Standard Recommended Practice

Transcription

1 NACE Standard RP Item No Standard Recommended Practice Recommended Practice for the Design, Fabrication, and Inspection of Tanks for the Storage of Petroleum Refining Alkylation Unit Spent Sulfuric Acid at Ambient Temperatures This standard represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone, whether he or she has adopted the standard or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not in conformance with this standard. Nothing contained in this standard is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This standard represents minimum requirements and should in no way be interpreted as a restriction on the use of better procedures or materials. Neither is this standard intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness of this standard in specific instances. NACE International assumes no responsibility for the interpretation or use of this standard by other parties and accepts responsibility for only those official interpretations issued by NACE International in accordance with its governing procedures and policies which preclude the issuance of interpretations by individual volunteers. Users of this standard are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determining their applicability in relation to this standard prior to its use. This standard may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this standard. Users of this standard are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this standard. CAUTIONARY NOTICE: standards are subject to periodic review, and may be revised or withdrawn at any time without prior notice. requires that action be taken to reaffirm, revise, or withdraw this standard no later than five years from the date of initial publication. The user is cautioned to obtain the latest edition. Purchasers of standards may receive current information on all standards and other publications by contacting the NACE FirstService Department, 1440 South Creek Drive, Houston, Texas (telephone +1 [281] ). Approved South Creek Drive Houston, Texas / ISBN ,

2 Foreword Sulfuric acid (H 2SO 4) is the largest-volume commodity chemical in use today. It plays some part in the production of nearly all manufactured goods. One large use of concentrated sulfuric acid is as a catalyst for refinery alkylation units. In these units, C 3-C 5 olefins such as propylene or butylene are reacted with isobutane to form gasoline-blending components such as isoheptane and isooctane. These products are used to boost octane for automobile and aviation fuels. Most refineries have an alkylation unit that uses either hydrofluoric acid (HF) or sulfuric acid as the alkylation catalyst. This standard deals with spent sulfuric acid associated with the sulfuric acid alkylation process only. Refineries using sulfuric acid alkylation typically require tanks for the storage of fresh (not yet used in the alkylation process) and spent (used in the alkylation process and in need of regeneration) acid. Design, fabrication, and inspection of fresh sulfuric acid tanks are covered in NACE Standard RP This standard covers additions and deviations from RP0294 that apply to spent sulfuric acid storage tanks. Large vertical sulfuric acid storage tanks are usually built in accordance with API (1) Standard or API Standard 620, 3 and horizontal cylindrical tanks are built in accordance with the ASME (2) Boiler and Pressure Vessel Code, Section VIII, Division 1. 4 While these codes and standards are sufficient for design strength and toughness considerations, they do not address the peculiarities of corrosion by alkylation unit spent sulfuric acid service. In addition, alkylation unit spent acid can contain dissolved hydrocarbons and hydrogen that release into the vapor space of these tanks and potentially produce an explosive environment. Thus, special care must be taken to deal with vapor leakage from the vapor space of these tanks and air intrusion into the vapor space of these tanks. Carbon steel corrodes moderately when in contact with alkylation unit spent sulfuric acid. If tanks are properly designed and adequately maintained, use of carbon steel is an economical option for the storage of these acids at ambient temperatures. However, accelerated corrosion can occur in various forms, and a catastrophic failure of a spent acid tank in Delaware City, Delaware, U.S.A., has focused attention on the hazards associated with corrosion, vapor space leakage, and hot work on or around alkylation spent sulfuric acid tanks. The Chemical Safety Board (3) report 5 can be reviewed for more details regarding this failure. This standard is to be used in conjunction with NACE Standard RP0294. It is intended for use by owners/operators and fabricators of alkylation unit spent sulfuric acid storage tanks. This standard was prepared by Task Group (TG) 300 on Petroleum Refining Spent Sulfuric Acid Storage Tank Requirements: Standard. TG 300 is administered by Specific Technology Group (STG) 34 on Petroleum Refining and Gas Processing, and sponsored by STG 36 on Process Industry Materials Performance in Chemicals. It is issued by under the auspices of STG 34. These committees include representatives of companies involved in the production, transportation, and use of large quantities of spent sulfuric acid. In NACE standards, the terms shall, must, should, and may are used in accordance with the definitions of these terms in the NACE Publications Style Manual, 4th ed., Paragraph Shall and must are used to state mandatory requirements. The term should is used to state something good and is recommended but is not mandatory. The term may is used to state something considered optional. (1) American Petroleum Institute (API), 1220 L St. NW, Washington, DC (2) ASME International (ASME), Three Park Avenue, New York, NY (3) U.S. Chemical Safety and Hazard Investigation Board (CSB), 2175 K St. NW, Suite 400, Washington, DC i

3 Standard Recommended Practice Recommended Practice for the Design, Fabrication, and Inspection of Tanks for the Storage of Petroleum Refining Alkylation Unit Spent Sulfuric Acid at Ambient Temperatures Contents 1. General Corrosion Concerns Inspection Design, Construction, and Maintenance... 3 References... 3 Figure 1: Steel Corrosion by H 2 SO ii

4 Section 1: General RP Spent sulfuric acid, generated by refinery alkylation units (spent acid), may differ from fresh sulfuric acid (fresh acid) in several ways. The most notable differences between alkylation unit fresh acid and spent acid are the acid concentration, water content, possible contaminants from the alkylation process, temperature of the spent acid entering the tank from the alkylation unit, and the presence of dissolved hydrocarbons and hydrogen. These differences must be taken into consideration when developing inspection strategies, setting inspection intervals, conducting an external inspection, and/or internally inspecting a tank that is in spent sulfuric acid service. This standard was developed to address these differences between fresh acid and spent sulfuric acid as they may have an impact on the integrity of spent sulfuric acid storage tanks. 1.2 NACE Standard RP0294 shall be followed for all aspects of spent sulfuric acid storage not covered by this standard. 1.3 This standard presents additions to and deviations from NACE Standard RP0294 that apply to alkylation unit spent sulfuric acid storage. 1.4 HF alkylation unit spent acid is not within the scope of this standard. 2.1 See Paragraph 2.8 of NACE Standard RP Upset Conditions Owners/operators should be aware that alkylation unit upsets or operation outside normal ranges of acid concentration and water content and contaminants and temperature, as described in Paragraphs 2.3, 2.4, and 2.5, can be the primary cause of the corrosion concerns for alkylation unit spent sulfuric acid tanks. The owner/operator should have a method to identify these conditions and notify those involved with spent acid storage tank integrity in a timely manner. 2.3 Acid Concentration and Water Content Spent acid is typically lower in sulfuric acid concentration (88 wt% to 90 wt%) than fresh acid (95 wt% to 99.5 wt%) Section 2: Corrosion Concerns ( and can contain more water and be more corrosive. Water in spent acid should be less than 5 wt% to avoid corrosion. Common corrosion rate references for sulfuric acid corrosion include the sulfuric acid isocorrosion curves and corrosion versus concentration curves for stagnant conditions and ambient temperature (Figure 1). Caution should be taken when utilizing these references to assure that the appropriate acid concentration is used, because water content can have a large impact on the effective acid concentration The effective acid concentration should be calculated in accordance with Equation (1). For example, a spent acid containing 89 wt% sulfuric acid, 4 wt% water, and 7 wt% organics, the effective acid concentration would be: [ wt% water + wt% organics] ) 100 effective acid concentration (wt%) = (1) ( 100 wt% organics) 95.7% = ( or [ 4 + 7]) ( 100 7) 100 1

5 Inches Penetration Per Year (1.00 in. = 25.4 mm) Temperature F (ºC = 5/9 [ºF - 32]) FIGURE 1: Steel Corrosion by H 2 SO 4 6 (Source: General Chemical (4) ) 2.4 Contaminants Hydrocarbon contamination, including conjunct polymer (soluble hydrocarbon) contamination, is generally not corrosive, but can lead to a stratification of the hydrocarbon and acid layers that can cause a horizontal ring of corrosion commonly called bathtub ring corrosion if water is included with the hydrocarbon Sulfurous acid can cause rapid attack of carbon steel spent acid storage tanks. Sulfurous acid can form when moisture from air ingress or rainwater entry into the tank reacts with sulfur dioxide (SO 2) in the storage tank vapor space. Alkylation unit runaway reaction by spent acid in the storage tank can cause accelerated corrosion of carbon steel because of acid dilution, excessive temperatures, and turbulence caused by the evolution of gases such as SO 2, hydrogen (H 2), and other gases that may be hydrocarbon-based. Measures should be taken to prevent and/or monitor this type of attack. 2.5 Temperature Increases in spent sulfuric acid temperature can increase metal corrosion rates. The acid temperature can be influenced by alkylation unit conditions and/or (4) General Chemical, 90 East Halsey Road, Parsippany, NJ

6 atmospheric conditions. Figure 1 should be used to estimate corrosion rates for stagnant conditions at different temperatures. 2.6 Velocity Velocity can have adverse affects on alkylation unit spent sulfuric acid containing equipment, including storage tanks. See NACE Standard RP0294 Paragraphs 2.8, 3.2, and 3.3, and their subparagraphs, for more details. Section 3: Inspection 3.1 See Section 5 of NACE Standard RP The possibility of elevated corrosion rates above those rates found in fresh acid service may require shorter inspection intervals for tanks in spent acid service. These inspection intervals should be set utilizing all information available to the owner/operator, including, but not limited to: Tank inspection history, Inspection history of tanks in similar service (at the same site), Process data (i.e., temperature, concentration, velocity, upsets, etc.), Technical assistance from an experienced and knowledgeable corrosion specialist, and NACE Standard RP0294, API Standard 653, 7 API 510, 8 API RP 571, 9 API RP 579, 10 and API RP A more detailed inspection should be performed for tanks in spent acid service than for those in fresh acid service because of the corrosion concerns previously mentioned. The areas most susceptible to corrosion are: All nozzles and manways, including the tank shell area around the protrusions, Any identified acid/hydrocarbon/water interface elevations (if water is entrained in the hydrocarbon, a bathtub ring type of corrosion may occur at this interface level), Vapor space corrosion by sulfurous acid (normally most pronounced at the liquid/vapor interface level), Any acid sludge/acid interface elevations, and Tank shell and floor plates adjacent to any internal fill lines or dip tubes. Section 4: Design, Construction, and Maintenance 4.1 See Sections 3, 4, and 5 of NACE Standard RP Consideration for hydrocarbon entrained in spent acid should be an integral part of the design of spent acid tanks and tank venting system. As a result of specific gravity differences, hydrocarbons entrained in spent acid during the alkylation process have a tendency to separate and rise, creating a hydrocarbon layer on top of the acid. Whether this hydrocarbon layer is disturbed (such as during the filling operation) or there are light hydrocarbons present, the hydrocarbons can produce an atmosphere in the tank vapor space above the lower explosive limit (LEL) if oxygen is present. This condition shall be mitigated by maintaining an inert gas blanket above the liquid level or otherwise assuring that an explosive mixture is not present (by using eductors, etc.). This is an important aspect of the design, construction, maintenance, and inspection of spent acid tanks, as it is an integral part of the safety system of the tank. References 1. NACE Standard RP0294 (latest revision), Design, Fabrication, and Inspection of Tanks for the Storage of Concentrated Sulfuric Acid and Oleum at Ambient Temperatures (Houston, TX: NACE). 2. API Standard 650 (latest revision), Welded Steel Tanks for Oil Storage (Washington, DC: API). 3. API Standard 620 (latest revision), Design and Construction of Large, Welded, Low-Pressure Storage Tanks (Washington, DC: API). 4. ASME Boiler and Pressure Vessel Code: Section VIII, Division 1 (latest revision), Rules for the Construction of Pressure Vessels (New York, NY: ASME). 5. Investigation Report: Refinery Incident (1 Killed, 8 Injured, Offsite Environmental Impact), U.S. Chemical Safety and Hazard Investigation Board Report, DE, October MotivaIR pdf (October 2002). 3

7 6. Recommendations for the Storage and Handling of 60 Be, 66 Be and 98% Commercial Sulfuric Acid, General Chemical Corporation Technical Service Report 50.78, June 1997, p (April 1, 2004). 7. API Standard 653 (latest revision), Tank Inspection, Repair, Alteration, and Reconstruction (Washington, DC: API). 8. API 510 (latest revision), Pressure Vessel Inspection Code: Maintenance Inspection, Rating, Repair, and Alteration (Washington, DC: API). 9. API RP 571 (latest revision), Damage Mechanisms Affecting Fixed Equipment in the Refining Industry (Washington, DC: API). 10. API RP 579 (latest revision), Fitness-for-Service (Washington, DC: API). 11. API RP 580 (latest revision), Risk-Based Inspection (Washington, DC: API). ISBN