Flow Orifice in Parallel of Pressure Control Valve


Thread Starter



I work in a EPC company and came across a P&ID of Nuovo Pignone Gas Turbine (not in our scope of project but was in supporting documents) and in this I found a PCV in lube oil circuit with a Flow Orifice in parallel to PCV.

A brief explanation on oil circuit. Oil from Main Lube Oil Pump get regulated (pressure not shown) then it goes through Oil Coolers and Oil Filters; and then it gets regulated again with PCV set at 1.72 barg with Flow Orifice in parallel; and then goes to Lube Oil Header.
I worked in Oil&Gas (Solar Gas Turbines) and fertilizer industry as mechanical engineer in maintenance department before this company, and this is the first time I came across this scenario. I have seen bypass valves across PCV or Two PCVs in parallel but never one with Flow Orifice in parallel. I tried searching internet before posting here but to no avail.

The only reason I can come up with is that its there to provide a minimum oil flow in case PCV gets stuck. If there is a specific reason which I am overlooking then please guide me.

Also I found same logic at Nitrogen Supply to Dry Gas Seal. PCV with Flow Orifice in parallel after filters.

Since it was in supplementary documents I do not have any other information on GT like model, make etc.

Thank you
I presume the lube oil pump is a positive displacement pump : such a pump must have a discharge overpressure protection device not to break anything. The flow orifice in parallel to the PCV may play that role ; as you suggest, it may also be there to maintain a minimum oil flow.

The Main L.O. Pump of many GE-design heavy duty gas turbines--and gas turbines made by Nuovo Pignone were licensed from GE for many years, until GE eventually bought NP--(the older models with gear-driven pumps) are positive displacement pumps. The Auxiliary L.O. Pumps on these same turbines are usually AC motor-driven centrifugal pumps, and are used in start-up, shutdown and loss of Main L.O. Pump conditions.

I was told MANY years ago, and have always believed, that the primary reason for the orifice in the bypass around the PCV was to ensure some flow would make it to the bearings in the event the PCV failed. I don't have any P&IDs at this time, but if I recall correctly there is a pressure relief valve on the discharge of the Main L.O. Pump--something which many people find odd, until they are told the Main L.O. Pump is a positive displacement pump. That pressure relief valve is constantly relieving under normal operation to limit the L.O. pressure going into the L.O. system. It's not a lot of flow, but there is flow through the pressure relief valve. (How else would one "control" (limit) the discharge of a positive displacement pump? Simply and easily, that is.) So, the first pressure regulation the original poster mentioned is usually a pressure relief valve functioning as a pressure control valve, not an actual pressure control valve....

Most people also find "fault" with the fact that the L.O. is cooled before it is filtered--since warm L.O. is "easier" to filter than cool L.O. It's GE's philosophy that a LOT of dirt can collect in the L.O. cooler (heat exchanger) and that since filters don't capture every bit of dirt that passes through them (by design) that dirt can build up and eventually be dislodged and if there's no filter after the cooler (heat exchanger) it can make its way into the bearings and hydraulic system (since L.O. is used as the hydraulic fluid on most GE-design heavy duty gas turbines). So, in the GE philosophy, the L.O. is cooled THEN filtered, reducing the chances of a sudden large quantity of dirt entering the oil systems.

Hope this helps!
HI Safaq, your understanding with respect to orifice in parallel to PCV is correct. it is indeed provided as a protection mechanism in the event of PCV failure (stuck close).