Control valve inlet & outlet reducer/expander

[tkdhwjd]

Dear Colleagues,

When the required body size of a control valve is lower than the inlet and outlet line sizes, inlet reducer and outlet expander are normally provided. Under what circumstances do we use eccentric reducer/expander? Likewise, under what circumstances do we use concentric reducer/expander? If eccentric type is required, under what circumstances do we use bottom flat orientation (or top flat orientation)? Are there any industry standards? Thanks in advance for your help.

 

[katmar]

This topic can arouse quite heated discussions. It may sound crazy to you, but people get very passionate about whether the flat side goes up or down.

My take on the subject is that mostly it doesn't matter and a concentric reducer is fine. If you have solids present that may settle out, then (I believe) it is best to use an eccentric reducer and to put the flat side down. On the other hand, if you have bubbles in a liquid then putting the flat side up eliminates a potential spot where gas could accumulate. In the overwhelming majority of cases the velocities are adequate to keep the solids and the bubbles moving and flushed out of the system when you use concentric reducers.

In a recent thread (see thread408-190009) BigInch gave a link to the Fisher Design Manual. If any reference anywhere has something sensible to say on this subject it will be the Fisher book.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[djack77494]

The vast majority of control valves I've seen are located near low points in a piping circuit. Often a pipe up in a pipe rack is diverted to belt buckle height for a control valve. This allows easy access to the valve; e.g. for maintanance. In these situations, I would always use a concentric reducer because 1) they really are "the standard", 2) they are cheaper, 3) they are more readily available, 4) less chance for error, and 5) the pressure drop will be less than an eccentric orifice. The only situations where I would use eccentric reducers are for slurry systems and where it is imperative to totally drain ALL condensed fluid. In both these situations, eccentric reducers can avoid small "dead spots" that exist behind concentric reducers. The only place I'd use flat side up eccentric reducers would be in pump suction line to avoid accumulation of gas pockets.

To Mr. Katmar - guilty as charged.
Doug.

 

[25362]

I admit being one of those that took part in the heated confabulation mentioned by Katmar about reducers on centrifugal pump suction lines.

An old ROT on CV sizing I remember: never less than two sizes below size line and never less than half the pipe size.

The general recommendations, from a process point of view, to avoid adversely altering the CV characteristics and capacity are:

1. Avoid arrangements that can result in a nozzle-effect into the CV
2. Avoid closed-coupled inlet block valves reduced from line size
3. Avoid contorted manifolds to achieve accesibility, in particular in block-and-bypass arrangenments
4. Use straight-line pipe sections into and out of the CV as needed for the CV, as originally established by the control engineer. A ROT for reducers and expanders: 12
nominal pipe diameters (NPD) upstream and 5 downstream. For elbows on the same plane 18 NPD upstream, 30 NPD if not on thesame plane, etc., etc.

These are ideal situations not always achievable. Piping designers, who determine the actual layout, and process-control engineers should cooperate on this subject.

 

[ash9144]

If specifying eccentric be sure to specify top of pipe or bottom pipe for flat side orientation to the fabricators and field installers. That one has cost me some time and money

Also if tying into an existing piping system the existing pipe elevations could dictate need for concentric or eccentric.

 

[tkdhwjd]

Thank you all for your valuable comments.
It appears that there are no "generally accepted" standards, but you apply common senses and do what is logical on a case by case evaluation. Once again, thank you.