Steam pressure question

it needs to be "internal syphon", that protects it from steam. Or you can just plug it and use a tee or cross and put it with your other gauge.

Or better yet plug it with a short nipple and a cap, that will be easier to get out if you ever want to use that tapping for something else.

You still need a 0 to 30 psi gauge for code and insurance purposes. The gauge must have at least twice the range of the setting of the pressure relief (safety) valve, which for a low pressure steam boiler is 15 psig by code.

Yes, I'll quote myself:

They all fail fast and you don't need them except apparently for code purposes

Failure usually has to do with it getting tweaked, if you bend the case while levering on something or drop it you are very likely to throw it far out of calibration or just plain get it stuck at some reading.

Nope, they die all on their own very fast

A few years ago I looked into the design of pressure gauges with low pressure ones in mind. Manufacturers stated different pressure capabilities, but it was somewhat common that gauges are designed to withstand 130% of the full scale (6.5 psig for a 0-5 psig gauge).

In the discussion about low pressure range gauges one option is to have both gauges installed, but to be able to isolate the low pressure gauge so that if for some reason the pressure does exceed the 130%, the gauge would not be damaged or rupture. Keep it isolated as mentioned previously and only have it in service when you are present and need to know the reading.

Chris J has lots of comments over the years of using a Magnehelic low pressure gauge that can withstand the 30 psig pressure.

If there is no 30 psig and something goes wrong, your insurance company and/or lawyers might have an issue with covering damage (if they even know the code).

Chris J has lots of comments over the years of using a Magnehelic low pressure gauge that can withstand the 30 psig pressure. 

If there is no 30 psig and something goes wrong, your insurance company and/or lawyers might have an issue with covering damage (if they even know the code).

No one has ever said remove the 30psi gauge and replace it with a low pressure gauge

Interesting. If the pigtail actually holds enough water to create a seal, then the void space between the seal and gauge would probably contain moisture (i.e., humidity) at the saturation level at the temperature and pressure at the time it was filled. If the overall temperature changes enough and goes below the temperature at which the pigtail was filled, it could go to vacuum.

If anyone has nothing to do, they could try a test, using water, then use a liquid that won't evaporate and run the test again to see if there is any measurable difference. Or install an oil filled manometer in parallel with the small gauge at the boiler pigtail.

A bench test might be easier than an in-place test.

@dabrakeman Any idea how much of a vacuum you get?

Just shut the isolation valve to the LP gauge when the boiler is off.

Or if you really want to be nerdy put a tee between the gauge and its isolation valve put a 1/4 ips x 1/4 flare fitting with a shrader port in the flare. Close the isolation and bleed the shrader.

That's a fascinating thought about the water vapour in the space between the gauge and the pigtail condensing and causing a vacuum. It's also wrong, unless the pigtail is closed off from the boiler. If any vacuum were to be created momentarily, it would just raise the level of the water seal on the gauge side relative to the boiler side with no change in pressure once the levels had adjusted.

As others mentioned, if you really want to see a better view of the normal operating pressure and your boiler is not the typical oversized for the attached EDR, put on a low pressure Dwyer Magnahelic Gage. Many new old stock on eBay. You will have to determine which range is best for your system there are many pressure ranges and center zero versions to choose from. The downside is the scale is usually in Inches of Water Column not PSIG. My system never goes over 2 Inches of Water Column.

Since the Gage can tolerate up to 15 PSIG I don't bother with a shut off valve. As an example in the picture below the Gage goes to 30 Inches of Water, which is just over 1 PSIG. 27.68 Inches of Water Column equals 1 PSIG (Red arrow on Gage scale).

This valve may be a code violation. For just the reason shown, the valve appears closed in the picture rendering the gauge that is there for safety purposes useless. You may know to open the valve, but others may not.

The 0-30 PSIG Gauge should have a Internal Syphon. The Internal Syphon acts like a miniature internal version of an external pigtail or water trap. Another method that may work to protect the Gauge is have a tall vertical nipple (probably greater than 12") to trap a buffer of air between the steam and the Gauge.

Does this Gauge have an Internal Syphon ?

@Jamie Hall If the space between the gauge contained 212F steam and cooled to 70F, what would happen to the water level, given steady state on the boiler side of the pigtail? As we all have discussed many times, steam vs. water specific volumes are significantly different….saturated steam at 212F occupies 26.799 ft3/#, water at 212F occupies 0.016719 ft3/#, and water at 70F (data for 68.939F) occupies 0.016048 ft3/#. A factor of ~1700 in round numbers.

The water level would rise, but since the volume of the piping and gauge are so relatively small, it nevertheless might not be detectable. But it does exist.

The dynamics on the boiler side have bearing as well.

Now this all gets wiped away to some extent, because it is unlikely the gauge side of the pigtail sees 212F steam. The initial conditions on that side makes all the difference. If there is no "steam", just air and a minimal amount of moisture in the water, all bets are off and the effect is less. Just for the heck of it, the specific volume of dry air at 212F is 16.93ft3/# and 70F dry air is 13.35 ft3/#. So as the air cools down it would occupy about 3/4 of the space. These numbers are based on 14.7psia atmospheric pressure.

Anybody got a headache?

Have a great holiday weekend and thank you for your service to all those who have served in the military.

The water level would rise, as you say, @SteamingatMohawk — but only far enough so that the height difference was enough to counteract the pressure difference.

And that woul only be if there were steam in the space between the pigtail and the gauge. If it is just condensing water vapour from the air in there — which is all that there should be in there — the air volume change will be very small.

The point is that depending on how much air, steam and water vapor is in the space determines the amount of "shrink", I think. Maybe the easiest way is to use a clear manometer instead with the gauge on the vent side and the boiler on the other.

@mattmia2 Even better use some boiling water. Then you get the shrinkage I mentioned.

@Pumpguy In the range we are talking about 1 inch of water is .07+ inches of mercury. On a 30 inc scale that's pretty small. But it would be interesting to actually see the real vacuum level.

Yes, that's a different situation you suggest. The check valve behavior of the isolation valve is mysterious. Assuming on the cooldown a vacuum is created as the steam condenses in the piping, etc. there would be some amount of vacuum, which would eventually go away as long as there is an open path to air via the vents.

I just find it interesting to noodle on what happens to the gauge side of the pigtail if the shut valve is actually tight.