We have multiple valves thoughout the plant that has a smaller bonnet bleed valve. The question came up as to what postion these valves should be in. Does one leave the valve open at all times; or do you open the valve prior to operation and then shut it?

For example we have a 8" Newco gate valve (feedwater control valve outlet) and there is a smaller 1" globe that taps into the bonnet and goes to the downstream side of the larger valve.

I am not an expert in this area; however, small changes in temp can creat huge pressures for fluids in confined spaces.  If bonnet is isolated and liquid packed when valve is closed, and then there is an increase in the trapped fluid temp, excessive pressure in the bonnet will lead to seal or bonnet failure.  

The small globe valve should be open when the gate valve is closed if fluid can be isolated in the bonnet when the valve is closed.

I believe that swagelok makes some of the small PSVs that I have seen on some bubble tight valve that have isolated pockets when the valve is closed.  I don