Methods for Determining the Amount of Hydrates Formed During Blowdown of Natural Gas Compressor Station

Natural gas hydrates could form during blowdown of compressor station yard piping, even if the moisture composition is within the allowable range of up to 65 mg/st.m³. This is because the temperature of the gas drops well below the vapour-hydrate equilibrium. If sufficient hydrates form, they have the potential to impede the path of the gas to the blowdown stack exit. To evaluate this risk, it is important to determine the conditions at which hydrates could form under gas blowdown situations and accurately determine the quantity that would form as both gas pressure and temperature drop during the blowdown process. This paper first compares the hydrate equilibrium conditions for different moisture contents obtained with a publicly available model to published measured data for some alkanes present in natural gas. A gas blowdown scenario establishing the gas conditions (P and T) is then presented based on the worst case scenario of adiabatic expansion of the gas. Based on these conditions, two methods are developed to quantify the amount of hydrates that could form during the blowdown process. These methods are demonstrated on a gas blowdown event of compressor station discharge yard piping where the gas was assumed to have moisture contents of 65 mg/st.m³. The potential amount of hydrates formed and the implications on the gas path to blowdown exit are discussed.