In this paper, we focus on the phenomenon that occurs when gas is directly vented to the atmosphere and its modeling in simulation software. This phenomenon is called blowdown. Depending on the size of the pipes and on the characteristics of the equipment used to vent, the blowdown time can widely change, so as the gas flow obtained at the top of the torch. Our goal is to develop a simulation module modeling blowdown to help network operators to insure short emergency time response and safety. The main question is how to model high speed flow while satisfying those two constraints: good reliability and short computation time? Thanks to laboratory and field-case results, a simple, accurate and efficient model has been developed. It highlights the main characteristic of the blowdown phenomenon: the very-high-speed flow generates significant pressure drops even in the smallest obstacles. At present, this model is being implemented in a CRIGEN1 simulation software, used daily by network operators in the GrDF.
Gaz réseau Distribution France is the main French gas distribution operator. It maintains, develops and operates more than 110 000 miles of pipelines over 75% of the whole territory. About 9 000 municipalities, mainly big French cities, are supplied with gas by GrDF. Consequently, one of its most important duties is safety. In case of an incident with gas emissions, the priority is to minimize the emergency time response, i.e. the arrival time on site and the time needed to make the zone safe. These safety measures consist in:
closing the valves to stop the gas flow
opening other dedicated valves to vent the gas to the atmosphere thanks to a specific equipment.
We call it: blowdown. Therefore, two indicators are crucial for field crews:
the time required to let the inside pressure decrease down to the atmospheric pressure: the blowdown time.
the maximal induced flow rate obtained at the top of the torch used to blowdown.
Controlling these two indicators is important for both incident management and communication with authorities. On one hand, the time information indicates when repair work will begin. On the other hand, the maximal flow rate information determines the radiation zone and thus the security perimeter to be installed. Unfortunately, field crews have no means at present how to estimate the blowdown time and the flow rate. In this paper, we present the steps involved in the development of a simple but accurate model for the blowdown phenomenon dedicated to network operators needs. In the first part, we describe the laboratory and field studies we have done, to get a better understanding of the phenomenon. In the second part, we propose a simple model and discuss its accuracy, comparing simulations and field-case results. Finally we detail the platform on which the model is planned to be integrated.
First, let us describe how blowdown is carried out during an incident.