As I recall, flange bolting is required to hold 1/6th of max pressure per bolt or stud. Re-torquing to standard is used when initial torque is expected to 'settle', as in engine heads. If re-torquing to standard (dry or lubricated, as per bolt size) does not do it for you, you need to replace the gasket or lap the surfaces for un-gasketed contact, (3X boiled linseed oil works well here). Other than normal PPE, safety is not an issue. Just use the regular tightening sequence, with normal torque values.

Carl

Why are you trying to do this? Are the flanges leaking?

It is possible to retorque flanges under load. But then you need to consider what your torque setting should be given the pressure load on the joint. You don't want to overload the bolts as they will yield.

Hi...

I say NO...........Tightening flanges already under pressure is a tricky business. Its proper to take a shut down and retighten the flanges with a new set of gaskets.

If there is no option it's better leave it to the experienced. However at times the experienced fail. On one sunday one of my supervisors tried his best to tighten the leaky flange of a coal tar pipeline. He ended-up in damaging the gasket further due to which the pressurised tar got sprayed over a huge area. We were lucky not to have a major fire accident as well.

Anil Tiwari / New Delhi

It is most unwise. If the system can be depressurised and the joint inspected and replaced prior to tightening up the bolts, then this is by far the better way to go.

Oil and gas installations have strict procedures for flange bolt tightening, BTW.

thanks to all u guys for your valuable comments.

Point taken..the work should be done on dese flanges after de-pressurising only. But since flanges are bein tested by he-Ni test,the flanges are maintained at high pressure (apprx 90 bar). Hence i would like to know till which pressure we can work on these flanges safely and not yeild the bolts.

Hello Freaky Raga,

I understand what you're asking: You'd like to know how much more you can tighten the bolts in case leakage is encountered during the leak test. It's a common concern. However, the answer isn't as easy as I'm sure that you'd like it to be:

The problem with "hot bolting" is that unless the proper steps were initially taken, you have no idea how tight the bolts already are when the leak is encountered. Perhaps the yield point of the bolts is close or has already been exceeded. Maybe you're close or you've already applied more bolt load than either the gasket or the flanges can safely handle. Therefore, any additional bolt load can cause significant damage to either the bolts, the flange or the gasket at 90 bar, 100 bar, 75 bar or even at 2 bar

On the other hand, if you know how tight the bolts are, and if there is reserve capacity, careful application of further bolt load in defined steps and preferably applied simultaneously to a number of studs in the vicinity of the leak may be emplyed in an attempt to stop the leak. While doing this, bolt load must be monitored to ensure that no single bolt is over-tightened. To determine if reserve capacity is available, you'd also have to know what the maximum bolt load is for that particular application. This depends upon the particular gasket, the bolts, and the flange. Although many would wish to do so, one cannot determine this value simply based on bolt material and bolt size.

A warning: People usually say "No problem: I know how tight my bolts are because x ft-lb (or Nm) of torque was applied. I can therefore apply an additional y ft-lb (or, Nm).

Wrong.

Torque is not an indication of bolt tightness. It is only a measurement of the resistance to force felt when turning a nut. This means that a fastener can either be too tight or too loose when tightened to the same torque.

The "proper steps" referred to above include load verification based on elongation measurement. If you've taken "no-load" reference lengths of all fasteners, you can compare these measurements with the values measured at load. This gives you a precise indication of "how tight" the bolts actually are and provides you with a reference point for additional tightening.

If the above steps haven't been taken or cannot be done, bring in the specialized contractor to squirt the magic epoxy goop into the flange, hope that it holds until the next shutdown, clean it out, re-machine the flanges and, start over - this time properly

Good luck!

In case of any tests (at least at our place whether on hydraulic lines in liquid or pressure vessels (gas)) we depressurise, if required tighten and them re-test.

Since you are working in high pressure gas (compressible medium) it is extremely hazardous-much more than liquid medium.