P-No.4 materials shall not be specified for pressure retaining equipment or pipework components where the design temperature is in excess of 900°F (480°C). Exceptions to this are heat exchanger tubes and standard weight (or thinner) steamgenerating boiler and fired heater tubes. Where a benefit can be demonstrated in using these materials for design temperatures in excess of 900°F (480°C), the recommendations specified in API Publication 938 shall be followed, subject to specific review and approval by PURCHASER, using lower strength Class 1 materials to minimize potential fabrication problems and in-service cracking.