Monel 400 Life Limiting Criteria 1

[ahafeez3131]

I am a maintenance engineer in a power plant. We have heat exchangers in our process water cooling circuit. The heat exchangers have Monel 400 material tubes. These heat exchangers have given us excellent service for last 30 plus years. Now we want to assess the tube material condition to check their healthiness and estimate when to replace the tube bundle. In this regard we have pulled out one tube out of heat exchanger. Mechanical tests were done on this aged tube. It shows an increase in yield strength, Ultimate tensile strength and hardness , and a decrease in %age elongation.
Based on this result how we can deduce the condition of tube material. Is there a limit of increase in say yield strength, after which it becomes mandatory to replace the tube bundle with a new one. In short is there any Life Limiting Criteria exists on metals Yield Strength, UTS, %age Elongation or/and Hardness.

 

[metengr]

ahafeez3131;
Normally, for Monel 400 tubing in heat exchangers this material can last up to 50 years of service because we have several high pressure feedwater heaters in steam service that have lasted this long. The life limiting condition for this material will be external corrosion damage in the form of wall loss. Mechanical properties of service tubing must be compared to original properties as specified in the material test report – using published values will not provide accurate results. The main concern is to verify wall thickness and determine if corrosion is occurring in service.

 

[MJCronin]

The best methods are those that have withstood the test of time.

The primary method of establishing remaining useful life is evaluating tube wall thickness. Corrosion is always the main culprit

In the past 30 years, has there ever been a survey performed to evaluate tube wall thickness ?

You have originally paid a good price for premium quality tubes. You should have many years of service left.

You do not mention the other components of these heat exchangers or other possible performance issues.

Has there been any vibration ? Any leakage or erosion at the tubesheet ? What about wall thickness on other water side ? components ?

 

[stanweld]

The product tensile test results need not fully reflect the results on the original CMTR. They should reflect the requirements of the ASTM/ASME materials specification. As both metengr and KJCronin have stated, wall loss due to corrosion is the chief gauge of remaining service life unless the tubes operate in the creep regime.

 

[EdStainless]

Your greatest concern should be the risk of pitting or other localized corrosion. Your best plan is to clean the unit and have it Eddy Current tested. Be prepared to pull a couple of tubes if you find indications in order to validate the testing and analysis.
It no pitting I don't see why these tubes won't out live you.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[ahafeez3131]

Thank you very much for the kind replies.
No appreciable denting or wall thinning is visible on the the pulled tube.Vibrations are in safe zone. We have 5 tube leakages in a bundle of 1875 U-Tubes in 30 years history.
However, the tubes are as per ASTM B 163 (UNS N04400), which gives Y.S 193, UTS 483, % elongation 35, Rockwell Hardness B75(max). The tube we have pulled out gives this data, Y.S 383, UTS 569, % elongation 32, Rockwell Hardness B83.
Based on this deviation from standard i.e. from ASTM B 163, how can I deduce the healthiness of my Monel Tubes.
My question again is, Is there a limit to the above four properties after which change is imminent. My Regulator is saying your YS & UTS are on high side, you should think of changing with new tube bundle. Kindly provide me a reference.
Regards.
A hafeez

 

[metengr]

My question again is, Is there a limit to the above four properties after which change is imminent. My Regulator is saying your YS & UTS are on high side, you should think of changing with new tube bundle.

No, there is no limit. Second, mechanical properties can come in higher than the minimum requirements as stated in specifications. So, can you prove that the material properties have degraded in service or was the material originally received in this condition?

 

[stanweld]

The fact that the actual tensile properties exceeded the minimum specified requirements is essentially axiomatic. Decreased tensile properties are a better indication of degradation.

 

[ahafeez3131]

Thanks again.
What about the other two properties, the %age elongation and Hardness. The % age elongation has decreased and the hardness has increased from the maximum permissible in ASTM. Does the rise in hardness and decrease in % age elongation shows a decrease in ductility. So now the tube material is becoming more brittle. Is going to brittle side not a sign of less strength against any vibrations causing en-mass tube ruptures.
Please do not mind my questions, i am just learning.

A Hafeez

 

[metengr]

A Hafeez;
Please review the attached information regarding Monel 400. Depending on original fabrication, yes, this material can age harden over years of service. Your concern should be corrosion not worrying about age hardening. Please review the information carefully, and it should help you to understand how this alloy performs.

http://www.specialmetals.com/documents/Monel alloy 400.pdf