Safely purging

American Oil Company s experience indicates that tall blown stacks are safe with no more than 6% oxygen 25 feet from the top. This oxygen level is roughly half that required for flammable mixtures with hydrocarbons. Hence, flammable conditions would be limited to less than the top 25 feet of stack. The gas rate required to establish the 6% level 25 feet from the stack top is therefore defined as the minimum safe purge rate. Minor increases in rate are required for the special application of short stacks and hydrogen-rich fuel gas. 

Using these values and setting H-h equal to 25 feet, safe purge gas flows have been calculated as a function of stack diameter for gases having molecular weights from 2 to 28. The results are plotted in Figure 6-19. 

Required steam rates for safe purging are governed by heat losses, A rate of 7.5 pounds per minute per foot of diameter appears adequate for any weather. However, stack oxygen content is related to stack wall temperature. Thus, a more economical and safer operation would be one in which rate is automatically controlled by a differential thermocouple. 

Maintain good operating and cleaning procedures Provide fixed water spray, if appropriate Use inherently safe material, where possible Provide inert purge... 

Blowing down Verify flow path before starting the flow. lines for cleaning., Blow (purge) lines to safe location which protects the operator and environment Wear proper personal protective equipment (PPE) Follow proper sampling procedures Use safe blow-down design CCPS G-22 CCPS G-23 CCPS G-29... 

Must be air purged, explosion proof, or intrinsically safe to be used in hazardous areas. 

Procedural Checklist Ensures Safe, Complete Pressure-Testing and Purging of Process Equipment... 

Of the instmments described only some are suitable for use outside the laboratory. Where any instrument is carried into a flammable environment it should either be certified as intrinsically safe for exposure to the flammable atmosphere or isolated from the atmosphere such as by keeping it within a purged enclosure. Any probe connected to the instmment must be separately considered as a possible ignition source. Electrometers are described in [ 1531. A more general review of electrostatic instruments is given in [ 136]. 

While some video display screens such as liquid crystal, gas plasma or vacuum fluorescent displays do not present the same charged screen hazards as CRTs, this does not imply that they are safe for use in hazardous locations. This requires special design and certification for use with a given flammable atmosphere. Non-certified equipment used in locations classified as hazardous under Article 500 of NFPA 70 National Electrical Code require a purged or pressurized enclosure to control ignition hazards as described in NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment. The screen in this case is located behind a window in the enclosure. 

Sfart-up/shutdown Provide for safe start-up, including purging if necessary Provide for safe shutdown Normal By a trip On standby In various emergency situations, etc. 

Close each end of hose until reconnected unless made safe by draining and purging. 

Most refinery process units and equipment are manifolded into a collection unit, called the blowdown system. Blowdown systems provide for the safe handling and disposal of liquids and gases that are either automatically vented from the process units through pressure relief valves, or that are manually drawn from units. Recirculated process streams and cooling water streams are often manually purged to prevent the continued buildup of contaminants in the stream. Part or all of the contents of equipment can also be purged to the blowdown system prior to shutdown before normal or emergency shutdowns. 

After testing for soundness it will be necessary to safely introduce gas into the pipework displacing the air or inert gas that is in it. Similarly, if pipework is decommissioned for any reason fuel gas must be displaced by air or inert gas. This is a requirement of the Gas Safety (Installation and Use) Regulations, Regulation 21. Guidance on recommended procedures is given in the British Gas publication Purging Procedures for Non-Domestic Gas Installations (IM/2). 

Provide for safe start-up, including purging if necessary... 

After treatment, the gases are evacuated either diieetly to the outside atmosphere or through a speeial exhaust system. Filtered, sterile air is then admitted either for a repeat of the vaeuum/air cycle or for air purging until the ehamber is opened. In this w, safe removal of the ethylene oxide is achieved ledueing the toxie hazard to the operator. Sterilized artieles are removed directly from the ehamber and arranged for desorption. 

Specify intrinsically safe equipment, or flame-proof enclosures enclosures with pressurizing and purging. 

Hazards attendant on use of ethylene oxide in steriliser chambers arise from difficulties in its subsequent removal by evacuation procedures, owing to its ready absorption or adsorption by the treated material. Even after 2 evacuation cycles the oxide may still be present. Safety is ensured by using the oxide diluted with up to 90% of Freon or carbon dioxide. If high concentrations of oxide are used, an inert gas purge between cycles is essential [7], The main factors in safe handling... 

Inerting begins with an initial purge of the vessel with inert gas to bring the oxygen concentration down to safe concentrations. A commonly used control point is 4% below the LOC, that is, 6% oxygen if the LOC is 10%. 

Purged and Pressurized Enclosures for Electrical Equipment, NFPA 496, National Fire Protection Association, Quincy, MA, 1982 Approval Standard Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II and III, Division 1, Hazardous Locations, Class No. 3610, Factory Mutual Research, Norwood, MA, Oct. 1979... 

We mentioned in Chapter 2 (Section 2.6.1) that a purge-and-trap procedure sometimes precedes an analysis by gas chromatography. An example of this procedure is found in the City of Lincoln, Nebraska, Water Treatment Plant Laboratory. Water treatment includes chlorination. When water is chlorinated, chlorine reacts with organic matter to form trihalomethanes (THMs), such as chloroform, bromoform, bromodichloromethane, and chlorod-ibromomethane. THMs in water are regulated by the Safe Drinking Water Act, and so the laboratory must analyze the treated water to determine their concentration. 

---