Hot Tap Floating Roof Tank 1

[quantexhung15]

Is there any requirements or precautions change on hot-tapping a floating roof tank versus fixed roof tank? I did not see any special notes in API-653 Par. 9.14 for Hot Tap. Please share your experiences. Thank you.

 

[Duwe6]

Massive change! With a roof, there cannot be the required 2 to 3 feet of liquid above the welding. If at all possible, DON"T DO IT 'wet'. Wait until the tank can be emptied, and the roof is on its landing legs. Then purge the space between the roof and floor with nitrogen or argon until the 'sniff' of the atmosphere gives a reading of 0.0% Oxygen. THAT will be safe. With the roof floating, there is no way to ensure that the weld area stays 'wet' until the weld has fully cooled. Nor is there a way to ensure that there will not be a film of air below the weld area during the addition of the nozzle. Not safe.

 

[quantexhung15]

Duwe6 - thanks for the BIG warning here. Can you please advise why with a floating roof the 3 feet liquid level above hot-tap location cannot be achieved? If OPS can control the roof level way higher than hot-tap location is there still a concern of trapping gas?

 

[Duwe6]

How would you achieve liquid several feet higher than the floater? I don't see how, but if it could be achieved, it would allow the vaporized gas to be carried off and quenched. That amount of 'cover' has been an API tank hot-tap rule-of-thumb for at least 50-years, so it indeed is OK.

 

[quantexhung15]

I should have made my question more clear. Sorry for the misleading.

For my case the hot-tap location will be at the bottom shell course not on the roof. Hot-tapping on the deck of floating roof is a No-No. This is clearly stated in API-RP 2201. In this case will the weight of floating roof raise any concern?

 

[valleytank]

If you are planning to do hot-work on the bottom course the product will have to be emptied regardless and all you will be left with is vapour underneath the floating roof which is sitting around that first horizontal seam typically. At that point I can't see anyone giving you the okay to perform hot work on it without it being water-washed, and made gas free first. At that point you might as well weld it in right with regards to your planned location and IFR seal height.

 

[Duwe6]

Gotta disagree with valleytank, with 3+ feet of cold product above the welding area, all vapor will be condensed & collapsed prior to reaching the roof membrane. Think about trickling steam into the bottom of a barrel of cold water.

Depending on the Specific Gravity of the tank's contents, there will be about 1/3rd to 1/2 psi of pressure per foot of liquid above the weld. A standard floater is made of 3/16" plate, at about 0.85 ounces per square inch. So with 4-ft of liquid and a floater, the pressure at the weld will be less than 5 psi. On piping, I don't start sweating until the internal pressure exceeds 150 psi. Tanks are just really big, really thin pipe.

UT for metal thickness and soundness [no laminations], triple-verify 3-ft of liquid above the top of the weld, and turn the welder loose. And I'm willing to stand beside that welder.