Steam curtains

Steam curtains are best used for diluting heavier-than-air releases of flammable vapors, not toxic materials. For flammable materials the level of dilution with air that has to be obtained is the lower explosive limit toxic materials could require dilution to 100 ppm range. Moreover, while steam curtains can provide the thermal effects that will help disperse flammable material, they hinder the absorption effects needed for toxic materials, especially materials that are water-soluble. 

Four design criteria will help to optimize the effectiveness of a steam curtain. 

a steam curtain should be designed to activate rapidly. As discussed in Chapter 2, a gas cloud develops very quickly after an accidental release, requiring immediate mitigation. With a steam curtain, this can be accomplished by connecting the hydrocarbon or gas detectors to the steam curtain controls and having them activate the system automatically. There should always be a backup system to activate the curtain manually. 

Second, a steam curtain should be set up to completely surround the area in which a flammable material release could occur. However, because of wind conditions, it might not be necessary to activate all sections of the curtain, just those downwind of the release. Therefore, it should be possible to control the sections of the curtain that are actually in service through remotely controlled, quick-acting valves. Such control provides the capability to quickly change active curtain sections as wind direction varies. Keep in mind, though, that capability requires good local instrumentation 

a steam curtain should be designed to allow quick activation of steam flow to the curtain s piping system. This means that the steam lines from the headers to the steam curtain sections must be well trapped to ensure that when the valves open, hot dry steam is supplied to the header. A delay while the condensate is drained from the system, or the unstable operation of the curtain that could result, is not desirable. Also, having an adequate supply of steam available for the time required to bring other mitigation techniques into play to secure the situation is essential. 

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Countermeasures Water sprays Water curtains Steam curtains Air curtains Deliberate ignition of explosive cloud Dilution Foams... 

We put a lot of effort into improving safety by adding protective equipment onto our plants, new and old gas detectors, emergency isolation valves, interlocks, steam curtains, fire insulation, catchment pits for LPG storage tanks, and so on. We also introduced new procedures, such as hazard and operability studies and modification control, or persuaded people to follow old ones, such as permits-to-work and audits. 

After the fire, the pump (and others) was relocated in the open air, under a canopy, so that small leaks would be dispersed by natural ventilation. It was surrounded by a steam curtain to disperse larger leaks. This would not have been necessary if the pump could have been located more than 150 m from sources of ignition. Gas detectors were installed to give early warning of any leaks. Emergency isolation valves (Section 7.2.1) were provided so that the pumps could be isolated safely from a distance [9]. What happened when another leak occurred is described in Section 7.2.1 (d). 

Seifert, H., et al., Steam Curtains—Effectiveness and Electrostatic Hazards, 4th Int. Symp. Loss Prevention Safety Promotion in the Process Industries, Harrogate, England, Sept. 12-16, 1983. 

Air and steam curtains, which can also act as a physical barrier to the vapor cloud and prevent its migration towards sensitive locations ... 

One steam curtain described in the literature (Guidelines for Vapor Release Mitigation, CCPS, 1988), is a 6-in. diameter pipe with a row of %2-in. holes spaced at 4-in. intervals. Steam was supplied to this curtain at 250 psig. It was effective in reducing concentrations by a factor of 30 when the steam flow rate was equal to the vapor release rate. 

Steam curtains consume steam at a high rate. One reference (Guidelines for Vapor Release Mitigation, CCPS, 1988) quotes 0.2 lb of steam per pound of material released, another quotes 100 lb of steam at 250 psig per hour per foot of curtain. Because of these high levels of steam consumption, steam curtains are usually confined to protecting a small area or places where small leaks might occur. 

Finally, although the generation of static electricity in steam curtains has been a concern, investigations have indicated that this is not a problem as long as the system is well grounded (Lees, 1980). 

Since a steam curtain may activate at any time, warning alarms and signs should be placed to alert personnel who may be in the area to the need for immediate evacuation. Also, personnel who may have to work close to... 

Inspection and Preventive Maintenance When a steam curtain is to be used to mitigate the consequences of a flammable material release, it must be treated as if it were part of the site s firefighting system. This means that a managerial system must be in place to ensure that the steam curtain system is inspected periodically. Deficiencies should be reported and repaired promptly. Follow-up investigations must be made to ensure that the inspections, deficiency reporting, and repairs occur in a timely manner. If the system must be shut down for the repairs, the same procedures that would be followed for a sprinkler or deluge system shutdown should be utilized. 

Many sites operate steam trap inspection programs as part of their energy conservation effort. Even if the traps for the steam curtain are included in this program, they should also be checked as part of the curtain inspection program. 

In addition to the inspections cited above, the steam curtain should be tested once a year. Operating the system will help to ensure that no faults were overlooked in the more frequent system inspections. 

An air curtain works somewhat like a water or steam curtain, but offers no potential to absorb a toxic material or increase dispersion from thermal effects. An air curtain merely provides air movement that promotes air dilution. 

Which safety measures are implemented against fires and explosions (explosion protection, inert gas protection, sprinkler, steam curtain etc.) ... 

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