Api publ 327 1994 scan (american petroleum institute)

SUMMARY OF PROCEDURES

Figures 2.1, 2.2, and 2.3 illustrate logic diagrams that depict the process for reducing the risk of bottom or shell leaks in both proposed and existing tanks The following discussion offers a concise summary of each diagram.

The initial decision depicted in Figure 2.1 involves determining if the tank is proposed or existing Although the evaluation processes for both types may be similar or even identical in some instances, the outcomes for a proposed tank offer options that impact design and construction choices In contrast, results for existing tanks typically result in repairs or modifications to service conditions.

To comply with the API in-service AST inspection STD 653 and RP 12R1, collectively known as the API Standard, it is essential to consider their influence on the proposed tank design A key aspect affected by the API Standard is the selection of the tank bottom, as illustrated in the accompanying logic diagram.

When proposing a tank, it is essential to follow the steps outlined on the left side of Figure 2.1 Compiling basic data allows for the identification of various tank bottom design options, which is a crucial aspect of the process To ensure optimal onstream time, it is important for the owner/operator to investigate how factors such as cathodic protection, leak detection, and linings affect inspection frequency.

Tank bottom design relies on established standards along with engineering and economic data The final section of Figure 2.1 outlines a method for choosing a bottom design when multiple configurations meet the API Standard.

The evaluation process for existing tanks, illustrated on the right side of Figure 2.1, is divided into three distinct paths This tutorial focuses on the Tank Bottom and Shell paths, which are detailed in Figures 2.2 and 2.3, respectively.

The evaluation of tank roofs is not covered in this tutorial; however, as illustrated in Figure 2.1, existing tank roofs can be assessed according to the methods outlined in the API Standard, allowing for repairs, alterations, or reinstatement to service.

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FIGURE 2.1: LOGIC DIAGRAM FOR TANK DESIGN & EVALUATION c a n p i l e B a r i c

EWalUate Need for Leak Oetcction Sect Ion 4.7 i

CAPI STO 653,'Secs 2 I 7 ) ( A P I RF 1 2 R 1 Sec 4 I 5 )

C A P I RP12 RI Sec S) o" Return 10 S e r v i c e u Updale R e c o r d s

The evaluation process for an existing tank bottom, as illustrated in Figure 2.2, begins with identifying concerns related to settlement, corrosion, or both This identification is achieved through a thorough tank inspection and careful analysis of the inspection data.

If a settlement problem is identified, then it should be evaluated as shown in Section 6.1

If found unacceptable, the problem should be added to the list of repair items

The evaluation of bottom corrosion is illustrated on the left side of Figure 2.2, with guidance provided by API RP 12R1, API STD 653, and Section 6.2 of this document If any damaged areas are deemed unacceptable, they must be included in the list of repair items.

After reviewing the inspection data, it is essential to calculate the internal inspection interval as outlined in Section 6.2, assuming all necessary repairs have been completed If the interval meets the required standards, the bottom repairs can then commence.

If the interval is deemed unacceptable, the bottom of the tank must be modified, repaired, or replaced prior to its return to service Section 4 outlines the procedure for assessing these options and provides illustrative examples.

FIGURE 2.2: LOGIC DIAGRAM FOR BOITOM EVALUATION

CAPI RP 12RI S e c 4 S ) CAPI STO 653, S e c U )

Figure 2.3 illustrates two evaluation paths for a tank shell, with the right path focusing on brittle fracture considerations This assessment should be conducted during the initial evaluation of the tank or whenever there is a change in service.

Shell evaluations unrelated to brittle fracture often pertain to issues such as corrosion, erosion, shell distortions, and shell settlement, as indicated on the left path Section 8 of this tutorial outlines the relevant paragraphs in the API Standard for these evaluations and demonstrates the application of the associated requirements.

The evaluation of an existing tank is crucial for assessing its suitability for continued use If the evaluation yields unacceptable results, the tank must undergo repairs, modifications, or be considered for a change in service.

FIGURE 2.3: LOGIC DIAGRAM FOR SHELL EVALUATIONS

BASIC DATA REQUIREMENTS

Before utilizing the API documents mentioned in Section 1.2, it is crucial for the reader to gather specific fundamental data This information is necessary to determine the most suitable aboveground storage tank for a particular scenario The essential data required are outlined in the left column, with further details available in the documents referenced in the right column.

Operating Data which RP 12R1, Sections 2 & 4 relate to above items STD 650, Appendix M

BOTTOM SELECTION

This section describes procedures and considerations for selecting a tank-bottom corrosion protection design that conforms to the API Standards

Site and subsurface investigations are presumed to be finalized, addressing key factors such as tank size, corrosion, and bottom type, including cone-up, flat, and cone-down configurations.

Choosing the right corrosion protection system for tank bottoms is crucial and should be based on factors such as the soil type beneath the tank, anticipated internal and external corrosion rates, the tank field's location, the maintenance program in place, and the tank's intended design life.

Requirements for internal inspections have the greatest affect on tank bottom design Internal inspection can be done only if the tank is taken out of service and entered

Owner/operators work to minimize out-of-service inspections because they are time- consuming, expensive and may generate residual material and emissions

4.1 Tanks built to API Standard 650 Specifications

API STD 653 connects out-of-service internal inspections to the thickness of tank components, particularly bottom plates According to Section 2.4.7.3, the minimum remaining thickness (MRT) of these bottom plates must meet or exceed the specified thickness outlined in Table 4.1 of API STD 653 by the conclusion of the in-service operational period.

The following equations are included in API STD 653 and are considered to be an acceptable method for calculating MRT:

MRT, = T, - GC, - StP, - UP, - (StP, + UP, + GC,) O,

MRT, = To - GC, - StP, - UP, - (StP, + UP, + GC,) O,

MRT, = minimum remaining thickness due to average internal pitting and maximum

MRT, = minimum remaining thickness due to maximum internal pitting and average external pitting, in inches external pitting, in inches

TO = original plate thickness, in inches

GC, = average depth of generally corroded area, in inches

StP, = average depth of internal pitting, in inches

StP, = maximum depth of internal pitting, in inches

UP, = maximum depth of underside pitting, in inches

The average depth of underside pitting is measured in inches, with a maximum internal pitting rate specified in inches per year The StP value is set to zero if the tank bottom is internally lined, as referenced in API RP 652 Additionally, the maximum underside pitting rate is also zero if the tank bottom is cathodically protected, according to API RP 651 The maximum rate of general corrosion is expressed in inches per year, alongside the anticipated in-service operational period measured in years.

Production tanks differ significantly from API Standard 650 tanks in size, operating conditions, and objectives, which are tailored to the specific needs of production rather than refining, marketing, and transportation These distinct features are outlined in API RP 12R1.

API RP 12R1 requires two types of scheduled internal examinations, the scheduled Internal Condition Examination (conducted by appropriate field personnel) and the scheduled Internal Inspection (conducted by a qualified inspector)

At a minimum, a scheduled Internal Condition Examination should be made for the following events:

When a tank is cleaned for normal operational requirements;

When there is a change in tank location;

When the service is changed more than five years after an Internal Inspection; or When the tank is entered for any type of maintenance or modification

Scheduled Internal Inspections should be based on the corrosion rate life of the tank as given by:

Corrosion Rate Life (yeam) = - _ corrosion rate (incheslyear)

Where tcurrent is the measured thickness of the bottom plate and where tmlnlmum, for the Critical Annular Ring area, is 0.50 times t,,,, ,,, of the shell or a minimum of 0.062 inch

For small production tanks, the calculation is based on the structural consideration of the annular ring

At a minimum, inspections should occur at the beginning of the last predicted life when a minimum required plate thickness is still in place quarter of the

Three cases are shown below, with basic design data provided for each These cases are used as the basis for the examples in this section

Case I - Basic Design Data for 150,000 BARREL API STD 650 Tank

A 150,000 barrel API STD 650 tank is in the design stage The design data are:

Capacity 150,000 barrels Soil Condition Firm, little settlement and good drainage Stored Product Petroleum crude

General Corrosion 3 mPY Internal Bottom Pitting 20 mpy External Bottom Pitting 25 mpy bare steel

Size Height 48 feet, Diameter 150 feet

Case 2 - Basic Design Data for 10,000 BARREL Gasoline Storage Tank

A 10,000 barrel gasoline storage tank is proposed The underside corrosion is based on replacing the native soil below the tank The following design information is available:

Capacity 10,000 barrels Soil Condition Good drainage, moderate settlement Stored Product Gasoline

General Corrosion 4 mils per year (mpy)

Internal Bottom Pitting 6 mpy External Bottom Pitting 9 mpy

Size Height 24 feet, Diameter 55 feet Bottom Plates 0.25 inch

Case 3 - Basic Design Data for 500 BARREL Production Fluid Tank

A 500 barrel production fluid tank is set up at a new site The tank is internally lined and placed on a crushed stone foundation The design data are:

Stored Product Frequency of Internal Inspection

General Corrosion Internal Bottom Pitting External Bottom Pitting Design Life

Size Bottom Plates Minimum for Shell

500 barrels Crushed stone with retainer ring over loose fine Production fluids sand

40 years Height 16 feet, Diameter 15 feet, 6 inches 0.25 inch

Tank bottoms in contact with the ground are susceptible to corrosion, even after the native material is removed According to API STD 653, specifically in Paragraph 4.4.2.2, if there are no methods for detecting and containing bottom leaks, the minimum required thickness (MRT) must be at least 0.10 inch.

In Case 2, Example 4.1 highlights the necessity for corrosion protection for the 150,000 barrel tank, which is assumed to be resting on the ground To comply with API STD 653 requirements, both external and potentially internal corrosion protection measures are essential.

Example 4.1 - Ground/Bottom Contact - 150,000 Barrel Tank

In Case 1, the analysis of a 150,000 barrel tank reveals the Minimum Required Thickness (MRT) at the conclusion of the initial 10-year period, based on the guidelines set forth by API STD 653, under the assumption of no corrosion protection The MRT calculations utilize specific quantities relevant to the equations involved.

StP, & StP, = 0.0 for a new tank

UP, & UP, = 0.0 for a new tank

MRT, T, - GC, - StP, - UP, - (StP, + UP, + GC,) O,

Substituting into the equations yields:

MRT, = MRT, = 0.25 - O - O - O - (0.020 + 0.025 + 0.003) 10 MRT, = MRT, = -0.23 inch

The findings from Example 4.1 suggest that a hole will form at the bottom prior to the next scheduled inspection in 10 years, necessitating additional protective measures By applying a value of 0.1 O inches for MRT, the equation can be resolved for O.

To ensure the longevity of a tank's operational period, it is crucial to implement corrosion protection According to API STD 653, if a tank is put into service without such protection, an internal inspection must be conducted after approximately three years of operation.

Example 4.2 applies paragraph 4.4.2.2 of API STD 653 to the 10,000 barrel gasoline tank in Case 1

Example 4.2 - Ground/Bottom Contact - 10,000 Barrel Gasoline Tank

Given the tank in Case 2, determine the MRT at the end of an initial 10 year period assuming no corrosion protection Quantities for the MRT equations are:

MRT, T, - GC, - StP, - UP, - (StP, + UP, + GC,) Où

MRT, = MRT, = 0.25 - O - O - O - (0.006 + 0.009 + 0.004) I O MRT, = MRT, = 0.06 inch

API STD 653 requires a minimum thickness of 0.1 O inch; therefore, additional corrosion protection must be provided in order to maintain the IOyear inspection interval

An anticipated in-service-period of 7.9 years results from the MRT value of 0.10 inch

Example 4.3 applies the criteria of RP 12Rl to the proposed tank in Case 3

The calculated out-of-service internal inspection interval is 28 years; however, an Internal Conditional Examination may be prompted by one of the four factors mentioned in paragraph 4.2 before this period concludes.

Many production tanks may not remain in the same location for the duration of the internal inspection interval When tanks are relocated, it is essential to conduct an Internal Condition Examination and, if necessary, a Condition Assessment.

Example 4.3 - Ground/Bottom Contact - 500 Barrel Production Tank

Determine the out-of-service internal inspection interval for Case 3, using the procedure given in API 12R1 The corrosion rate is 0.005 inches per year

Determine tminimum where tminimum is based on the structural considerations of the annular ring

For the critical annular ring area, tminimum is 0.5 times tminimum of the shell (0.036 inch) or a minimum of 0.062 inches

Or, tminimum is the larger of 0.5 (0.036) = 0.018 inches or 0.062 inches

Last quarter begins 0.75 (37.6 years) = 28.2 years

Therefore, the calculated interval is 28 years

According to Paragraph 4.4.2.2 of API STD 653, a reduction in Minimum Required Thickness (MRT) is not permitted for cathodically protected tank bottoms However, Section 2.4.7.1 of the standard permits a zero value for external pitting if the tank bottom is adequately protected by a well-designed and maintained cathodic protection system.

Example 4.4 shows the calculations for the 150,000 barrel tank (Case 1) assuming the bottom is cathodically protected

Using the data given in Case 1 , determine the MRT at end of an initial 1 O year period assuming the bottom is cathodically protected Quantities for the MRT calculations are:

MRT, = T, - GC, - StP, - UP, - (StP, + UP, + GC,) Or

MRT, = T, - GC, - StP, - UP, - (StP, + UP, + GC,) O,

MRT, = MRT, = 0.25 - O - O - O - (0.020 + 0.0 + 0.003) 10 MRT, = MRT, = 0.02 inch

To prevent leaks before the next shutdown, additional protection is essential A nominal external pitting rate of 0.001 inches per year can be utilized for a more conservative estimate of the Minimum Repair Thickness (MRT) This tutorial does not cover the example of cathodic protection for the tank in Case 2, so readers are encouraged to explore this option independently For a conservative approach, it is suggested to set the external pitting rate to a nominal value, leading to an MRT of 0.14 inches when using a pitting rate of UP = 0.001.

API STD 653 permits a reduction in the Minimum Required Thickness (MRT) when a system for detecting and/or containing bottom leaks is implemented In this case, the MRT must be at least 0.05 inches For instance, Example 4.5 illustrates this criterion in relation to a 150,000 barrel tank (Case I).

Example 4.5 - Leak Detection/Containment System

Using the data given in Case 1, determine the MRT at the end of an initial

Basic Design Data for 500 BARREL Production Fluid Tank

Tiêu đề	Aboveground Storage Tank Standards: A Tutorial
Trường học	American Petroleum Institute
Chuyên ngành	Environmental Health and Safety
Thể loại	Publication
Năm xuất bản	1994
Thành phố	Washington, D.C.

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Số trang	76
Dung lượng	2,57 MB