Environmental issues are a concern during a shutdown because there is a greater potential for chemical spills as process piping, equipment, and vessels are drained and opened. The potential for unwanted environmental releases increases because changing pressures and temperatures can cause joints to leak.
Also, as systems are opened, materials that were not completely drained and purged from the system may contaminate the environment.
The first line of defense against emissions is to prevent spills and/or recover oil and other materials at the process unit before they have a chance to become a larger problem.
An emergency action plan for environmental releases should be in place, and an adequate supply of absorbents and materials for diking or otherwise confining spills should be readily available. Some waste materials generated during process equipment cleaning will be classified as hazardous wastes and must be disposed of according to environmental regulations.
Plans for storage of these materials and their eventual disposal must be developed prior to the shutdown.
When a unit is shutting down, hazards not normally present during routine operation will exist. Equipment temperature may change by hundreds of degrees, especially when fired furnaces are involved.
Rapid temperature changes may result in equipment damage or leaks due to thermal stress.
Shutdown procedures involving temperature control and cooling rates should be followed closely to prevent such problems. As systems are cooled rapidly and vessels are emptied, a vacuum can be created if changes proceed too quickly.
The vacuum can damage equipment not designed to withstand a vacuum or pull air into an unpurged system that is not vacuum tight, creating a hazardous situation.
Changes must be made slowly to avoid creating vacuums that damage equipment or create hazardous conditions, and system pressure should be monitored closely.
When steam is used to purge a system during clear up, a vacuum can easily be created as the system cools and the steam condenses. Vents should be opened to admit air (or some kind of gas) at a rate sufficient to displace steam as it condenses or water is drained from a vessel.
Other hazards may be created by pressure surges resulting from water added to a system with a temperature greater than the boiling point of water. If there is enough water, the vaporization of water to steam can generate enough pressure to rupture equipment and harm personnel. There is also the danger of air entering through an open vent or leaking flange and mixing with flammable materials. An ignition source, such as static electricity, could result in a fire or explosion. Procedures and training during shutdowns can prevent such situations.