Hazards of Pressure

high pressure can be almost any level prescribed for the equipment or system in use. For accident prevention purposes, any pressure system must be regarded as hazardous. Hazards lie both in the pressure level and in the total energy involved. Any employee on a processing unit—operator, instrument and analyzer technicians, mechanical personnel, contractors—that has pressurized lines or vessels is subject to the hazards of pressure. 

This chapter introduces workers in various industries to several more hazards they must guard against high pressure, steam, and electricity. Remember we said in Chapter 1 that risk was everywhere Maybe we should have added that multiple risks are everywhere. All work sites have multiple risks, from heights (stairs), electricity (office equipment), punctures (stapler, scissors, etc.), ergonomics, and many more. 

When the pressure of a fluid inside a vessel exceeds the vessel s strength, it will fail by rupture. A slow rupture may occur by popping rivets or by opening a crack. If the rupture is rapid, the vessel can literally explode, generating metal fragments and a shock wave with blast effects as damaging as those of exploding bombs. Boiler explosions are often disastrous. 

A boiler rupture occurs if steam flow output is prevented or restricted and the temperature and pressure in the boiler increases. If some form of safely device is not provided or is inadequate to limit the pressure to a safe value, the boiler will rupture. Boilers are required to be equipped with safety valves to relieve pressure if they exceed set values. Low points in some boilers are provided with fusible plugs. During normal operations, the plugs are covered with water which keeps them cool. If the water level drops and they are exposed, the plugs melt and create another vent for pressure relief. Boilers also have low-water shutoff devices which block in the burner fuel when the steam drum water drops below a certain level. 

Pressure vessels do not have to be fired to be hazardous. Heat input can occur in other ways. The sun can heat outdoor pressure vessels, such as portable compressed gas cylinders and sample bomb cylinders, some that contain gases at pressures up to 2,000 psi at room temperature. These vessels should be stored or housed in shaded areas. For example, the vapor pressure of liquid carbon dioxide is 835 psi at 70°F and 2,530 psi at 140°F. Pressure vessels (cylinders) inside buildings should not be located near sources of heat, such as radiators or furnaces. 

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Any use of a pressurized medium, such as hydraulic fluid, can be dangerous. Hydraulic systems carry all the hazards of pressurized systems and special hazards related directly to the composition of the fluid used. 

Application of Haddon s energy theory (Chapter 9) helps identify controls for pressure release hazards. Remember, a pressurized container has stored energy. Haddon s energy theory helps one think through the options for controlling potential hazards of pressure. Here are some approaches to consider. 

Chemical Safety Alert Rupture Hazard of Pressure Vessels (PDF) (6 pp, 107K) Issued Sept 2007... 

Example 9.1 A process involves the use of benzene as a liquid under pressure. The temperature can be varied over a range. Compare the fire and explosion hazards of operating with a liquid process inventory of 1000 kmol at 100 and 150°C based on the theoretical combustion energy resulting from catastrophic failure of the equipment. The normal boiling point of benzene is 80°C, the latent heat of vaporization is 31,000 kJ kmol the specific heat capacity is 150 kJkmoh °C , and the heat of combustion is 3.2 x 10 kJkmok. ... 

Wider passages are provided for vapours and the comparatively narrow tubes, which are usually fitted through holes bored in cork or rubber stoppers, are absent this considerably diminishes danger in violent reactions and also tends to give better results in distillation under reduced pressure as well as diminishing the hazard of choking. ... 

Polymerizations of methacrylic acid and derivatives are very energetic (MAA, 66.1 kj/mol MMA, 57.5 kJ/mol = 13.7 kcal/mol). The potential for the rapid evolution of heat and generation of pressure presents an explosion hazard if the materials are stored ia closed or poorly vented containers. 

In 1991, the European fabric softener market took a sharp turn. Producers in Germany, the Netherlands, and later in Austria and Switzerland voluntarily gave up the use of DHTDMAC (238) because of pressure from local environmental authorities, who gave an environmentally hazardous classification to DHTDMAC. A number of esterquats were developed as candidates to become successors to DHTDMAC (see Fig. 1). The ester group facihtates biodegradation. 

Most tanks store Hquid rather than gases or soHds. Characteristics and properties such as corrosiveness, internal pressures of multicomponent solutions, tendency to scale or sublime, and formation of deposits and sludges are vital for the tank designer and the operator of the tank and are discussed herein. Excluded from the discussion are the unique properties and hazards of aerosols (qv), unstable Hquids, and emulsions (qv). A good source of information for Hquid properties for a wide range of compounds is available (2). 

Explosibility and Fire Control. As in the case of many other reactive chemicals, the fire and explosion hazards of ethylene oxide are system-dependent. Each system should be evaluated for its particular hazards including start-up, shut-down, and failure modes. Storage of more than a threshold quantity of 5000 lb (- 2300 kg) of the material makes ethylene oxide subject to the provisions of OSHA 29 CER 1910 for "Highly Hazardous Chemicals." Table 15 summarizes relevant fire and explosion data for ethylene oxide, which are at standard temperature and pressure (STP) conditions except where otherwise noted. 

Pneumatic and hydraulic vibrating conveyors have as their greatest asset ehmination of explosion hazards. If pressurized air, water, or oil is available, they can be extremely practical since their drive design is relatively simple and pressure-control valves can be used to vaiy capacity either manually or automatically. 

Pb as the vessel burst pressure in bars. Other sources are Baker Explosion Hazards and Evaluation, Elsevier, 1983, p. 492) and Chemical Propulsion Information Agency Hazards of Chemical Rockets and Propellants Handbook, voT. 1 NTIS, Virginia, May 1972, pp. 2-56, 2-60). 

Example The combustion process in large vapor clouds is not known completely and studies are in progress to improve understanding of this important subject. Special study is usually needed to assess the hazard of a large vapor release or to investigate a UVCE. The TNT equivalent method is used in this example other methods have been proposed. Whatever the method used for dispersion and pressure development, a check should be made to determine if any govern-mentaf unit requires a specific type of analysis. 

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