Condenser issue

[picasa]

THIS IS THE SECOND FORUM THAT I AM POSTING THIS MESSAGE SINCE I THINK ENGINEERS HERE MIGHT BE HELPFUL.

An ammonia condenser has well water as cooling media on the tubeside. Condensing Ammonia on the shell side. After just ~6 years in service, tube leaks develop. Obvious first hand solution is to plug the leaking tubes with tapered plugs. Now during retubing, if you try to drill out the plug, existing pressure is noticed behind the plug! why is that? what is the mechanism for this to happen? Theoretically, leaking tubes should release to atmosphere when the exchanger is depressurized.

 

[25362]

Just a wild thought: is it possible that precipitated salts enabled trapping ammonia in the tube by closing the leak ?

 

[StoneCold]

My guess would be that the spits in the tubes act like check valves and you end up with some residual pressure in the tubes due to the orriginal hole closing up.

StoneCold

 

[rmw]

Often during tube plugging some tubes adjacent to the leakers, especially if bore scoping down the leakers indicates adjacent damaged tubes, tubes that aren't leaking are plugged as "insurance."

The plugs in these tubes can leak during service for a variety of reasons, but reseal and hold pressure that is evident upon drilling at retubing.

Some tube plugs are removed by other than drilling, and there have been occasions where they shot out of the tube when they released.

So, just because you have a plugged tube doesn't mean that it was a leaker.

I have encountered nuclear plants that had very elaborate procedures for drilling out plugged tubes due to potential contamination issues regarding what the tube might contain as well as personnel protection.

In any case, when you are plugging good tubes for insurance purposes, it is often a good idea to puncture the tube with a tubing cutter to prevent pressure build up.

rmw

 

[rmw]

Upon later reflection, I thought of one other possiblity.

The leaker that was plugged may have been leaking at the tube/tubesheet joint, while the tube itself was OK.

In this scenario, when the exchanger was put back in service, the tube could have filled up at the tube/ts leak around or behind the plug, creating a dead zone in the tube, and with subsequent corrosion build up in the area, potentially sealing the captured fluid inside the tube, and under pressure.

Just a thought.

rmw

 

[unclesyd]

I've seen several cases like the above scenerio as described by rmw on ice house condensers. At one time we had 12 ice house type ammonia condensers and a hard and fast rule for plugging a tube was that the tube be purposely ruptured to prevent a condition as you describe.

As we built and repaired the majority of our Hxs' we had this requirement on any tube plugging operation . To not rupture a tube when plugging required a MOC order signed of the plant manager. Any bundle where the tubes were not ruptured this particular tube bundle was duly marked as such.
A guest manufacturing company on site had a nasty accident because of a flying plug.

 

[picasa]

RMW, I dont understand why the tubes that are not leaking are plugged as "insurance". Please explain

 

[picasa]

How to purposefully rupture a tube if the heat exchanger is a fixed tubesheet type HX.

 

[MikeHalloran]

The "insurance" is based on the assumption that tubes near a leaker would soon become leakers themselves. That in turn is based on the reasoning that whatever localized flow/ chemical conditions compromised the leaker on its outside, non-inspectable surface, are probably similarly affecting nearby tubes.

You can purposely rupture them through the tubesheet using a tool that cuts from inside the tube, before inserting the plug.



Mike Halloran
Pembroke Pines, FL, USA

 

[rmw]

In addition to what MikeH said; or, if it is feared that the high pressure stream jetting out of the failed tube has possibly damaged adjacent tubes.

And, MikeH answered the second question. It is the same tool that would be used to cut the tube behind the tubesheet for removal, except that the tool is not completely rotated to cut the tube, only a partial arc is made, just enough to start the tool so as to puncture the tube.

rmw

 

[picasa]

Ok. So the problem is that ammonia is trapped in the leaking tubes as the corrosion products have sealed the leaks. Is it right to assume that the pressure in the tube is the shell side operating pressure? What is the physical condition of the stuff that is trapped in the tube? IS it partial vapor partial liquid? If a tube is in the bottom portion of the shell, I guess it is possible that some of liquid ammonia has gotten in before the corrosion products sealed the tubes.

A way has to be figured out to depressurize the tubes. Is condensing the stuff trapped in the tubes a possible solution? If so what is an effective way to do so? Ice-cold water in the shell? Is there any mechanical solution to this problem? Like getting the cutter through an open tube next to the plugged tube and cutting the good tube and the plugged tube by rotating the cutter. Is the pitch of these cutters adequate to do so? Does this sound like a good solution? This will sacrifice a good tube though.

The people who have ran into this problem before, how was it solved? How were the tubes depressurized before drilling the plugs out?

 

[MikeHalloran]

Neglecting for the moment the mechanical problems associated with actually doing so, it would make sense to mechanically restrain the plug prior to and during the drilling operation, and to make the first penetration with a relatively small drill.

E.g., you could make up a bar with a couple of slots toward the ends and a drill bushing in the middle. It would bear a gross resemblance to a steering wheel puller. Perhaps a generic puller could be adapted with a threaded bushing replacing the puller screw. Apply the bar over the plug to be drilled, and restrain it with expansion anchors in nearby unplugged tubes, or backed up with longer bars and clamps over the shell flanges.

Maybe it's too specialized a tool to make commercially, but you should be able to bang out something appropriate in the maintenance shop.



Mike Halloran
Pembroke Pines, FL, USA

 

[rmw]

No, you can't assume that it is only shell side pressure that might be trapped in the tube. Whatever caused this might just as well have been channel side pressure. You can't be sure.

And, the fear is that whatever corrosion, etc. that has the tube sealed, if it is at the plug, is just at the the strength threshold such that any torque applied to the plug by drilling might dislodge the plug and the pressure would launch it.

The key is knowing what the worst case scenario is and making sure that the process developed for tube removal protects the technican first.

A plugged tube under pressure partially filled with gas is much more dangerous than a tube filled with solid liquid under the same pressure.

rmw

 

[MikeHalloran]

I meant to say that the plug should be restrained before drilling. Perhaps that's not enough.

It might be wise to first set up and maintain a clear field of fire between the plug and a sturdy plate angled at 45 degrees above a trough of sand, and then massaging the plug a bit with a long handled hammer.

And of course instructing the crew to assume there is high pressure gas behind the plug, and respect it accordingly.





Mike Halloran
Pembroke Pines, FL, USA