Cap Screws in Piping Design

I assume by cap screws you mean soc head cap screws ? If it is Hex head cap screw it is more or less same as normal hex head screws.

I am not sure about the usage, since I have rarely seen the soc hd cap screws being used in flange joints.

These are basically used when

a) Space is constrained (diameter of soc hd cap screws are typically 1.5 to 1.6d against almost 2d across corner of the hex hd screws- in which you have to further accommodate the box/ D-spanner)

b) Strength requirement - Soc Hd screws are typically 10.9, 12.9 and above grades whereas hex hd screws are usually max upto 10.9 (though higher grades are available)

Normally flange joints rarely need these features, at least I have not come across.

And if you mean Hex Soc Head cap screw then it is almost same as bolt, with just a bit of closer tolerance (as per ASME definition)

Hi,

I am asking about hax. head cap screws only.

Hex Head Cap Screws are normally used where you are attaching a normal flange to a cast or forged body which has drilled and tapped holes. You might find such tapped holed on equipment such as a Compressor.

They are also used on certain "Lug" body Wafer Butterfly Valves where the Lugs are drilled and tapped. With this type of Butterfly valve installation you can remove the piping from one side of the valve and the valve is still firmly attached to the piping on the other side.

I'm not sure of this, but the corner between the shank and head of a bolt or cap screw may constitute a stress riser. If so, a stud/nut combination might be stronger than a bolt head. As I recall, TEMA standards call for flanged joints to be made with studs and nuts rather than with bolts or cap screws. BoltIntegrity could probably shed some light on this.

The corner between shank and head has a radius to ensure the stress concentration is within limit. In fact in a properly manufactured bolt, I am yet to come across the head failing in this particular area.

The main problem with these screws are the friction & the biting in at the head. For this we use a machined (hardened) washer below the head to protect both the screw/bolt and the flange/face.

As far as the OP is concerned, the Cap screw and the non-cap screw differences are very ambiguous now and the exact cap screw may be very difficult to distinguish, even if one follows the literatures. In fact we have now the practice to call - to remove any confusion- specifically as full threaded whenever we need it. Thread Tolerance grades are anyway taken care of in the specification.

And the usage of stud+nut , bolt/screw+nut and the screw (minus nut) actually depends on the flange configurations. (if one of them is threaded the user doesn't really have a choice)

A wafer type valves (including the butterflies) make the corresponding bolt availability difficult (due to length), there we go for stud.

In certain cases, it becomes a bit difficult to accommodate bolts and there too studs are in use (close bend/bulge near by).

Thank You PennPiper,

But after reading discussion I got a new question.

You wrote that Cap Screws are used for "Lug" type butterfly valve. So how to find from number of butterfly valves that any particular valve will need it.

Does we need to see the vendor data everytime we are going to put a butterfly valve or there is any rule of thumb to find it out. As after reading material discription in material spec it does not get clear that valve is of lug type or not.

Thank You.

Please look again at my first response, I said:

"They are also used on certain "Lug" body Wafer Butterfly Valves where the Lugs are drilled and tapped." This means "Threaded Lug" Butterfly only.

Your Piping Material Engineer (or who ever writes your Specifications) must specify one of four basic types of Butterfly Valves.

These are:

• Flanged end Butterfly Valve - See: http://www.tycoflowcontrol-pc.com/products_results.asp?Selection=Double+Flanged+Butterfly+Valve&m=1
• Wafer Butterfly Valve - this is the type of butterfly valve that fits totally inside the bolts. See: http://landee.manufacturer.globalsources.com/si/6008801516777/pdtl/Butterfly-valve/1023224712/Butterfly-Valve.htm
• Through Lug Butterfly Valve- This type has lugs that are drilled to the same diameter as the flange bolt holes. The Studs or Bolts will pass through the lugs. See: http://www.alibaba.com/showroom/lug_type_worm_gear_butterfly_valve.html
• Threaded Lug - These lugs are drilled and threaded to accept "Cap Screws"