Hi All,
In one of the petrochemical plant we have some issue with Furnace piping. The outlet manifold header of the furnace has yielded laterally and has permanently deformed. The root cause for the deformation/misalignment is already identified. The deformation is basically because of thermal load acting on one of the guide support which was under designed and broke. The broken guide support led to lateral displacement of the header and due to elevated temperature it got deformed/yielded permanently.
Main concern which I have is that there are vertical radiant coils (tubes) connected to outlet manifold. Because of header, tubes have also deformed. Tube are operating in creep temperature and I am worried that because of deformation there will be some bending stresses which may cause creep failure in tubes.
The furnace vendor has recommended replacing deformed piping and tubes (with mitigation for root cause) but we don’t have any backup calculation/ explanation for recommendation. Any suggestion how to back it up….

Estimate the stress that caused the deformed pipe. Plot this point on a stress versus cycles graph for the material in question. CAESAR offers some. The BPVC offers others (I think?).

Let's say for sake of argument that your plot point is 10. 10 cycles before failure.

This one-time deformation ate up a minimum of 5% of the overall life of the pipe (because it's bent, and not bent back into its original shape, it only underwent a half-cycle).

For an industry that expects 20, 30, 40, 50 year life from their components, a 5% lifetime reduction is significant. Offer to perform a lifecycle analysis to determine what % has been used thus far, and how much life is left.

CAESAR can help with that.

Khalidmf;

It is very difficult to visualise without a picture or drawing since you do not mention about the support types and locations for the furnace, manifolds and piping. Additionally the design information needs to be provided, such as pressure, temperature, operational loads etc... with pipe/tube properties.

In case the furnace is bottom supported and everything built on it you do not have luxury other than what furnace vendor says.

If the system is structurally top supported you may be able to do some, of course if you have all the design information especially pipe stress calculation.

I guess the best thing is to go to the original manufacturer or design vendor, and ask them to repair with no costing, if you can provide evidence that this is the design failure not operational, or they should come up with the root-cause.

Ibrahim and Michael,
Thanks for your reply. I had not provided much detail because my question is general on plastic deformation. I am not looking for root cause or why it happened. However I have attached sketch showing general arrangement of inlet/outlet manifold and radiant coils. Radiant section temperature is expected to be 1100 degree C and outlet manifold temperature is 800 – 900 degree C. Coils are suspended (Supported from Top).

The original manufacturer has recommended replacing bent manifold and coils because of plastic deformation. My question is how to support this recommendation with proper calculation or explanation. I understand that after plastic deformation, behavior of the piping system cannot be predicted.
Michael,
With regards to your suggestion to estimate stress in deformed pipe, I understand that after plastic deformation stress in piping will be released because of elastic shake down.
I can estimate stress in manifold and tube by applying measured deformation (displacement) and plot that stresses against S-N curve. But that will give me fatigue life. I may be missing something from your suggestion. Please correct me if my understanding is wrong.
Regards,
Khalid

I believe I was thinking out loud prematurely. The more I think about it, I'm doubting this kind of stress will even show up on the curve.

This makes me think it's as much of a materials property problem as it is pipe stress. If the materials specialist states buys into the new configuration with plastic strain hardening, and new loads with the deformed piping, then you could rationalize keeping the old material.

However, that's admittedly out of my expertise. Perhaps look into the Fitness for Service API standard?

It looks like it is a superheater between two manifolds. The main problem with superheater is the condensation of the steam in the tubing after the shut downs. If you do not slow heating the furnace, and of course the superheater to allow the water to evoporate slowly, the condensed water in the superheater tubes and in the heaters will cause hammering and lots of damage. Do you see the drain pots on the headers? Check the drawings. If they do not exist you may have the same problem on each fast cold start-up. If they exist so the control system needs to be checked with on/of valves.

I expect the damage has already been done on the tubes of the superheater, probably replacing them is the best option. However you need to investigate the healthiness of the tube connections on the manifolds, and the broken support locations on the header, if welded.

The best thing is to have drains on the lowest parts of the tubes as well in addition to drains on the heaters for the fast cold start-ups. But the furnace manufacturer should give directions for the start-up operations not to cause this kind of failures. Check your operations manual.

Ibrahim,
You nailed it, yes this is steam superheater. This is a good point which I had missed. I checked the drawing for header and could locate vent and drain points on outlet manifold.
But if this is the case, damage should happen during start-up only. But operations and turn-around team has not reported any problem during start-up and I was at site during recent start-up after T/A. I will discuss this point with operation. Thanks for your valuable input.
I agree with Michael that FFS will give some indication on fitness for use and remaining life of the heater piping but unfortunately we don’t have most of the input data required to perform FFS assessment.
Thanks again for your valuable inputs.

Regards,

If you do not see a drain on the inlet manifold I would additionally have suspicion on unsaturated water inlet to the superheater tubes through this header as well.

Check if all the start-up drain valves and control systems functioning. Check how fast the start-up is being done against the manufacturer operation manual.

Thanks Ibrahim,
I will check with T/A and operations team about start-up. For your information we have two radiant cells and problem is reported in one cell only.

Regards.