we have steam Boiler super-heated, we operate it 37 Kg/Cm2 at 417deg C. but after pressure reducing at 19 kg/Cm2 gauge, the temperature decreased at 373 degC. the distance between boiler and user is on 50 meter. steam line is fully lagged, we are confused how it possible? steam temperature decreased 44 C deg within 50 meter line with out any desuper heating.

 

if you have adiabatic pressure reduction the steam temperature is 404.8C at 19 kgf/cm2, ignoring heat loss in the valve. So now you are down to 31.8C temp loss in 50 m. If you do the heat loss estimate for your pipe, I think you'll find that the temperature drops pretty fast.

You made no mention of the line sizes or the mass flow rates, you can also have additional P,T losses with friction loss in the pipe.

 

Check steam tables.
At 40 barA, 400 degC, steam has an enthalpy of 3214 kJ/kg. If pressure is reduced to 20 barA, and the enthalpy remains the same, the temperature will reduce - interpolating I get 384 degC.

So steam does reduce in temperature if pressure is reduced through an isenthalpic process.