Hot Tapping Sulfuric Acid Lines?

[hokie96]

I'm looking at a temporary project option to hot tap "fresh" 98 wt% sulfuric acid lines (material 316ss). Everything I've found in industry literature (API 2201, internal corporate guidelines, etc.) lists sulfuric acid service as a service NOT RECOMMENDED for hot tapping due to hazards associated with acid decomposition at welding temperatures. Discussions with Hot Tap Contractors revealed that hot tapping was feasible, although the on-stream welding required is outside their scope of work. Would the thermal decomposition of the acid cause process problems (I expect the amount of acid affected to be very small) or am I missing another consequence of this thermal decomposition? Could the decomposition issue be associated with purity requirements for chemical plants?

I am also considering the use of mechanical (bolted) hot tap fittings to avoid the welding (if necessary), although I've had some poor experiences with the required sealants in acid service.

If you've had experience hot tapping such acid lines, what additional precautions or procedures were used?

Please note: In referenced literature, NOT RECOMMENDED (or "should not be performed&quot is a stronger cautionary category than "requires extra precautions".

Thanks in advance.

 

[tblaser]

hokie96 :

I would agree with the NOT recommended. I have never hot tapped a H2SO4 line before, so I do not have the definitive answer. But, the rate of corrosion will increase due to the higher temperatures from welding -- I would be very carefull. What is the operating pressure of the line?

tim

 

[hokie96]

The line operates at about 30 psig.

I agree the rate of corrosion will increase due to higher temperatures. However, the time at temperature is very short (the duration of welding) and very localized; so while corrosion rates may be temporarily quite high, I would expect measured metal loss to be insignificant to the life of the line. Flow in the line will provide cooling to keep pipe wall temperatures at the inner diameter below temperatures in the heat affected zone.

In any case, great care should be taken before proceeding.

 

[PAN]

I never check the reference standard for this application. However, in my company's specification, hot tapping is not allowed in stainless steel pipe.

 

[hacksaw]

Don't do it, the plants should be shutdown for that sort of work. Your piping materials are only rated for a certain temperature range while in contact with acid. With welding you create an enormous problem for the welder and the adjacent equipment.