Spec Breaks

[NPC5]

I have a line coming off of a 300# spec line, a block valve, another block valve, and then the line returns to the 300# spec line. Between the two block valves there is a branch, a block valve (normally closed) and then the branch flows into a sewer header (150#) that feeds into an atmospheric tank. Why is it okay to have a 300#/150# spec break downstream of the block valve in the branch line? I have attached a drawing to try to clear my description up. Thanks.

 

[KenA]

NPC5,
What your sketch shows looks fine to me, assuming that the "sewer" is open to atmosphere. The spec break is on the downstream side of the last isolation valve. If you close the valve then everything (including the valve) is rated for the upstream pressure. The downstream piping should never see higher pressures than 150# rating because flow rate can never be high enough to generated that sort of back pressure. (unless you get a blockage )

 

[europipe]

Why do You have a break there?
Is the schedule of the pipe different?

 

[JLSeagull]

Why not have a spec break there? You don't know what someone may add later. For example, if I add a variable area flow indicator to see how much I was draining to the sewer I would not need a Class 300 flow meter.

 

[NPC5]

Thanks a lot for the help. Europipe, the line eventually ties into a 150# header so I had to put a break somewhere.

 

[europipe]

It's allright but not neccessary when You add another 150#
continuing line ( you can lay the break there),
but when You want to add another 300# system later You have to demolish the line, (except when You can upgrade the pipe; p.e. when it's the same schedule)

Greetings

 

[scoho83]

I am not sure what Europipe is saying. It would appear that you would have to do allot more than just "change the pipe" if you tried to put a 300# system there instead. One thing you should do is to do some simple blow-down calcs to make sure that at choked flow through the block valves and going to atmosphere you don't generate a backpressure in the 150# system that exceeds its rating. (i.e. someone left the valve open when the 300# system is fully open, and then the pressure just downstream of the last valve turns out to be due to hydrauloics above 285 psi (or whatever you design pressure is on the 150# system).

 

[europipe]

I'm sorry for not being very clear.
English is not my native language.
But when I look at the picture it's only a piece of pipe with atmospheric conditions.
So why should You use a different pipespec. and maybe a different pipeschedule?

Greetings.

 

[scoho83]

You use a different pipe spec for relief analysis, safety systems and hydrotesting issues. It may in the end be the same pipe (materially and wall thickness wise) its more of a rating issue and what kind of safety systems you need around it.