I think you still can use or derive the allowable stresses with the guidance above. For the above ground piping you additionally need to take the environmental factor into consideration. Sun light thermal effect should not be ignored. Additionally you need to shield the pipe this material against other hot uninsulated piping or any other thermal effect which will increase the wall temperature to unticipated level.

HDPE can be safely used -20 deg C to +40 deg C in accordance with ISO and Australian standards. The Code Case N-755 allows to use it up to 60 deg C. If you use it you need to use different service factors other than water service. The Code Case uses service factor of 0.5 (derating factor) for water. It may go to 0.25 for extreme cases. Therefore this factor should be derived by the user of the pipeline if you cannot decide yourself easily. So you have to be careful in selecting or deriving the allowables. If you check the elasticiy modulus at this temperature it may discourage you to use HDPE at this temperature. You may have to support the pipe continuously. The poisson's ratio and elasticy modulus are very important factors in the design of the pipe, selection of span length. If you do not have concrete information about the material that you are using you need to get those from the manufacturer of HDPE including supporting distances etc...

I suggest you to go through the handbook/catalog of major suppliers to understand the material and the concept how to use it. Check Performance Pipe Handbook on the internet.

Regards,

Ibrahim Demir