Fitting difference

[jmiles]

Can someone explain to me what is the difference between a coupling and a weldolet or threadolet, are they essentially the same things?

My actual question is in relation to welding these on to a vessel, fom that perspective are they essentially the same with just different ends? (I know a weldolet and threadolet are the same except for the end connection coming off the vessel they would be welded on to, but i just want to make sure im clear on what the difference is between these and a coupling.)

 

[pennpiper]

A coupling is made to join pipe. It does not have any built-in reinforcement for the missing material of the vessel shell or pipe wall. Couplings are often used for small nozzles or pipe branches because they are cheaper but have the flexibility of use to join pipe or make nozzles.

O-Let fittings are engineered, designed and made for the specific purpose of making self-reinforced nozzles on vessels or branches on pipe. The shape of the O-Let fitting has the extra bulk to replace the missing material from the vessel shell or the pipe wall. O-Let fittings are a little more expensive but cannot be used for any other purpose.

 

[racookpe1978]

One small advantage of a coupling used on a pressure vessel or tank rather than an "O-let" fitting:

The O-let fitting is cast and forged to sit on the side of a pipe - which is usually relatively small bore: the O-let fitting is attached to an OD of between maybe 2.75" to to 14 or 16 inch diameter. So the base (bottom) of the O-let fitting is curved for those small diameters.

A tank or pressure vessel might be anywhere from 24 inch OD to 200+ inches OD. So the sharply curved bottom of the O-let fitting doesn't fit well over the relatively "flat" large diameter tank wall. A coupling inserted through the tank wall and fillet welded on both sides gives a better, (and faster!) double weld with less heat buildup and stress residuals than the large fill-in-the-heaping-gap weld required for a small pipe or vent on the tank.

 

[pennpiper]

racookpe1978
There are FLAT O-Let fittings.
Check here:

http://wfi-intl.com/WFI_catalog05.pdf

 

[jmiles]

Thanks for the clarifications much appreciated

 

[racookpe1978]

Good find, I'll update my (much older, paper-copied) versions of the Bonney Forge catalogs: Thank you.