Cup and Grip Seal Failures on Low Pressure Gasholders: Part 1. The Release Rate

The loss of a seal between two elevated lifts on a water-sealed gasholder, although a rare event, can occur. Such an event leads to the loss of natural gas from the holder, as the lifts gradually deflate. The release is self-limiting, in that the seal will re-enter the base water tank and be refilled with water to prevent further escape of the natural gas inventory. Nevertheless, it is important to have methods available to predict the consequences of such releases and this paper describes a model that can be used to calculate the rate at which natural gas escapes through the failing seal. The model is based on an analysis of the flow path from inside the gasholder to the external atmosphere. It uses conventional fluid dynamic relationships for the pressure losses associated with steady one-dimensional flow in order to derive an equation for the natural gas flow rate. It is noted that the model gives good agreement with the limited available data. From a risk assessment viewpoint, the model provides the initial conditions for any subsequent analysis of the dispersing or combusting gas.