Heat exchangers tubes retubing 4

[0707]

What is the rule of thumb practice for heat exchangers tubes retubing? One should replace tubes when they reach 1/3 of wall thickness? Or tubes should be replaced when they reach ½ of wall thickness?

Thanks

Luis Marques

 

[11echo]

I don't think there is a "rule of thumb" as to when to replace equipment. It depends on the system rating, commodity, and “turn-around” time. When ever they cleaned and checked the tube bundle on the exchangers I've deal with, if they found a bad tube(s) is was plugged and put back in service. And when the bundle was "too bad" the whole exchanger was replaced. Waiting for a new tube bundle would cause too much down time, and I've never seen a “on the shelf replacement” on any to the exchangers I came in contact with. My $0.02 anyway. Good luck! ...Mark

 

[0707]

After inspection if we found bad tubes our practice is also to plug them. Because of thermal transference we avoid to have exchangers with more than 10% of tubes plugged. If for some reason we have to plug more than 10% of the tubes we do it. In this case we make previsions for tube replacement as soon as possible until we have time to purchase a new bundle.

My question is related with eddy current and iris inspection. What are others practice? Replace tubes when they reach 1/3 of wall thickness? Or tubes should be replaced when they reach ½ of wall thickness?

Thanks

Luis

 

[epoisses]

I'm not an expert as to whether there are any design guidelines, if not the decision would be based on economics and risk. Would leakage result in a cross contamination problem (e.g. reactor feed/effluent exchanger) or just a minor loss of energy efficiency? Would there be a way during your busy shutdown to sit down to establish the degradation mechanism and predict the decrease of thickness over time and see if you meet the next shutdown?

 

[Lhenrique]

Luis
Epoisses is right, depends on your need to keep plant running, specially when your company get a higher price for the produced commodities. In my country people wait up 3/4 wall looses to retubing.

Luiz

 

[0707]

Heat exchangers tubes thickness is based in BWG. Most of the carbon steel tubes are 2.8mm thick. So, when you have exchanger tubes with nominal thickness of 2.8mm you replace them if you loss about 2.1mm of the thickness, which is a little bit less conservative than when tubes reach 1/3 of nominal wall thickness. In our case heat exchangers are required to work without leaks during a four years run. Heat exchangers lives are based on expected corrosion rates. In some services we have exchangers working during 20 to 16 years without problems and in some few cases we have exchangers, which don’t withstand a four years run. In services where you have generalized tubes corrosion is easy to schedule heat exchanger retubing but when your corrosion is localized is almost impossible to predict heat exchangers tubes replacement.


Thank you for your sharing

Luis

 

[bsme3]

The "rule of thumb" in the refinery industry relies on the process engineer and the efficiency of the exchanger. If the corrosion rate is calculated and the exchanger has no other issues besides the wall loss you mentioned it would be a safe assumption to say that you are missing some key variables in calculating what to do with your tubes. First thing you need to determine is what is the mechanism causing the failure. 1)Is the loss due to corrosion of the tubes? 2)What is the corrosion rate? After you have determined these two variables you or your process engineer would have to land on the acceptable efficiency of the exchanger. If it is agreed that you can plug say 10 - 20% of the tubes and still have the desired temperature of the product on the outlet side, then go ahead and plug the tubes that are thinning if the outlet temperature is within this range. Usually the reliability engineer will come up with a standard ratio to plug. I have seen where plants will plug tubes that are over 60% wall loss and others that plug only the ones that have 80% wall loss. This is a decision that will be made when you calculate the corrosion rate of your tubes. If this mechanism is erosion you will have a different animal since it will be hard to determine the amount of solids in the flow at any given time. In some cases I have seen MIC cause tube failures as well. For safety sake use the corrosion rate formula to calculate date of failure so that you can plug the tubes that will not make the desired run time. For instance it you have a tube that say has a new thickness of .180 and you run it for say 5 years then measure the thickness of that tube and find that it is down to .130". You can safely say in 5 years you have lost .050" of wall thickness. You will have to determine the failure mechanism. This calculates to .005" per year. If the pressure of the tube controls the thickness, lets say the minimum thickness of the tube is .100" then you will only be able to run that particular tube for 6 more years. Remember that tubes in a heat exchanger do not corrode at the same rate since there are so many phase changes in the exchanger depending on design. You are, at a minimum, going to have a vapor space and a liquid space somewhere along the tubes particularly on the back side of the internal baffles where fluid is not present so keep that in mind when you determine tube remaining life. Fretting can also be the cause of your failure where in you will need to stabilize the member causing the fretting. Tell your iris or eddy current crews to pay particular attention to the baffle areas for fretting. If you need more info you can email me direct at bsme3@msn.com. I will be glad to help if I can. Also visit TEMA.ORG to get information on the formulas used to design the tubular components. I hope this helped.

Thanks,

LV

 

[0707]

My question is related with Eddy Current and IRIS inspection during a turnaround inspection. What are others practice? Replace tubes when they reach 1/3 of wall thickness? Or tubes should be replaced when they reach ½ of wall thickness?
Heat exchangers tubes thickness is based in BWG. Most of our carbon steel tubes are 2.8mm nominal thick. In our case Heat exchangers lives are based on theoretical expected corrosion rates from RBI analysis. In some services when your corrosion is localized such as pitting or erosion is almost impossible to predict heat exchangers tubes replacement. When those few exchangers which don’t withstand a four years run, a root cause analysis will be required to determine or not a material upgrade.

Thanks for your sharing
Luis

 

[wifi]

Can anybody tell me what is heavy liquid and light liquid, and how actually a liquid is pumped somewhere (say to a flash tank) on flow control (or level control)i.e. would you describe the process itself?
Thank you in advance.

 

[unclesyd]

imfwat,

Start a new post and you will stand a better chance of getting an answer.

 

[jnleroux]

Luis,
About 12 years ago, I spent several years working as a contractor performing IRIS inspections on heat exchangers. At the time, the majority of the refineries took action when they saw 50% wall loss. The action varied, based on time in service (corrosion rate), number of tubes that needed to be addressed, cost, failure consequence, and repair philosophy of the engineer. Typically, tubes were plugged or bundles retubed. When plugging, most planned to retube or reinspect at the next outage.
John