Models and Methods of Determining the Minimum Stack Metal Temperature During Sonic Blowdown of Natural Gas

During a natural gas blowdown event, the flow through the blowdown stack is either sonic or supersonic at the stack exit. In this case, the temperature of the gas at the stack exit can drop significantly as the gas enthalpy is converted to kinetic energy. Depending on the initial pipeline temperature, it is possible for the gas temperature at the stack exit to drop below the minimum metal temperature specification of the stack steel grade. Traditionally in this case it is assumed that the blowdown-stack metal temperature follows the gas temperature; therefore, it would also drop below this minimum temperature. However, analogous to the decrease in the temperature of the gas as the velocity increases, the gas will increase in temperature as the velocity decreases near the wall of the stack due to the boundary layer. As such, the blowdown-stack wall temperature will not decrease to the same extent as the bulk gas temperature during the blowdown event. This phenomenon is referred to as wall temperature recovery. This paper describes the various models and methods for determining the extent of this temperature recovery via a parameter known as “recovery factor” which is a function of the flow Reynolds number, Prandtl number, Mach number, etc. These methods are evaluated and compared for several pipeline conditions, and the most suitable model is determined. It is recommended that fundamental blowdown testing on natural gas be conducted on well instrumented full-bore blowdown stack to validate the predictions by these methods.