Exchanger Inspection 101 Visual Inspection of Shell and Tube Heat Exchangers The inspection of heat exchangers is the one area of equipment inspection where I firmly advocate the use of a check list to make sure you got everything. The reason that I do so is that in my opinion, with the parts scattered and the confusion endemic to the bundle slab, it’s easy to miss a part, or to only partially inspect a part. I’ll include sample checklists, as well as a sample Excel type sheet for those unfortunates that might be tasked with multiple exchangers, and a typical wall chart for exchangers at the back of this book. On to the inspection!!! **Hopefully, by this time you already know what to look for on a gasket surface, but just in case, here’s a list: Actual gasket seating surface – cuts, dings or gouges that extend across greater than 25% of the surface. Severity weighting should be given to those which start from the pressure side, are particularly deep, or which have displaced, raised metal which might interfere with the gasket sealing. If the “defect” doesn’t go deeper than the serrations, it generally won’t cause a leak, except in high pressure steam service. Watch for nibbling away of the actual seating surface outward from the stagnant area and if there’s a nubbin, check that the nubbin is intact and “square”. The best way to do this is with your fingers as well as your eyes.

• Serrated stagnant area - The remaining serrations. Corrosion, pitting, cuts or gouges which might lead process fluids to the aforementioned gasket seating surface Stagnant area – The area inward from the actual gasket seating surface, through the knuckle. Corrosion in this area is often difficult to quantify, and gets ignored, since there’s lots of metal there, but the corrosion allowance doesn’t say everywhere except…

• Pass partition gasket seating surfaces – Should be straight and true on both sides. Some degradation of the edges is normal, but there needs to be at least an eighth inch of flat, smooth surface from end to end to ensure the gasket seats.**

• Channel Cover: Mostly open area, pitting and corrosion are

• common. The gasket surface 360o and across the face for the pass partition is of primary importance. Persistent leakers may be warped and should be dialed in by a machinist.

• A WORD ON THE NUBBIN• Metal jacketed and double jacketed gaskets• When used on a tongue and groove joint

without a nubbin, the gasket should be installed so that the tongue bears on the seamless side. When a nubbin is used, the nubbin should bear on the seamless side. Nubbins are usually 1/16th inch wide, by 1/32" high. If the height varies significantly (by touch), remachine!!!

Channel:• I look at channels in as methodical a

method as I can. I try to look at the channel cover end first, followed by the barrel/shell, then the shell cover end. The nozzle internal attachment welds and the nozzle bore are next. A mirror is a must for this inspection. . Also, special attention should be paid to any bull plug vents. They may be corroded out, with all the threads gone except the threads holding the plug. These vents are usually 3000# or 6000# couplings, and are difficult to hydroblast free of deposits. Buster Powell says; “They should remove the bull plugs before blasting.” That applies to blind flanges, too.

**Cooling Water Service Tubeside -

Cooling water deposits are bad…

Make them blast the channel clean

of deposits, but, if possible, without

disturbing the black oxide scale.

This black scale is a corrosion barrier,

and with a little luck on the part of the

client, more will form where the Micro-

biologicals in the deposits have been

blasted away.

Don’t inspect a dirty channel,

especially if there are tubercles

/carbuncular deposits. There is

no way to predict how deep the

corrosion under deposits may be.

Process Tubeside - The inspection is usually a lot more boring, but any

corrosion found is usually taken much more seriously, for obvious reasons. Again, don’t let’em talk you into inspecting it without cleaning, too many things can be masked by even a thin coating of “black nasties”. **

Primary (Stationary) Tubesheet - Channel End Tubesheet on a removable bundle, or on a multi-pass fixed tubesheet. Again, I look at the gasket seating surface(s), first and foremost, then the tubesheet itself. Tubesheets are subject to a variety of corrosion, from simple corrosion under deposits, to service induced cracking, surface checking and galvanic or turbulence induced corrosion at the tube perforation.

They should not be thinning to knife edges, or corroded below the tubesheet surface. Any mechanical damage should, of course, be noted, as well as the number of plugs installed.

Tube ends - Tube ends should be intact and square, with no corrosion taking little bites out…

Tube ID’s -This is one of the few areas where Iuse “Weasel Wording” on a regular basis. “That portion of the tube ID’s visible…”, ‘Cause you can’t see very much, and what you can see, you can’t quantify. I’ve had old timers from Mobil Oil tell me that they can tell how deep ID pitting is with a “Pit Picker”, but I don’t have the touch, I guess, and that’s still only a few feet down the tube. As the following pictures show, the only good way to survey the inside of a tube is with a video boroscope. Flashlights are horrible. Reflected sunlight is better, but still very limited, and even a boroscope sucks if the conditions aren’t just right. Be that as it may, when doing a visual, try to use sunlight. Note any visible pitting and scale. Note any damage at the tube rolls, and any longitudinal die marks (these are a prime spot for corrosion to start).

• Secondary Tubesheet – The other one… If there’s more than two, it’s too weird for me to inspect. On a fixed tubesheet exchanger, they’re identical. On a floating head, the secondary tubesheet is small enough to pass through the shell, and will probably have a seating angle for the split backing ring. (Don’t forget to check this area for humps, bumps and other things that might make the split ring not seat right. Check the split rings for twisting that might make them not seat right, since this can effect the seating of the floating head, too.)

Bundle - Again, may not be the best way, just the way I do it. I try to find the angle that I can look through the bundle, and slowly walk around the bundle, tapping tubes, tie rods and baffles, but primarily looking through the bundle, checking for fouling and/or distorted tubes.. If the baffles or tubes sound at all loose (and they almost always do), I stop and look to see if there’s hole enlargement and/or fretting damage to the tubes.

I then walk around the bundle again, looking at the tube OD’s. I check closely at the tubesheet to tube junction, and the “U” bends, too., as well as the length of the tube. I look at the impingement plate, plate attachment tacks and the surface of the tubes around the plate. If there isn’t an impingement plate, I try to find indications of inlet/outlet locations.

Floating Head – The floating head requires no special inspection, treat it like any other component. Do look closely, as always.

The Shell and Shell Cover – (For the sake of this book, we’re going to pretend that all

shell covers are left in place. This is a fairly common occurrence, with both pro’s and con’s, but if the shell cover is dropped, you treat it just like any other component.)I approach the shell like any other internal inspection, with this proviso; I REALLY don’t like to inspect wet shells. If you inspect shells immediately after hydroblasting, you WILL miss stuff. Maybe not today, but eventually, if you continue to do it, you will. All wet shiny stuff looks wet and shiny, and that’s all it looks like.First, I look at the bolt circle, flange face, gasket surface, nubbin if present, stagnant area and attachment weld.A note on bolt circles… Try as I might, I can find nowhere in the codes that says you can lose ANY of the thickness of the bolt circle. I’ve put what I have found in the back of the book…• On to the shell…

The variations in design seem endless, and a certain amount of “Figuring it Out” is frequently necessary.

First, shadow the shell from the outside

(presupposing that entry is possible). You

should be able to see, if not quantify, a lot of

what you are going to find from right there. From

the flange, you can see; one of the two prime

areas for deposit collection, which are right up

against the back of the tubesheet, and at the

back of the shell, below the bundle. You can see

the primary location of any extraction damage,

i.e., gouges to the shell internal surface, as well

as any damage to the skid bars. Any bulges

or distortions of the shell will be most evident

when shadowing from the end, and any severe

baffle wear on the shell will be very evident.

Remember though, EVERYTHING looks worse when it’s shadowed. As you crawl the shell, all the items that were visually noted from the flange should be checked out for validity and quantified, as well as any corrosion that wasn’t visible from outside. I try to pay particular attention to any phase change marks (like those in a drum) on the shell that might lead me to expect different corrosion characteristics, the nozzles internal surfaces and the

•bottom and top centerline, as well as all the welds. The shell to shell cover joint, whether the shell cover is integral or not, is a prime location for corrosion, as is the shell immediately adjacent to the weld and the

shell cover bottom third or so.