Depressurization and Blowdown

In the event of a process plant fire, or an impending fire /explosion /collision or any such threat, it is absolutely necessary that any gaseous or liquid hydrocarbons contained in pipelines or vessels to be transported out of the endangered area. This is intended for following reasons,

- To avoid vessel and pipeline rupture caused by heat weakening of the containment materials
- Rupturing vessels and pipelines generate projectile fragments which endanger people and also cause further damage to equipment and structure
- Combustibles containing in rupturing vessels and pipelines further fuels the initiated fire.
- Rupturing vessels may release toxic /harmful materials which endanger humans and the environment

If the involved hydrocarbon (or any such combustible /hazardous /or pressurized material) is gaseous, this removal process is called “depressurization”, and if the involved hydrocarbon (or any such combustible or hazardous material) is a liquid, then the removal process is called “Blowdown”. However it is noted that some people tend to use these two terms interchangeably (especially in Scandinavia).

Note that many process vessels have pressure safety valves (PSVs) which shall activate in the event of a high pressure development in the system. PSVs shall release pressurized material into a safer area if the predetermined pressure limits are exceeded.

But, in the event of a fire, PSVs can render useless due to the reason that a vessel can rupture at a pressure far below its PSV set point (and even under normal working pressure of the vessel) due to the reason that material strength of the vessel is severely reduce due to the heat exposure. That is why depressurization and blowdown (D&B) is very important irrespective of other pressure safety arrangements.

The depressurization and blowdown philosophy is a very important part of any Emergency Preparedness Plan related to a process facility that contains large quantities of combustibles or pressurized /hazardous materials. This plan should elaborate which areas /vessels to be depressurized in which sequence. Usually, a typical process plant’s depressurization and blowdown capacity is limited at a certain rate. Hence, the D&B process should be sequenced for different fire scenarios with mostly endangered areas are prioritized.

There are many considerations to be made when developing a Depressurization and Blowdown Philosophy. We will discuss some of these aspects during forthcoming posts.

A fire ball