Steam Rings on flanged joints 1

[Dpearcey]

We have had problems with leaks on the flanged connections on our vertical platforming unit combined feed exchangers. We have steam rings installed to manage any leaks when they occur. The gasket is a double jacketed grafoil filled. Anyone else out there with a similar problem? Are steam rings common elsewhere?

 

[hokie96]

My experience is on the downstream side. In any case, steam rings are regularly used to temporarily mitigate hydrocarbon leaks from exchanger girth joints (or other flanged joints). If you're having a chronic problem with these exchangers, an upgrade in gasket material is probably in order. The problem with double jacketed grafoil filled gaskets is that the grafoil is really on the wrong side of the gasket.

As I'm unfamilar with upstream equipment and the temperatures and pressure involved at your specific location, I would suggest you consult a gasket manufacturer (Flexitallic) for some recommendations. Three gasket types that have worked well at our refinery are spiral wound, corrugated metal graphite covered (CMGC or "Graphonic&quot and grooved metal graphite covered (GMGC or "Kammprofile&quot. Personally, I limit use of CMGC gaskets to Class 300 or less. Each of these gaskets has the grafoil on the outside which allows it to flow between the reinoforcing element of the gasket and the flange faces helping effect a seal. Typically we purchase these gaskets in a 304ss/grafoil combination.

I'd also be remiss not to point out that an effective controlled bolting program should be considered if exchanger leaks are a chronic problem. Good Luck.

 

[kbander]

I've had the same experience in this service and found that these exchangers seem to be notorious for flange leaks (cometimes leading to small fires).

Newer designs don't seem to include this flange. Instead the shell is equipped with a burn zone...i.e., you have to cut it to dismantle.

Regards,


Bob

 

[Bolting]

Exchanger flange leaks are common, particularly on channel to shell connections. The reason is that this flange sees the greatest temperature fluctuation and has two gaskets under a common bolt load. Simply put, flanges leak when there is insufficient stress on the gasket to maintain a seal during its operation. When a flange is first bolted, often using manual or impact means, the bolt load compresses the gasket and helps to maintain a specific gasket stress. When the exchanger is put into operation, the gasket will tend to relax due to the heat, resulting in a relaxation of the bolt load. This often gets worse during a temperature transient, resulting in a leak during operation. The key areas to watch to maintain a seal:
1. Ensure the gasket sealing face is flat and to proper surface finish
2. Install a gasket type that is more resistant to creep druing its operating life
3. Provide controlled bolting. Meaning bolt load is acheived to the correct level and are uniform around the flange
Preventing leaks is possible with the right control

 

[EdStainless]

I believe that MTI has recently done a study of the reliability of flanged joints. They werre looking at flange condition, gasket types and bolting procedures.

http://mti-global.org/mti/t/template/pub,Index.vm

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[jsummerfield]

Can anyone comment on successfully preventing leaks in hot cyclic applications with hydraulic tensioning?

John

 

[Bolting]

EdStainless, Is the document available to the public or do you need to be member of MTI? Can you advise how to get a copy? Thank you

 

[Bolting]

John,
I have had good experience preventing leaks on hot cyclic service with hydraulic tensioning. It's not a gaurantee, because there are many factors leading to cause of leaks. Hydraulic bolt tensioning, if done properly, provides a uniform and accurate bolt load. With traditional bolting there is a wide bolt load scatter around the flange. With bolts greater than 1" in diameter, there is also a tendency for the overall bolt load to be lower than what is required. When cyclic flanges heat up the gasket load and bolt load relaxes. If the bolt load is not as high as it needs to be, as well as not uniform, the bolt relaxation will lead to leakage. The term "controlled bolting" refers to a process like tensioning or ultrasonic measurement to control bolt loads so that the user knows the load is high enough AND uniform.

There are several technical papers on this. The attached link describes the use of ultrasonics to control the bolt load to prevent leaks.

http://www.integra-home.com/h_bolt_stress.htm

If you are having leak problems you need to have a more holistic approach to preventing leaks that includes the gasket and flange, in addition to use of controlled bolting techniques such as tensioning.
Gord

 

[Dpearcey]

Since I posted the original message we determined the cause of the flange leak was excessive stresses imposed by the inlet piping. The piping has been redesigned and the flanges refaced. We haven't had a problem since.

 

[Dpearcey]

Since I posted the original message we determined the cause of the flange leak was excessive stresses imposed by the inlet piping. The piping has been redesigned and the flanges refaced. We haven't had a problem since.

 

[InspEngr]

We've also had flange leaks at the locations mentioned, as well as various other flanges in similar (high heat) systems. One of the problems is believed to be the relaxation of the bolts due to thermal cycling (startup/shutdown, rain, etc). We now specify Bellville washers in these applications. These washers help maintain a compressive load on the flanges. They've done well enough for us (although not 100%) that we use them in Hydrotreating and Hydrocracking service as well.

Here's Belleville's website

http://www.bellevillesprings.com/

-InspEngr

 

[aybee]

This will stir up a hornets nest....
Bellville washers do nothing to "maintain the load"..the load is applied by the elasticity of the bolts which act like big springs, when they relax (which they do)the load drops off and the flange can leak if there is not enough residual load left in the bolt tension to provide the sealing and pressure retaining load. Having an extra set of springs under the nut does nothing that the bolt does not already do
The bellville washer spring load must be the same as the bolt load until the washer flattens out whereupon it becomes a bearing load like any other washer. Once the bolt load reduces through relaxation, the washer load also in balance reduces to equal the bolt load and evantually if there was not enough tension applied to provide residual load the flaneg leaks. I believe that Bellville washers may have some application in providing flexibility but have been unable to track down literature on this...Aybee

 

[EdStainless]

Bolting, sorry that I missed your question.
MTI usually publishes material for sale about one year after it has been avaiable to members.
Check mti-global.org , or find someone who is a member.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[unclesyd]

Properly engineered and applied Bellevilles can and do offer a means to maintain the proper loading on a gasket.

We have used them for more years than I like to remember on 48" dia Flanges (12) in continuos operation between FV and 250 psig with a 200? C temperature swing every 45 minutes. The flanges go cold every 6-10 days for 8 hours. The gasket is modified spiral wound graphite.

We also have them stacked on a 30" 316H vs 825 flanges with 718 bolting operating at 250 psig @ 1250?F. This equipment had about an 80% utility. This flange uses a recessed, rabbet, spiral section only gasket for sealing resulting in the two flanges being face to face. The only leak in 25 years of operation on this flange was when Inconel/Graphite was substituted for Inconel/Mica.
An interesting note on this flange is that this substitution occurred several times and the joint didn't leak as the spirals alone sealed the flange.

http://www.keybellevilles.com/apps.htm

You might want to look at this website for a method of getting the proper tension on a bolt.

http://www.surebolt.com/