2.1 API NORMATIVE STANDARDS API Manual of Petroleum Measurement Standards, Chapter 19.2, “Evaporative Loss From Fìoating- Roof Tanks” MPMS Chapter 19.3, Part F, “Evaporative Loss Factor
Definitions
3.1.1 data acquisition: The process of receiving signals from the sensors, determining the values corresponding to the signals, and recording the results
The deck of a floating roof is essential for providing buoyancy and structural support while covering most of the liquid surface in a bulk liquid storage tank It features an annular space around its perimeter, allowing for vertical movement as the tank is filled or emptied, preventing any binding against the tank shell This space is sealed with a flexible rim seal, and the deck may also include penetrations secured by deck fittings to facilitate various operational functions of the tank.
Deck fittings are essential devices that effectively seal openings in the deck of floating roofs used in bulk liquid storage tanks These openings are generally designed to support various functional or operational features of the tank.
Certain internal floating roofs are made with deck sheets or panels that are mechanically joined at the seams, which can lead to deck seam loss Additionally, other types of floating roofs, whether internal or external, are constructed from metal materials.
1ASTM International, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania 19428
2 CHAPTER 1 EVAPORATIVE TIVE Loss MEASUREMENT sheets that are joined by welding Such seam devices do not have an associated deck seam loss
The evaporative loss factor is a key metric that characterizes the rate of evaporative loss for floating roof devices To determine the standing storage evaporative loss rate of a bulk liquid storage tank with a floating roof, it is essential to adjust the evaporative loss factor based on specific climatic conditions and the properties of the stored liquid.
The characteristics of the stored liquid are expressed as a vapor pressure function, a vapor molecular weight, and a product factor
A floating roof is a device designed to float on the surface of stored liquids in bulk liquid storage tanks By substantially covering the liquid product's surface, it effectively minimizes evaporation exposure The construction of floating roofs includes a deck, a rim seal, and various deck components.
An indicator is a device that displays or records signals from a sensor, converting them into useful measurement units For instance, an electronic signal received in volts may be shown as pounds Often integrated into electronic data acquisition systems, indicators can be pre-programmed to record data at specific intervals, analyze the received data, and electronically store the results.
3.1.8 instrument: A device used in the measurement pro- cess to sense, transmit, or record observations
An internal floating roof is a type of floating roof that remains protected from environmental conditions due to its placement within a bulk liquid storage tank that features a fixed roof at the top This design differentiates internal floating roofs from external floating roofs, as the fixed roof shields the internal structure from exposure to the elements Typically, the design of internal floating roofs adheres to the guidelines outlined in Appendix H of API Standard 650, Welded.
Steel Tanks for Oil Storage
The product factor is a key element that characterizes the evaporative loss of a specific liquid product To calculate the standing storage evaporative loss rate of a bulk liquid storage tank with a floating roof, the product factor, vapor pressure function, and vapor molecular weight are multiplied by the total equipment loss factors.
A rim seal is a flexible device designed to close the annular space between the tank shell and the perimeter of a floating roof deck Effective rim seals not only seal this space but also accommodate irregularities between the floating roof fittings and the tank shell, ensuring proper centering of the floating roof while allowing for its normal movement.
A sensor is a device that detects specific attributes or measurement data during a measurement process It transmits this information to an indicator for display or recording purposes.
Standing storage evaporative loss refers to the evaporation of stored liquid stock that occurs past the floating roof under normal service conditions This type of loss excludes evaporation from liquid clinging to the tank shell during emptying (withdrawal loss) and vapor loss when the liquid level is low enough for the floating roof to rest on its support legs However, it does encompass evaporative losses from the rim seal, deck seams, and deck fittings.
The vapor pressure function is a dimensionless factor utilized in loss estimation, representing the ratio of the vapor pressure of the stored liquid to the average atmospheric pressure at the storage site To calculate the total standing storage evaporative loss rate of a bulk liquid storage tank with a floating roof, the vapor pressure function, stock vapor molecular weight, and product factor are multiplied by the sum of the loss factors from individual floating roof devices.
3.1.15 weight loss test method: The method of deter- mining a loss factor by measuring the weight loss of a test assembly syF*b!y (?\;er &TAe 2s Eqdid e\/q.I?or&=s k(?F* Lhe
Units of Measurement
This standard utilizes inch-pound units from the English system, referencing values from the U.S National Institute of Standards and Technology (NIST) While the standard does not provide equivalent International System of Units (SI) values, guidance for converting to SI and other metric units is available in Appendix C, Metric Units.
The primary units of measurement include length, mass, force, time, temperature, and electromotive force Length is measured in miles (mi), feet (ft), or inches (in) Mass is quantified in pounds (lb), while force is expressed in pound-force (lbf) Time can be represented in hours (hr) or years (yr) Temperature is indicated in degrees Fahrenheit (°F) or degrees Rankine (°R) Lastly, electromotive force is measured in volts (V).
The unit of reporting rim-seal loss factors is the pound-mole per foot of tank diameter per year, designated lb-mole/ft yr
The rim-seal loss factor, K, is not measured in pound-moles of vapor loss over time; instead, it serves as a dimensionless factor that, when multiplied by the tank diameter and other dimensionless coefficients, helps calculate the actual pound-moles of evaporative loss for a specific liquid product To convert the rim-seal loss factor from pound-moles per foot of tank diameter per year to a loss rate in actual pound-moles, K is multiplied by the dimensionless coefficients P*, which depend on the product vapor pressure and atmospheric pressure, and Kc, the product factor.
A pound-mole (lb-mole) is defined as the mass of a substance in pounds that corresponds to its molecular weight To convert pound-moles per foot of tank diameter per year into pounds per year for a specific liquid product, the loss rate (K,P*Kc) is multiplied by the tank diameter (D) and the molecular weight of the liquid in its vapor phase (M), with the molecular weight measured in pounds per pound-mole For further details on this calculation, refer to API Publications 25 17 and 25 19, as well as API MPMS, Chapter 19.2.
The unit of pressure is the pound-force per square inch
Nomenclature
Table 1 provides a description of the symbols and units Note: See Section 3.2 for definitions of abbreviations for the units
This standard outlines a test method that employs a weight loss procedure to quantify the rate of evaporative loss A test assembly, equipped with a test rim seal and suspended from load cells, features spacers that establish a specific rim-seal gap area between the seal and the simulated tank shell The area beneath the test rim seal is filled with a volatile hydrocarbon test liquid, such as normal-hexane or isohexane, at a designated height Over time, the weight loss of the test assembly is recorded, and the data is adjusted for temperature and atmospheric pressure fluctuations This allows for the calculation of a loss rate, which is then modified based on the properties of the test liquid and the length of the test rim seal to derive an evaporative rim-seal loss factor for the specified seal gap area.
This test method outlines a procedure for determining the evaporative rim-seal loss factor of rim seals utilized on internal floating-roof tanks Testing must be conducted in a laboratory approved by the API, following the guidelines set forth in the API MPMS, Chapter 19.3, Part G.
Rim-seal gap area in2
Constant in the vapor pressure equation Constant in the vapor pressure equation
Rim-seal gap area factor in2/ft
Lc Length of the test assembly shell plate ft
Ls Length of test rim seal ft
Kr Rim-seal loss factor lb-mole/ft yr
L, Rim-seal loss rate lb/yr
Mv Molecular weight of test liquid vapor lb/lb-mole
P Vapor pressure of the test liquid psia pa P* t
Atmospheric pressure Vapor pressure function Time
Stock liquid temperature Weight loss of the test rim seal psia dimensionless hr
Chapter 1 discusses the measurement of evaporative loss, emphasizing the importance of the Loss Factor Testing Laboratory Registration The values obtained through this method must be assessed in line with the API MPMS, Chapter 19.3, Part F, which focuses on evaporative loss.
Factor for Storage Tanks Certification Program,” to assign
Manufacturers of floating roof rim seals can obtain API-certified loss factors for their proprietary designs through a comprehensive procedure that includes laboratory approval, specific test methods, and evaluation techniques.
Evaluation of Results
The results of this test method are not intended to be used apart from their evaluation in accordance with API
LowLossRates
This test method is invalid for rim seals with a loss rate that falls below the instrument's specified tolerance or the observed drift range of the load cells.
If the loss rate of the test rim seal is below the detection limit of the instrumentation or the drift rate of the load cells, the test results report must indicate a minimum value for the rim-seal loss factor, which is determined by the instrumentation's detection limit and the load cells' drift rate.
TestApparatusSchematic
The test apparatus, illustrated in Figures 1, 2, and 3, is designed to obtain measurements essential for developing a certified evaporative rim-seal loss factor for an internal floating roof's test rim seal This apparatus includes specific test equipment and instrumentation organized within a test room and a data acquisition room.
TestRoom
The test room must be sufficiently spacious to accommodate the necessary test equipment, instrumentation, and personnel, while the data acquisition system is located in a separate room as outlined in Section 7.4 Additionally, the test room should be designed to maintain the air temperature within ±5°F of the chosen test temperature throughout the testing duration.
The test room should be insulated to aid in the control of the air temperature within the test room
The test room shall have a dedicated temperature controller for maintaining the air temperature in the test room The test room may also have a dedicated heater and air conditioner
The test room must have an air ventilation system to ensure adequate airflow, preventing the accumulation of evaporated test liquid It should include a ventilation blower to maintain a consistent stream of air However, the ventilation air flow rate should be restricted to approximately two air changes per hour to avoid creating conditions that differ significantly from the vapor space above an internal floating roof.
The test room must have a large equipment access door to facilitate the installation and removal of test assemblies, along with a smaller personnel access door for inspecting the test assembly during testing.
The test room must include a support frame designed to hold a test assembly throughout the testing process This assembly will be suspended from load cells that are securely attached to the support frame.
It is advisable to place a spiil pan under the test assembly to collect any spillage of the test liquid that may occur during fiiiing and emptying operations.
TestAssembly
To conduct testing on a rim seal, it must be installed on a test assembly equipped with a minimum of three suspension points, allowing it to be suspended from load cells for accurate measurement.
Figure 3 illustrates a test assembly designed to replicate a segment of the rim of an internal floating roof, consisting of a bottom plate, rim plate, shell plate, and end plates To ensure the shell plate maintains its proper contour, a stiffener plate may be incorporated The shell plate is specifically curved to mimic a tank shell with a 50-foot radius Additionally, the test assembly can be made from aluminum to minimize weight, and its length must accommodate a test rim seal section with a minimum arc length of 10 feet.
The test assembly shall be fitted with a thermocouple to measure the temperature of the test liquid within 3 inches below the test liquid surface
The test liquid is contained in the simulated rim space that is between the shell plate and rim plate
The rim plate height must ensure that the test liquid surface is maintained within 11 inches below the top of the rim plate, aligning with the industry-standard level for the rim seal being evaluated.
The test assembly must include a minimum of three support points to ensure it can be suspended from load cells, allowing for orientation adjustments to maintain a horizontal position during testing.
Testing rim seals with low loss rates necessitates the use of load cells that can detect minimal weight changes, unlike those needed for seals with higher loss rates Consequently, this may lead to the adoption of load cells with lower load capacities, which in turn restricts the overall weight of the test assembly.
The rim seal to be tested must be installed on the simulated floating roof rim following the manufacturer's assembly and installation procedures An exception to these procedures is the sealing method at the ends of the test rim seal, which must be securely joined to the end plate of the test assembly to prevent emission end effects It is essential to document the sealing details for these end joints in the test results report, ensuring that their installation does not impact the emission characteristics of the test rim seal.
The test rim seal and assembly shell plate must be thoroughly cleaned to ensure they are free from oil and any substances that could influence the rim-seal loss factor test outcomes.
A leak-tightness test is conducted on the test rim seal to verify the absence of leak paths at the end joints This test involves applying a slight gas pressure in the vapor space beneath the rim seal, followed by the application of a leak detection liquid, usually a soap-like solution that produces bubbles at vapor leaks The absence of bubbles indicates that there are no significant emissions from the end joints.
The test liquid must be kept at a level that aligns with standard industry practices for the rim seal being evaluated.
The test liquid used must be normal-hexane (n-hexane) or iso-hexane of technical grade or higher It is crucial that the temperature of the test liquid does not exceed its normal boiling point during the test Additionally, samples of the test liquid should be collected both prior to and following the test period to assess the Reid vapor pressure of the mixture, in accordance with ASTM D323 standards.
To minimize the required quantity of test liquid, it can be floated on top of water The layer of test liquid must be deep enough to fully cover the water's surface throughout the test duration Additionally, this depth should prevent the vapor pressure of the test liquid from decreasing by more than 5 percent due to the evaporation of lighter hydrocarbon components during the test.
When testing rim seals that extend into the liquid product of a storage tank, it is essential to install them in a way that allows the test liquid to flow freely to all areas beneath the test rim seal For instance, during the evaluation of a mechanical-shoe primary seal, the test liquid layer floating on water should reach below the bottom of the shoe.
All penetrations and attachments to the test assembly, including those for filling and emptying, must ensure a leak-tight seal It is essential to provide a method for indicating the liquid level in the test assembly for both initial filling and monitoring during tests The recommended approach for liquid level indication is a sight tube or window, although alternative methods that prevent the loss of test liquid or its vapors may also be acceptable.
Data Acquisition Room
The data acquisition room must be spacious enough to accommodate the data acquisition system and the personnel needed for the testing method It should be designed to maintain the air temperature within ±15°F of a specified room temperature throughout the entire testing period.
The data acquisition room should be insulated to aid in the control of the air temperature within the room
The data acquisition room will feature a dedicated temperature controller to regulate air temperature effectively Additionally, it may be equipped with a specific heater and air conditioner to ensure optimal climate control.
The data acquisition room shall be equipped with a fan that circulates the air within the room so as to reduce air tempera- ture variations within the room
6 CHAPTER 1 EVA EVAPORA TIVE Loss MEASUREMENT
This test method evaluates rim seals for internal floating-roof tanks using full-scale samples that adhere to the manufacturer's standard practices The samples must incorporate all features typical of actual usage to ensure accurate testing results.
The rim seal under evaluation must be affixed to the test assembly in a way that mimics its actual installation on a floating-roof rim Additionally, test rim seals that typically extend into the liquid product on a floating roof should be mounted similarly on the test assembly.
8.3 TEST RIM SEAL END CONNECTIONS
The ends of the test rim seal shall be sealed to the end plates of the test assembly A suitable sealant or caulk may be used for this purpose
Install the test rim seal on the test assembly, as described in
7.3.2 and 8.2 Suspend the test assembly from the load cells
To prepare the test assembly, fill it with the test liquid to the specified level as outlined in section 7.3.4 Next, place rim-seal gap spacers between the test rim seal and the shell plate to establish the necessary rim-seal gap area, as detailed in section 1.1.
Start the test room air temperature control system and adjust the test room temperature to the required level
Start the data acquisition room air temperature control sys- tem and adjust the data acquisition room air temperature to the required level
Begin the data acquisition system to monitor the weight loss of the test assembly over time until a consistent weight loss rate is established After this initial stabilization phase, the data collected by the system will serve as the basis for calculating the evaporative loss factor.
Accuracy
To accurately measure each parameter, it is essential to utilize a sensor, an indicator, and a data recording method This article outlines the necessary instruments, measurement techniques, and accuracy standards Calibration procedures are detailed to reduce systematic errors or biases in the instruments A summary of the instrument requirements can be found in Table 2.
To minimize random errors in the measurement process, specific procedures are established for critical steps, particularly in the indication and recording of observed values This process, known as data acquisition, involves receiving signals from sensors, determining their corresponding values, and documenting the results A programmable electronic data acquisition system is utilized to ensure that the frequency and precision of observations adhere to defined tolerances The accuracy of the sensors is verified through the readings provided by this system, ensuring both the indicator and sensor are reliable Calibration standards must be traceable to national measurement reference standards maintained by NIST.
Data Acquisition System
The data acquisition system must effectively record all sensor data and include a chronometer with a maximum interval of one second and an accuracy of 0.1 percent It should be programmable to capture individual sensor readings at specified frequencies and capable of recording multiple readings within a designated time frame to calculate the mean and standard deviation Additionally, the system should provide real-time display of observed values to promptly identify and address any out-of-specification conditions Finally, the software must be verified using the data acquisition system during sensor calibration.
Weight Measurement
The test assembly's weight will be measured using high-precision load cells, which will transmit the weight measurement signals to the data acquisition system These load cells are designed to detect weight changes as small as 0.01 percent of the test assembly's total weight.
Rim seals with low evaporative loss rates may necessitate the use of sensitive load cells to detect minimal weight changes or may require an extended test duration.
8 CHAPTER 1 EVA EVAPORA TIVE Loss MEASUREMENT c