FFS Forum: Minimum Pressurization Temperature (MPT) Curves: Why and How they are Going to Change

Since about 1987, the various pressure vessel and piping design codes have included a toughness check. They require that the design either meets certain minimum toughness requirements (the exemption curves) or that Charpy testing is performed. This is generally adequate for the design conditions to which it is applied. But these design conditions typically do not represent the transient conditions encountered during start-up and shut down. For these more complicated conditions, a more detailed assessment is required. This is the domain of the Minimum Pressurization Temperature (MPT) curve.

The basic problem in most pressure system brittle fracture problems is that, for carbon and low alloy steels (i.e., ferritic steels), the fracture toughness of the material decreases at lower temperatures. The fracture toughness at -20F is significantly lower than it would be at +100F. So, in a start-up scenario, where the pressure may rise faster than the metal temperature, you can wind up with material at the design stress value when the metal temperature is still far below the design temperature value; that is, a high stress, low temperature scenario.

MPT curves represent one particular area within the more general topic of brittle fracture assessment (BFA). Brittle fracture assessment is an area that is currently getting a lot of attention in the technical community. A plethora of problems related to foreign flanges and forgings, hydrogen charging, and other problems, has raised BFA to a high level of visibility. There has been significant activity in many code committees in recent years on this topic and the procedures for BFA will likely evolve over the next few years. That said, the goal of this article is primarily to demonstrate how an MPT curve is developed under the current API 579 rules.

BFA has its root in fracture mechanics, so we’ll start with a brief introduction to the basic fracture mechanics principles.