Hello Forum members,

Consider a hot reheat line that is 37.50 inches O. D. with a wall thickness of 1.906 inches. The line operates at 565 psig and 1005 degrees F. The material is A335 P22. The line has been in service continuously since 1992. In 1998 the staff at the Utility's Generating Station noticed that the constant support hangers in the basement (one level below the turbine operating floor)that support this hot reheat line were all sagging and at the bottom of their travel. The "fix" that the Utility's staff came up with was to add more supports. My contention is that the line is no heavier now than when it was first installed and what is being seen is the irreversible effects of thermal creep.

I am inclined to think that the line should have random metallurgical inspections of the grain structure (by replication) to ascertain what the degree of creep progression in the metal. These inspections can be at planned outages every year. If it is determined that there is still 150,000 to 200,000 hours before advanced creep damage sets in, then continuing to add supports to the reheat pipe might be a solution until advanced stages of creep damage are spotted. When advanced stages of creep damaged are picked up by metallurgical micrography then you replace the pipe.

Does any of this make sense and is this a prudent approach ?

I would apprciate this forum's comments


Thanks


Pat LaPointe

Hi Pat,

Is this seam welded piping?

Some thoughts: Sixteen years is a bit early for that degree of creep elongation. Investigate the rest of the hangers. Before (steam flow direction) the HR piping comes to the horizontal run you are describing it likely comes down vertically from the reheat outlet header. At some elevation on that vertical riser there should be a rigid support (likely a trapeze). Strip some insulation and look that rigid support over closely for damage or distortion. If there is no rigid support on that vertical riser (“full floating system”) you will have to look at all the rest of the HR spring hangers. The hangers on the horizontal run might be badly affected by heat (springs “relaxed”) and perhaps they are no longer providing the proper amount of reactive force (can you test one or two with a dynometer?).

I believe your thoughts about “this pipe has NOT gotten heavier” are correct. If you add spring hangers, you will further upset the balance unless you can quantify the current amount of supporting reaction the old hangers are providing and supplement that with spring hangers that are well chosen to provide the SAME support reaction as was there originally. Be sure to check the spring hangers at the HR inlet (to the turbine) valves to see if they have also “bottomed out”. If only one spring hanger has failed, the entire balance would be upset. Any chance that a stop valve was added after original construction? Could here have been a new branch connected?

One outside possibility is that the top of the horizontal HR pipe somehow was deluged by “cold” water at some point and is now “bowed” due to plastic deformation due to differential expansion (top to bottom). Or conversely, a puddle of condensate formed on the inside of the pipe on shutdown and bowed the pipe (in the other direction) before the "unwetted" upper part of the pipe could cool.

Regards, John.

Dear Pat,

My hunch is that the constant spring hangers had been wrongly selected at the start, as just in six years in a power plant for P22 piping to creep so much at 540 deg C is unusual. With the fix of additional support ( must be constant or soft variable spring type) if it is working fine, you can just revise the piping stress calc with this as-built condition & revise turbine stability calc/load comliance & document the change for future reference.
regards,

sam

Normally that section of the line is a long flat section. The line is very difficult to control. Hysteresis of the hangers, poor selection, poor drainage etc result in this sagging. Our approach is normally to do level surveys, hanger readings, engineers walk throughs, NDT, replication, rerun stress analysis, take some of the hangers for testing and evaluate. Normal solutions include total realignment of the line, rotation of the pipes, replaement of hangers, recalibration of hangers, addition of drains, stabilization of the system by adding rigids and variable springs. Control hangers have been tried also. Expensive and requires maintenance. With a quick thinking I cannot remember a case where we would have added hangers.

These P22 thick lines are much more difficult to get right than X20 (of that time). Your hangers are most probably high hysteresis type. Possibly in wrong setting in addition. Do you know the actual wall thicknesses? I doubt.

My bet is:
- you have some creep damage
- your drainage is poor to non existing
- hanger hysteresis is over 10% (New should have 2% used 4% or better)
- some of the hangers may have failed if no maintenance has been done, broken springs, seased bearings, broken straps, relaxed springs, rusty sliding surfaces, ash etc on cams...
- stress calc says that long flat section should move down and up (which it doesn't do)
- you pipes are deformed
- if hangers were added then loads may be out. Just check if ancillary masses were added. In situ adjustment without load cells doesn't work.

You do not do random metallurgical evaluation. Locations are selected very carefully or you have to have plenty time and money.

If your system has no rigid hanger between the boiler and the turbine then as far as I am concerned the system is wrongly designed and will never work.

We have done this type of work some 25 years. Orders accepted.

"the line is no heavier now than when it was installed"

Yes, beyond a doubt. And it's a HR line, so even if the insulation got waterlogged it will dry out.

But the question you need to answer is, "Is the line heaveier than the line that was analyzed?" Mill tolerances on the wall of seam welded pipe is pretty tight compared to extruded pipe (see the applicable ASTM spec, and the plate spec it references for details), but even 1/16" on your pipe is 3%. If there is no vertical control point in the riser, or if the control point has failed, this is more than enough to cause a bunch of constants to bottom out.

Also, adding supports without a lot of analysis could raise hell with your nozzle loads. You'll need to do a lot of analysis to consider what has happened to the HR nozzles on the turbine, and what might then happen if you attempt to return the HR line to its design elevations.

Finally, you need to check to make sure that the sag has not created a low point within the system, else the potential puddling problem noted above by John Breen could have bowed the line.

All in all, a rather fine mess. Good luck!

We ran across a HR system that experienced such sag. Turns out the original analysis had two "oversights" that allowed the creep sag - 1) the boiler connections were modeled as rigid restraints (with imposed displacement where necessary) & 2) wall thickness was not updated for bent pipe. These two errors predicted lower loads down at the bottom and the constant effort supports down there were undersized. In the beginning the pipe (and turbine?) carried the load but creep eventually allowed the supports to bottom out. Number 1) made the system too stiff at the top so the top of the model carried too much deadweight load. Number 2) ignored the added weight of the heavier pipe.

Tony Paulin has shown (In his recent Webinar) the simple fact of the matter is that ignoring nozzle flexibility is not "conservative" as some in the industry feel (such as the power people). It often times simply presents an incorrect load distribution.

I Encourage all of you to sign up for his webinars!