Cutting boiler reheat pipe in cold position 1

[mrgaee]

A reheat pipe has been operating for approx. 30 years between boiler and turbine. It is proposed to cut the pipe at the turbine end to replace a nozzle. Design of restraints is required to maintain the cold pipe position for rewelding.

The initial construction required cold pulling. My question is whether the initial cold pull will have relaxed after this time and whether, in the cold state, the pipe stresses are due only to pipe self weight.

 

[stanweld]

You can never really tell. In the past few years we have made 4 or 5 such cuts. Each one has been different in terms of movement after cutting, from less than an inch to nearly 2-feet. We always design our rigging for complete restraint during the cut. Rigging designs are made by a P.E. after site visit.

 

[davefitz]

It depends if it is a hot reheat pipe of cold reheat pipe. The hot reheater P22 ( or P11) piping would have creep relieved nearly all of the original cold springing plus any other shakedown stresses by now.

Often times there are provided in the supports near the turbine an ability to install pins in the supports , to allow removal of pipeline stress from the turbine flange when undergoing the 7 yr turibne overhaul and casing split.

However, I remember one case ( TVA Bull Run) where they forgot to remove the pins after the restart, and the tremendous pipeline thermal expansion stresses that were generated were creep -relieved by the piping in the form of ratcheting. The pipe baically grew an additional 12 inches, and from that day forward the supports were never quite right. Some creep cracks here and there, as well.

 

[JohnBreen]

Hello mrgaee,

A good question and you have some good answers.

Whatever the cold spring was at the time of installation, it will not be THE SAME now. But there will be some "self springing" added (algebraically) to the cold springing and there is little or no way to know either the magnitude or the direction of the total resulting springing. What I am trying to say is that some of the self springing ("relaxation" or creep) may have "cancelled out" some of the original cold spring. If the original designer was accurate in calculating the total expansion that he wanted to compensate with cold springing, the amount of self springing would be less.

I think it is conservative to say that the total springing that is in the pipe now (either hot reheat or cold reheat) will be less that the original cold spring was. If you perform an analysis at ambient temperature WITH the original cold spring numbers (at each location) you will have usable resulting forces at the restraint points. The best way to design the restraints that you will need (to hold the pipe close to its cold position when you cut it) is to NOT try to restrain any moments - use multiple translational restraints to break the moments into force couples. You are gonna end up with a lot of 10 ton chain hoists ("come alongs") holding the pipe in place.

When in doubt, build it stout. Good luck.

PS. Be very careful about placing travel stops in the vertical riser hangers - with just a little temperature change, you can have much of the weight shift to very few hangers and severely damage them.

Regards, John.

 

[mrgaee]

John:

Take your point on the moments. Since the new restraints are acting on the displaced shape of the pipe, most piping analysis programs (linear elastic and undeformed geometry) may not be able to determine the restraint forces required. How is this done?

 

[RobertACookPE]

in calculating the total displacement at the "cut position" (from cold position of the entire pipeline to operating temperature of the entire pipeline and attached eqipment) remember to account for the movement of the equipment nozzles.

Turbine inlet nozzle I was at a few weeks ago moved 1.125 inches axially and .75" "up" from its cold position to the hot position.

Depending on where your new "cold position" cut in the pipeline will be with respect to the nozzle, the amount of cold spring travel will change because of movement of that nozzle as well. Nozzle movement from cold to hot might increase, or decrease, the amount of cold spring in the pipe.