Inspection of Heater Tubes

[Farestt]

Inspection procedure of heater tubes of our 40,000 BPSD Crude Refinery consists of the visual inspection, O.D measurement, Ultrasonic thickness measurement and Hardness Testing. I want to know the maximum permissible hardness of tubes (which should become the replacement criteria). There are four heaters and material of their tubes is ASTM A 335 P5, P9, ASTM A 161-37 and SS 316 L respectively.

Also if someone can help me how to calculate minimum required thickness of heater tubes??? I tried calculations in API 530 Annexure "A" but results were not satisfactory possibly because our heaters operate at low temperatures (425 to 530 deg.C) where creep phenomenon will be negligible???

[ashfaqanwer]

Regarding thickness calculation of heater tubes as per API, I have provided you a link.
I hope you would be getting some help from this document.
http://www.thepetrostreet.com/database/ ... _tubes.pdf

[ben]

The upper limit of hardness in case of P5 and P9 tube materials must be set at 250-270 HB.
Specify the grade of A161.
For SS316L, the hardness must stand below 220 HB.
Normally, the heater tubes available commercially stand within the hardness values of 150-220 HB. 150 HB for stainless steels and 220 for low alloys like P-9, P91.

[Farestt]

@ashfaqanwer. API 530 is meant for calculating design thickness of heater tubes. I wish to find out the retiring thickness of heater tubes i.e. when they are placed in service and corrodes with time, so what should be the minimum thickness after which they should be replaced??

@ben. I think the hardness you specified is for new tubes/pipes. During the service, the hardness values increases with time. Is their any upper limit after which tubes should be replaced. P5 and P9 tubes of our heaters have been in service for about 10 years and their current hardness is in the range of 400 BHN to 450 BHN. What should be their replacement criteria???

[mechcolor]

Tube life assessment / Retiring thickness
For tube life assessment or calculation of retiring thickness you can also follow API 579. Meanwhile, I would provide you a standard procedure in light of API 530 to estimate the remaining life of tubes.

Hardness
The hardness values of tube ideally should not be rising during operation. Rise in hardness values indicate carburization. Also note that the hardness values must be taken after grinding the surface a bit. Please share the hardness values for each material so that I can comment accordingly.

[Farestt]

Yes, we carried out replica of one of the tubes with maximum hardness 420 BHN (Material A 335 P9) and the results also showed carburization. But the question still remains, WHAT SHOULD BE THE LIMIT? HOW MUCH CARBURIZATION / HARDNESS WILL BE THE LIMIT FOR REPLACEMENT??


For A-161 carbon steel tubes, we go for replacement when the hardness reaches 350 BHN. The P5 and P9 material hardness have reached around 450 BHN and we are planning to replace them now, but their must be some calculations / recommended practice / best engineering practice for setting an upper limit on hardness???

I think the hardness values will, and does rise during service because every tube material has a life, and in some cases during heater operation, if flame impingement and overheating occurs, the hardness values will rise. API 530 Annexure "A" gives a procedure for estimating remaining life of tubes but it do so taking into consideration the effect of CREEP. If the heaters operate at low temperatures, below the creep range, then creep phenomenon will not be present and only way for reduction in wall thickness will be internal / external corrosion and in that case, we cannot make use of API 530.

[ben]

Farooq,
Hardness of the heater tube is not a direct controlling factor for the replacement. Hardness is a measure which provides you a direction as to what is happening to the heater tube. The increasing values of hardness like in your case indicates carburization the extent of which determines the replacement of heater tubes. Now, the extent of carburization has not been mentioned in any of the code / standard / engineering practice however, the thickness loss resulting from carburization must be noted down after every operating period and then plotted as per the guidelines provided in API 530.
API 530 Annex. A then helps you in determining the limiting temperature and pressure by using larson-miller parameter. Now its up to you and your management that which limiting temperature and pressure values would ask for the replacement of tubes.
Hardness value can go down also with the passage of time which indicates creep phenomena becoming mature and may lead to failure any time.

Now taking the whole case based on experience, what I have done in my total experience with heater tubes is "replace the tubes as and when carburization mechanism reach a level where I have to submit new limiting temperatures and pressures to the plant operating staff". Not in every case the hardness values go that much high but yes if still one asks for a upper limit, I would write as follows:
Carbon Steels - 330 BHN
P5 - 400 BHN
P9 - 450 BHN
Stainless Steels - 350 BHN

I would conclude that hardness readings must not be taken as a basis of heater tube replacement directly. And more importantly, if hardness of the tubes is going that much up as in your case, you must consider taking some actions to mitigate this increasing trend.

[arcpro]

There are many ways in determining the remaining life of the heater tubes that is:
1. Larson Miller parameters as per API 530
2. Metallographic Observations
3. Omega method
4. Reliability approach (taking inspection data, history of failures and making functions)
At our site, we believe in metallographic observations and conclude on the basis of following:
(a) No cavities. The primary carbides precipitate along the grain boundary. The corresponding life fraction is within 20% of the remaining life.
(b) Few cavities appear in the grain boundary of the inner side of the tube. The corresponding life fraction is about 20–40% of the remaining life.
(c) String of cavities found along the grain boundary. Few cavities form micro-cracks. The corresponding life fraction attains 40–60% of the remaining life.
(d) A few cavities are linked forming microcracks. Some microcracks are linked with cracks caused by carbonization in the inner surface of the tube. The consumed life fraction is about 60–75% of the life of the tubes.
(e) The microcracks link with each other, forming a macrocrack propagating toward outer surface. The crack length is about two-thirds of the thickness. The tube is postulated to fail.

Now coming to the hardness issue, for P9 material any hardness reading above 300 BHN is the first alarm for us. Last time when we replaced the tubes, hardness readings were at 330 BHN although the replacement was done on the results of metallography.

[kamran]

As far as my limited experience goes, I have not seen heater tubes getting replaced based upon their hardness values and if that is the case some NACE standard has to be referred to which should be mentioned by OEM of heater.

However, high hardness values of P9 material in your case make up for a serious issue. You need to sit with your Operations team and understand the way they handle/operate the heater.

At our site, we have tubes that have been in service for well over 25 years with no significant increase in hardness - last inspection records show 300 BHN as the highest reading.
However, we did replace some tubes in the past in another heater due to corrosion and carburization issues.

Kamran

[ashfaqanwer]

Hello Farooq,
I had a detailed discussion with some of the experts on this issue and have following inputs for you:
1. For P9 material, the limiting hardness must be 240 BHN above which you must think of controlling this by some means.
2. The hardness values which you have observed is too high which can cause failure of tubes at any time.
3. There is a way to reduce the already increased hardness which may or may not be successful. If you are interested in knowing that, do let me know.
4. Some mechanical tests must be made for tensile and bend properties to see if the tubes are mechanically fit to operate. This would also help in confirming whether this high hardness is on the surface only or not.

Do let me know for any clarification you need.