What boiler controls are used on a completely sub-atmospheric system

Unhippy

As the title says....what pressure controls are/can be used when the system is operating totally in vacuum....say up to 1"Hg from 6"Hg cold.....

I live in New Zealand and am looking at putting together a steam heating system for my house that for legal reasons needs to stay sub-atmospheric....anything steam pressure here needs a inspection once a year that starts at $1500 plus you need a steam boiler operators ticket to run it.....old rules that have never been changed

I spoke to the inspector about it and asked if a vacuum steam system is exempt....answer: "if i was to open a valve and air gets sucked into the system?.....hmmmm.....no, there is no rules covering that.....no chance of it exploding so there is no problem"

So i am good on that front....however being in New Zealand, i will have to build everything that is too large to be shipped at a reasonable cost....meaning radiators and the boiler itself.....however.....my trade qualification is (ta-da!!!) as a boilermaker/fitter/welder so thats not an issue.

I asked a few of steam parts suppliers from a former workplace about vacuum pressure controls.....and got blank looks.....most of the stuff they deal with is in the 150psi area.

all the vapourstats i can find online (State supply etc) switch from 0-16-oz or 0-4 psi....not really what i need when i'm never going to get to 0-psi......some of the dwyer switches are inches of water column but still measure positive pressure only unless i've missed something.

Is there something off the shelf that will work or will i have to build a controller as well.....(i already have a design drawn up that will reference boiler pressure to vacuum condensate receiver pressure).

i intend to get a vapourstat and have it set to activate a manual reset if the pressure ever reaches 0-psi as a secondary shut-off

Cheers
Callum

jumper

I believe that if safety valves are at zero psi boiler can't blow up either. But I've been over ruled by government officials and machinery insurance inspectors. I'd explore using thermostatic control instead of pressure. Probe into water instead of twisty tube?

Steamhead

Search on vacuum controllers and you should find something. They use these on large vacuum pumps so there have to be replacements available.

For a recent discussion of running a steam system in vacuum, go here:

http://forum.heatinghelp.com/discussion/154462/vacuum-pump-for-steam-system

Unhippy

The inspector i asked about it is the government official that decides on issueslike this for this area......lol and lets just say that insurance people don't have the influence here that they seem to have in the states.

i take it none of the easily available controls work below atmospheric?......thermostatic control might work if i can find something with a narrow enough deadband.....i shall look into it

RobG

With the restrictions placed upon you in NZ why are you not going with a hot water system (sorry Frank but I have to ask)? It just seems that it would be allot more practical in your situation.

Unhippy

Two reasons, the first...because i can.....i've done hotwater heat install in a previous house and while that worked really well it was kinda boring to run.....so i want something different.....and i will probably have the only one in the country.....which is about normal for me.

the second being that for some reason hot water systems here eat pumps....2 winters is considered a good run for a grundfos circ pump.....gets a bit expensive not to mention annoying and steam gets me away from having circulator pumps

RobG

Interesting, you must have water quality issues. There are water treatment chemicals to combat that. Another thing to think about is that you will have the same issues with steam so you must take precautions against destroying the boiler with poor water. I don't know your situation but if you are the only person around with steam it may make home resale a problem as the purchasers will have no one who can service the system. Good luck with whatever you choose to do and everyone here is happy to help in any way we can.

Steamhead

RobG said:

With the restrictions placed upon you in NZ why are you not going with a hot water system (sorry Frank but I have to ask)? It just seems that it would be allot more practical in your situation.

Unhippy said:

Two reasons, the first...because i can.....

Same reason I designed and helped build a one-pipe steam system some years ago!

Steamhead

Unhippy said:

The inspector i asked about it is the government official that decides on issueslike this for this area......lol and lets just say that insurance people don't have the influence here that they seem to have in the states.

Had a thought- since hot-water systems run under a certain amount of pressure, if you design a steam system to run and have the safety valve blow at less pressure than hot-water, would he accept that? Because under the right circumstances, a hot-water boiler can explode too.............

Unhippy

Steamhead said:

Had a thought- since hot-water systems run under a certain amount of pressure, if you design a steam system to run and have the safety valve blow at less pressure than hot-water, would he accept that? Because under the right circumstances, a hot-water boiler can explode too.............

i did point that out to him that if i was run positive pressure it would be at 1psi or less compared to the 10 to 15 psi my hotwater system ran at......steam is treated differently due to a few boiler blowouts back in the late 1800's and early 1900s when the laws where made....and " they have worked well up till now so why change them....."

lol looks like i'm going to be making my own control as well...all the temperature switches i can find with a narrow enough hysteresis are mega dollars

Steamhead

Which is why I said "under the right circumstances". HW boilers do explode..... check out the ones in this article from the Hartford Insurance Co.:

https://heatinghelp.com/heating-museum/explosion-reports-from-the-early-1930s/

Steamhead

Which DOES happen. Dan wrote an article about one that did- I think it was called "Death by Dual Valve" but I can't seem to find it now. It overheated, the circ started, the cold return water came in and flashed to steam and it almost knocked the house off its foundation. Fortunately, no one got hurt.

The notion that a hot-water boiler can never explode is just not true. That's why there are limit controls, low-water cutoffs and safety valves on hot-water as well as steam boilers. Relatively few boilers explode these days, but the possibility is always there. Never forget that.

Steamhead

Here are a couple HW boilers that exploded. The second one turned out to not be a gas explosion:

http://www.dli.mn.gov/ccld/BoilerIncidentsWater1.asp

http://www.nj.com/news/index.ssf/2011/09/emergency_crew_responds_to_gas.html

http://www.dli.mn.gov/ccld/BoilerIncidentsDayCare.asp

Unhippy

I built my last boiler (hot water) with twin thermostats on it specifically so that if the primary failed the second one would shut the boiler down until i had sorted out why it had gone over temperature....from memory i had them set for 180f and 190f.....also had a PSV on it set to 20lb with a one-shot thermal switch set inline with its outlet......logic being that if i had a low water issue it would make steam, pop the PRV and the thermal switch/fuse would trip and shut the whole show down.....it worked when as a test i deliberatly drained water off while the boiler was running

jumper

What narrow dead band? Say 210° high limit and 200° or 190° low? Standard hydronic stat can do that.

Unhippy

Ok i know nothing on domestic heating with steam apart from what i have read on here and one or two other places but i have had a bit to do with industrial steam heat of still's, cookers and process stuff.....so i am baseing what i am designing on what i know.....but instead of 150psi i'll have a 'just' subatmospheric pressure (aiming for 1"Hg) and a pumped vacuum condensate reciever (set at 5"hg)....altho my line lengths will be orders of magnitude shorter

The 4"Hg pressure differential allows the condensate to be pushed back to the reciever by the higher pressure in the boiler and regulated by the radiator traps into individual return lines back to the reciever....the condensate is cooled to ambiant temperature before it gets back to the reciever.....the reciever is set to be about half full and has a vacuum pump draw vacuum until the switch trips and has the boiler feed pump drawing from it.....sealed system

The systems i worked with are identical in principle (just bigger) to the mini-tube system that Iron-Fireman came out with in the 50's and Gerry Gill has brought back......small diameter lines and high velocitys.....except in my case the 'small diameter lines' were usually 3/4" at 50psi+ and the returns 1/2" disk trapped with heat exchangers on them for other parts of the process.

One thing you learned real quick that that the steam pressure/reciever pressure ratio had to stay within a very narrow range or the condensate would stop returning to tank due to line resistance....or elevation in one case......and as return line sizes got smaller the ratio range got narrower.

Hence why i am after something that will control boiler pressure within a tight range realitive to reciever pressure.

A 5f temp range (210f-205f) gives me a 2.8"Hg range......i think that with the small pipe sizes of the mini-tube that variation would be pushing my luck....altho the copper tubes lower flow resistance compared to steel steam line will help

Pumpguy

To get back to your original question, Dwyer does make 0-30" Hg. Mercury type vacuum switches. Is Mercury allowable in NZ?

The Dwyer 31-2-2 is their Normally open switch. The Dwyer 31-3-2 is their Normally Closed switch.

The minimum range of these Dwyer Mercoid Mercury type vacuum switches is around 3-4" Hg. vacuum.

PMJ

I don't know anything about the rules in NZ but logically simply controlling a steam system to run sub atmospheric wouldn't eliminate the danger of a runaway boiler in a failed controls situation. Essentially that is the reason for all steam system inspections - a professional verifies that the controls and safety devices are in place and working. Maybe in NZ your sub atmospheric idea would get you out of an inspection but logically it shouldn't if inspections are required for residential steam at all.

Unhippy

Logic and rules are often found pointing in different directions around here.....usual story when the politicos making the decisions have no idea what they are making rules about and the people advising them are pushing their own agenda and protecting their own little fiefdom's.

But yeah the failed controls scenario can happen.....after a little thought i had designed in a 6" flanged port with a lightweight hinged lid that seals on the flange with a silicone seal.....vacuum holds it closed normally, if there is any pressure over atmospheric it will lift off the seal and allow the pressure to escape.....i also found pair of non resetting pressure relief valves, one set for 2psi and the other for 3 psi as backups....mainly just to stop anyone that might inspect it from totally hitting the roof.

I did get told by the inspector yesterday about one glaring anomally in the "All Steam stuff must be certified" mantra......hot waterblasters are exempt from any initial and/or annual inspection's.....my one at work is able to heat the water up to 140C and is 2500psi.... its literally a steam-cleaner and doesn't need any inspection and had been right under my nose the whole time.....finding that out has set the cogs turning.....i almost feel the worlds most 'badly' designed "steam-cleaner" coming on

izhadano

Hello, Unhippy.
What's the status of your system now? Run into this blog (with long delay, - sorry). Still interested in vacuum heating?
I may help - please check
https://www.pmmag.com/articles/97633-a-new-look-at-vacuum-heating
https://forum.heatinghelp.com/discussion/161394/vacuum-boost-retrofit-for-1pipe-and-2-pipe-steam-systems#latest.
Igor
my email izhadano@gmail.com