Required Pipe Diameters Up and Downstream of Control Valves
Required Pipe Diameters Up and Downstream of Control Valves
(OP)
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.
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.





RE: Required Pipe Diameters Up and Downstream of Control Valves
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.
David Simpson, PE
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
RE: Required Pipe Diameters Up and Downstream of Control Valves
RE: Required Pipe Diameters Up and Downstream of Control Valves
I agree with you and I offer the following Blog for your information and amusement.
http://pipingdesigners.com/contents/blog/130-blog-...
Sometimes its possible to do all the right things and still get bad results
RE: Required Pipe Diameters Up and Downstream of Control Valves
Do you recall where you read this practice?
RE: Required Pipe Diameters Up and Downstream of Control Valves
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
RE: Required Pipe Diameters Up and Downstream of Control Valves
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.)
RE: Required Pipe Diameters Up and Downstream of Control Valves
RE: Required Pipe Diameters Up and Downstream of Control Valves
RE: Required Pipe Diameters Up and Downstream of Control Valves
RE: Required Pipe Diameters Up and Downstream of Control Valves
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.
RE: Required Pipe Diameters Up and Downstream of Control Valves
"Tell me and I forget. Teach me and I remember. Involve me and I learn."
- Benjamin Franklin
RE: Required Pipe Diameters Up and Downstream of Control Valves
RE: Required Pipe Diameters Up and Downstream of Control Valves
Tolerance is the virtue of the man without convictions -GK Chesterton
RE: Required Pipe Diameters Up and Downstream of Control Valves
This the methodology I use as well. I believe it is something like 3D upstream and 6D downstream.
RE: Required Pipe Diameters Up and Downstream of Control Valves
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
RE: Required Pipe Diameters Up and Downstream of Control Valves
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-involved/honor...
Reno King and Sabin Crocker were the editors prior to Nayyar taking over the role.