A blowdown is the purposeful venting of natural gas to the atmosphere during well operations and/or during pipeline operations or maintenance to relieve pressure in the pipe. Here are examples of when a blowdown is used:
During oil well operations, wells may be shut in for a period. When this occurs, the well will frequently develop a cap of natural gas that has percolated through the oil. This gas is often vented prior to restarting the well.
When maintenance is to be performed on natural gas pipelines or related equipment such as compressor stations, it is often necessary to remove the gas prior to performing the work. This is often accomplished by blowing down the gas in the affected facilities to the atmosphere.
Compressor stations and regulator stations often use blowdown stacks as a means of pressure control. If pressure goes above design levels due to equipment failure or other operational issues, the equipment is designed to automatically vent gas to the atmosphere to reduce the pressure.
An essential factor in the design of blowdown systems is to ensure that the equipment can handle the cold temperatures associated with the rapid depressurization of the gas as it is released to the atmosphere. Depressurization results in a drop in the temperature of the gas due to the Ideal Gas Law. Thus, the equipment must have sufficient resistance to brittle fracture to handle the cold gas temperatures.
Because natural gas is a significant greenhouse gas, the industry is working to reduce emissions of methane into the atmosphere. Some pipelines and distribution companies are developing techniques to reduce the amount of blowdown gas released.