There are seveal problems with the statement that you have made. First is the pressures, If your line was only tested to 6750psi. That means by b31.1 requirements, your maximum allowable working pressure is only 5625. Do the math, 6750/1.2 = 5625. once you tested to this pressure, and the material was certified to this pressure, if you go above that pressure you have violated/ voided the certification of the material. Next you cant violate the test pressure by more than 6% (sec I) or you have effectively "destroyed" the material by certification. It has to be scrapped. Meaning if you certifed the pipe to a MAX of 5625, tested at 6750, if you hit 7155+ you have to throw the material away. SO having said all that. If you want run the material over 5625psi, i would suggest you retest the material. In the pressence of an AI. Its a certification problem not an an engineer function. The engineer can say Yeah it will hold more, but the AI says the paper says this, go above that, and ill take it out of service! hope you understand.
You and i know it will hold more, but its not what we know, its about what we can prove on paper! And as for allowable maxes, sec I for all other section is 90% of yeild strengh ( 2% offset) at test pressures. also 137.1.4 at no time during the test shall and part of the piping system be subjected to a stress greater than that permited by 102.3.3(B) During pressure tests performed in accordance with 137 the circumferential (hoop) stress shall not exceed 90% of the yield strenght (2% offset) at test temp. blah blah. the sum of long stresses due to test pressures and live and dead loads at the time of test, excluding occasional loads, shall not exceed 90% of the yield at test temp.
anyways i hope that helps.