A standard-length piping spool (typically 600-1000 mm long) is installed between a blowdown valve (BDV) and its associated restriction orifice (RO), as a good engineering practice. This is done to avoid brittle failure of the upstream piping due to low temperature ‘creeping’ into it from the downstream piping. This is particularly relevant during blowdown of hydrocarbon inventory from high pressure, where the downstream piping is subjected to extreme sub-zero temperatures. It is sometimes not possible to provide this required spacing due to layout constraints, particularly in offshore installations. No calculation or software tools, apart from Computational Fluid Dynamics (CFD), are currently available to ensure that any reduction in this length will not adversely affect the integrity of blowdown lines.
Although Process Simulation Software typically used in the Oil and Gas Industry include modules for performing blowdown calculations, these do not have the capability to estimate the length required between the BDV and RO, as they do not model the effects of axial heat transfer. This paper presents a simple method to estimate the cold temperature creep, based on solving a 2nd order non-linear differential energy balance equation, which includes the effect of radial as well as axial heat transfer. Attempts have been made to minimize the assumptions in deriving the equation. The dynamic results of the MMDT Study (viz., fluid or metal temperature, upstream and downstream of RO) are used as boundary values for solving the derived equation.
The use of this method is demonstrated in various case studies from past projects executed by NPCC and the results of this equation are validated using Computational Fluid Dynamics (CFD). The equation derived and the method developed is very useful to quickly verify if the required spacing can be lowered due to layout constraints, without compromising safety and integrity during blowdown and without resorting to sophisticated tools like CFD.