Fluid Compressibility Effects in Steam Hammer Analyses

In the power industry, steam hammer piping analyses (and fluid transient loads) are often based on incompressible flow principles. Consequently, it is common to use either computer programs based on the Method of Characteristics for incompressible flow or simple hand calculations such as that described by E. C. Goodling (ASME PVP, Vol. 149–157, 1989). Goodling’s paper provides a simplified method to visualize and estimate transient loads resulting from stop valve closure in a main steam system. To account for compressibility effects, the calculated loads are sometimes adjusted by using a correction factor directly applied to the fluid loads with the fluid sound speed assumed constant in time and space. For example, Goodling’s paper suggests that a load increase of 5% may be used. In this paper, a sample steam hammer problem is solved using the Method of Characteristics for compressible flows, where the fluid is assumed to behave as a perfect gas. The effects of steam compressibility are then discussed. Specifically the shortening of the characteristic valve closure pressure wave length, resulting from the increasing magnitude of the sound speed as the pressure wave moves upstream from the closing valve, and resulting higher loads in straight pipe segments shorter than the pressure wave length, are then discussed. It is shown that in these shorter pipe segments fluid transient loads may almost double those calculated using the MOC for incompressible flows or the Goodling methodology (without correction factors) if the distance upstream from the closing valve is of sufficient length.