Basically Valve is Final Control Element in any circuit.It may be controlling air,water,oil,gas,chemical or compatible flow media where Vaccum use of Valve is Unavoidable.
For Optimum selection of valve, several important factors are involved.

Flow Media.

Process Temperature.

Pressure

Preference for Valve Type.

End Connection Preference.

Material of Construction.

Duty-ON/OFF or Regulating.

Type of Actuation.

Application.

The information indicated above can be used to assist in selection of Valve

Flow Media,Process Temperature and Pressure helps in selecting body trim and rating of the valve if the class ratings is not specified.

In case if size is not specified with the data of flow rate and inlet pressure, one can calculate Cv and select the size of the valve.The calculated Cv should always be less than Valve Cv to accomodate the End user requirement.

End connection if not specified,depending upon type and size Valve can be recommendedsuch as Screwed End/Flanged End or Butt Weld.

Duty and Actuation preference-It is better to provide preference according to the requirement keeping in mind the technical compatibility and commercial vaue.