Low-pressure tanks

Low Pressure Tanks. Low pressure ia the coatext of tanks means tanks designed for a higher pressure than atmospheric tanks. In other words, these are relatively high pressure tanks, designed to operate from atmospheric pressure up to 15 psig (101.4 kPa). 

Low-Pressure Storage Tanks Low-pressure storage tanks are fragile. Even an eggshell can withstand more pressure and vacuum (Sanders, Don t Be Another Victim of Vacuum, Chemical E/ig. Prog., September 1993, pp. 54-57). Low-pressure storage tanks do not require much pressure difference between the inside of the tank and the atmosphere to buckle the relatively thin tank walls. Pressure... 

Pressurized storage vessels are either refrigerated tanks (low pressure) or horizontal vessels and spheres for storing lighter material under pressure so they remain liquids. 

Gases (spherical pressure gas tanks, low-pressure gas containers such as disk-type gas tanks and floating-bell gas tanks). 

This carbon dioxide-free solution is usually treated in an external, weU-agitated liming tank called a "prelimer." Then the ammonium chloride reacts with milk of lime and the resultant ammonia gas is vented back to the distiller. Hot calcium chloride solution, containing residual ammonia in the form of ammonium hydroxide, flows back to a lower section of the distiller. Low pressure steam sweeps practically all of the ammonia out of the limed solution. The final solution, known as "distiller waste," contains calcium chloride, unreacted sodium chloride, and excess lime. It is diluted by the condensed steam and the water in which the lime was conveyed to the reaction. Distiller waste also contains inert soHds brought in with the lime. In some plants, calcium chloride [10045-52-4], CaCl, is recovered from part of this solution. Close control of the distillation process is requited in order to thoroughly strip carbon dioxide, avoid waste of lime, and achieve nearly complete ammonia recovery. The hot (56°C) mixture of wet ammonia and carbon dioxide leaving the top of the distiller is cooled to remove water vapor before being sent back to the ammonia absorber. 

Quenched and Tempered Low Carbon Constructional Alloy Steels. A class of quenched and tempered low carbon constmctional ahoy steels has been very extensively used in a wide variety of appHcations such as pressure vessels, mining and earth-moving equipment, and in large steel stmctures (see Tanks and pressure vessels). 

For tank work, inches water column (in. wc) or ounces per square inch (osi) are commonly used to express the value of pressure or vacuum in the vapor space of a tank. These pressures are usually very low relative to atmospheric pressure. The common measures of pressure are compared as follows ... 

Fixed-roof atmospheric tanks require vents to prevent pressure changes which would othei wise result from temperature changes and withdrawal or addition of liquid. API Standard 2000, Venting Atmospheric and Low Pressure Storage Tanks, gives practical rules for vent design. The principles of this standard can be applied to fluids other than petroleum products. Excessive losses of volatile liquids, particularly those with flash points below 38°C (100°F), may result from the use of open vents on fixed-roof tanks. Sometimes vents are manifolded and led to a vent tank, or the vapor may be extracted by a recov-eiy system. 

API STD 2000. 1992. Venting Atmospheiic and Low-Pressure Storage Tanks, Nonrefrigerated and Refiigerated. American Petroleum Institute, Washington, D.C. 

Pressure-Vacuum Relief Valves For apphcations involving atmospheric and low-pressure storage tanks, pressure-vacuum relief valves (PVRVs) are used to provide pressure relief. These units combine both a pressure and a vacuum relief valve into a single assembly that mounts on a nozzle on top of the tank and are usually sized to handle the normal in-breathing and out-breathing requirements. For emergency pressure rehef situations (e.g., fire), ERVs are used. API RP 520 and API STD 2000 can be used as references for sizing. 

Low-Pressure Tanks (below 15 psig) Low-pressure storage tanks for highly hazardous toxic materials should meet, as a minimum, the American Petroleum Institute (API) 620 Standard, Recommended Rules for the Design and Construc tion of Large Welded, Low-Pressure Storage Tanks (API Standards). This standard covers... 

Examples of Vacuum-Related Accidents Figure 26-47 shows a jacketed tank, where the jacket was designed for low-pressure steam. When the steam was turned off and the drain valve and trap were closed, the steam condensed, causing the jacket to collapse. The jacket should have been designed for full vacuum, or a suit le vacuum relief device should have been installed on the jacket. 

Storage tanks are generally designed based upon the vapor pressure of their contents, and can range from low pressure (API-type) tanks to high pressure tanks for compressed gases or pressurized liquids. 

API Std 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks, Ninth ed., February 1996. 

API Std 2000, Venting Atmospheric and Low-Pressure Storage Tanks Nonrefrigerated and Refrigerated, 4th ed., September 1992 (ANSI/API Std 2000-1992). 

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