Valve placement for maintenance 3

[yama21250]

A piping contractor recently made a comment on our piping saying that we should add a spool piece in between valves for "maintenance purposes". Has anyone else heard of this? If the valves allow I have always put them flange to flange to avoid extra pipe and fittings.

 

[MortenA]

Depends on pipesize and pipeclass. If the pipe is very stiff a spool piece (or maybe just a ring spacer) could be a good idea. I have, however, more commonly seen it if i wanted positive isolation.

 

[LittleInch]

valves flange to flange implies no vent or drain and potential for getting high pressure locked in / thermal expansion.

what type of valves?

Access and operability of the valves ( handles clash etc) would seem to me to be more important.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zdas04]

I have never bolted valve to valve that sometime in the future I didn't regret it. At the very least valves are heavy and pipe moves--a combination that can make it impossible to get a valve back into the spot it came out of.

Also if I put a 6D spool piece between valves, I can put a vent, drain, or both on the spool piece to allow for double-block-and-bleed, chemical injection, purge gas injection, or check valve testing. No one ever had too many access points to a piping system.

Finally, if I drop the spool out for maintenance and find it impossible to get it back into the space it came out of (this has happened twice to me), you can make up a shorter spool to fill the gap.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[yama21250]

The contractor mentioned that in any case you shouldn't put valves back to back regardless of size, spec, pressure, etc. I see the point of getting pressure locked in between, therefore I understand why on a steam header you would want to space the valves (plus this allows the hand-wheels to be spaced better). But on a lower pressure piping system (cooling water, etc) we commonly have cases where we have a butterfly up against a strainer, or a ball valve and a check valve together. Does anyone see an issue with this?

 

[zdas04]

I frequently see wafer-style butterfly valves inserted between a strainer flange and a pipe flange. When you break the flange the valve falls onto the ground. There are times where that is fine (the real block valve is elsewhere and the wafer valve is just there to isolate the strainer for cleaning), other times it is not so fine.

I find it best to take a moment and think about as many scenarios that I can come up with and assess to appropriateness of my desired valve placement in each scenario. What you do not want to do is increase your lifetime costs by saving a little bit of up front capital.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[pennpiper]

yama21250,
Can you give us some information about the specific situation that initiated this comment?
In the overall picture of a typical Process Plant (Refinery Unit,Chemical Plant Unit) or Power Plant there are normally not a lot of Valve to Valve situations.



Sometimes its possible to do all the right things and still get bad results

 

[lilliput1]

Also must have union if pipe fittings are threaded.

 

[yama21250]

I would say the two most common situations are valves up against a strainer and a shut off valve up against a check. (I realize the valve up against a check can be another discussion)

We are constantly getting beat up for reducing our fitting counts and welds in our design. Although I see you point to this I think it would be a bigger deal to have to answer to why we added 2 more welds, pipe, and two fittings and an extra gasket if not needed.

It also surprised me that the contractor brought up this comment and not the plant maintenance guys. We have done work on multiple plants for this client and it has never been mentioned in the past. There was some talk about the bolts are difficult to put in and take out. I wonder if it is more of a inconvenience that they cant use a air gun to tighten all of these valves during fabrication?

 

[zdas04]

My answer to people who count welds and fittings is "this budget will be behind us in a few weeks, the result will be in the field for the next 30 years." The last time I got talked out of what was right (in that case it was a block valve at the end of a well-tie line early in my career), I got cussed for having to shut in 45 wells and blow down many miles of pipe to get a check-valve flapper out of an ESD, it cost us several million dollars to avoid a $600 4-inch valve (with the proper lateral block valve I would have only had to shut in one well and blow down 80 ft of pipe). I never did it again.

My priority will always be maintainability. I really want to take every one of those KPI-spewing idiots and put a wrench in their hands in a driving blizzard, standing on a frozen mud hole that will always thaw enough to break the first time you are in an awkward position. I've never been able to, but I've always dreamed of it.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist