Agenda Item 653-153 10/26/04 Page 8 of 9
API AGENDA ITEM 653-153
Operation at Elevated Temperatures
HANDLED BY: Steve Braune
Tank industry Consultants
13146 Midlothian Turnpike, #312
Midlothian, Virginia 23113
Phone: (804) 897-7176, Fax: (804) 897-7178
Email:
PURPOSE: To add rules for evaluation of tanks that continue to operate at elevated temperatures or are to undergo a change of service to elevated temperatures.
ORIGIN: SGD discussions Fall 2001
STATUS: Originated Fall 2001 SGD
Spring 2002, briefly discussion at SGD meeting
Fall 2002, held on agenda, Braune absent from meeting
Spring 2003, presented for SCLB consideration
Fall 2003, held on agenda, Braune absent from meeting
Spring 2004, revised and discussed for SCLB
Fall 2004, discussed, revised and MSP to SCLB
BUSINESS IMPACT: Tank integrity will improve with addition of consistent technical requirements.
TECHNICAL JUSTIFICATION:
There are no requirements in the current standard for the evaluation of API storage tanks that operate at temperature of over 200F. Likewise, the standard does specifically address the requirements to change the service to operation above 200F.
There are many tanks that were placed into elevated temperature service before the rules of API-650, Appendix M were written. These tanks continue to operate safely; however, there is not a specific guidance in API-653 in regards to the allowable shell stresses for the evaluation of such tanks. As such, there are probably inconsistencies in the way these tanks are evaluated for continued service.
There are no rules for allowable stresses in API-653 for tanks constructed originally to the requirements of API-650, Appendix M. This again leads to inconsistencies in the way that these tanks are evaluated for continued service.
In addition, there is not a set of rules for a change of service from ambient conditions to elevated temperatures. While the effects of elevated temperature must be considered, no specific guidelines are given for existing tanks.
The proposed rules will address allowable stresses at elevated temperatures and other important issues such as butt-welded annular rings.
Background
API-650 requires that a new AST be designed with a reduced allowable shell stress that is based upon the operating temperatures. Section M.3 addresses this by requiring a calculation of allowable design stress based upon two-thirds of a reduced material yield strength. The yield strength reduction factors are listed in Table M-1.
Lacking specific guidelines for the evaluation of tanks for elevated temperature service, there is a risk that a tank owner may mistakenly use the basic allowable stresses from API-653 without the appropriate reduction for operation at elevated temperatures greater than 200F.
To illustrate this problem, consider an AST in asphalt service, designed per API-650, Appendix M for an operating temperature of 300F. The basic allowable design stress, Sd would have been determined to be 20,000 psi for ambient conditions. Designing to the Appendix M requirements would have resulted in a reduced allowable stress of 17,600 psi. When this tank was later inspected and evaluated, the owner could mistakenly select the basic allowable stress of API-653 as 23,600 psi and 26,000 psi for lower and upper shell courses, respectively. Although not expressly addressed in API-653, I do not believe that this is the way the SC PV&T intended for an existing asphalt tank to be evaluated.
Although non-Appendix M stresses are based upon calculations that consider both yield and tensile strengths, the Appendix M calculations only involve yield strength. This is due to the fact that there is not typically any significant reduction of tensile strength until temperatures reach 700F. Since Appendix M is limited to 500F, a reduction in tensile strength will not become a design issue.
Currently, API-653 uses an allowable stress that is based upon both the yield and tensile strength of the shell material at ambient temperatures. With regards to evaluating ASTs originally designed per the requirements of API-650, Appendix M, it seems reasonable to follow the same process in API-653. Therefore, this proposal is based upon the current API-653 allowable stress criteria, but to require a reduction of the yield stress for elevated temperature operation.
Approval of this proposal will result in an industry-wide uniform approach to the evaluation of existing tanks that were originally designed for elevated temperature service. As proposed, the allowable stress increase between API-650 and API-653 for the evaluation of existing tanks will be uniform for both ambient and elevated temperature service tanks.
A brief comparison follows...
Y = Yield Strength, T = Tensile Strength
For API-650… Sd = lesser of 2/3 Y or 2/5 T
For Appendix M… Sd = 2/3 Y x Reduction Factor (per Table M-1)
To illustrate, apply this to A283-Grade C, ( Y = 30,000 psi )
From API-650, Appendix M, Table M-1:
No Yield Reduction Factor at T 200F
Yield Reduction Factor = 0.91 at T = 201F
Yield Reduction Factor = 0.88 at T = 300F
Yield Reduction Factor = 0.85 at T = 400F
Yield Reduction Factor = 0.80 at T = 500F
The following table shows how this would affect an existing asphalt tank originally built to API-650, Appendix M using A283-Grade C material.
Operating Temp. (F) / API-650Stress, Sd
(psi) / API-653
Stress, S
(lower 2 courses) / API-653
Stress, S
(upper courses)
Up to 200 / 20,000 / 23,600 / 26,000
201 / 18,200 / 21,800 / 24,000
300 / 17,600 / 21,100 / 23,200
400 / 17,000 / 20,400 / 22,400
500 / 16,000 / 19,200 / 21,100
In order to incorporate these revision into the Standard, Section 6.3.3.1, could be revised to include additional definitions of the allowable stress, S. This approach was considered, but not used as it would make an already busy section busier. The addition of two new sections was chosen as a means of inserting this new material.
This proposal also requires the use of yield strength reduction factors in order to determine the value of Y at elevated temperatures. For convenience of future maintenance of this document, reference to API-650, Appendix M, Table M-1 is made. The factors are not proposed for inclusion into this Standard.
The following cases are proposed for consideration.
Case 1… Existing Appendix M Tanks
These tanks were originally designed for an allowable stress, S = 2/3 Y, where Y = yield stress. Y is reduced due to elevated temperature effects per Appendix M, using the yield strength reduction factors of Table M-1.
API-653 uses S = 0.80 Y for non-elevated temperatures.
It is proposed that these tanks be evaluated for future service using S = 0.80 Y. As for new tanks per Appendix M, Y is reduced for the elevated temperature. The increased stress level will result in higher rotations at the base of the tank; however, tanks over 100 feet in diameter are designed with butt-welded annular rings.
For non-elevated temperature service, API-653 uses an allowable stress of 0.90 Y for UPPER shell courses. This additional margin is not proposed for upper courses of Appendix M tanks as there is often undetected external shell corrosion beneath poorly maintained insulation systems. This will provide an added margin of safety in an area of the shell that is somewhat difficult to inspect.
Case 2… Conversion from Ambient to Elevated Temperature Service Conditions
These tanks are to be rated for the new service using the rules of API-650, Appendix M. This would require that the need for butt-welded annular rings be addressed and the allowable shell stresses of Appendix M not exceeded.
API-650, Appendix M requires the use of annular rings if the diameter exceeds 100 feet. Existing tanks larger than 100 feet in diameter will require the installation of annular rings. A hydrotest would therefore be necessary, as this is a major alteration.
An alternative to this requirement would be that the tank be evaluated for the effects of liquid head and temperature cycles using the Karcher’s procedures given in M.4, or similar. This analysis may show that the addition of a butt-welded annular ring to an existing tank is not required.
As a practical matter, the allowable shell stresses of Appendix M are typically lower than the allowable stresses for ambient conditions. This fact combined with the higher specific gravity of asphalt (1.0 to 1.03) may result in less than full shell fill heights for these tanks.
Case 3… Existing Tanks in Elevated Temperature Service, But Not Meeting the Appendix M Requirements.
These tanks were built prior to the issuance of Appendix M. Appendix M was first published in the mid-to-late 1970s. Guido Karcher of Exxon wrote a series of papers that outlined the basis and development of Appendix M. Karcher’s papers are currently referenced in Appendix M.
I would expect that most tanks in this group are pre-1980. Also, tanks in this group are probably basic API tanks, designed for S = 21,000 psi, E = 85% and G = 1.0 (min). The original design criteria will not likely present much of a problem. However; we often find that these tanks were not equipped with butt-welded annular rings.
For tanks greater than 100 feet in diameter, the lack of a butt welded annular ring will be an obvious deficiency. However, further analysis using the methods proposed by Karcher in M.4 would be a suitable means of addressing this issue.
This proposal is based upon the following evaluation guidelines.
· Do not require the retrofit of these tanks with an annular ring unless conditions justify it.
· Consider successful operating experience very heavily. Successful service experience should be based upon present conditions and would be indicated by the following conditions. (Note… These items have not been written into the agenda items as they are probably too specific for all cases. Since the Standard already considers a change of service as requiring detailed considerations of all conditions, this listing is probably not required in the Standard. I would expect that the Storage Tank Engineer would develop a similar list.
· No cracks in the interior shell-to-bottom weld. MT examination of the entire perimeter would be required to verify this.
· No cracks in the lap welded bottom seams within a distance of 24 inches of the shell. Use MT for inspection.
· No severe corrosion of the sketch plates is present, especially in the critical zone.
· Edge settlement, if present, must not exceed the limits of Appendix B.
Note… These items have not been written into the agenda items as they are probably too specific for all cases. Since the Standard already considers a change of service as requiring detailed considerations of all conditions, this listing is probably not required in the Standard. I would expect that the Storage Tank Engineer would develop a similar list.
With regards to an allowable shell stress for continued operation, it is proposed that the same as Case 1 be used.
Agenda Item 653-153 10/26/04 Page 8 of 9
Proposed New Section 4.3.10
4.3.10 Operation at Elevated Temperatures
Tanks that operate at elevated temperatures shall be evaluated for suitability by one of the following provisions. The requirements of this section are based in part on the requirements of API Standard 650, Appendix M and are applicable to operation at temperatures exceeding 200OF, but not more than 500OF. These rules address only tanks of welded construction.
4.3.10.1 Continued Operation at Elevated Temperatures
A. Existing tanks that were originally designed and constructed to the requirements of API Standard 650, Appendix M shall be evaluated for continued service, as follows.
1 The tank shell shall be evaluated in conformance with 4.3.3 of this Standard, except that the allowable stress (S) for all shell courses shall not exceed 0.80 Y. The value of Y shall be taken as the minimum specified yield strength of the shell material multiplied by the Yield Strength Reduction Factor in Table M-1 of API Standard 650, Appendix M. When the value of Y is not known, the evaluation shall be based upon an assumed yield strength of 30,000 psi.
2 If the critical zone has been reduced in thickness beyond the provisions of the original tank bottom corrosion allowance, if any, the shell-to-bottom joint shall be evaluated for elevated temperature, liquid head and thermal cycles. The simplified analysis technique recommended in API Standard 650, M.4 may be used to satisfy this requirement.
B. Existing elevated temperature service tanks that were not originally designed and constructed to the requirements of API Standard 650, Appendix M, but have a successful service history of operation shall be evaluated for continued service as noted in 4.3.10.1 (A) above. If the tank diameter exceeds 100 feet and the tank was not constructed with a butt welded annular ring, the additional analysis of B above is required. In addition, the maximum operating temperature shall not exceed the temperatures at which the tank has operated successfully in the past.
4.3.10.2 Conversion to Operation at Elevated Temperatures
Existing tanks that were not originally designed and constructed to the requirements of API Standard 650, Appendix M shall be evaluated for a change to service to elevated temperatures as follows.
a. The tank shell shall be evaluated in conformance with API Standard 650, Appendix M. The allowable shell stresses of this Standard shall not be used.
b. The need for a butt welded annular ring shall be determined in conformance with API Standard 650, Appendix M and installed if required.
c. The shell-to-bottom joint shall be evaluated for fatigue conditions. In addition, the adequacy of the critical zone shall be based upon the requirements of this standard.