Flange Connection DN200-150# with Spiral Wound Gasket-Gasket Seating_2 2

[saplanti]

Dear Members,

Some time ago I launched the following thread;

"Flange Connection DN200-150# with Spiral Wound Gasket - Gasket Seating" with the shortcut:

http://www.eng-tips.com/viewthread.cfm?qid=204661&page=1

I have found an invaluable paper (Bolt Preload Stress for ANSI Raised-Face Flanges Using Spiral-Wound Gaskets-Sealing Technology & Plant Leakage Reduction Series) in EPRI website which is supporting my claim, and I have provided the comment below from this paper for your information:

"It is unfortunate that the low pressure class (150-300) flexible-graphite filled gaskets, which require the highest bolt preload stresses to achieve near full compression, are matched up with the ANSI B16.5 flanges having the least ability to accommodate the higher bolt preload stresses.
This situation is a result of many of these gaskets being too stiff to meet the load-deflection requirements of ANSI B16.20. It is desirable for the manufacturer to work at bringing the load-deflection characteristics of these gaskets into conformance with the ANSI B16.20 requirements in the future so that a good technical case can be made to reduce the recommended preload stresses, or provide greater margin against leakage for the preload stresses recommended above."

In the same paper there was a report called "EDF PAPER: VARIABILITY IN COMPRESSION AND LEAK-TIGHTNESS CHARACTERISTICS OF SPIRALWOUND GASKETS", and this attachment had the following conclusion:

"The mechanical characteristics as well as the leak-tightness performance of spiral-wound gaskets have been found to contain a great deal of scatter. Temperature or pressure transients and time could significantly change the gasket sealing characteristics, perhaps further increasing the scattering. As such, it is very difficult to get clear variation laws from the tests we have done.
The mechanical characteristics are important to properly tension the gasketed joint, and they determine its future behavior during operations. For this reason, the amount of scatter encountered during the testing is unfortunate. Relative gasket leak-tightness is also an important value to know when the medium is corrosive, inflammable, explosive, or otherwise dangerous.
The results seem to indicate that asbestos-filled spiral-wound gaskets or those flexible graphite gaskets plus mica and used with standard flanges may not be the best gasket of choice for low pressure classes (150 and 300) as their bolt stress may be too low to obtain suitable gasket deflection and a good leak-tightness level under most circumstances. Flexible graphite-filled gaskets with an inner ring and compressed up to the full compression offer the best characteristics.".

I suggest everyone involves in the flange calculation and selection of gaskets to read this EPRI paper to get some deep understanding.

Kind regards,

Ibrahim Demir

 

[papiesz]

Mr Demir


I've recently found your post on this forum. I foun this because I searched answers for the same question you asked year ago. It was a problem why bolt stress for seatnig condition for class 150 ASME flanges is in most flange sizes above allowable.

I work for company where that fact is commonly known. We have excel spreadshhet and old DOS based programs which utilizes ASME Sec. VIII Div.1 App.2. All those programs indicate bolt overstress when a spiral wound asbestos filled gasket is used with ASME B16.5 #150 flange. Folks from my company used to say: "don't even look for bolting stress".

More recently I had oportunity to come across German Standard AD Merkblatt 2000 and came back to old ASME problem. It seems that one of the reason is that they use lower safety factor for bolt stress. In ASME for bolts made out from A 193 B7 (<2 1/2") we have allowable stress 25 ksi and min. yield stress 105 ksi (tensile strenght is 125 ksi). It comes to safety factor more than 4 while for flange or pipe material we have factor 1.5 or 1.6 (allowable stress 2/3Sy or 5/8Sy).

By the way I found another interesting EPRI paper.

http://mydocs.epri.com/docs/public/TR-104213.pdf.

This paper expresses explicitly the need for stressing bolts higher than allowable eg. 45 ksi. But I still wonder why this situation is not reflected by the Code. And more why this is not even mentioned.

Best regards

Pawel

 

[LSThill]

Ibrahim Demir RE your LINK is not working

saplanti (Mechanical) 21 Dec 07 3:17
This is onother one with a different sealing approach in the flange connections;

http://www.standard.no/pronorm-3/data/f/0/02/42/5_10704_0/L-005r1.pdf

Kind regards,

Ibrahim Demir

 

[JerryDowler]

When I went to work where I am presently employed, we had a big problem of our maintenance department mechanics doing a poor job at following flange bolt-up pattern procedure. In the past, I saw problems of over torque of RW sprial wound gaskets whch actually caused the spiral to enter into the actual pipe flow. I suggested RWI's with the inner ring. Therefore you cannot really over torque the bolts to ruin the gaskets themself. The inner ring prevents the gasket from compressing into the actual process flow. Also on the plus side, we are a FDA approved sight, so the actual sealant does not come in contact with the process. Call me crazy, but so far in approx. 10 years we have not had gasket leak problem in most of the various processes.