BY PASS SIZING FOR BUTTERFLY VALVES

Butterfly valves (BFV) are very used in irrigation lines as a guard for service valve maintenance. They must be capable in closing at full flow but open at balanced pressure. For balance of pressure water must be allowed to fill downstream side of BFV, by using a by-pass line. My question is:
Is there any relation between size of BFV and size of by-pass line? Is it (size of by-pass line) related also to rated pressure of the line?
Any help will be highly appreciated.

 

[ChasBean1]

MASOUD - I don't know that there's any set rule for a bypass line, although I don't own ASME standards because they're too many and too expensive. If the system downstream is closed, any sized bypass will fill the loop, but size will determine how fast this will be accomplished. Are you sure it's necessary to bypass a butterfly valve? I thought the forces acting on the disc cancelled (or nearly offset). -CB