So if you use a needle valve, and the Cv is right, why use a by-pass valve at all? As its name suggests, the by-pass valve bypasses the regulator
, or in my case the needle valve. A needle valve by nature, can adjust from closed (depending on the seat type) to full open. I normally operate the 1/4 turn bypass (ball type) valve slowly so as not to shock the membrane and turn it off after every use so as not to start up under load. If this is the SOP, then why not just use the needle valve?