Welded End Valves (1500# and 300# Ends)

[Angsi2]

Situation:

A. Have a 1500# carbon steel actuated ball valve with butt weld ends in B31.3 piping. A pressure class break occurs after that to 300#.

Question:

B. Should one side of the butt weld end valve be specified for the required wall schedule at 1500# and the other end for 300#?Or should both sides be specified for 1500# and leave it to the pipes being welded to make the adjustment?

o Should the 300# piping match the wall thickness at the valve end?
o Or can a valve end with Sch 160 be welded onto the 300# pipe with lets says Sch 40? Is this permitted by Code/B31.3?

Thanks in advance.

 

[XL83NL]

Put the spec break behind the valve, i.e. if the valve can feel the Cl 1500 pressure rating (please note its Cl. as a designator, not #, for flanges), rate it at that pressure.
I believe there's some rules/guidance on this in B31.3 (2010 edt); para 302.2.5.

Furthermore, B31.3 has details on trimming/taper, see fig 328.4.3. Where the Cl. 300 piping starts, take a sch160 (if thats your schedule size at the butt weld end of the valve) piece of pipe, weld it to the valve, and downstream, taper the pipe at a 2nd weld end to sch40 to continue with your Cl 300 rated system.

 

[LittleInch]

I agree with xl83nl. I assume you are looking at a drain / vent isolation valve as a ball valve cannot control pressure accross a design pressure break. I can only assume there are safety features in place to avoid the potential overpressure that could exist in such a large presusre rating change.

The valve should be a single pressure rating including any end connections, apart from anything else you would find spare / replacement valves difficult to source and you never know if the valve could be welded in the wrong way around. Always work on the basis of if it is possible and not able to be checked at a later date, it could happen and then if it fails it becomes your fault, not the pipe fitters....

A tapered spool at not more than 1:4 should be used. Again if you can't seen the internal weld bead change, you can't check it.

I have seen choke / control valves with different flange ratings on inlet and outlet before, but weld ends are much more difficult

My motto: Learn something new every day

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