Studs vs Bolts into valve block 2

[RangerBill]

We have an application on a current project where we are using 1/2" bore double block and bleed valves at high points in a subsea manifold piping system (2-1/16" API 6A 10K flange faced, Duplex material). The compact valves are machined out of a single block of Duplex material with 10K flange faces and tapped for studs. The valves are mounted onto 'Weldoflange' type reinforced branch connections with SBX gaskets.

There is a project 'suggestion' to use bolts through the mating branch flange into the tapped holes in the valve blocks instead of pre-fitted studs and then nuts.

I am against this for several reasons and worries: The threads in the expensive vlave block become sacrificial rather than the stud/nut interface being the 'consumable'. The bolts need to be very accurately sized to ensure that a minimum strength engagement occurs prior to bottoming out in the full thread section.

Are there any known gauling problems with L7M bolts into Duplex?

Real piping engineers thin on the ground here..... i.e. nil

Any thoughts much appreciated.

 

[Scipio]

No experience with L7M into Duplex, I have however, had to repair stripped threads out of a cast-in 10" 1500# flange on a carbon steel reactor once, after two 193 B7 studs gaulled on removal. Apparently they were 'stuck' when the crew tried removing them, so they solved the situation by applying a hammer wrench and a 20 lb maul, stripping the threads out of 2" taps. The only repair was to over-drill the two damaged taps to something like 2-1/2", retap, and get the whole repair registered as a vessel modification.

My current view - avoid studding outlets at all costs where possible, if not, make sure once the studs are installed they never, ever have to come out. The reasons you've listed are at the top of my justification list. Another is that if you've got a nut & stud combination, you've doubled your chances you can disconnect the joint. If a bolt seizes, you might have no choice but to drill it out. If a nut on a stud seizes, you can cut the nut off, or remove the entire stud.

 

[BRT549]

I agree with Scipio's comments, and would add another point in favor of the stud idea. Studs allow a more exact torque to be applied, since with bolts there is the additional twisting force as you tighten them as well as the downward squeeze.

If you ever see cylinder heads being installed on a high performance auto engine, studs & nuts are always used rather than the bolts that come from the factory. More strength and more precise torque application.

As far as galling is concerned, the L7M [assuming this material to be ASTM A320 (L7, L7M, L7D) Covers 4140 Alloy Steels in Bolting Material] has a little chrome and moly in it. Duplex alloys generally don't have much nickel, which is the galling culprit in many stainless steels. So, just as a guess, galling shouldn't be a show-stopper.

 

[Scipio]

Ahh, good point on the torquing, I'd forgotten about that. It leads me to another advantage, you can actually eliminate the twisting force that torquing produces on bolts completely with studs by hydraulic tensioning instead of torquing. Might not be a consideration in this particular instance, but always a good 'back-pocket' argument.

 

[ccfowler]

If blind tapped holes are involved, the use of studs will permit maximum thread length engagement for better distribution of thread load stresses. Another consideration is the practicality of using "tighter" thread fits in the valve block to reduce potential thread wear problems. Depending on temperatures and environmental conditions, it would probably be wise to use a suitable anti-sieze compound on all of the thread surfaces.

 

[Metalguy]

BRT549,

Studs/nuts do *not* allow more accurate torquing. What they do allow is the use of tensioning devices, like Scipio noted.

The big problem with torquing isn't the twisting-it's the high friction between the underside of the bolt head or the nut and the stationary item. Tests have shown that up to 90% of the torque can be used up by this friction, leaving 10% for the vital PRELOAD.

If special bolts are used which have accurately-machined parallel surfaces between the top of the head and the other end, an ultrasonic "bolt gage" can measure the actual bolt stretch-far more accurate than torquing. But even with well-preloaded bolts you lose the "double-chance" of removing a stuck fastener.

 

[RangerBill]

Thanks for all the advice folks. Out of my hands now.....

I still can't believe we are actually calling up bolts(instead of studs and nuts) making up a duplex material, tapped 1/2" bleed valve block onto an API 10K flanged branch fitting. (FYI: This large subsea oil production manifold structure wil be immersed in salt water for ~15 years)

Hope the guys in the Fab shop don't strip a thread in any of these expensive blocks! (lots of them) There is no room for hyd. tensioning gear in these locations either so its a torque wrench at best assembly job. If one leaks in operation its a diver who has to remove and repair it.

:-( Unfortunately looks like the ISO's are stiil going out AFC with bolts :-( called up.... Groan, whats happening to engineering these days............?

 

[Scipio]

Just wait until there's a problem and someone comes looking for an engineer to blame for not talking them out of bolts

 

[hacksaw]

believe the studs and nuts are more reliable than nuts and bolts. Has to do with how the bolt head is formed and the stress concentration.

 

[11echo]

I'm sure by the time of this posting this is a "done deed", BUT I think you need to think about the maintenance aspect of this. IF you were to use studs, you would have to disassembly the entire leg of the piping to get the valve out for maintenance issues. If you used bolts, this would not be necessary. Also if you used the copper or graphite based anti-seize thread compound, this would eliminate any galling issues you would have. Good Luck!

 

[varunmahajan]

It is my opinion that go with Class 5 interference fit on studs. In this way you can be assured that studs don't come out of the tapped hole. Also, play with H or D limit of tap to get things worked with class 5 interfernce fit. Only drawback with class 5 interfrence fit might be that your stud might be difficuly to put in.

 

[Scipio]

That reminds me, last week we had to remove a compressor cylinder and have it sent away for machining - guess why? Cap screw into the cylinder flange face stripped the threads out of the cylinder.

 

[RangerBill]

11Echo,

These bleed valves have a flanged face on one side of the block only. The other side is a bleed port only, the valve has a 2-1/16" flange face but is only 1/2" bore, there are no clearance issues removing these blocks if studs were fitted into the block as it lifts off a 'weldoflange' branch fitting. (BTW: Weldoflange is a component we use in the UK a lot which incorporates a flange, nipple and weldolet all in one forging, in case anyone here is unfamilier with this terminology, I guess they are used in the US as well?)

Bolts with nuts could never be used in this situation as they are blind stud pockets off the flanged face of the DB&B valve. Its the proposed use of bolts to make up an API flange to this blind tapped block that makes me shake my head.

 

[11echo]

RangerBill
...Well to be honest I'm not familiar with "weldoflange", but I'm in the states, dealing in the oil field and we don't see much of the "good stuff"! What we do see is like a threaded lug butterfly valve used on a tank nozzle. We use Grade 5 cadmium II coated, Hex. Hd. bolts on both sides. When we spec out the length, we try to keep a 1/4" to 1/2" clearance.
One question I have for you, ...on your blind threaded holes in your valve, are the threads chased all the way to the bottom? ...Mark

 

[RangerBill]

Mark,

Not fully chased Mark, the blind threaded holes are supplied spec'd with a minimum 'full thread' only though this should be quite accurate as the manufacturer (Pacson) produces top quality kit. In theory it should be possible to use corect length bolts instead of studs and get away with assembly and hydrotest. Its a diver ever having to fix a leak subsea and hoping firstly that the bolt head is still intact and that the bolt hasn't welded itself into the blind threaded pocket. Oh well we can only try........