Gasket Selection and Minimum Required Bolt Preload
Gasket Selection and Minimum Required Bolt Preload
(OP)
Hello All,
Although my background is in engineering, I'm definitely "green" when it comes to pressure vessels, so I thought I'd reach out to the experts for a little guidance. Basically, I have been tasked with getting a very large horizontal pressure vessel ready for re-certification. I'm mostly looking at replacing all of the blind flanges, gaskets, nuts, bolts, etc... The vessel is very large (~30k gallons) and is only actually filled and pressurized (with water @ 150 psi) during testing. Most of the flanges already welded to the vessel are 2" 150# SA 105 RF, although a lot of the blinds I have been pulling off are actually FF. So my first question is, shouldn't RF flanges also be mated with other RF flanges? Also, is it recommended to only use ring-type gaskets with RF flanges instead of full face gaskets? The only reason I'm considering full face is that they seem like they will be easier to align and keep in place during assembly. Also, how do I go about determining what type of bolting configuration to use (studs, bolts, lock washers, etc..)? Is the normal process to select the type of gasket that will best suit your system's needs first, then to calculate the minimum preload required to seat that gasket along with containing the internal pressure (Wm1)?
Thanks in advance. Any and all information is appreciated!
Although my background is in engineering, I'm definitely "green" when it comes to pressure vessels, so I thought I'd reach out to the experts for a little guidance. Basically, I have been tasked with getting a very large horizontal pressure vessel ready for re-certification. I'm mostly looking at replacing all of the blind flanges, gaskets, nuts, bolts, etc... The vessel is very large (~30k gallons) and is only actually filled and pressurized (with water @ 150 psi) during testing. Most of the flanges already welded to the vessel are 2" 150# SA 105 RF, although a lot of the blinds I have been pulling off are actually FF. So my first question is, shouldn't RF flanges also be mated with other RF flanges? Also, is it recommended to only use ring-type gaskets with RF flanges instead of full face gaskets? The only reason I'm considering full face is that they seem like they will be easier to align and keep in place during assembly. Also, how do I go about determining what type of bolting configuration to use (studs, bolts, lock washers, etc..)? Is the normal process to select the type of gasket that will best suit your system's needs first, then to calculate the minimum preload required to seat that gasket along with containing the internal pressure (Wm1)?
Thanks in advance. Any and all information is appreciated!





RE: Gasket Selection and Minimum Required Bolt Preload
http://flexitallic.com/
review the references, it will explain many items you've not thought about. Yet.
Why have you decided to replace old blind flanges - other than the RF/FF criteria?
RE: Gasket Selection and Minimum Required Bolt Preload
I appreciate the link as well. I've seen a lot of similar information on other companies' sites (Lamons, etc...), but I had not run across Flexitallic until now. So I suppose the first step is to actually determine the specific gasket material/manufacturer then either just torque within their suggested limits, or obtain a more precise range by calculating Wm1 then determining the torque(the upper torque limit for A 193 B7 seems to be a standard ~120 ft-lbs to achieve no more than 60ksi bolt stress). I would like (probably need) some of my own calculations/analysis to back up the minimum applied gasket stress and bolt preload/torque for our system parameters. I have yet to see anything in the ASME codes that covers the use of lock washers, which I believe may help retain bolt stress due to our cyclic operating conditions). I think I did see something in the ASME PCC-1 that recommended through hardened washers on both sides of the joint for better torque translation. (I don't have access to ASME BPVC, but I can get some of the other documents through our tech library).
What are some of the items you were referring to that I have yet to consider?
Basically the majority of the blinds that are currently on the tank were all fabricated in house and unmarked. I believe they are all carbon steel, but I think they are also thinner than the minimum required ASME thickness for 2" 150# as well, and they are experiencing surface corrosion. We need all bolts, nuts, flanges properly marked and identified to pass our inspection.
RE: Gasket Selection and Minimum Required Bolt Preload
Cleanliness (no scratches nor residue on the gasket faces) and makeup of the joints are important. Torque methodology, training (should be a routine, but you have to check, and keep checking, and let the crew know you are going to keep checking) for first torque, intermediate torque and star pattern of bolting, and final torqueing.
RE: Gasket Selection and Minimum Required Bolt Preload
RE: Gasket Selection and Minimum Required Bolt Preload
Follow the tightening requirements of PCC-1 and you will have no problems.
As stated above, be careful of gasket faces
MJCronin
Sr. Process Engineer
Venture Engineering & Construction
www.VentureEngr.com
RE: Gasket Selection and Minimum Required Bolt Preload
I was just responding to your question about why I'm replacing all the flanges. The bottom line is that all our current blinds (~16) need to be replaced, it's just a matter of making sure I am getting the right ones. I understand the torque methodology and sequence, I'm more concerned about defining an accurate value of minimum torque to safely seal the flanges.
TGS4,
Well, the PV is actually cyclic in both temperature and pressure. Temperature control ranges from room (~70°F) to about (100°F), and pressure ranges from 0 psig to 150 psig. The tank (PV) is used to simulate hydrostatic pressure up to approximately 300 ft below sea level.
MJCronin,
Thank you for the info, and I'll be sure to follow the PCC-1 procedures when installing, but I would still like to calculate a ballpark torque range for a little more accuracy. That's really my main objective right now. I understand the recommended torque ranges will more than likely suffice, but I would still like the peace of mind that goes along with actually approximating a unique value based on all of our specific variables.
RE: Gasket Selection and Minimum Required Bolt Preload
If you have flange faces in a poor condition then a soft material such as expanded graphite or even an expanded PTFE might be needed to accommodate some imperfections if compressed fibre is too firm. The ASME PCC-1 document tends to aim high in terms of bolting and is more geared towards spiral-wound gaskets. - You could try an EN1591 calculation for greater load requirement accuracy (the traditional ASME calculation often under-calculates especially on gas applications). Note of course that torque tightening is not that accurate as friction can vary considerably - you are measuring turning power not bolt load. However you are only talking about class 150 here, so pressures are not that high anyway - note that 3" and 8" are perhaps the worst flanges for bolt load to gasket area ratio.
Most ESA and FSA member companies have good technical support and can probably assist you locally with material selection
RE: Gasket Selection and Minimum Required Bolt Preload
RE: Gasket Selection and Minimum Required Bolt Preload
Be aware the over tightening and plastic flange rotation is very hard to avoid on most 150# flanges. A FF gasket surface, with the proper sheet gasket, or one of the corrugated designs might be a solution to that potential problem.
RE: Gasket Selection and Minimum Required Bolt Preload
RE: Gasket Selection and Minimum Required Bolt Preload
RE: Gasket Selection and Minimum Required Bolt Preload
RE: Gasket Selection and Minimum Required Bolt Preload
I'm more than likely going to go with Garlock 5500s for my flanges. I contacted their engineering department just to verify that ring gaskets and full face gaskets can be used interchangably with RF flanges, and they confirmed. I kind of prefer full face gaskets with the bolt holes for alignment purposes since most of my flanges are all oriented vertically, and they will also help keep foreign matter from getting inbetween the flanges...just in case anybody was still wondering, and since I never really received a definitive answer on this thread. Garlock also has quite a bit of information and resources on their site pertaining to gaskets, and they are very easy to contact if anybody here has any future questions related to gasket determination.
RE: Gasket Selection and Minimum Required Bolt Preload
And I have to take exception to rickets "... most 150# flanges lack sufficient strength to properly maintain the gasket/bolt loads required for normal spiral wound gaskets." Spiral-wound "Flex" gaskets are industry standard in refinerys and petrochem plants for Class 150 flanges, at least in the USA. I havn't run any flange calc's*, I've just used, and seen used, Flex's for 32 years. They work, and work well. To include drip-tight hydrostatic tests run at "Full Flange Hydro" pressure; i.e. 1.5 x 70°F MAWP of the flange. For A-105 c/s Class 150# flanges, Full Flange hydro is 420 psi. For 304/316L s/s, it is 345 psi. Flex gaskets and B-7 studs, using torque values from the Flexitallic literature.
*Aerodynamicists in the 1950's (?) ran calc's on bumblebees. Proved that they were unable to fly. Took a lot of improvements in theory and computing 'horsepower' [IIRC - 1995] to be able to get calc's on bees that agreed that they actually could fly.
RE: Gasket Selection and Minimum Required Bolt Preload
DUWE6, your exception is noted, but I worked for Flexitallic for ten years as part of their field service group, and I can tell you that most of the larger (10" and above) 150# RF flanges in nearly every major petrochem facility in the US is warped due to overloading of the bolts. This is the reason Flexitallic began to make their low stress version. At one Houston refinery, the plant had to replace nearly seventy 10" and larger 150# flanged joints when the deformation created a 1/8" stand-off of the gasket face before the bolts were installed.
RE: Gasket Selection and Minimum Required Bolt Preload
RE: Gasket Selection and Minimum Required Bolt Preload
Regards,
Mike
RE: Gasket Selection and Minimum Required Bolt Preload
RE: Gasket Selection and Minimum Required Bolt Preload
Under high external loading and cyclic operation I would think additional bolt loading would be beneficial to maintain the seal.
RE: Gasket Selection and Minimum Required Bolt Preload
Regards,
Mike
RE: Gasket Selection and Minimum Required Bolt Preload
http://proceedings.asmedigitalcollection.asme.org/...
This first paper outlines the theory and calculation basis.
http://proceedings.asmedigitalcollection.asme.org/...
This second paper provides an application based on the theory.
For some of the Class 150 flanges, they have, by design, insufficient bolts. So, you need "excessive bolt stress" to generate enough bolt force to seal on the gasket. But, the flanges aren't "designed" for the high bolt loads.
You can get flange rotation without cupping. I would define rotation as what results from pivoting around the end of the raised face, which is in contact with the centering ring. It is caused by rotation around the hub-to-pipe connection. Cupping, on the other hand, results from deformation in the flange ring itself. Often it is observed as plastic deformation (permanent), but there is an elastic component to it - both of which is captured in the EN-1591 calculation methodology.
The general approach by those who troubleshoot flange leakage for a living, is that more bolt load is better. Yes, some flanges may deform (even permanently), but the failure mode of flange is leakage - it takes an awful lot of bolt load to actually "break" a flange. Some flange rotation can be good, because it increases the contact pressure. Likewise, some cupping can be good. It becomes detrimental when the flanges don't actually seal. Whether a flange seals on the spiral windings, or the centering ring (and actually, the spiral windings are designed to compress down to the height of the centering ring, so you need a very minimal amount of rotation or cupping to engage the centering ring). There's been lots of good papers at the PVP Conference by Dr. Warren Brown on the subject.
RE: Gasket Selection and Minimum Required Bolt Preload
I was thinking relatively small displacement. Assuming large displacement (permanent plastic) how much would be acceptable (even beneficial) and when would you classify as too much cupping (potential for undetected leakage) that should be replaced?