Higher Rating for Small Fittings 3

[McDermott1711]

Hi everybody,
In all of the projects that I was engaged in, small fittings (<2") have higher rating with regard to larger ones. Does anybody know the reason for this?

 

[LittleInch]

Physical strength.

See the post on 31.3 structural strength which covers similar things. Also a #900 flange is the same dimensions as a#600 for 2" and below. There is so little difference in price it is easier to have one size of pipe and flanges to cover all pressures.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[McDermott1711]

Thanks Littleinch for your reply, but I didn't get "See the post on 31.3 structural strength which covers similar things". I'm not thinking about price but rather want to technically know why when our line is 300#, the higher rating (say 3000#) is used for small (welded or threaded) fittings.

 

[LittleInch]

As I said it's all about structural strength not technical pressure ratting. You also need a minimum thicknesses for screwed joints and socket weld which you don't get if you calculate thicknesses based on pressure containment only due to the small diameter. It also means the support distance can be bigger and you have more resistance to external forces (people climbing on it etc) than if it was thinner.

That's all there is to it I'm afraid, there's no magic formula or greater technical reason, just years of past experience.

LI

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MikeHalloran]

I have seen a requirement like that in specs for equipment bound for offshore oil rigs. The thinking, as I conjecture it, includes:
- In small sizes a 3000# fitting doesn't cost a whole lot more than a 150# fitting, but:
- It's less likely to break if stepped on.
- It's less likely to break if something heavy falls on it.
- It's not going to be perforated by corrosion anytime soon.
- It will be useful when the rig crew needs to kluge up a hydraulic system to deal with some emergency, but they have no stock of loose fitings, so they have to cannibalize your product.



Mike Halloran
Pembroke Pines, FL, USA

 

[donf]

A lot of it has to do with what's commercially available. For example - for a small bore, low pressure, welded piping system, it's quite common to see class 150 flanges, class 800 socket weld valves and class 3000 socket weld fittings. In each case, that's probably the lowest pressure rating commercially available component.

donf

 

[moltenmetal]

donf has it right. In carbon steel, you're likely to see 150# flanges, 800# or 1000 WOG SW valves and 3000# SW fittings in a welded spec < 2", since the 150# fittings are only available in threaded cast MI (not weldable). In stainless steel you're likely to see the same except with the 3000# fittings replaced with 150# CF8M cast fittings because they're available- as long as nobody is worried about sensitization during welding since CF8M is 316 not 316L.