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.
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 .
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?
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.
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).
