A masoneilan (41312) 3 inch pressure control valve is installed on a propane line, the valve is passing when fully closed (N.C.). Put in a new seat and plug but the valve is still passing. any suggestions?
How much seat load do you have? Is it ATO or ATC actuator? With new trim and proper seat load it should seal that TFE seat.
Your valve model listed is not leak tight with Class IV or V Leakage rates. At a minimum you need a valve rated for Class VI or better.
If the cage-guided valve you have is selected to handle pressure drop/noise, then you need a separate valve in series.
With combustibles there should be a double block valve arrangement. At burner fronts, a double block and bleed to insure leak free shut-off.