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baffling pressure readings after new gauge tree

Binnacle Member Posts: 126
I'm preparing for a new Vaporstat which will replace the existing Pressuretrol.  Was curious about the accuracy of the old gauge and so put in the new tree and gauges with the old Pressuretrol for comparison.  They say curiosity killed the cat.

In theory the new piping should behave the same, but strangely it does not and I am baffled.  Hoping someone can see what I'm missing.

Two differences:

1) Formerly the Pressuretrol would cut-out at 2psi and cut-in at 1psi per the old gauge.  The Pressuretrol would always break the circuit about one minute before the thermostat was satisfied.  Now the Pressuretrol is *never* satisfied and even from a dead-cold start pressure never quite makes it to 2psi per the new gauges.  Set to minimum pressure and differential.  On normal cycles pressure appears to max at around 10 ounces, way below where it apparently was.  No leaks in the new tree and no changes anywhere else.

2) When the boiler cools down the low-pressure gauge shows something like three ounces of vacuum.  I'm not getting how this happens, though I do understand the LWCO is not the best place to connect pressure sensors and am planning to move it to the top of the water level gauge.  I am 100% sure the vacuum is real as both gauges register it and every time I break the seal on the union to the Pressuretrol the low pressure gauge zips straight back to a perfect zero.  As soon as I seal the union the pressure goes slowly back to the same three ounces of vacuum.  Seems bizarre.

If anyone has insight into what's happening, please chime in.


My belief is that in both the before and after scenarios, the pressure gages are giving correct (or in the case of the old gauge reasonably correct) readings.  It seems to me something about the piping is distorting pressure.  The system sounds and behaves the same and I think that the pressure in the pipes is closer to what the old gauge tree indicated when it was in place.

A Ray Porous snubber is installed just below the cross.

The new pigtail was filled with water.

The old piping had two elbows and a horizontal section just above the top of the LWCO, the new piping goes straight up from the pressure port.

The old piping had a 3/16 inch orifice on the elbow that attached to the LWCO port.  The new piping has something more like a 5/16 inch inside-diameter.

Recently finished replacing old Dole 1A vents with Maid-O-Mist Jacobus vents (these are temporary, Gorton's later).  I did verify that the Pressuretrol was ending the cycle since that happened but perhaps the gradual closing of the MoMs has changed the character of back-pressure in the system.  The Dole 1As close and open absolutely, in binary fashion.  On the other hand one radiator vent was never closing and was constantly blowing steam--this was corrected with the MoM install.

Recently added a Gorton #2 at the end of the 40' main.  This #2 does not seem to be leaking steam at 2-or-so PSI as some are said to.

After removing the old piping I noticed that the water level in the LWCO is high enough to flow out the 1/4 inch left-side port when the boiler is filled so the level gauge shows 1/3 or so.  Made a point of lightly flushing away the bits of rust in the threads by filling the boiler enough to cause constant flow and gently scraping.

In light of the previous paragraph, I ran a cycle with the boiler water level low enough to assure that water should have drained out of the bottom of the pigtail and that vapor or air should have been present in the bottom of the pigtail.  This made no difference whatsoever to the pressure readings.


  • Binnacle
    Binnacle Member Posts: 126
    edited December 2013

    I noticed that it appears the new copper nipple projects about 1/8 inch further into the LWCO threaded port than the old steel elbow--perhaps 1/4 inch total.  Still looks like the nipple does not project inside the housing but I'm not absolutely certain.

    Doesn't seem like it should matter but I'm trying to include every detail in case someone has seen something like this issue.
  • Binnacle
    Binnacle Member Posts: 126
    edited December 2013
    snubber was bad idea

    On a guess I removed the snubber and now the new gauge tree is behaving the same as the old gauge tree.  Everything is working as before and apparently the old gauge was fairly accurate since the Pressuretrol does in fact cut-out at two PSI.

    The snubber was slowing down the pressure reading so much that the gauge tree was, by the end of each steam cycle, 1.5 PSI behind the actual pressure in the boiler.

    The 3oz of vacuum is a previously unmeasured effect of the boiler cooling phase on the LWCO.  The snubber was maintaining the vacuum in the pigtail when the Pressuretrol union was opened.  Now with the snubber gone simply cracking the LWCO flush valve clears the vacuum.

    The new low-pressure gauge demonstrates about +/- 1/2 oz of oscillation from boiling water pulses, but that doesn't appear to be a problem.

    So I would have to say using a snubber, at least a porous bronze snubber, is a bad idea for boiler pressure control tree.  After some reading and pondering, it seems to me that a snubber is designed to service a single gauge or control and so is expected to pass a small fraction of the one milliliter or less of air or vapor found in the gauge or control cavity.  The cross nipples and fittings in the upper part of the gauge tree must hold 20 or even 100 times the volume as a gauge does and so a snubber won't work in the position below the cross-fitting.

    If one must use snubbers, separate ones for the low-pressure gauge and the Vaporstat should probably be installed even though this could lead to pressure divergence.

    Also I observed that the porous bronze snubber already appeared to be clogging with rusty sediment after just one day, and this was at the top of eleven inches of brand-new nipple and pigtail tubing that had been cleaned with acetone just before installation.  So a self-cleaning piston-type snubber would be correct choice for a boiler pressure control if one is forced into using them.

    At this point I don't see sufficient benefit to justify it.  A little gauge jitter is not a big deal.
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