ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
Inorganic acids play a major role in the chemical process industries (CPI). They are used as raw materials, catalysts or finishing and pH control agents in the manufacture of a wide range of chemical products, from fertilizers to detergents, and even foods. Given their widespread use, a major issue in the CPI is the proper and safe handling of the acids, the adequate materials selection for the pieces of equipment, piping and fittings used in the process, and the correct storage and even disposal of these materials.
These are important factors that need to be taken into account from the design phase throughout the operating life of a facility, in order to ensure there will not be integrity problems that may negatively impact project economical turnover, personnel safety or the environment.
This article covers the most important inorganic acids: sulfuric acid (H2SO4), nitric acid (HNO3), phosphoric acid (H3PO4), hydrogen chloride (HCl) and hydrochloric acid, and hydrogen fluoride (HF) and hydrofluoric acid; providing general guidelines on their physical properties, safety data, appropriate materials, storage, pumping and other common issues encountered when handling such fluids in the CPI.
Some acids are naturally present as liquids (H2SO4), some are solids at ambient conditions (anhydrous H3PO4), and others are gases (HCl, HF). Acids are very soluble in water and thus also widely available as aqueous solutions at different concentrations. Some of these solutions are also enhanced by dissolving additional compounds (for example, fuming sulfuric acid is made by dissolving SO3 in sulfuric acid).
Given that there are several available grades, the knowledge of physical properties for each one is important in order to avoid freezing, the formation of hazardous fumes, or other problems when storing and handling these materials. The physical properties of the acids covered in this article are briefly presented here. These properties for common available grades are presented in Table 1. Figure 1 plots their vapor pressures at different temperatures.
Sulfuric acid. Sulfuric acid is the single most important inorganic chemical in tonnage produced and in use. H2SO4, can be described as a colorless, oily, hygroscopic liquid with no odor; it is the largest inorganic chemical manufactured and one of the most widely used inorganic chemical in the manufacture of many other products. By the year 2004, North America and Asia were the biggest producers of sulfuric acid, recording almost 60% of world total production. Sulfuric acid is manufactured by the combustion of sulfur with dry air to form sulfur dioxide (SO2), then sulfur trioxide (SO3) is produced through a catalytic conversion. Finally, sulfuric acid is obtained after absorption of SO3 in water.
Sulfuric acid is a strong acid and a strong oxidizing agent; therefore it reacts violently with bases, combustible, reducing materials, water and organic compounds with the evolution of heat. It is highly corrosive to most common metals and forms a flammable/explosive gas.
Sulfuric acid is mostly used in the manufacturing of fertilizers, organic pigments, explosives and more. As a strong electrolyte it is used in electroplating baths for pickling, and for operations in the production of iron and steel. Moreover, it is extensively used as a solvent for ores and as a catalyst in the petroleum industry.
Nitric acid. HNO3 is a solution of nitrogen dioxide (NO2) in water; it is a colorless to light-brown fuming liquid with an acrid suffocating odor. Nitric acid is the second most important industrial acid; it is a highly oxidizing agent, used in the manufacture of chemicals, explosives, fertilizers, steel pickling and metal cleaning. However, the largest use for nitric acid is for the production of fertilizers.
Nitric acid is a strong acid that reacts violently in the presence of strong bases, reducing agents and combustible fluids, such as turpentine, charcoal and alcohol. It is corrosive to metals, forming flammable or explosive gas. Nitric acid also reacts violently with organic compounds.
Phosphoric acid. H3PO4 or orthophosphoric acid is a white solid with a melting point of 42°C, which is highly soluble in water, non-toxic and a relatively weak acid.
H3PO4 is the third most important acid in industry. It is used mostly in the production of phosphate fertilizers; but also in the manufacturing of agricultural feeds, soaps, detergents, waxes; and in the food industry as preservative, acidifier, clarifier or flavor enhancer; among other uses.
H3PO4 has two main methods of production: the wet process and electric furnace. It is commercially available at concentrations of 75, 80, 85 and 87 wt.% of PO3. Higher concentrations, such as 105 wt.% (superphosphoric) and 115 to 118 wt.% (polyphosphoric) are also available. “Pure” or “technical grade” phosphoric acid is usually found at 85 wt.%.
Hydrogen chloride and hydrochloric acid. Hydrochloric acid is a solution of the gas hydrogen chloride; it is a poisonous, highly corrosive, hazardous liquid that reacts with most metals to form explosive hydrogen gas. Its appearance varies from pale yellow to colorless, according to purity. Hydrochloric acid has many applications in the production of organic and inorganic compounds such as fertilizers, chlorides, dyes and more. HCl plays an important role in pickling of steel, acid treatment of oil wells, chemical cleaning and processing, and ore reduction among others.
When boiling all aqueous solutions, HCl forms an azeotropic constant boiling mixture that contains 20.24% HCl and boils at 110°C (230°F).
Hydrogen fluoride and hydrofluoric acid. Anhydrous hydrogen fluoride (AHF) is a clear, colorless, corrosive fuming liquid with an extremely sharp odor. It easily dissolves in water to form hydrofluoric acid.
HF forms dense white vapor clouds if released. Both liquid and vapor can cause severe burns to all parts of the body. Specialized medical treatment is required for all exposures. HF occurs naturally in volcanic gases and may result from industrial activities, such as coal-burning, and the manufacture or production of aluminum, phosphate fertilizer, steel and other chemical derivatives.
Commercially, HF is used to manufacture fluoropolymers, pharmaceuticals, aluminum, stainless steel, high-octane gasoline, electronics (microchips and printed circuit board cleaning) and uranium isotopes. It is also used to etch glass or metal.
Safety and emergency response
Because acids are mostly hazardous chemicals, their toxicity levels and incompatibilities need to be taken into account when storing and transporting them, as well as how to respond in the event of a spillage.
Permissible exposure limits (PEL) for hazardous materials are given by the U.S. Occupational Safety and Health Administration (OSHA) regulations: 29 CFR 1910.1000, 29 CFR 1926.55 and 29 CFR 1915.1000 for the general, construction and maritime industries, respectively. Other toxicity levels, such as the Recommended Exposure Limit (REL) and Immediately Dangerous to Life and Health Concentrations (IDLH) are published in the U.S. National Institute of Occupational Safety and Health (NIOSH) Pocket Guide to Chemical Hazards. The chemical incompatibilities, health effects and other concerns when handling or storing hazardous chemicals are also given in the NIOSH Pocket Guide.
In the U.S., transportation of these acids or other hazardous materials is subject to the U.S. Department of Transportation Pipeline and Hazardous Materials Safety Transportation regulations. Transportation of hazardous materials in various forms (bulk, pipeline or tank cars) is subject to Title 49 of the Code of Federal Regulations (49 CFR).
In the event of spills of these acids or other hazardous materials, only properly trained personnel such as firemen and policemen (or properly trained plant personnel) should be involved in the emergency response and containment of the product.
The Emergency Response Guidebook 2008 (ERG2008) provides guidelines for managing emergencies when hazardous chemicals are involved. This guidebook is available in printed form, and can also be downloaded in convenient electronic form, including applications for smart phone that allow for quick searches of the chemicals and their associated guides. A new version of the Emergency Response Guidebook is scheduled for release this year (2012).
The chemical safety data for the acids covered in this article, including toxicity levels, incompatibilities and emergency response guides are summarized in Table 2.
The materials of construction, as well as any lining or internal coating requirements should be determined by a materials expert based on the acid, its concentration and storage conditions.
Aqueous acid solutions are very corrosive, and usually require special materials depending on the temperature or phase.
Some recommendations are given regarding the correct material selection depending on acid, such as in the following reference for H2SO4: NACE RP0391 — Materials for the Handling and Storage of Concentrated (90 to 100%) Sulfuric Acid at Ambient Temperatures; HF: NACE 5A171 — Materials for Storing and Handling Commercial Grades of Aqueous Hydrofluoric Acid and Anhydrous Hydrogen Fluoride.
Depending on the acid and storage, transport or process conditions, interior coatings or linings could also be considered. For instance, rail tank cars transporting concentrated sulfuric acid should be internally coated according to NACE SP0592 — Application of a Coating System to Interior Surfaces of New and Used Rail Tank Cars in Concentrated (90 to 98%) Sulfuric Acid Service
Tables 3–8 list some common metal alloys used in the CPI, along with their general acceptable use ranges (concentrations and temperatures) for each of the acids covered in this article.
Usually aboveground storage tanks (ASTs) are used to store acid as they facilitate accessibility to tanks and ancillary equipment for inspection and maintenance. The storage tank should be sized for at least 50% more volume than required. Tanks for acid storage are usually built of either metal (lined or nonlined), or fiber reinforced plastic (FRP). Metal tanks offer a higher durability, and can also resist higher stresses or impacts; whereas FRP tanks are economical, usually chemically inert, and can be a good alternative for low-volume, short storage times.
The mechanical design of tanks for acid storage usually follows either of the following codes:
- API STD 650 — Welded Steel Tanks for Oil Storage: for vertical tanks with flat bottoms and operating pressures less than 0.14 barg (2.5 psig)
- API STD 620 — Recommended Rules of Construction of Large, Welded, Low Pressure Storage Tanks: for vertical tanks with flat bottoms and operating pressures between 0.14 barg and 1.03 barg (2.5 psig and 15 psig)
- ASME BPV Code, Sect VIII, Div 1: for other operating pressures
Special design criteria, such as particular corrosion allowances or nozzle design, are also considered in acid storage tanks — either by special company or supplier criteria, or from professional associations. For instance, concentrated sulfuric acid tanks design should follow NACE SP0294 — Design, Fabrication, and Inspection of Storage Tank Systems for Concentrated Fresh and Process Sulfuric Acid and Oleum at Ambient Temperatures.
Tanks should allow access to the top nozzles and the vent system, and offer an appropriate facility for sampling. Periodically, it is necessary to homogenize the contents of the tank, because the acid that remains on the surface establishes a vapor-liquid equilibrium in which toxic and corrosive gases are released, so a recirculation system is recommended.
Special attention should be given to the acid physical properties in storage to prevent freezing, high corrosion rates or vaporization.
In general, corrosion rates increase at higher temperatures, so acids should be stored at the lowest possible temperature without freezing the acid. Higher corrosion rates could also result from heating of the metal surfaces due to sun radiation, so the tank exterior should be painted with a radiation reflecting color, such as white. Another regular measure to maintain acids at an appropriate temperature is coating the tank with an adequate material such as vinyl-based materials.
In places where the storage temperature could be below the acid freezing point, storage tanks and vessels should be provided with heating facilities, such as plate coils mounted on the outside of the tank wall, or external heat exchangers connected to the tank. Internal heating coils are not recommended, because excess temperature in the coil walls accelerates corrosion and could cause leaks. Also, high-pressure steam is not recommended as a heating medium since heat exchange surfaces could exceed 100°C, causing severe corrosion.
Pressurized storage is required when the vapor pressure exceeds the atmospheric pressure at the storage temperature.
Common guidelines for acid storage tank design are summarized in Chem. Eng. May 2008, Facts at your Fingertips: Acid Storage.
When storing acids above ground, containment is also an issue. Tanks should be properly diked, or double walled, to contain spills. In general, containment should be at least for one tank volume (if not properly drained), or less provided there is adequate drainage to an acid neutralization pit, with blockage valves accessible to operators. Local code requirements should also be addressed when designing acid-tank containment; for instance, the U.S. State of Florida has specific requirements as given by Rule 62-762.891 — Mineral Acid Storage Tank Requirements.
The design basis should be set before selecting a pump, that is, the operating conditions such as temperature, suction pressure, acid concentration, and so on. A primary issue that must be taken into account while pumping acids is safety, so, the selected pump for the system cannot leave place for leakage; this is an advantage regularly offered by vertical submerged pumps over horizontal pumps. Also, material selection guidelines shall be followed to avoid casing, impeller or other internals damage.
Piping and fittings
Selecting pipe material and designing the pipe system is a very important issue in a plant, especially while handling acids. The system must ensure the acid is transported safely and efficiently. Piping should have as few flanges as possible, so the chance of having leaks becomes negligible.
In order to select the piping material, the following aspects have to be defined: acid concentration, transport temperature, phase, fluid velocity, type of flow, impurities in the acid and solids present.
Corrosion is often related to an acid’s velocity. In order to maintain a low velocity of the fluid, a bigger pipe diameter is suggested.
Valves are used for various functions, including the following:
- For blocking, gate valves or plug valves are regularly used. However, plug valves are preferred for this service, to ensure proper valve operation.
- For control, globe or butterfly valves are suitable; they can be manually operated or be fitted with actuators.
- Materials for different parts of the valves (disk, stem and seat) should be selected according to the acid concentration and operating conditions, by consulting the valve manufacturer.
Some common materials according to the acid to be handled are presented in Tables 9–13.
Sulfuric acid. Sulfuric acid must be stored separately from combustible and reducing substances in a well ventilated environment at temperatures below 23°C (73.4°F). Concentrated acid needs to be isolated from water, as it may react violently, releasing heat. If sulfuric acid needs to be diluted or combined with water, then it has to be added to water carefully. To manipulate sulfuric acid, proper personal protective equipment, such as gloves, a vapor respirator when ventilation is inadequate, face shield and full suit shall be used.
Nitric acid. Nitric acid must be stored separately in a corrosion resistant location, avoiding contact with powders, carbides, hydrogen, sulfide, turpentine and strong bases. Along the same lines it is important to mention that nitric acid’s storage requires special conditions, such as adequate ventilation and especially low temperatures to ensure a cool environment for the solution, because heat may cause containers to burst and result in escape of poisonous gases; so it should not be stored above 23°C (73.4°F), and the container must remain dry and locked up. Nitric acid and its vapors can cause severe damage during its handling to persons who have contact with it; the severity of the damage is related to the time of contact or exposure and the acid concentration.
Every process that involves nitric acid handling or storage must contemplate an adequate ventilation system that ensures airborne levels below the safety exposure limits allowed, not only this measure needs to be taken into account but also workers should be aware of the risks arising from management of nitric acid.
Phosphoric acid.Phosphoric acid can be described as a stable chemical, because it is not subject to thermal decomposition. However, the design criteria for its handling should be based on the acid concentrations and operating temperatures. The most important issue about this acid is the variation of its freezing point according to its concentration; the freezing point of standard concentrations are 0.5°F at 75%, 40.2°F at 80% and 70.01°F at 85%, therefore it becomes necessary to heat phosphoric acid at high concentrations in order to maintain the acid as a liquid
solution. Unlike other acids, phosphoric acid does not react violently with metals; reaction occurs slowly and progressively with hydrogen as a product, so, caution should be exercised because the vapors formed are flammable.
HF acid is a very hazardous material, both in liquid and vapor phase. It can cause severe burns, which may not be immediately painful or visible. HF is a strong irritant to the skin, eyes and respiratory tract.
The fluoride ion easily penetrates the skin and generates destruction of tissue and severe bone damage. Package sizes range from 500–1,000 mL for analytical products,
to 10,000-L ISO containers. HF is delivered commercially in concentrations of 98 wt.%, 48–51 wt.% and 40 wt.%. Due to HF’s nature, strict measures shall be taken when handling the acid in industrial facilities. Such measures include administrative controls (for example, work permits); engineering controls (instrumentation: detectors, relief valves, emergency dump systems); and personal protection equipment (appropriate clothing).
HF forms an azeotropic constant boiling mixture that contains 35.6% (by weight) HF and boils at 231.8 °F.
Hydrochloric acid. HCl must be stored in a corrosion resistant location. Even though the acid is nonflammable, when it is heated hydrochloric acid fumes are released, which can compromise the safety and toxicity levels allowed, therefore storage tanks need proper venting that shall be directed to a safe location and treatment facility.
Operators handling hydrochloric acid must wear protective equipment and it is advisable for them to take a shower and gargle with
When boiling all aqueous solutions, sodium bicarbonate after manipulating the acid in order to avoid teeth corrosion in other activities performed by the operator. Undesirable reactions can take place between hydrochloric acid and the following compounds: chromate, permanganate and sulfate. Such reactions generate chlorine gas as a result. A subsequent reaction occurs with metal peroxide forming its corresponding chloride. When storing hydrochloric acid, proper ventilation has to be ensured in order to maintain the acid concentration in air below the permitted limit of exposure.
Source: Chemical Engineering: Acids Handling by Alberto Baumeister Sebastiano Giardinella Mayhell Coronado Ecotek, October 1, 2012
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