The case of Ray versus the A dimension

RayWohlfarth

In this week's case, there is no mystery to solve. It was when I finally got it, when I finally figured out what the A dimension is. I've heard @DanHolohan and others talk about it for years but I couldn't understand it. Sorry Dan!

In my commercial world, we used pumps to inject the condensate into the boiler so my lack of knowledge of the A dimension never really mattered. There I admit it. They say confessing is good for the soul. The video is this Friday at 6am EST

ethicalpaul

I'll look forward to your video since I have come to the conclusion that the A dimension doesn't matter in a simple boiler within reason--it only comes into play when there are complications involved such as multiple boilers, or valves, pumps, or check valves, etc. A clogged return will also make the water rise at the far end, but that's got nothing to do with the A dimension (but maybe has fooled some people over the years).

In a simple steam boiler, the water level is consistent at the boiler, in the near-boiler return, and at the far end of the return, regardless of the presence of the "equalizer". The real equalizer is the main itself.

This diagram, by the otherwise excellent Peerless, is a myth. There is a very slight level variance of maybe an inch, but the A dimension is hugely over-stated. It's easy to verify with a sight glass at the far end.



The "start up condensate" is especially funny to me...do they think the condensate appears at the far end without a corresponding reduction in water level due to creating the steam that led to the condensate??

Intplm.

RayWohlfarth said:

In this week's case, there is no mystery to solve. It was when I finally got it, when I finally figured out what the A dimension is. I've heard @DanHolohan and others talk about it for years but I couldn't understand it. Sorry Dan!

In my commercial world, we used pumps to inject the condensate into the boiler so my lack of knowledge of the A dimension never really mattered. There I admit it. They say confessing is good for the soul. The video is this Friday at 6am EST

@RayWohlfarth
I like your style.
I too have been mostly, but not entirely, a commercial steam guy.
"A" dimension! Whatszat?!
Over the years getting a great education from @DanHolohan, lifes been good. Thank you, Dan.
And thank you @ethicalpaul for posting that very nice reminder pic above.

ethicalpaul

The picture is helpful to illustrate what the A dimension is, but it's a complete liar haha. There is no typical simple steam system as illustrated where the steam pressure at the end of the main is 1/2 PSI different from the pressure at the boiler. Also easily observable with a gauge there.

EBEBRATT-Ed

@ethicalpaul

Sorry but I don't agree on this. It all depends on the pressure you're running at and the size of the building and more accurately the pressure drop in the piping.

Had a two-pipe air vent system with gravity returns. This was in the guard house for the main building at a famous gun manufacturer in Springfield, MA. The building was short of radiation or maybe it wasn't but the guards were constantly in and out with doors left open, so they complained about being cold.

When they got cold they would call maintenance who would go down and crank the pressure control up to 10psi. The condensate would then stack up into the radiation and leak out all the air vents. The radiation was waterlogged, and they got less heat than they had before. The radiation was probably8' above the boiler water line so 4psi would get it there.


We would go there and shut the boiler down, knock the water out of the vents or install new ones. Lower the steam pressure and all the radiation was red hot.

I admit this is an extreme example of a system with other problems, they needed to add a couple steam unit heaters or more radiation.

And for the average residential job what you have found is probably correct.

But it doesn't apply to every system and apartment building would be a better example.

ethicalpaul

There are indeed a lot of variables. For example, in your story if they were able to raise the pressuretrol and see 10 PSI as a result in their system, then the boiler was dramatically oversized. We also don't know what other variables might have been at play. You have so much excellent experience that I truly respect and I know you have seen some "stuff" out there!

But I made that video showing basically zero pressure drop at the far end of the main (on my admittedly small system) and I have never seen any other evidence presented, so that's where I'm at. Even with an oversized boiler, you can see the drawing....any high pressure is applied both to the bottom/water side, and it's equally applied to the top/main side so there is no water rise at the far end.

If you or anyone else would care to install a gauge at the far end of the main and prove me wrong, that would be valuable!

Jamie Hall

Oh dear. @EBEBRATT-Ed has put it pretty well -- but to put it into a nutshell, if you have dry returns the A dimension matters. A lot. The dry returns are at atmospheric pressure, not steam pressure (or at least they should be; if they aren't you have other problems) and the water in any drips coming from the wet return to a dry return will indeed rise to the A dimension above the boiler water level. If that stays below the dry return itself, that's fine. If it rises into the dry return, havoc.

@ethicalpaul 's argument is pretty much correct -- for the steam mains or one pipe systems. it is NOT correct for two pipe steam systems.

ethicalpaul

Thanks Jamie, but what would the pressure at the boiler be in such a system? I'd still love to see a pressure gauge showing a pressure difference...just for my knowledge, the part of system shown in Peerless's drawing above is the same for one pipe or two pipe in that there is a main, and a drop at the far end to the waterline?

mattmia2

In a 2 pipe system the mains are sealed with a steam trap or vent but the returns are either open to the atmosphere or have a vent that should never close so if the mains build pressure there is pressure in the main but the pressure in the return is atmospheric pressure(or a vacuum in certain other systems).

ethicalpaul

Thanks Matt! So there's not a direct open line from the header to the far end of the main where it drops to the water line?

mattmia2

ethicalpaul said:

Thanks Matt! So there's not a direct open line from the header to the far end of the main where it drops to the water line?

No. The returns act as the vent in a 2 pipe system so there can not be steam in the returns or it will prevent some of the emitters from venting. Some systems accomplish that with a steam trap that connects the main to the return, others with a connection that drops from the main to a wet return below the water line(and usually a vent on the main), still other use a combination of the 2.

ethicalpaul

OK great, so that would be something to add to the list of possible complications that I mentioned in my original response, thanks for the knowledge!

it only comes into play when there are complications involved such as multiple boilers, or valves, pumps, or check valves, etc.

EdTheHeaterMan

I love this discussion, It gets @ethicalpaul to voice his very strong opinion on why the "EQUALIZER" on a boiler is not properly named. So Go ahead Paul, ask that question about what the Equalizer makes equal. Is it pressure, is it water level, is it something in the word of dark matter? Will the Space / Time Continuum be irreparably damaged. Will we survive this conundrum?

Please post the link to the video here on Friday, After Ray post's his!

EBEBRATT-Ed

It really depends on the condensate return system and the pressure in the boiler that determines the need for the condensate to stack.

You can have a 2-pipe air vent system with radiator vents and no traps and the condensate from each radiator drops to a wet return which leads back to the boiler with no feed pump. In that case the steam supply pressure helps push the condensate back.

You can have any two pipe system and a drip from a steam main can drip to a wet return with no trap because the water makes a seal.

For the most part if you have traps they cut off all the steam supplies from reaching the returns (as long as the traps work) so the condensate can only return to the boiler by "stacking" to overcome the boiler pressure. The only alternative is to use a Steam powered or electric condensate pump or one of the old Boiler return traps.

Jamie Hall

To make matters even more convoluted (are we confused yet?) many vapour systems had (and should still have...) assorted ingenious devices to assist condensate return if the boiler pressure got too high. The dreaded Hoffman Differential Loop is one of the simplest of these, but there are others...

RayWohlfarth

Wow lots of discussion on this. Like I said, I just couldn't get it how the A dimension works then it hit me. It's all about pressures. If the steam leaves the boiler at 2 psi and during its journey to the end of the steam main, it loses some pressure. Let's assume for ease of math, the pressure is 1 1/2 pounds. How can the condensate being pushed at 1 1/2 pounds of steam overcome the two psi inside the boiler? We dont have a pump so we have to rely on the weight of the water in the vertical return pipe. If the water rises 28" above the boiler line, the pressure at the bottom of the 28" is one pound. Add that pound to the 1 1/2 psi leftover steam and we now have 2 1/2 pounds at the bottom of the vertical pipe. That extra half pound of pressure will overcome the 2 psi in the boiler and return the condensate. I can't believe I was stumped for years on this. And to think the old timers figured this out without an iPhone or internet. Here is the link to the video
https://youtube.com/watch?v=BaxgX9uuF80

RayWohlfarth

Here is an illustration

dko

Thank you for the lesson

RayWohlfarth

@dko Most welcome

ethicalpaul

It's an interesting theory that is definitely widespread in the industry, but in practice there is no measurable pressure drop along the main. There is no rise in water level at the A dimension. I'm talking about common typical residential (and simple commercial) systems like in the drawing you posted. I have installed a sight glass and an accurate gauge at the far end of my main to verify this.

I think that in residential/small commercial steam, there just aren't the flow rates and pressures that would cause any pressure drops. The entire system is always the same pressure with only very small variance, too low to measure with typical gauges and no rise in water level is noticeable.

This fits with our target of very low operating pressures. The pressure to move steam quickly throughout a system is 1" WC or less. And once pressure starts building, there is really no chance for any pressure variance throughout the system. The whole thing at that point is like a full balloon.

PS: Ray, your video is very good and does show exactly what would happen IF we saw any measurable pressure variance in a residential steam system. But for example at 3:50 in your video, that will never happen regardless of pressure achieved, because at those pressures (and really any measurable pressure), the pressure in the whole system will be the same, so the main is pushing the water column down at the far end equally to how the boiler is pushing it up via the wet return.

And just to make Ed happy, I'll add this is regardless of the presence of the "equalizer" because it is the main itself that is the actual equalizer.

RayWohlfarth

@ethicalpaul I believe those systems were designed for a 1 ounce pressure drop per 100 feet. That doesn't factor in if its wet steam and premature condensation In reality there is probably more like an few ounce difference in pressure between the boiler and return

Jamie Hall

The error is in focusing on a steam main or runout. In any system in which there are traps (or water seals) between the steam mains and the wet returns, or between steam mains and dry returns carrying only air or condensate, there is, in fact, a pressure differential between the dry return or outlet of the trap and the wet return. Most residential systems with this arrangement -- which are very common -- have the dry return at atmospheric pressure. Many industrial systems also have this, but some differ in having vacuum pumps.

At some risk of raising the dead here, this also ties into the problem which arises in distinguishing steam main extensions of one kind or another from dry returns. The former do have steam pressure in them. The latter do not. We have seen many examples over the years here on the Wall where the distinction wasn't made, or wasn't understood, with confusion over everything from A dimensions to venting to where they connect -- if the do -- at the ends.

ethicalpaul

RayWohlfarth said:

@ethicalpaul I believe those systems were designed for a 1 ounce pressure drop per 100 feet. That doesn't factor in if its wet steam and premature condensation In reality there is probably more like an few ounce difference in pressure between the boiler and return

Thanks Ray! They may have been designed for something, but I can guarantee you in practice there is roughly a zero ounce difference between the boiler and return.

My video below is too long and it rambles a bit, but it will show what I'm talking about. Try to get to the end where I close down my supply valve at the boiler to force a pressure differential between the wet return and the main, it's worth watching I think. Jamie, I'd love to see a gauge showing this pressure difference in practice. You have a two pipe system, right? I can bring the Magnahelic!

https://youtu.be/6ZLg3fqU1MQ

RayWohlfarth

@ethicalpaul I always thought the equalizing pipe was used to avoid a sudden loss of water in the boiler under pressure when a return pipe leaked. I was told the equalizing line would equal the pressure between the top and bottom so if there is a leak, the water level would drop down to the level of the hartford loop. Now this was before we had electric controls and no quick shutoff of the boiler. But I could be wrong LOL

EdTheHeaterMan

We should always have these discussions. When someone actually measures the actual performance of an assumption made by some dead man that maybe had a unique situation and therefore applied that "RULE" to every system regardless of the differences in the systems, we can see the "Myth Buster" at work. Thanks Paul for your comments and video. You actually taught me something so many months ago after our extensive discussion on that subject.


The fact that the "Dimension A" is part of Steam textbooks means that at some point there was an issue, Could it have been on a system with steam mains that were too small and therefore there was a significant pressure drop off, or were there other factors …only the dead men know. But the explanation Ray offers does have its basis in physics of water and pressures and the like. So understanding that IF you happen to come across a one pipe system that does have that pressure drop issue, the Dimension A will resolve it. IF (and that is BIg IF PAUL) you find it, you can see the fix in your mind's eye without the need to install glass pipes and pressure gauges all over the place. But I really like the Weil McLain glass boiler and @ethicalpaul's glass pipes.

ethicalpaul

Yes, Ray, some others had said that was a purpose of it (for in case of a rapid return leak), but in The Lost Art it is stated that it keeps the water from being pushed out of the boiler during normal operation. Maybe a misinterpretation or misunderstanding of it on my part but I don't think so.

And I find the "rapid leak" purpose of it to be very dubious...just how often were return pipes exploding in the old days? And it's just a short delay before the boiler is drained due to steam anyway, so the whole idea sounds like an afterthought.

RayWohlfarth

@ethicalpaul Thank you for that. That makes more sense. I did enjoy your video
@EdTheHeaterMan I wish Mythbusters was still on so we could have them test this myth

RayWohlfarth

Now I want to know why most steel boilers do not use an equalizer 

Jamie Hall

Repeat after me. Or, if that doesn't work, write it on the blackboard fifteen times.

The A dimension is of critical importance in understanding the water level in a drip to a wet return in systems where there is a dry return -- that is, at atmospheric pressure -- as part of the system. This would apply to any two pipe system other than ones with vacuum pump assisted dry returns or condensate receivers vented to the atmosphere (they have some truly interesting dimensions)

The A dimension is not of importance in understanding the water level in a drip to a wet return from a steam main. This would apply to any one pipe system.

Not surprisingly, The Lost Art is correct -- but you have to understand which type of system is being discussed.

ethicalpaul

Sorry Jamie, but it's not us you have to convince, it's Peerless (see their graphic I attached in an above post), and it's TLAOST itself.

I refer to pages 44-46 (revised edition) where you will see basically the same graphic that Peerless used, with no mention of one vs two pipe. If you go back a bit to page 42 (sort of the preamble to the "A" Dimension section) you'll see that it even explicitly references it as a "simple one-pipe steam system".

I quote a pertinent paragraph here:

"In a typical gravity-return system over 100,000 Btuh (a gravity-return system has no condensate-or boiler-feed pump), the returning condensate can rise up about 14 inches. You really have no control over this; it's a result of the pipe-sizing charts the old-timers used."

And here from page 127:

In a one-pipe gravity-return steam system, "Dimension A" has to be a minimum of 28 inches.

If you can find the part in The Lost Art that states it like you did, Jamie, where the A dimension only applies to two pipe systems, I would welcome to see it. And if anyone wants to try my observations to see if I'm wrong about one-pipe systems, I'd love to see that (honestly!)

Jamie Hall

I don't have my copy of the Lost Art handy, but I'll believe you, @ethicalpaul . It all, however, reinforces my determination to make sure that people really understand the differences (and this is not the only on!) between one pipe systems and two pipe systems with dry returns., and the distinction between dry returns and steam main extensions.

The distinction keeps coming up, and it really is important in several different ways...

EBEBRATT-Ed

My feeling is that the Dimension A probably is more applicable to larger systems than typical residential stuff.

There are a lot of old building using 1 pipe steam that are a lot bigger than a typical house. Bigger systems usually +more pressure drop.

@RayWohlfarth

Not just steel but CI as well. A lot of the old CI boilers were 1pipe out (supply) and I Pipe in (return with a swing check)

Commercial stuff usually has (or is more likely to have) a condensate return tank or boiler feed tank so it doesn't need a Hartford Loop.

Equalizers have to be on a CI boiler now because they throw water into the header. (would be much better if the water stayed in the boiler like it did with old boilers) Steel boilers like a Scotch or Steel fire box boiler have a large area inside to release the steam from the water. On a CI boiler that space is all cut up into individual sections ....by the sections. Every section is an individual boiler. Steel boiler are just a huge tank of water with fire tubes running through them. No restriction on water or steam flow. CI The water and steam have to navigate all these passageways in the sections and the sections will run at different temperatures. A CI boiler with a power burner will always get hot and steam the rear sections before the front.

That's why on CI we need all these equalizers and external headers that a steel boiler can survive without.

And yes. The Hartford Loop was originally supposed to keep water in the boiler if the return line leaked but the best it could do is buy them some time. Coal fired there was no low water control. A lot of the old coal CI boilers had a fusible plug above the combustion chamber. If the boiler finally got low on water the plug would melt when the water got low then it would dump the remaining water on the coal fire in hopes of putting it out.

jmho

Even a huge steel boiler only has 1 supply weather it is 8, 10, 12" whatever no boiler header, no equalizer

mattmia2

Some 2 pipe air vent systems are also excepted from having returns at a significantly different pressure than the main.

As far as the pressure drop in the mains, in 1 pipe systems the mains are sized to allow flow of steam and condensate in the same pipe so they are larger than what sizing them for pressure drop would require. In residential systems the mains are generally pretty short too.

ethicalpaul

Aren't 2 pipe systems of any kind supposed to run at a few ounces of pressure maximum anyway? I wish I had one to experiment on.

Sorry to repeat myself but the notion of pressure drop along a main is greatly inflated in people's minds vs reality. Put a gauge on your largest system some time and see what I mean.

Jamie Hall

Oh dear. Now we are confusing two pipe systems with vapour systems. ( @mattmia2 's comment about two pipe air vent systems is quite valid -- fortunately there aren't too many of those). There is nothing to say that a two pipe system is intended to run on vapour type pressures -- I dare say there are a good many out there which are intended for the typical 1.5 to 2 psi running pressures. The nature and operation of two pipe systems has nothing to do with pressure, but rather with how the air is supposed to get out of the system when it starts, and how the condensate is intended to return.

It is not all that difficult to demonstrate that a true two pipe system runs with the dry returns at atmospheric pressure: many of them were originally built with the dry returns actually open to the atmosphere (sometimes vented into the chimney through a nice vent pipe). Many more were built with and intended to operate with just one main vent location -- no vents anywhere else on the system. And many of these intended to run on vapour system type pressures had various devices to ensure that the pressure differential between the steam mains and the dry returns never exceeded some convenient low value. But the intention was, and the operation depends on, the dry returns being at that pressure differential.

Which means that the A dimension (which, technically, is referred to the pressure differential between the steam mains and the dry returns, not to the gauge pressure -- a consideration when the dry returns are placed under a vacuum as a few systems do) ) is applicable to the drip connections from the dry returns to the wet returns or the boiler itself.

Solid_Fuel_Man

I'm convinced that I am not smart enough to do steam. Hot water is so much simpler, only phase change I need will be fireside in the boiler. 

Gsmith

I think perhaps there is a lot of truth on both sides of this issue. Equalizers may not in fact be needed in many or most cases and most of the time on one pipe systems. But, engineers in particular, and system designers and installers before engineers got involved when these steam systems were first developed, often include equipment to prevent problems which might occur only occasionally or under non-normal operating circumstances. It seems to me that an equalizer does provide some benefit in, as stated above, returning any water carried up into the steam mains quickly to the boiler, in case the risers were undersized or only one used when two were needed, and under some conditions can in fact equalize the pressure when for example there are large losses in the system piping (long mains, big system etc). And most of these rules of thumb design features were included to allow quick and standard system configurations that would handle most cases so each system didn't need to be carefully and particularly designed for its own individual circumstances. Seems like pretty cheap insurance for the cost of a couple of fittings and a few feet of pipe. Why have a pressure relief valve if you have pressuretrol, or vice versa? They both provide the same protection, same concept, relatively cheap insurance for unlikely events/conditions/situations.

ethicalpaul

This isn’t the thread about the equalizer not equalizing (although that is a little related).

this is the thread about the A dimension

i think it’s greatly overstated in two pipe steam too. If there were enough pressure to raise the A dimension in a system that has its dry return open to the atmosphere as Jamie described above, it would be spewing live steam into the building.

but hook up a gauge someone and shut me up 😅

Gsmith

same concept on the A dimension, might not be needed in most cases, but doesn't hurt or cost much as a "standard" way of designing/installing most systems.

ChrisJ

Gsmith said:

same concept on the A dimension, might not be needed in most cases, but doesn't hurt or cost much as a "standard" way of designing/installing most systems.

Actually it did hurt in my situation.
It would've been nice to have my boiler 2 or 4" above the floor but i didn't put it on blocks because I was concerned about not having enough A dimension.

Fact is my boiler could be a foot off the floor and not have an issue.

All well.

Now I've made changes to make it not matter but I'm sure my situation isn't unique.

Jamie Hall

No, sorry, a dry return will NOT spew steam into the building. If it does, one of two things is wrong: if this is a vapour system without traps, the pressure in the steam main is too high, allowing excess steam to pass the orifices or other control devices. If the system -- vapour or not -- has steam traps, as most do, one or more traps has failed (or related, the pressure in the steam mains is high enough to blow one or more water seals).

One more time around O'Houlihann's barn: A dry return in a two pipe system which is operating correctly, never has steam in it. Ever. Never mind pressure.

I don't have a gauge on Cedric's dry returns. I do have main vents at the boiler. They never close -- always wide open.

(I won't confuse the issue with a dissertation on how the Hoffman Differential Loop works).