flow control philisophy flow control philisophy bayan1999 (Mechanical) (OP) 20 Mar 17 05:39 Hi, i am curious to know why some seal gas flow control design have an orifice in parallel to the valve ? http://files.engineering.com/getfile.aspx?folder=275c449e-068e-430d-8aed-8b RE: flow control philisophy MortenA (Petroleum) 20 Mar 17 09:33 Seems like some kind of minimum flow or a simple split range control? RE: flow control philisophy LittleInch (Petroleum) 20 Mar 17 11:45 Agree. It's a guaranteed min flow amount which can then increase as required. Given the valve is a FO design, indicates that excess flow or pressure is not a big issue, but low flow / pressure in. I've seen this also done by mechanically limiting the control valve to a certain limit of closure. Remember - More details = better answers Also: If you get a response it's polite to respond to it. RE: flow control philisophy rotw (Mechanical) 20 Mar 17 19:58 In an ideal world the orifice would suffice (while being sized) to cover the compressor operation at the nominal/fixed point. Because the requirements of DGS can however be higher (operation off design / normal point - reference to the need to maintain a certain min. flow velocity / lift off effect between seal faces), the differential control valve is installed in parallel with the orifice, that is to say purposely to cover any surplus. RE: flow control philisophy bayan1999 (Mechanical) (OP) 21 Mar 17 07:51 Hi ROTW, if we eliminate the orifice, can we just size a valve for that purpose. The intent is to achieve the same with less parts and material. Pls share me your thoughts . RE: flow control philisophy LittleInch (Petroleum) 21 Mar 17 08:53 You will need to set a minimum flow / opening stop (mechanical stop is normal) on the control valve to fix min flow to the same as the orifice. Downside is it will not seal shut, but neither did the orifice. Remember - More details = better answers Also: If you get a response it's polite to respond to it. RE: flow control philisophy MortenA (Petroleum) 21 Mar 17 12:57 Unless the RO is tagged as safety critical i would assume that it is a pretty simple item to maintain? Given the consequences if "somebody" forgets why the limits were there on the valve in the first place and decides that these are just "extra maintenance" then you may be better of with the current design? Best regards, Morten RE: flow control philisophy rotw (Mechanical) 21 Mar 17 19:41 I think the reasoning has to do with the fact that at the contractual point, it might be needed to "guarantee" the flow at a single value. So the orifice provides a way to match this requirement (i.e. choked flow). The pressure control valve will cover operation in the area where performance are expected. Anyway, this is my guess -> to be on safe side, double check with someone who knows better the reason. RE: flow control philisophy georgeverghese (Chemical) 22 Mar 17 00:00 Are you sure this PdCV is FO and not FC ? It is not safe to have this dPCV go wide open on instrument air failure to avoid very high seal gas flow. When the control valve is FC, the bypass RO may enable seal gas flow at a controlled rate for some time when there is a general plant power failure and the instrument air supply pressure has dropped off rapidly. When seal gas flow / pressure has also dropped off, the compressor should be auto depressured ( if it hasnt already by this time). You could probably achieve the same result if you had the dPCV fitted with a min stop (either mechanical min stop or soft stop in DCS may do).