The installation of gas turbine meters is more govern by AGA Report 7 and they take a little differnet bent. They're not perscriptive as API so vendors have different recommendations for turbine meters. For smaller meter you could use a confirguraion like that shown in the attachment...
Man 500°C really puts you out there. Are you sure about that spec? Hydrocarbons tend to coke at these temperatures. This is the reason that you can't find reliable fills above 300°C they break down and release vapor which is death to pressure measurement. The gas law takes over at that point...
The amount of straight run is a function of what is happening prior to the run. If you have a couple of elbows then you have trouble. You’ll get swirl which is a flow vector that is not orthogonal to the pipe axis. This will reduce your accuracy. API MPMS 5.3 allows 5D upstream for a standard...