One pipe steam system revisted

Dave Grabowsky

I have a situation where I have hooked up 3 small one pipe steam coils to an air handler. This air handler is the farthest heating device in the building which is 4 stories above the Boiler room. Being a one pipe steam system there is only one 2”line which serves this coil for both steam supply as well as condensate return. There is lack of heat transfer and considerable banging in the lines. I will use one of my coils as an example. This coil is sized for 1600 CFM and 80,000 BTUs through the coil. This should be a delta tee of 45 degrees. The most I can reach is delta tee of 25-30 degrees. In addition, the banging of the piping is so bad I have replaced the air vent after 2 days of use. I have the steam coil installed with the steam headers above and below the coil. The piping and the steam/condensate connection is 2”. I have on my steam/condensate opening a full port 2" belimo slow acting modulating valve located below the header which takes appoximately 3 minutes to open. Of course this should allow steam to flow up into the coil as well as condensate to fall into the pipe. I also have 2 connections at the top of the coil. On one side which is on the top of the coil and directly above the 2” drain I have installed an air vent. The other side of the top of the coil has an 1/2 " connection with piping running down and connecting to the 2”steam/condensate piping. Within this ½ “line is a slower one minute closing Belimo full port valve which ties in below both the 2” steam valve and into the 2”steam/condensate line. Therefore we have a 2” main steam /condensate line up feeding into the coil and a ½” relief at the top of the coil which ties into the steam/condensate line below the main 2” valve. This 1/2" valve is considered an on/off valve but will take one minute to open. As the main 2” steam valve begins to modulate open it is approximately 1/3 rd. open when the 1/2" valve is fully open. As the main 2” steam valve opens halfway there is minimal noise. As this main 2”valve opens from halfway to full open there is considerable hammering and shaking of the coil. The steam pressure entering the coil is 3#'s. To help correct this, my steam consultant had me remove the air vent and in its place put an Armstrong rv4-s one pipe steam valve with a sv-12 air vent and lv-4 operator. This didn't seem to help much. His advice to me is to better synchronize the 1/2' secondary valve to open quicker in relation to the 2” steam valve. I need help ASAP.I have an area that cannot heat properly and is being water hammered to death. What should I do?

Steamhead (in transit)

Dave, you need to repipe it

There is no way steam and condensate can keep out of each other's way in that coil. That's what causes the uneven heating and banging.

Bring your steam into the top of the coil. At the bottom, run a pipe of at least the same diameter as the coil to the wet return, with a tee near the coil outlet for an air vent.

Remember that with the zone valve, unless a 1/2" bypass is used around the valve, the "A" dimension will turn into a "B" dimension when the valve is closed. This may cause water to back up into the coil, which will cause banging on the next start-up.

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Christian Egli_2

More pipes, more pipes

A long way away from the boiler, this upfeed one pipe condensate counter flow is not big enough. For a quick glance The Lost Art on page 89 gives a 40,000 BTU/h capacity for the 2 in line. Just for one coil, you’re already half sized.

I am puzzled, this seems to be an adaptation in an old building where previous heat needs were met by the current 2 in line. Why would your heat needs increase so much today? Perhaps cutting down on the air flow in the handler would solve some of your problems. I’d really look into this again.

I don’t quite follow all of your designed operation, but here are some considerations. I view your coil in a similar way we’d look a standing radiator. The 2 in steam enters and leave on the lower left side. Lower right side - nothing. Upper left side - the air vent. Upper right side - a 1/2 in pilot steam line.

The air vent breaks fast. Well, I don’t find it is particularly well located. It would do much better nearer the lower right side, perhaps situated on a cross link of pipes between the upper and lower connections. Don’t forget either the all important vacuum breaker.

I think things work more or less okay while only the 1/2 in pilot line is in operation. Steam enters coolly at the top right, and showers down the condensate towards the slightly opened 2 in line at the bottom left - steam is still not entering too hard from the bottom connection and condensate can wiggle away.

It probably works alright because the 1/2 in valves limits the steam throughput to what your 2 in riser can deliver and steam and condensate cross each other politely up and down the four floors. Once the season is fully open, I can see the condensate running for it’s life.

What to do quick.

Ideally figure out why you need so much steam up there and pipe a suitable riser. Meanwhile, and this might be easier, remove all dripping condensate from the riser, this will about double the capacity; so if we’re talking 80,000 BTU/h per 2 in line, we might still be undersized a bit but close enough for operation.

This means a whole new condensate line has to be fed down to the basement and into a wet loop seal or a wet return (but, I don’t know what kind of return system you have, you‘ll need to think this through for sure)

I would also pipe the supply to the coil on the top left side, with your 2 in electric valve, let the condensate drizzle down inside the coil and pour into the 1/2 in electric valve I’d place on the lower right side and from there go downstairs in a new drain line (in a pinch, you could use a suitable flexible line, but I don’t know what kind of building this is either). Steam would now be free to rise without having to elbow it’s way up.

You must know that zoning on one pipe is hard to control, but keeping your current way by opening the return side slightly faster than the big supply line, you’d get nice operation.

You also must keep the air vent (near the bottom) and an imperative vacuum breaker.

If all three coils feed off the one poor 2 in line, a serious recalculation is in order and a new riser as well.

There, I hope this does not spoil your Sunday. Here it’s raining and getting cold.

Christian Egli_2

Beaten in the rise

Steamhead was way quicker on this than I was, so now there's two of us with the same diagnostic. Good for you Dave.


Ooooh, and one more idea.

If the riser is proved to be completely inadequate, you could also build a new express riser somewhere else in the building, go up to the top floor where you are, and turn the current 2 in riser into a down fed line. This bumps up capacities by a whole bunch. It also gives lots of flexibility as to where you pipe your express line.