VALVE SIZING

hi.... i have a condition where to design a valve with only the input parameters of p1=69.64bar(a)&
t1=545c. The valve flow coefficient is not known and so the outlet pressure and temperature. I have referred the standard IS 75-01 & 02, it has both coefficient and pressure drop in the same equations. in my case both are unknown. i guess someone can help me out with this valve sizing. The flow rate is about 260000kg/hr and inlet and outlet line sizes are 10 inch. the valve is immediately followed by a silencer with an elbow fitting before the silencer for an vertical silencer stack.