Required Pipe Diameters Up and Downstream of Control Valves 9

[JTPiper]

Hello all. This is my first question posted and it's probably something that varies with each situation, but is there a standard requirement for pipe diameters up and downstream of control valves? I have a
3" line that drops approximately 15' down from a pipe rack and am leaving 3 diameters of pipe on the upstream side and 3 pipe diameters downstream of a 3" control valve at grade. I have read that leaving 3 diameters upstream and 3 diameters downstream of a control valve is a good practice, but have never seen it mentioned anywhere else. Is this a standard practice or does it vary with different scenarios? Thanks in advance for any advisement in this matter.

 

[zdas04]

JTPiperGRSA,
There is a lot of minimum straight pipe stuff in the measurement and control literature. In nearly every case the underlying intent is to prevent a swirling flow from "screwing" itself through a narrow opening and overriding the control function (e.g., two elbows out of plane can create a long-lived swirl that finds it easier to get through the orifice in a differential producer than a fully developed flow and can understate dP enough to understate "measured" flow by over 30 percent).

Pipe of the '30's and '40's was far rougher than today's pipe, and had a chance to dampen swirl. A persistent swirl in modern steel pipe will last for a few thousand pipe diameters. In plastic pipe it is a few tens of thousands of pipe diameters. Adding 3-6 pipe diameters upstream and/or downstream of a control valve has zero practical impact. Having said that, a LOT of people don't believe me (story of my life), and will fuss at you if you don't provide SOME straight pipe around a control valve. I find that life is awfully short to fight this battle on every control valve and if the client wants some straight pipe I rarely try to educate them, I just call out "X diameters of straight pipe before and after control valve" on the drawings.

[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

 

[JTPiper]

Thanks for that explanation David and if it helps, I believe you.

 

[pennpiper]

zdas04,
I agree with you and I offer the following Blog for your information and amusement.

http://pipingdesigners.com/contents/blog/130-blog-1l-who-is-beauregard-gustafson

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

 

[Gator]

I would ask whoever demands such a straight flow to the control valve to point out where in the vendor documentation this is specified. Always read ALL vendor material where it's relevant to installation requirements.

Do you recall where you read this practice?

 

[DGrayPPD]

Piping Handbook 7th Edition by Nayyar

Page B.85 on Piping Layout

Control Valves

All control valve stations should be designed with the valve stem in the vertical upright position and a minimum of three diameters of straight pipe both upstream and downstream of the control valve, in order to reduce the turbulence entering and leaving the valve and to provide space for removal of the flange studs or bolts. Where applicable, this straight pipe will include the usual reduction in pipe size required to match the control valve size. Space must be provided for flange stud bolt removal where control valve bodies are designed for through-bolt installation.




"Tell me and I forget. Teach me and I remember. Involve me and I learn."

- Benjamin Franklin

 

[Artisi]

Pennpiper: thanks for the link, finally solves the problem of his real name. And as pointed out,the bugger crops up everywhere, not just engineering.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[JTPiper]

Gator, I have never read it in a vendor supplied document before. I am getting this "practice" from the quote of the book mentioned in DGrayPPD's reply.

 

[moltenmetal]

In this case, Nayyar is providing a rule of thumb advice that is baseless in my opinion. Upstream and downstream lengths are required ahead of flow measurement devices, not flow control devices. 3 diameters won't be enough to meaningfully settle down the jet exiting the trim from most control valves in the partially open position, and since control valves are often one or more pipe sizes smaller than the running line, you will still get a jet at the line size expansion on the discharge side. Best to put flowmeters upstream of control valves in most cases rather than trying to settle down the jet out of the control valve discharge by means of straight length alone.

 

[JTPiper]

Thanks for all the great advice and replies. I am gathering from most of you that if it isn't specifically required from the vendor documents, it's pointless because 3 diameters wouldn't settle the turbulence down enough to make a difference (which makes a lot of sense). I will probably take it out and see what the client has to say in our next model review. If they request "X" diameters up and downstream, I'll just add it in. Thanks again.

 

[Gator]

I've asked Mohinder to comment on that portion of the chapter, but he's retired now and I haven't heard from him for a few years.

That particular chapter was written by three people who are not Mohinder Nayyar.

Which reminds me to mention that Elsevier has published the second edition of The Planning Guide to Piping Design, which contains my new chapter on CAD and Design Automation in Piping Design.

 

[DGrayPPD]

Nice plug Gator haha.

"Tell me and I forget. Teach me and I remember. Involve me and I learn."

- Benjamin Franklin

 

[Gator]

_{Shhh. I don't think anybody noticed and I got away with it.}

 

[Krausen]

Although one could argue it's minor, you will de-rate the performance of your control valve from the standpoint of how much pressure it can throttle without cavitation if you do not provide adequate straight pipe upstream & downstream. This is according to the Fisher control valve handbook for liquid applications anyway. If you step through the sizing calcs in this handbook, you will run across the factor for having straight pipe upstream & downstream (I can't remember the designation of the top of my head). My experience has been this effect is relatively minor, but it does exist (at least according to the control valve manufacturer) and should therefore not be ignored in my opinion.

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[MFJewell]

Although one could argue it's minor, you will de-rate the performance of your control valve from the standpoint of how much pressure it can throttle without cavitation if you do not provide adequate straight pipe upstream & downstream. This is according to the Fisher control valve handbook for liquid applications anyway. If you step through the sizing calcs in this handbook, you will run across the factor for having straight pipe upstream & downstream (I can't remember the designation of the top of my head). My experience has been this effect is relatively minor, but it does exist (at least according to the control valve manufacturer) and should therefore not be ignored in my opinion.

This the methodology I use as well. I believe it is something like 3D upstream and 6D downstream.

 

[pennpiper]

Gator,
You wrote:
"I've asked Mohinder to comment on that portion of the chapter, but he's retired now and I haven't heard from him for a few years."

Let's think about this a bit, Mohinder L. Nayyar wrote Chapter A1 and A4 in the first Edition of this book. The Book then first published in 1930. If Mr. Nayyar was even as young as 30 years old when he wrote those chapters then he would be 117 years old now. I suspect he was much older when he was involved in the writing and publishing of The Piping Handbook.
At this point I don't think he is available to answer your phone call.

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

 

[Gator]

Hi pennpiper,

Nayyar was alive and kicking as of 2012. The same might not be true for the authors of the Piping Layout chapter.

https://www.asme.org/about-asme/get...s/mohinder-nayyar-honored-contributions-codes

Reno King and Sabin Crocker were the editors prior to Nayyar taking over the role.