Table of Contents
1 INTRODUCTION & BACKGROUND
Tank gauging plays an important role in the smooth functioning of the Petroleum Supply Chain. Some gaseous petroleum products like LNG and LPG (despite being part of the gas supply chain) are also measured by tank gauging while in the liquid phase at the time of custody transfer or fiscal metering.
Industrial storage tanks usually contain large volumes of liquid product with significant fiscal value – the larger the tank larger is the financial impact of any incorrect operations. Tank gauging is critically important for measuring liquids in large storage tanks. The ultimate objective is to accurately determine the value of volume and mass of the product in the tanks. The oil and gas industry generally uses static volumetric assessments of the tank content. However, mass measurements are also implemented in China.
Tank gauging involves liquid level, temperature and pressure measurements. Since liquids undergo volume changes, depending on individual thermal expansion coefficient, there is an additional requirement for proper volume compensation during calculations of volumes at varying temperature conditions. Often, more significantly, for high vertical tanks and pressurised tanks, pressure measurement of the liquid head is important and included in calculating the pressure correction factor. This, in turn, provides an accurate assessment of the average observed density for converting volume to mass.
In addition, dimensional correction factors for variations in tank structural volume under changing operating parameters are applied to maximise liquid product quantity measurement accuracy. Tank gauging scope varies and is dependent on 1) type of tank, 2) type of liquid, and 3) the purpose storage tank is used for.
1.1 Types of Storage Tanks
Tank gauging configurations vary with changes in basic designs of storage tanks that usually come as:
- Cylindrical fixed roof tanks (Atmospheric or pressure less than 15 psig)
- Cryogenic pressurised cylindrical tanks (LNG pressure < 4psig)
- Cylindrical floating roof tanks (Atmospheric or pressure less than 15 psig)
- Pressurised tanks of spherical design
- Pressurised tanks of horizontal cylinder (bullet) design.
- Cryogenic Pressurized Land-based LNG Tanks
- Cryogenic Pressurized LNG tanks onboard marine vessels
- Crude / Petroleum products tanks on marine vessels
1.2 Types of Liquids Stored
Tank gauging configurations vary with changes in products contained in storage tanks that include:
- Very Heavy Crude (often heated tanks)
- Medium to Light Crude and Condensate
- Nonvolatile petroleum products (heated tanks for products like coal tar and bitumen)
- Volatile petroleum products
- Pressurised liquid gases
- Liquified gases
- Excessively Corrosive and Hazardous Products
1.3 The Use of Storage Tank
Tank gauging configurations vary depending upon the usage and purpose of information from a tank gauging system. The most common are:
- Petroleum (Oil, LPG, LNG) movement and operations
- Inventory control
- Custody transfer
- Loss control and mass balance
- Material balance or volume reconciliation
- Overfill prevention
- Leak detection
2 AUTOMATIC TANK GAUGING
The traditional tank gauging technique is hand gauging which can only be performed on atmospheric tanks. A specially designed metallic measurement tape measures ullage or innate (liquid level). The ullage is the distance from the reference point of the tank down to the liquid surface. The tank level is calculated by taking the reference height minus the measured ullage.
Tank gauging is swiftly moving towards automatic tank gauging systems. These are not just instruments on the tank. Rather, they combine field instrumentation, data acquisition, data processing, data communication, data storage, and retrieval. Automatic tank gauging systems are based on reliable data communication between control room servers and field instrumentation over large field bus networks, often wired and wireless. When buying and selling large volumes of liquids, automatic tank gauging systems serve as the main input for establishing correct invoicing and taxation. Certified tank gauging systems can provide more accurate transfer assessments than dynamic metering (LACT or ACT Systems) when performing large transfers from a tanker ship to a shore tank.
Float gauges, servo gauges and radar gauges are most commonly used in automatic tank gauging systems. As part of automatic tank gauging systems, all these gauges are configured to determine accurate values of ullage, innate, and water cut – where required. The transmitters attached to the tank gauge send the tank level values through signal cables (or in the form of radio signals) to the control room. Among the various tank gauges in today’s industry, radar level gauges are usually considered better because they have no moving parts and require no regular maintenance. However, proper use of technology is important while selecting automatic tank gauging systems based on radar gauges. Another characteristic associated with radar devices is their ability to carry out tank fluid measurement in dry mode – they do not require direct contact with the liquid. This unique feature makes it possible to use a radar gauge on various liquids such as corrosive chemicals, heated heavy asphalt, cryogenic liquefied gases like Liquefied Natural Gas (LNG), liquid Oxygen, Liquid Nitrogen, etc. etc.
2.1 Radar Tank Gauges
Radar tank gauges are commonly referred to as microwaves – ultrasound and capacitance level sensors have faded with time due to the effectiveness and accuracy of microwave-based radar gauges. Their initial applications were for automatic tank gauging of marine petroleum tankers but were further developed to fit onshore storage tanks.
For high-performance requirements of custody transfer accuracy in tank gauging applications, the Frequency Modulated Continuous Wave (FMCW), also referred to as “Synthesized Pulse”, signal processing method is used in radar gauges. The FMCW method sometimes goes under the. FMCW can deliver an instrument-level gauging accuracy better than a millimetre over a 50+ meter range. The latest design has been miniaturised to small enclosures. The power requirements are reduced so that radar tank gauges can be made intrinsically safe and require only a 2-wire bus for power and communication.
FMCW technology makes a radar tank gauge accurate, but this is not enough on its own. Precision radar gauges must also have specially designed microwave antennas to deliver instrument accuracy and installed accuracy required by custody transfer standards. One important property of radar antennas is that they should be designed to drip off any condensation quickly. Therefore, antennas inside tanks require sloping surfaces to avoid the accumulation of condensate liquids. To understand this application, the reader is referred to American Petroleum Institute Standard (API MPMS ch. 3.1B, ed. 1) related to antenna design with no horizontal surfaces.
3 COMPLIANCE AND STANDARDS
Several international standards are relevant for tank gauging. The main purpose of these standards is to serve as guidelines for both users and manufacturers of tank gauging equipment. The members of the working groups behind the development of these documents are, in most cases, experienced users from the petroleum industry or manufacturers with large tank gauging knowledge. The present trend is to avoid technology-specific standards as much as possible and specify the requirements on equipment for a certain application, thus leaving the door open for any technology to confirm if it can be proved that it fulfils the requirements.
However, to prove compliance to a standard is not always easy since there must be an independent authority/body available with the knowledge and resources to test a tank gauging system. ISO (International Organization for Standardization) and API (American Petroleum Institute) are responsible for the most important standards within tank gauging but do not have their test organisation and are not organised as typical test institutes.
National metrological authorities in a country with this expertise are required with legal requirements for tank gauging equipment. There should also be a department within a metrological organisation that handles tank gauging equipment’s legal aspects. Depending on how custody transfer based on tank gauging equipment has been implemented in their country, they have (to a varying degree) the necessary experience, skills and resources. Therefore, it often works as a step-process with the following sequence:
- The government is responsible for the law (the legal requirements on tank gauging), and they issue accreditation to a national test institute through an accreditation body.
- The test institute must show the accreditation body that they can perform the testing and define a test procedure.
- After approval from the accreditation body, the test institute is granted the right to perform testing and issue a test report. An approval can be issued if the test report conforms to the legal custody transfer requirements.
Fortunately, the national institutes that perform testing cooperate within OIML (International Organization of Legal Metrology). Several test procedures are defined in this organisation, and there is a special procedure defined for tank gauging equipment called OIML R 85 (Recommendation 85).
A tank gauging system that has been tested by an accredited OIML R 85 institute in one country may not need to repeat the same test in another (exceptions occur if the evidence is lacking that the R 85 procedure has been duly followed as intended).
The API standards provide useful experience-based facts and solutions to daily tank gauging problems by summarising know-how from practical investigations performed by research departments major IOCs. Some important API standards in MPMS related to tank gauging are listed below:
- API MPMS CH3.1A Ed. 3 (2013) – Manual of Petroleum Measurement Standards – Chapter 3.1A: Standard Practice for The Manual Gauging of Petroleum and Petroleum Products
- API MPMS CH3.1B Ed. 2 (2001/R2016) – Manual of Petroleum Measurement Standards – Chapter 3 – Tank Gauging – Section 1B – Standard Practice for Level Measurement of Liquid Hydrocarbons in Stationary Tanks by Automatic Tank Gauging
- API MPMS CH3.3 – Manual of Petroleum Measurement Standards – Chapter 3 – Tank Gauging – Section 3 – Standard Practice for Level Measurement of Liquid Hydrocarbons in Stationary Pressurized Storage Tanks by Automatic Tank Gauging
- API MPMS CH3.6 Ed. 1 (2001/R2011) – Manual of Petroleum Measurement Standards – Chapter 3 – Tank Gauging – Section 6 – Measurement of Liquid Hydrocarbons by Hybrid Tank Measurement Systems
- API MPMS CH7 – Manual of Petroleum Measurement Standards – Chapter 7: Temperature Determination
- API MPMS CH7.3 Ed. 2 (2011/R2016) – Manual of Petroleum Measurement Standards – Chapter 7.3: Temperature Determination – Fixed Automatic Tank Temperature Systems
- API MPMS CH16.2 Ed. 1 (1994/R2012) – Manual of Petroleum Measurement Standards – Chapter 16 – Measurement of Hydrocarbon Fluids by Weight or Mass – Section 2 – Mass Measurement of Liquid Hydrocarbons in Vertical Cylindrical Storage Tanks by Hydrostatic Tank Gauging
3.1 Tank Calibration
Tank gauging system accuracy is fundamentally dependent on accurate input of the tank’s dimensional values into the data processing system at system commissioning. Tanks undergo dimensional variations due to maintenance operations, accidents, or upgrades. In such instances, tanks must be re-calibrated, and new dimensional values are ascertained for input to the automatic tank gauging system.
When variations occur due to any of the activities above, storage tanks require verification of Dimensional Measurements and generation of Tank Capacity Tables (also called Strapping Table, Calibration Table, Tank Chart, Tank Calibration Chart, Tank Gauge Chart, Gauge Table, Dip Charts). This is required to maintain the accuracy of conversion of liquid level measurement in the tank to more meaningful volume or mass values. As per standards, physical calibration requirement or “Wet” calibration is required to meet final product metering accuracy requirements – wet calibration is the physical dimensional measurement and recording of tank sizes after its complete installation at site. Tank calibration is performed independently of the actual tank gauging system components to minimise intrinsic error. Following are the recommended tank calibration methods:
- API MPMS Chapter 2.2A – Measurement and Calibration of Upright Cylindrical Tanks by Manual Tank Strapping Method
- API MPMS Ch 2.2B / ISO 7507-2:2005 – Optical Reference Line Method
- API MPMS Ch 2.3 / ISO 7507-3:2006 – OTM, Optical Triangulation method (Laser)
- API MPMS Ch 2.4 / ISO 7507-4:2010 – EODR, Internal Electro-Optical distance ranging method (Laser)
- API MPMS Ch 2.2G / ISO 7507-5:2010 – EODR, External electro-optical distance-ranging method (Laser). ()
- API MPMS Ch 2.2E / ISO 12917-1:2002 – Calibration of Horizontal Tanks. (Petroleum and Liquid Petroleum Products—Calibration of Horizontal Cylindrical Tanks—Part 1: Manual Methods supersedes API 2551)
- API MPMS Ch 2.2F / ISO 12917-2:2002 – Calibration of Horizontal Tanks. (Petroleum and Liquid Petroleum Products—Calibration of Horizontal Cylindrical Tanks—Part 2: Internal Electro-Optical Distance-Ranging Method)
- API 2552 – Calibration of Spherical Tanks.
- API MPMS Ch 2.7 (Also required API MPMS Ch 2.8A & 2.8B) – Calibration of Barge Tanks.
- API Std 2554 (R2012) – Measurement and Calibration of Tank Cars.
- API 2555 Ed. 1 (1966/R2009) – Method for Liquid Calibration of Tanks