1、 Standard Practice Design, Fabrication, and Inspection of Tanks for the Storage of Petroleum Refining Alkylation Unit Spent Sulfuric Acid at Ambient Temperatures This NACE International standard represents a consensus of those individual members who have reviewed this document, its scope, and provis
2、ions. 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 NACE International standard is to b
3、e 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
4、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 Internat
5、ional assumes no responsibility for the interpretation or use of this standard by other parties and accepts responsibility for only those official NACE International interpretations issued by NACE International in accordance with its governing procedures and policies which preclude the issuance of i
6、nterpretations by individual volunteers. Users of this NACE International 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 NACE International standard
7、 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 NACE International standard are also responsible for establishing appropriate
8、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: NACE International standards are subject to p
9、eriodic review, and may be revised or withdrawn at any time in accordance with NACE technical committee procedures. NACE International requires that action be taken to reaffirm, revise, or withdraw this standard no later than five years from the date of initial publication and subsequently from the
10、date of each reaffirmation or revision. The user is cautioned to obtain the latest edition. Purchasers of NACE International standards may receive current information on all standards and other NACE International publications by contacting the NACE International FirstService Department, 1440 South C
11、reek Dr., Houston, TX 77084-4906 (telephone +1 281-228-6200). Reaffirmed 2010-07-29 Approved 2005-10-15 NACE International 1440 South Creek Drive Houston, TX 77084-4906 +1 281-228-6200 ISBN 1-57590-203-6 2010, NACE International NACE SP0205-2010 (formerly RP0205) Item No. 21107 SP0205-2010 NACE Inte
12、rnational i _ Foreword Sulfuric acid (H2SO4) 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, C3-C5olefins such as
13、propylene or butylene are reacted with isobutane to form gasoline-blending components such as isoheptane and isooctane. These gasoline-blending components are used to boost octane for automobile and aviation fuels. Most refineries have an alkylation unit that uses either hydrofluoric acid (HF) or su
14、lfuric 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 a
15、lkylation process and in need of regeneration) acid. Design, fabrication, and inspection of fresh sulfuric acid tanks are covered in NACE SP0294.1This standard covers additions and deviations from SP0294 that apply to spent sulfuric acid storage tanks. Large, vertical sulfuric acid storage tanks are
16、 usually built in accordance with API(1)Standard 6502or API Standard 620,3and horizontal cylindrical tanks are built in accordance with the ASME(2)Boiler and Pressure Vessel Code, Section VIII, Division 1.4Although these codes and standards are sufficient for design strength and toughness considerat
17、ions, they do not address the peculiarities of corrosion by alkylation unit spent sulfuric acid service. In addition, alkylation unit spent acid may contain dissolved hydrocarbons and hydrogen that release into the vapor space of these tanks and potentially produce an explosive environment. Thus, sp
18、ecial 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 c
19、arbon 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, v
20、apor space leakage, and hot work on or around alkylation spent sulfuric acid tanks. The Chemical Safety Board(3)report5provides more details regarding this failure. (1)American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005-4070. (2)ASME International (ASME), Three Park Avenue, New Y
21、ork, NY 10016-5990. (3)U.S. Chemical Safety and Hazard Investigation Board (CSB), 2175 K St. NW, Suite 400, Washington, DC 20037-1809. SP0205-2010 ii NACE International This standard is to be used in conjunction with NACE SP0294. It is intended for use by owners/operators and fabricators of alkylati
22、on unit spent sulfuric acid storage tanks. This standard was prepared by Task Group (TG) 300, “Petroleum Refining Spent Sulfuric Acid Storage Tank Requirements: Standard.” It was published in 2005 and reaffirmed in 2010 by TG 300. TG 300 is administered by Specific Technology Group (STG) 34, “Petrol
23、eum Refining and Gas Processing,” and sponsored by STG 36, “Process IndustryMaterials Performance in Chemicals.” It is issued by NACE International under the auspices of STG 34. These committees include representatives of companies involved in the production, transportation, and use of large quantit
24、ies 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. The terms shall and must are used to state a requirement, and are considered mandatory. The term should is used to s
25、tate something good and is recommended, but is not considered mandatory. The term may is used to state something considered optional. _ SP0205-2010 NACE International iii _ NACE International Standard Practice Design, Fabrication, and Inspection of Tanks for the Storage of Petroleum Refining Alkylat
26、ion Unit Spent Sulfuric Acid at Ambient Temperatures Contents 1. General 1 2. Corrosion Concerns 1 3. Inspection 3 4. Design, Construction, and Maintenance . 4 References 4 Figure 1: Steel Corrosion by H2SO42 _ SP0205-2010 NACE International 1 _ Section 1: General 1.1 Spent sulfuric acid, generated
27、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
28、 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, performing an external inspection, and internally inspecting a tank
29、 that is in spent sulfuric acid service. This standard was developed to address these differences between fresh acid and spent acid as they may have an impact on the integrity of spent sulfuric acid storage tanks. 1.2 NACE SP0294 shall be followed for all aspects of spent sulfuric acid storage not c
30、overed by this standard. 1.3 This standard presents additions to and deviations from NACE SP0294 that apply to alkylation unit spent sulfuric acid storage. 1.4 HF alkylation unit spent acid is not within the scope of this standard. _ Section 2: Corrosion Concerns 2.1 See Paragraph 2.8 of NACE SP0294
31、. 2.2 Upset Conditions Owners/operators should be aware that alkylation unit upsets or operation outside the normal ranges of acid concentration, water content, contaminants, and temperature, as described in Paragraphs 2.3, 2.4, and 2.5, can be the primary cause of the corrosion concerns for alkylat
32、ion 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 2.3.1 Spent acid is typically lower in sulfuric acid concentration (8
33、8 wt% to 90 wt%) than fresh acid (95 wt% to 99.5 wt%) 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 co
34、ncentration curves for stagnant conditions and ambient temperature (see Figure 1). Caution should be taken when using these references to ensure that the appropriate acid concentration is used, because water content can have a large impact on the effective acid concentration. 2.3.2 The effective aci
35、d concentration should be calculated in accordance with Equation (1). As an example, for a spent acid containing 89 wt% sulfuric acid, 4 wt% water, and 7 wt% organics, the effective acid concentration would be: SP0205-2010 2 NACE International )()( organicswt%100organicswt%waterwt%100(wt%) ionconcen
36、tratacid effective+= (1) or )()( 71007410095.7%+= Temperature: C = 5/9 (F 32) Corrosion Rate: 1 in/y = 25.4 mm/y Figure 1: Steel Corrosion by H2SO46 (Source: General Chemical Corporation(4) (4)General Chemical Corporation, 90 East Halsey Road, Parsippany, NJ 07054. SP0205-2010 NACE International 3 2
37、.4 Contaminants 2.4.1 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 corrosioncommonly called bathtub ring corrosionif water
38、 is included with the hydrocarbon. 2.4.2 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 (SO2) in the storage tank vapor space. Alkylation unit “runaway”
39、 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 SO2, hydrogen (H2), and other gases that may be hydrocarbon-based. Measures should be taken to prevent
40、 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 atmospheric conditions. Figure 1 should be used to estimate corrosion rates for stagnant c
41、onditions at different temperatures. 2.6 Velocity Velocity can have adverse affects on alkylation unit equipment that contains spent sulfuric acid, including storage tanks. See NACE SP0294 Paragraphs 2.8, 3.2, and 3.3, and their subparagraphs, for more details. _ Section 3: Inspection 3.1 See Sectio
42、n 5 of NACE SP0294. 3.2 The possibility of elevated corrosion rates, above those corrosion rates found in fresh acid service, may require shorter inspection intervals for tanks in spent acid service. These inspection intervals should be set using all information available to the owner/operator, incl
43、uding, but not limited to: Tank inspection history; Inspection history of tanks in similar service (at the same site); Process data (e.g., temperature, acid concentration, velocity, upsets); Technical assistance from an experienced and knowledgeable corrosion specialist; and NACE SP0294, API Standar
44、d 653,7API 510,8API RP 571,9API RP 579,10and API RP 580.113.3 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 manwa
45、ys, 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 th
46、e 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. SP0205-2010 4 NACE International _ Section 4: Design, Construction, and Maintenance 4.1 See Sections 3, 4, and 5 of NACE SP0294. 4.2 Considera
47、tion for hydrocarbons entrained in spent acid should be an integral part of the design of spent acid tanks and tank venting systems. 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 light hydrocarbons are present, the hydrocarbons can produce an atmosphere in the tank vapor space above the lower explosive limit (LEL) if oxygen is present. This condition s
