Control Valve Station Piping

I would install a reducer at the inlet and outlet to the regulator. The one-size smaller regulator usually has more than enough capacity to serve the downstream piping. Any other reduction in valves or piping would not be wise, because of excessive pressure drop, either now or in the future. Hope this helps.

In general, the block valves are installed at full line size. the only reason why the reducers are installed around the valve, is so that the desired min and max flow rates at predetermined pressure drops are controllable by the control valve. usually the pressures are fixed by the material handling process and magically, the control valve must be selected as such to satify everyone.

the line is sized as according to the design criteria, current capacity and future requirements. so, you could have more than one reducer upstream of the control valve. but the block valves should be procured and installed as according to the line size and not to follow the control valve sizes.

For this reason, I recommend to use ISA Recommended Practice for selecting size of control valve and all dimensions of piping controlling circuits to adapt with your piping layout.

The Recommended Practice ISA RP75.06 for Control Valve Manifold Designs, printed in USA by the Instrument Society of America, ISBN 0-87664-656-9.

This Recommended Practice ISA RP75.06 includes 6 Tables and 6 Figures showing type of control valve manifold dimensions for ASME class 300.