Minor Gasket Leakage During Cooldowns 2

[vpl]

This may qualify for stupid question of the year, but...

Does anyone know how cooling down a system (dropping both temperature and pressure) affects leakage through gaskets as compared to full power operation? My practical experience is that when a system is heating up, minor gasket leakage tends to disappear and generally does not reappear. However, I haven't been able to find anything about minor leakage that shows up during shutdowns - if that leakage would be there during full power operations. This is on a dual gasketed pump where it's not possible to check for leakage while at power. I've done the Google search, and read through all the information I have on hand, so I'd appreciate any leads.

Thanks much

Patricia Lougheed

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[saxon]

VPL, Familiar with thermal coefficients of expansion? It explains why systems leak on cool down. The problem is normally tied to improperly torqued flange bolts at the lower temps.

Hope this Helps.
saxon

 

[saxon]

vpl, Oh yeah! BTW, if the seal design happens to be self energizing, like delta rings, system pressure drop can also account for leakage.

saxon

 

[Metalguy]

If correctly-sized spiral-wound gaskets, esp. "Flexicarb" or equiv. are installed correctly against flange faces which do not have radial scratches, etc, they do not leak hot or cold.

 

[pipeman]

As it was mentioned, thermal expansion could be a problem. One of the things to look is material used for FLG and Studs (as SS studs versus CS FLG or other way around). That is very problematic on "cold boxes" ( Al heat exchangers). Operators told me that it took them 2 shuts downs to stop leakage withhout bolt tensioning after startup.

 

[aybee]

the initial bolt load must allow for thermal expansion and relaxation in service so they need to be higher than the minimum design stress. also they MUST be tightened tp a controlled procedure using Star type pattern or Hydratight ot torque controlled hydraulic tools, dont leave it to ol' Joe to just pullem up..needs done correctly. lubricate the nut and the stud, put a washer under the nut, make sure the studs are clean and not all dinged or corroded . if they are replace them

 

[tothepoint]

If you're using any type of sheet gasket, the thermal expansion of the pump casing and flanges will add additional compressive forces to the gasket, and compress it further. The load remains on the gasket while the system is hot so there is no leakage. Cool the system to ambient and the gasket is now thinner than when it was initially compressed. Depending upon the degree of bolt stretch obtained during assembly, there may (and usually is) insufficient bolt load remaining to maintain a seal. This situation is very common with steam systems. Once they're operating and then cycled to ambient, sheet type gaskets often leak during subsequent start-up.
If you use a "soft seating" spiral wound gasket, and install it correctly so the flanges are squeezing against the outer guide ring, you have a rigid metal to metal joint. The thermal expansion of the system is taken up elsewhere- not by the gasket. Voila! the gaskets stay tight during operation, cool down and restart. Assembly torque values can be very misleading, so unless you know exactly what your friction factor is, you may have difficulty knowing if you have compressed the gasket all the way down to the guide ring. Take a look at a used spiral wound gasket. If it was installed corretly, you should see marks from the flange raised face biting into the guide ring all the way around it on both sides.
There are BIG differences between spiral wound gaskets. Make sure you specify and receive a low stress design.