue to supply whatever minor steam is consumed by condensation in the steam chest, but once the system shuts down and the steam mains and boilers are filled with air, wouldn't this be a moot point?



Got another question for Boilerpro too.  (meekly inquiring.  I don't mean to question the master.)   Have you looked into the efficiency of the Peerless running at 50%?  If it is decent, wouldn't 2 staging be an option instead of 2 boilers?

The Steam Whisperer (Formerly Boilerpro)

Haven't tested yet

I hope to mess around with the one I recently installed to test it at low fire.  However, it is 2 hours away, so I can't just drop by.

The other reasons for going to two boilers are: built in backup and being able to supply larger systems while keeping equipment standardized for easy repairs and inventory.

The Steam Whisperer (Formerly Boilerpro)

Hmmmm......

Not quite getting what you're driving at re Pressure.

I have a some modulating burner steamers out there that run quite nicely, so that is always an option.  However, the noise of a power burner is a problem in the the early 1900's apartments and condos I am seeing a whole lot of.  Power burners are out.  I am thinking of "from scatch" applicaitions  for outputs up to maybe 1.5 million, which would cover probably 80 % of the complexes in Chicago I am starting to look at (up to about 50 units)

Besides the simplicity of atmospheric is a real plus for maintenance and repairs and built in back up.

RE: pressure: This is how I see a two stage low pressure heating plant working.   A feed pump would be used to feed each boiler with check valves on the steam supply from each boiler that drops to the common header.  On a call for heat the first stage would come on and begin suppling heat to the system at whatever pressure is needed (probably a fraction on an ounce, after the check valve is open).  If this was all that was needed, then it would shut down.  However, if it is cold out and getting colder in the space, the second stage would be activated.  The second boiler would fire and begin building pressure behind the check valve and eventually pop it open.  Now there would be a surge in pressure initially (I suspect it wouldn't be as big as with power burner boiler, since water content.....and therefore heat content....is relatively much smaller), that may or may not trip the common high limit.  Even if it did, both boilers would shut down and then come back on together once the steam is consumed. 

An alternative to the two stage stat is a simple outdoor thermostat that brings the second boiler on line at a preset outdoor temp.  While really primitive, this probably wouldn't be too bad for the applications I am thinking of since there typically is no setback to deal with, so load will follow outdoor temp fairly well.