Airfin Exchanger Inspection Plugs?

[lastone]

Why are airfin exchangers built with plugs for inspection purposes? The ones I have dealt with over the last few years, (which is only about 20), always seem to be prone to leakage at these inspection plugs. The plugs and exchangers are stainless. I have been told it is difficult to get a good seal, or it is easy to damage threads with stainless-stainless contact. All of the exchangers I have worked with are fairly large with hundreds of these plugs in the channel covers. I have never heard of anyone actually removing all of these plugs during a turnaround for inspection, with the reasoning always being "it is too much work" or "the ones we take out will just be more prone to leak when we put them back in". With that being said, why not just build a solid cover that can be removed when necessary for inspection of the tube banks? Is there some other reason these exchangers are built this way? I'm asking this question in this forum because I am just a process engineer, and I am sure you mechanical guys will know the answer.

 

[Montemayor]

lastone:

The plugs are used to inspect and rod-out (clean) the tubes in the bundle. Without the plugs (or something similar) you would not have access to the tubes.

The plugs were furnished on the unit because someone in your organization specified them or did not object to them when they were quoted with the unit. Header cover plates are a more expensive option and definitely the practical way to gain access to the tubes - especially when you have a stainless steel header with stainless plugs. Stainless steel galls very easily and I would accept a stainless steel threaded part on a similar alloy stainless parent metal.

I don't agree with your statement that process engineers don't know about such things. Process engineers are expected to specify the materials of construction - especially in a corrosive atmosphere or when exotic metals are involved. And the specification of the parent metal can, as you have noted, affect the operation and maintenance of the equipment. You may not have the experience with stainless steel, but I would recommend you spend a few hours in the maintenance shop when the guys are machining stainless on the lathe. It pays to learn all you can about the metals you specify on major equipment because it ultimately affects everyone if the specifications prove to be flawed. For example, chloride stress corrosion in stainless steel is something all engineers should know about.

I hope this explains the plugs and your options next time.

Art Montemayor
Spring, TX

 

[speco]

lastone,

Actually, there is also a very practical reason for using shoulder plugs. Besides allowing for inspection or rodding out the tubes, they are also used for access when roller-expanding the tubes into the headers.

It's true that cover plate or bonnet type headers can be used in some applications. Some refineries prefer them, in fact. However, these types of headers are only practical for low pressure applications, typically under 150 PSIG or so. Above that, the header plates, especially the cover plates, become extremely heavy. There are two reasons for this. (1) It is not possible to use any kind of stay plates in a coverplate design, so the plates must span the entire width (or height) of the header box. (2) The cover plate must be able to withstand both the internal pressure and the additive stress of bolting around the perimeter.

Also you might look into using some kind of anti-sieze compound when inserting the shoulder plugs back into the the header. If they are tapered plugs and not shoulder plugs with straight threads, you should look for another cooler manufacturer.

Regards,

speco

http://www.stoneprocess.com

 

[lastone]

Thank you both very much for the answers. Our exchangers do operate under high pressure, and stainless metallurgy is required due to corrosivity of the process. I guess the short answer is; if the exchanger is operating at pressures above 1000 psi then a removable coverplate would probably have to weigh more than the exchanger itself.

And don't worry Montemayor, I was only joking. I try and learn as much as I can about everything. That's one of the reasons I was asking y'all to start with.