I am involved in a project where Customer specifies globe valve vee-reg type disc for throttling service and additionally requests leakage test complying with BS EN ISO 1266, rate A, i.e., NO VISUALLY DETECTABLE LEAKAGE FOR THE DURATION OF THE TEST. It has been hard to find a vendor to provide rate A, but rate B is not a problem. My opinion is that Customer has a too strict position but I haven't been able to persuade them to accept rate B. Has anyone dealt with such situation? 

It is difficult to find a regulating valve which will also give tight shutoff. I usually install a shutoff valve upstream of the regulator ( typically a soft seated ball valve).
The vee reg valve can the be left in it's throttling postion with no need to readjust when the line comes back on stream.
If we're talking small diameter pipe here ( less than 2" say),
the cost of the extra valve is negligible.

For what it is worth........... control valves are for control, isolation valves are for isolation. Each has its specific purpose and generally do not perform the operation of the other very well for very long! How long, one wonders, will a control valve maintain its ability for tight shut off after it has been controlling the flow for a decent period of time? Is not the control valve flanked on each side by an isolation valve?

Inevitably someone is trying to cut costs and I say that it is false economy.

Metal seated, triple offset butterfly valves might be an exception, but that is not what we are talking about here and whilst they can be a modulating valve I do not believe that they have a fine control capability.

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