Studs vs Bolts into valve block
Studs vs Bolts into valve block
(OP)
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.
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.





RE: Studs vs Bolts into valve block
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.
RE: Studs vs Bolts into valve block
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.
RE: Studs vs Bolts into valve block
RE: Studs vs Bolts into valve block
RE: Studs vs Bolts into valve block
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.
RE: Studs vs Bolts into valve block
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.
RE: Studs vs Bolts into valve block
RE: Studs vs Bolts into valve block
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.
RE: Studs vs Bolts into valve block
RE: Studs vs Bolts into valve block
RE: Studs vs Bolts into valve block
RE: Studs vs Bolts into valve block
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.
RE: Studs vs Bolts into valve block
...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
RE: Studs vs Bolts into valve block
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........