I have been asked to select a steam trap for installation after a desuperheater. I have been led to believe that it is usual to install a collecting pocket with a trap in such circumstances. I believe the desuperheater will be a spray type.
What load do you base the sizing of the trap on? The steam flow rate is 150 ton/hour and the pressure is 1 bar g.

If it's a big line or if you're trying to get close to saturation, you may want to consider a drain pot with level switches and an air operated valve instead of traps.  

You also have very little differential pressure across the trap, so if you go with a trap, it's likely going to be quite large.

Using an operated valve(ball valve) allows you to make a conservative assumption as to how much water you might overspray.  The most conservative thing to do is to assume that you need to remove all the water that the desuperheater can introduce into the pipe.  That approach is often not practical and you'll need to develop a logical overspray scenario to size your trap or valve.  

I can't give a simple answer, but factors to consider are whether the line is in continuous or intermittent operation, the turndown requirements/capabilities of the desuperheater, the location of the temperature feedback probe, whether the desuperheater control valve has a tight shutoff isolation etc.  

Our normal practice is to size a drain pot for no less than 2 pipe sizes smaller than the line size, minimum 4".

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