Tell Tale Gauge Typical Installation 2

[ms323]

Does anyone have some specific guidelines or rules of thumb for installtion of a tell tale gauge between a Rupture Disk and PSV?

I am aware that they are required by code. I am also aware that each of the rupture disk manufacturers will try to sell you their own version of a Tell-Tale assembly. But, I have seen several of these things, and they are not special, just simply a nipple, Tee, Pressure Gauge, and isolation/bleed valve. Sometimes there will be an excess-flow vent, but this is not always included, and they always have a nice logo and shiny paint on them. But, I see no reason why these parts could not be individually bought and put together in-house as part of a potential cost savings effort.

I think that the API code is (intentionally) very vague, to give the end user some flexibility as to the installation of these pressure sensing devices. However, my concern is that, without some sort of 'standard installation guide,' not only will each of these assemblies look different, but that there could be some potential hazards associated with a sloppy installation. If, as an extreme example, someone were to put a 5 foot piece pipe, with a large-faced pressure gauge, and 4 stop valves, I would be concerned that the weight of this entire assembly would be too much for a small NPT connection to handle the stress, especially if there is any vibration on the system.

I've done some web searching for allowable stresses on small-diameter pipe, but all that I have been able to find is rules of thumb which state that you shouldn't have more than a 6' run of 3/4" pipe without multiple pipe supports. This is a slightly different situation.

Is this even a valid concern? Or is small diamater pipe actually strong enough to hold a large amount of weight? I am not a mechanical engineer, so I don't know what 'allowable' stresses on such a small piece of pipe would be. But, my gut feeling is that there is some weight limit and that it could be small enough that a giant vibrating pressure gauge would possibly be able to exceed it. Any thoughts? Has anyone seen any kind of failure like this in the field? Or am I being overly concerned?

Thanks in advance for the help.

/XLS

 

[don1980]

That's a very valid concern, and that's the reason I never use that port. Instead I locate the gauge or pressure transmitter on a pipe spool between the rupture disk and the PSV.

 

[Latexman]

Sounds like a good opportunity for a company engineering standard.

Good luck,
Latexman

 

[moltenmetal]

A gauge not looked at offers no safety whatsoever. A pinhole leak through the disk will prevent the disk from rupturing, rendering your relief system useless. To me, that's a critical potential failure which warrants instrumentation at minimum, or even better, eliminate the failure risk by venting leakage rather than hoping you detect it in time.

Install a check valve with a predetermined crack pressure which is a small fraction of the disk burst differential pressure- unless the disk and valve are gigantic, even 1/4" tubing size is OK for this, so you can install this on a tee on the tell-tale port on the discharge of the rupture disk, with the check valve discharging into the relief valve's tailpipe. You also need to install a pressure gauge, pressure switch or transmitter to sense pressures up to the cracking pressure of that check valve. As to how to do this, you have to judge whether you do this with tubing or pipe. Tubing makes installation and maintenance easy so it has my vote, again unless the disk and valve are huge.

 

[nmiller127]

MS323,

I would agree with moltenmetal's statement. I have long wondered what the purpose of putting a gauge with an open end would help prevent? Most of the systems I have seen are not set up to detect leakage through the rupture disk. They are only set up to show when a rupture disk has failed. I feel that putting a gauge with a check valve with a higher spring cracking pressure (but much less than the PSV and rupture disk setting) would allow one to detect leakage through the rupture disk. Then what would happen when the rupture disk leaks, but does not fail, the gauge will read up to the spring cracking pressure of the valve (let's say 10 psi) and bounce around that 10 psi reading as the check valve is bleeding off the leakage. One could even put in a pressure switch, instead of a gauge, in this setup and make the system able to be remotely monitored.

nmiller127

 

[MortenA]

More "tell tail":

When specifying rupture discs (i rarely do that by on some heat exchangers on the cooling media side it happen) i specify two discs in series, a slight overpressure inbetween and a PALL/PAHH in the ESD/SCADA system. Yes thats quite a setup and not cheap - but then again its rarely used.

I know other industries (or places) rupture discs of often used instead of PSV's.

Best regards

Morten

 

[MortenA]

oops should have been tell tale

Best regards

Morten