4 replies to this topic

[processji]

Normally it is a practice to have the pipe size is one size higher than the control valve size.When engineering starts in a project ,initial control valve sizes are based on the inhouse programs the company has.Subsequently the vendor checks and confirms the sizes.

If the vendor suggested control valve size that matches the line size Is it required to still increase the line size by one size add reducers?Or a control valve can be same as line size?



Thanks
Regards
K Suresh

[Samir Gupta]

Dear Suresh
The use of a control valve having one size lower than the pipeline is not a mandatory requirement. It all depends on the Cv value of the control valve and the allowable pressure drop thru the valve. Normally, the allowable pressure drop thru a control valve is considerably higher than what would have been thru a pipeline. This delta P allows better control of the fluid. To accomodate the pressure drop, the control valve is normally sized lower than that of the pipeline and hence you require a reducer.
If the valve manufacturer specifies a control valve of the same size as that of the pipeline, I donot see a reason in increasing the pipeline size and then putting a reducer. However, you should check that the pressure drop thru the line is as per your requirement and not more than that.
Samir

[Art Montemayor]

Suresh:

The way I do hydraulic design and the way I've worked with hydraulic design in/with major design engineering organizations and projects in the past is that the line size is first calculated and fixed - not the other way around as you describe it. Perhaps that is how you've been taught, but the logic seems contradictory. Perhaps it is a language or translation mis-interpretation, but you state: "Normally it is a practice to have the pipe size is one size higher than the control valve size" and this is not correct engineering design. Normally you first find the appropriate pipe size for the velocity and pressure drop you can tolerate and then you calculate the corresponding control valve sizes - just as Samir has indicated - allowing for a pressure drop acorss each valve.

It is rare to find a hydraulic system that is composed primarily of a control valve and negligible piping. Just as Samir points out, if the resulting control valve turns out to have a miserable Cv value (indicated by it being the same size as the piping, then you either have a valve with severe Cv characteristics, the wrong valve size, or you haven't allowed for enough pressure drop (& control) across the valve - something that is commonly done by engineers without sufficient design experience. I suggest you obtain the free copy of the Fisher Control Valve Handbook that can be found at their website and which I have recommended in the past. It is, by far, one of the best texts on control valve design and characteristics.

[processji]

Dear Mr Samir and Mr Art
Thanks for the response.The valve is globe type, Fisher make.Model EWT.THE IN/OUT LINE SIZES ARE 12".The pressure up/down stream are 94/20 psig.Flow is 29 MMSCFD.Temp is 79 Deg F and MW is 20.42.SG is 0.705.Service is gas.

Fisher has recommended 12" valve with WIIIA3 trim.CV at 72% opening is 331.Actualy the inlet outlet of 12" is adequate from velovcity and DP point of view.

My concern was on valve performance without reducers.
Maybe i worded my query in vague terms.Sorry about that.
Now i am convinced i do not have to increase the line size.

regards
K Suresh

[djack77494]

I would again caution everyone concerns avoidance of any "one size fits all" approach. I have seen properly sized control valves at full line size and at least two sizes (and I believe three sizes) smaller than line size. The control valve's purpose is to control the process variable and it is sized to purpose. There are situations where very little pressure drop is available and fine control is not needed. That could result in a line size valve. (My current project has several full size butterfly control valves that mainly function to balance flows in parallel paths.) If you have long circuits and lots of equipment, you could experience large hydraulic losses in a circuit. You still need to provide a proportionate amount of dP "to burn" in the control valve. Such a valve may be several sizes smaller than line size. Read the literature, exercise caution, and, if you don't know, ask.
Doug