Fluid pressure gauges -- how do they always face the right way? [Non-electrical]

[rs20]

Using my yearly non-electronics-related post quota*, as I think this is a reasonably interesting question, it has to do with pipes (not the internet), and someone on this forum knows the answer.

Suppose you have a pressure gauge with a standard 1/4" BSP or NPT thread. You screw it into the female BSP/NPT receptacle that you have, and tighten it up nice and tight. Here's my question: if you tighten it up until it feels nice and tight, won't the gauge be facing in an arbitrary direction? How do the people who set up industrial equipment always get the gauges facing the right way?

Is there really 360 degrees worth of rotation range that still gives an acceptable seal? Or do all pressure gauges have the threads starting off at the same point? Or do they see that it's at the wrong angle, take it off, and stuff another few layers of teflon tape on?

I'm sure this question has an easy answer known by almost anyone in the trade, but I can't for the life of me find it by Googling.

* No, there is no such thing 

[fubar.gr]

I don't have experience with high pressure piping, but I do have some plumbing experience.

The answer is this:

Is there really 360 degrees worth of rotation range that still gives an acceptable seal?

Threads are sealed with natural hemp fibers or (nowadays) teflon tape or other adhesives. This ensures a good seal over a wide rotation range.

[Gyro]

You get some leeway (on those fittings) between where you feel it start to tighten and when it's too tight.

As the old adage goes: 'Tighten it up until the threads just strip and then back off half a turn'. 

[Tomorokoshi]

Here is the technique I use:

1. Make sure the threads are clean on both the gage and the fitting.

2. Coat the threads of the gage with teflon paste. Work it down into the threads. Don't overdo it.

3. Wrap the teflon paste with teflon tape. About 2 wraps.

4. Start the thread by hand.

5. Torque it in with a wrench on the square block of the gage and another wrench on the fitting. Don't stress the other lines.

6. As you torque it in get a sense for the feel of it. The force required of course goes up as it turns in. Decide if you can make it or not.

7. Don't back off if you go too far. Bring it around, replace the other fitting, exchange it, etc.

8. If you have several fittings in "series", for instance by reducing the size, placing the direction is much easier.

9. Low-pressure, let's say to 200 PSI, is fairly easy. Be extra careful with high-pressure.

Like soldering, it might take a few times before you get a sense for the feel of it.

[BradC]

You get some leeway (on those fittings) between where you feel it start to tighten and when it's too tight.

Yeah, this. Taper threads deform as they are appropriately tightened, and how much they deform is entirely up to your arm strength (or the appropriate torque setting if you are doing something critical). Personally I use Loctite 567 pipe sealant and turn it finger tight, then however much is required to get the alignment right with whatever sized wrench is required to get it around that far. If it's a particularly valuable gauge and there is any possibility of me ever wanting to remove it in the future, I put it into a sacrificial brass fitting with 24hr Araldite and nip it up tight, but not so tight as to deform the threads. Let the glue to provide the seal, then I just tighten the brass fitting into whatever I want to attach the gauge to. When I want to remove the gauge, I heat the fitting in boiling water to soften the Araldite, take it off and throw it in the scrap bin. That only works if your media is compatible with the adhesive. Air, gas or water are fine.

[rs20]

Wonderful, thanks folks! Thanks to your advice, my little arrangement of Tees, ball valves and gauges* has gone together with everything facing the way I envisioned.



Regarding thread deformation; does that mean that these fittings have a very finite life? For the record, the medium is a vacuum (/air).

* Gauge hasn't actually arrived yet, end cap piece substituted for now.

EDIT: Fixed broken image link.

[sarepairman2]

get swagelok!!!

you use a NPT to swagelok adapter.



npt is evil bullshit!! swagelok has a go-no go gauge so you know exactly when its tightened properly. and you can choose how the parts will be oriented BEFORE you tighten the fittings.

for NPT you want to use either a appropriate loktite or three layers of teflon tape. for swagelok you don't need jack shit, its good for at least like 1000 PSI in high diameters.

[Shadetreeprops]

, take it off, and stuff another few layers of teflon tape on?


* No, there is no such thing 

i did aprentice plumbing till i discovered i did not like getting human fecal matter on me. Which caused involintary upchucking, and the loss of a many lunches. So i took up truck driving...and now i see that EE is alot like plumbing with electrons, and no human fecial matter to cost me a lunch..


but teflon tape was what i used to make sure pressure gagues were A on with a tight seal, and B made sure the gauge faced the correct way. or i would use, the blue seal stuff get it tight but not too tight facing the right way, and wait for the blue seal to cure up before applying pressure.

had to use to a few times with some water filtration systems that would go POP if they were exceeeded pressure. and the area was notorious for water pressure fluxuations. so if the gague when above safe lvl, it would kick a valve off to shut off water flow.

[CatalinaWOW]

The rules have changed over the years.  Modern brass fittings are made with little or no lead content for fear that lead will make us all stupid or dead.  As a result the friction levels have gone up, and more importantly the fittings are very brittle.  While you used to be able to get a good seal with the desired orientation, it is now common to split a fitting when attempting this.  There are many solutions to this.  All of those mentioned above work to some extent, all have their own advantages and disadvantages.

[SeanB]

Yes NPT has a definite limit on the number of times you can use a fitting, as each time you are both removing metal from the threads by friction and are stretching the thread as well. Parallel threads however rely on either a rubber or nitrile oring to provide the seal, either in a recess for higher pressure or just between the end faces for low, or use a crush washer to provide a deformable seal that is the leakage stop, not the threads which only maintain the pressure on the seal.

You might get 100 cycles at low pressure, but high pressure ones like used in hydraulic fittings often only are rated for under 5 cycles, being designed to be used a single time and often with a thread lock to reduce leakage, which also means it will break before coming loose.

[ndunnett]

I'm an E&I technician, so this sort of stuff is my bread and butter. It's generally something you just feel out, in most cases you can torque it up reasonably well while having it face the right direction. Tapered threads with sealant are pretty forgiving, at least at the pressures we work with (up to 1000 kPa). If you can't get it quite right the first time, you can vary the amount of thread tape you use to adjust the position it tightens to. It looks like you've figured it out anyway.

With regards to Swagelok, whilst it is awesome gear it is also very expensive, so for home projects I wouldn't bother unless you don't have to pay for it or you have no other options.

[CatalinaWOW]

Swagelok is awesome.  But also not idiot proof. I have watched this demonstrated often over the years.