One Pipe Systems and Zone Valves

Steamhead

get in touch with me. I'm in Baltimore too.

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Ken Heselton

One Pipe Systems and Zone Valves

I knew there was a substantial number of details about low pressure steam heating systems that I didn't know. However, since I was mainly dealing with high pressure boilers in my many years with Power and Combustion I didn't worry much about it. Then, on May 19, 2005, I walked into a church in west Baltimore and found myself confronted with one of those one-pipe steam systems that I had read about in several of Dan's articles among others. It didn't take long to see what the problem was, a contractor that didn't understand how those systems work had revised it to really make a mess of things. While I could see what some of the problems were, and knew what to do to correct them, I was surprised to find steam zone valves in the system and I had no idea how they were supposed to work. I knew Dan Holohan had written his book "Lost Art of Steam Heating" so I decided this was a good time to buy it for reference. That way, I would have a good reference to check against other possible problems so I could really get the church's heating system working again.

Regretably the only reference to zone valves in a one pipe system advised putting check valves in the returns. Maybe that would prevent condensate backing up into the piping when a zone valve closed but ....

The church has a parallel flow upfeed system with a wet return and that wet return (which they had just replaced) is buried under concrete and flooring. Not only do I not have the heart to tell them they have to dig some of it up to install check valves; I know those checks will be a source of constant maintenance.

I was really looking for Dan's stories to confirm what I planned to tell them to do would work. Will it? That's what I want to know.

I pictured their problems of boiler flooding and water spitting out vents being associated with development of a vacuum in the zone piping faster than the air vents could react to it. Even the main vents would not open right away to prevent it because of their thermal inertia. What I was going to recommend before reading the book is to put a check valve in as a vacuum breaker after every zone valve so the respective zone wouldn't draw up condensate as it shut down. I would specify resilient seated valves that would always open with a pressure differential of about one half psi. I would also extend piping from the checks so they would drip to a drain when they had the occasion to leak.

Has anybody done it? Can you think of any reason why it wouldn't work?

Ken Heselton, KEH Energy Engineering, 105 Haverhill Road, Joppa, Maryland 21085, 410-679-6419

DanHolohan

This should help:

Motorized valves

Ken Heselton

One Pipe and Zone Valves (continued)

Dan,

Bought the book and love it. I enjoyed your explanations although my engineer's mind kept saying "the real reason this happens is...." Nothing in it explains why a one pipe parallel flow and upfeed system apparently worked with zone valves. I say apparently because there's nobody around that knows when they went in. The problems are the bain of a new owner.

I figured I will get the condensate back by allowing air to enter the zone thereby preventing a vacuum lifting all the condensate into that shut off zone. They have a minimum 'A' dimension of about 7 feet but high set and upfeed radiators that can flood as vacuum builds in the system. That's the only reason I can come up with for their description that the steam pressure suddenly jumps to 15 psig (he said 25 psig but we know better) and water spits out of all the vents. Something had to get that water up over thirty feet to the third floor radiators and the only thing that can do that is a combination of boiler and atmospheric pressure. I'm still waiting for a requested copy of the instructions for the new vents he installed.

I understand what you're saying about the speed of the steam, but it can't go any faster if the pressure difference doesn't change. It will make the boiler short cycle, that I know. Short cycling is inefficient, but I don't think it's as inefficient as heating a large chapel all week (when it's only used on Sunday) just to keep the offices warm.

Bill Burton suggested his Danfoss TRV's but I figure they'll do the same thing as shutting off a zone, for a while. Eventually the air will drop down the risers, (it is heavier than the steam)out the main vents, and the radiators would be heating anyway unless we force the boiler pressure to drop to zero at intervals. I believe your thing about steam filling the rest of the system faster will happen if those TRV's are all closed. I could just tell the church to block all the vents except a couple to see what happens!

The church isn't ripe with cash so I'm looking for an inexpensive solution that will solve the problem they had before the contractor put in the condensate pump that didn't work. I figure the vacuum breakers right after the zone valves would eliminate the condensate holdup, water hammer, and boiler flooding but cause short cycling. If I can quiet the system down the next step would be high-low firing the boiler to eliminate rapid cycling. Has anyone ever tried it?

Steamhead

Ken, what I'd do

is pipe a 1/2" (or larger depending on size of valve) bypass around each zone valve. The bypass should run above the zone valve so it won't fill with water. Your problem is not (or not entirely) vacuum- the closed zone valves are turning the "A" dimension at the end of the main into a "B" dimension, which allows water to back up into closed zones. This type of bypass will mean some steam will leak into closed zones when the boiler is running, but this will maintain the "A" dimension and it shouldn't be enough to cause the closed zone to heat up the first couple rads.

And I'll bet that pump and tank is piped so it doesn't allow air to vent down the formerly wet returns and out the tank vent. If it did you wouldn't be getting vacuum.

Let me know how you do- and you could always sell the pump on eBay......

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Ken Heselton

To: Rick Mandel
Dan Holohan

I think I better try to explain this because both of you came back with a suggestion that I can't picture working. One of my first thoughts was simply drilling a hole in the disk of each of the zone valves, somewhat cheaper than piping in a bypass, but I quickly dismissed it. That small hole, or a bypass valve, will not prevent the system sucking up. I've had enough experience seeing what vacuum does in high pressure steam plants to realize its potential in these little heating systems. Lock up steam in a pipe that's losing heat and condensate will flow back up it until it's over thirty feet high. Then break the vacuum and that head of water produces a differential of 13 psi.

Shutting a zone valve isn't like shutting down the boiler in a one pipe system. When the burner is shut off there's still plenty of energy left in the boiler and the boiler continues to generate steam by flashing off the water as the pressure drops. The pressure differential across the mains drops as flow decreases so the necessary static head on the condensate return (the 'A' dimension) has to be less. The main remains balanced so the condensate continues to flow back to the boiler.

What we have in the church I'm dealing with is a one pipe system with three zone valves off the boiler header and main risers to parallel flow mains with drops into wet returns (recently replaced) that join in a concrete floor then return to the boiler room, ending at an elevation equal to the bottom connection of the boiler, and rise up to the boiler's Hartford Loop. In reviewing my photographs I noticed that not all of the radiators are above the mains. Most are up-feeds but one section, in a recreation room under the chapel, are down feeds to a dry return that's approximately seven feet above the wet returns. If you'd like, I'll try to make an isometric drawing. Now, to the operation of the zone valves.

When a zone valve shuts there's no source of steam to keep the pressure balanced and all the components in the zone lose temperature rapidly. The drop-off of the vents doesn't necessarily happen because they're still hot enough to stay closed for a little while, that's thermal inertia. Even if all the vents did open they couldn't admit air into the system fast enough to prevent a vacuum forming. Condensate in the connected wet return flows up to fill the rapidly forming vacuum, quickly flooding the dry header, risers, and lower radiators of this system. It may still be hot enough to keep the vents closed and the vent float floats in the condensate to help keep it closed. The rush of condensate into the shutoff zone results in the boiler shutting down on low water and firing intermittently as the feeder forces more water in.

The vents finally cool and admit air into the zone (or open fully to let it enter faster), filling what little bit isn't full of condensate, so now there's a tall standing head of water with atmospheric pressure on top of it. We don't have the little column of water known as the 'A' dimension, we have several feet of water and even at 212 degrees it produces a static head of several pounds. That's when all the vents on the lower radiators start spitting water as they cool and the floats drop out. Also, the boiler floods because all the water that moved up to fill the vacuum is now free to flow down the return. That's the only way pressures well in excess of the pressuretrol setting can be observed at the boiler. If it happens as the zone valve opens back up there's water hammer galore!

Of course there's the solution of putting a check valve in the return of each zone so condensate will not back into the zone that's shut down. In the case of the church that's a considerable expense because of the buried wet returns. It's also introducing a maintenance headache keeping those valves operating. When they don't operate the scenario described happens. I think the system needs a check valve on each zone, but not in the return.

Consider what happens if we put a vacuum breaker in right after the zone valve. I'm not talking about a weeny here, I would go for a one and a quarter to one and one half inch check valve, maybe larger. Once I calculate the weight of the steel and iron and figure out how quickly the steam condenses I'll know how fast the vacuum can form and about how big to make the valve so the vacuum can't exceed 2 inches of mercury, the equivalent to a reverse pressure of 2 psi. That means the condensate will only back up into the zone that's shut down about four and one half feet - which shouldn't reach the bottom of the lowest steam main or dry return. Admitting the air that's heavier than the steam at the beginning of the main allows it to push the remaining steam up to the radiators to finish heating. The air doesn't interfere with the flow of condensate (it's just another gas).

When the zone valve opens again the check is shut and the system fills with steam, pushing the air ahead of it just like the normal startup of a one pipe system. The only variation is a quicker shutdown with the zone valve because the zone is isolated from the boiler. Just in case the check sticks open or doesn't close completely the air inlet is piped from a point just over a floor drain.

Yes, Dan, the boiler should be a modulating type because the exposed radiation will vary and the boiler will short cycle when a zone valve is closed; even more dramatically if two shut down. I intend to be certain that the last zone operating only shuts down the boiler and doesn't shut its zone valve. That way one of the zones will always be absorbing the residual heat in the boiler. If it didn't then the boiler's safety valve would probably be popping regularly.

A hole in the zone valve disk or a little bypass can't stop that vacuum forming. And, if we allow enough steam through to keep the pressure above a couple of inches in the zone we're still heating it. Think through the scenario above and tell me it won't work.

One thing that always makes me sweat when I look at these little teakettles (to me it isn't a boiler unless I can climb into the furnace and stand up) is that one, lonely, little old safety valve. I've seen enough of safety valve failures to believe every boiler should have two no matter what size they are. I know of many an instance when the operating control, high limit, and safety valve all failed so triple redundancy isn't enough as far as I'm concerned.

It goes along with my contributing to the pitch to have ASME CSD-1 require two low water cutoffs on steam boilers. As far as I'm concerned they should both lock out. When a system such as the one at this church has to rely on regular operation of the low water cutoff to protect the boiler it makes me shudder. If that low water cutoff has to work every time a zone shuts off that's just asking for disaster. An average of 381 heating boiler failures per year are due to low water, 48% of the failures on average. If I had my way every heating boiler would, in addition to two non-recycling low water cutoffs, have to be fitted with a special high capacity vacuum breaker to prevent goofy things happening that could allow the water to be forced out of the boiler into a vacuum. The Hartford Loop only limits how much drains back, leaving the water at a low level where a little more evaporation starts exposing hot heating surfaces. And, when the water comes rushing back, those hot surfaces are exposed to thermal shock with potential rupture.

I'm surprised one of the dead men hadn't done this already. In a manner of speaking I'm rapidly approaching their status so I may soon be the one that came up with a way to zone a one pipe system. I've given considerable thought and calculation to this application and I believe reliable vacuum breakers with on-off zone valves will work safely provided we have a 'C' dimension that ensures a zone will not have it's dry sections flooded on shutdown.

Steamhead

Ken, you make some good points

part of the problem might be vacuum (which is why I said "not entirely" in my earlier post) but I am reasonably sure the zone valves are also at least one part of the problem.

Why not install the vacuum breakers first, and see what happens? Hint: If there are no main vents on the steam mains, installing properly-sized vents will not only speed up steam distribution, but the vents will also pop open under vacuum especially if they are Gortons. So you'll kill the vacuum and save the church some fuel too.

If the VBs don't have the desired effect, the next step is to drill or bypass the zone valves as described above.

BTW, my name's Frank- I'm the computer-literate one. And I look forward to meeting you sometime and talking steam!

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