Hi,
I need suggestion from forum member on a general problem. I have a piping system with normal operating temperature is ~180 Deg. C , Design ~260 deg. C and excursion temperature is 375 degree C. Considering cost of the piping I have designed piping with maximum axial displacement as ~350mm in excursion temperature case. Code stresses are with allowable range, no interference with other piping or structure and longer shoe to take care of axial displacement.
Many companies have thumb rule to restrict axial movement to maximum 6”.
My question is if we qualify the entire code requirement and take care of other criteria, Can we have axial displacement (near loop) more than 350 mm specially for excursion case which may happen for very short duration in entire life of plant? I have earlier designed piping loop to absorb more than 800 mm expansion (400 mm on both side) and system is working fine for years.
Do we have to follow thumb rule irrespective of case (excursion / upset), size of piping(large bore above 60”) or material (exotic materials)?

Rules of thumb are guidelines based on experience or common practice, and thus are not hard requirements.

Check areas of high displacement. Does it crash with other obstructions with the displacement?

Is it reasonable to assume that someone won't follow you and use the rack spacing charts to locate a pipe in that same area that's clear today?

I don't care how long or short a duration your upset condition is, if your pipe is trying to move through the space of another pipe, bad things can happen.

have your shoes design for the displacements, no interference?

Also, have you used any hangers? slanting angle for hanger limited to 4 degree.

If you checked things that likely happen, and it's within the criteria, then it's ok.

For power plant, high temperature stainless pipe line such as sampling line could be expanded more than 200mm.

Many company set max. displacement standard only for normal case/design. for your case it can be treated as special case and your company have to accept it. if they insists you to follow their rule of thumb, then they have to add more expansion loops or to use expansion joints. These items will increase their cost and not really justified. it is better for the pipe to move 350mm provided all the interferences have been taken care rather than spending more money for the non-technical reason.