Tank Gauging Systems

What Is a Tank Gauging System?

Tank gauging means determining the quantity of product in large storage tanks with an accuracy that can serve as the basis for commercial and operational decisions. Level information alone is not enough for this task: because hydrocarbons expand with temperature, the same level corresponds to different quantities of product at different temperatures. Commercial transactions are therefore based on net volume and mass corrected to a reference temperature. A tank gauging system combines precise level measurement with multi-point average temperature, the free water level at the tank bottom and, where needed, vapor and liquid pressure measurements.

What sets the system apart from a process level transmitter is that the measurements are combined and calculated tank by tank. A process transmitter usually provides a single signal to a control loop; a tank gauging system converts level into volume using the tank capacity table, applies temperature correction, generates inventory and movement data and provides an overfill prevention layer that is independent of basic process control. In configurations intended for custody transfer, the radar gauge and the connected data collectors can be used together with type approvals from legal metrology bodies.

The solution can be adapted to a wide range of applications — onshore facilities with fixed roof, floating roof, internal floating roof and pressurized tanks, as well as ballast, fuel and draft measurement on ships.

Tank Gauging System Architecture and Working Principle

A modern tank gauging system works with a layered architecture from the field to the control room. Each layer passes both raw measurements and calculated values to the layer above:

1.      Level measurement by radar: The radar level gauge uses the FMCW (frequency modulated continuous wave) method: a signal swept linearly around roughly 10 GHz is reflected from the liquid surface, and on its return a small frequency difference appears relative to the signal being transmitted. This difference scales directly with the antenna-to-surface distance. The non-contacting device has no moving parts and produces a new measurement every 0.3 seconds; the antenna type is selected according to the tank roof design, opening diameter and product.

2.      Temperature, free water and pressure measurement: Multi-point temperature sensors suspended in the tank allow the average liquid and vapor temperatures to be calculated, and an integrated water level sensor tracks the free water at the tank bottom. Pressure transmitters for vapor and hydrostatic pressure, and temperature transmitters with bus communication for single-point measurements, can be connected to the same structure; these devices are covered on their own product pages.

3.      Collection, power and calculation in the tank hub: The data collector at the tank (tank hub) both powers the field devices and communicates with them over a 2-wire, intrinsically safe tank bus. Based on FOUNDATION Fieldbus and FISCO-compliant, this structure allows more devices on the same segment without entity parameter calculations. The tank hub recognizes and addresses the devices automatically and performs average temperature, observed density and tank-table-based volume calculations locally.

4.      Data concentration in the system hub: The system data concentrator (system hub) continuously polls one or more tank hubs and, where needed, field devices from other manufacturers, and holds the data in its buffer memory. When a request arrives, the data for the tank group is sent without delay. Eight configurable ports can be divided flexibly between field and host communication.

5.      Inventory software and DCS/SCADA: Inventory software in the control room handles gross and net volume, mass, alarms and reporting. The same data can be sent simultaneously to a DCS, SCADA or existing operator displays via Modbus TCP, OPC UA or serial Modbus.

Inventory Calculation Chain: From Level to Net Standard Volume

The measured level is converted into total observed volume (TOV) using the tank capacity table (strapping table) established by calibration. Deducting the free water volume (FWV) at the tank bottom gives the gross observed volume (GOV). Multiplying GOV by the volume correction factor (VCF), which depends on the average product temperature and density, converts it into gross standard volume (GSV) at the reference temperature; subtracting the sediment and water (S&W) allowance yields the net standard volume (NSV) on which commercial transactions are based.

The tank hub can calculate TOV with a 100-point capacity table and API-corrected NSV at tank level. The system hub supports capacity tables of up to 5,000 points per tank for cylindrical and spherical tanks, runs real-time gross/net volume and mass calculations according to ASTM D4311 and API tables 6, 54, 24, 60 and A–D, and can calculate NSV at a user-defined reference temperature. When level, temperature and pressure are measured together, hybrid calculation also delivers density and mass continuously.

Overfill Prevention and SIL Architecture

The radar level gauge carries IEC 61508 certification as capable of SIL 2 and SIL 3. At a predefined liquid level, the safety function activates a separate alarm loop and trips the safety relay or 4–20 mA output in the tank hub; this signal is wired to the emergency shutdown system (ESD) or to an automatic overfill prevention system. SIL 2 is achieved with the analog output or the safety relay, whereas SIL 3 requires the 2-in-1 version.

The 2-in-1 gauge houses two independent, galvanically separated radar electronics in the same housing and uses a single tank opening. One unit can handle the inventory level and the other an independent high-high level alarm, so a single device serves two independent protection layers such as the basic process control system (BPCS) and the safety instrumented system (SIS). Comparing the signals of the two units in real time allows measurement deviations to be spotted early. The SIL 3 relay in the tank hub is closed (energized) in normal operation and opens the circuit when the alarm level is reached or a device fault occurs. This approach makes it possible to build overfill prevention solutions in line with API 2350.

Redundancy and Wireless Data Transmission

At critical terminals the system hub can be operated redundantly with two identical units: the primary unit is active and the secondary is passive, and if the primary fails, the secondary takes over and sends a fault message to the inventory software or the DCS. Redundancy can be applied to all or part of the system, from the control room down to the field devices; with two level devices on the same tank (a 2-in-1 gauge, two separate radars or two devices of different types), field values can be kept in hot standby, and field ports can back each other up in a ring structure.

For remote tanks where running cables is expensive, a wireless adapter connected to the tank hub transmits tank data such as level and temperature over a WirelessHART (IEC 62591) network to a gateway. The IEEE 802.15.4-based radio uses channel hopping in the 2.4 GHz band, and the user can select an update period between 8 seconds and 60 minutes.

Modernizing Existing Systems by Emulation

Many tank farms still operate mechanical, float or servo gauges from various manufacturers together with their associated data acquisition units. The tank hub takes the place of the replaced legacy gauge by emulating the tank gauging protocols of different manufacturers: the existing host continues to see the new radar gauge as if it were the old device. The site can therefore be modernized step by step while the tanks remain in service and the existing wiring is reused.

The system hub can also communicate directly with existing field devices, allowing old inventory software to be replaced by new software. With user-defined Modbus mapping, the hub mimics the register structure expected by the existing DCS or operator display, so no reprogramming is needed on the host side. Up to three device types among field devices communicating via RS-485 Modbus RTU can be defined and integrated into the system.

Floating Roof Monitoring

Floating roofs reduce the need for vapor recovery; however, a roof that sinks, tilts, leaks or collapses can lead to mechanical damage, overfilling, release of explosive hydrocarbon vapors and product contamination. Damaged or incorrectly installed rim seals, leaking pontoons, strong winds and insufficient drainage during heavy rain or snow can dangerously affect the position and buoyancy of the roof.

Non-contacting radars placed at equal intervals on the tank top monitor tilt by comparing their distances to reflector plates on the roof; this solution can be added to existing systems while the tank stays in operation, and buoyancy is also calculated when the liquid level is used as a reference. Alternatively, guided wave radars whose rigid probes extend from the roof into the liquid are installed directly on the roof; this configuration relies on battery power, wireless communication and the nozzles already on the roof. An integrated function in the system hub provides roof tilt detection, buoyancy calculation and status warnings with up to three tank-mounted tilt gauges; automatic alarms can be generated for tilt, buoyancy, a stuck roof, a blocked drain sump and hydrocarbon detection. On tanks with an internal floating roof, the roof position can also be monitored by non-contacting radar.

Electro-Pneumatic Tank Level Gauging at Sea

On ships, an electro-pneumatic system is used to measure ballast, fuel, miscellaneous liquid and void spaces, as well as draft. Controlled air is blown through an air pipe running from the measuring cabinet to the tank; the pressure needed to push the air out of the pipe equals the hydrostatic pressure at the pipe end. The liquid level is calculated from this differential pressure, compensated for pipe line resistance, temperature changes and measuring point offset. A vent pipe provides the atmospheric reference; on pressurized tanks the tank top pressure is measured through a separate line, or a 1:1 pressure repeater is used instead of blowing air.

The cabinets are offered in two operating arrangements: in the scanning arrangement, pressure transmitters are shared among six measuring channels and all channels are scanned in a cycle of less than 8 seconds; in the continuous arrangement, each channel has its own transmitter and tank data is updated without interruption. Draft is measured with pipes taken from several points on the hull bottom; an inclinometer for trim and list, as well as radar or guided wave radar level devices, can be integrated into the system via analog channels. Because the electronics are located away from the harsh tank environment, maintenance is easier.

Key Features

The features below summarize the main capabilities of the onshore tank gauging architecture — radar level gauge, tank hub and system hub — and of the electro-pneumatic system for marine use.

·         Millimeter-level accuracy: Under reference conditions the radar level gauge offers ±0.5 mm instrument accuracy and 0.2 mm repeatability; temperature stability over the −40 to +70 °C ambient range is typically better than ±0.5 mm.

·         Antenna options for every tank type: Parabolic and horn antennas for fixed roof tanks, a still-pipe array antenna for floating roof tanks, and a dedicated pipe antenna with a ball valve for pressurized or cryogenic liquefied gases. Because the same measuring head is used with all antenna types, fewer spare parts are needed.

·         Stable measurement in rusty still pipes: The still-pipe antenna uses a transmission mode with minimal losses that concentrates the microwave energy in the center of the pipe, so custody transfer accuracy is maintained even in old, rusty or product-coated pipes.

·         Redundant 2-in-1 measurement: Two galvanically separated radar electronics are combined in one housing on one tank opening; one handles the inventory level, the other an independent high-high alarm or a backup measurement.

·         Bus-powered, self-configuring field network: Field devices draw power from the 2-wire, intrinsically safe bus and can be daisy-chained. The tank hub addresses devices automatically, monitors their status and provides network termination internally.

·         Calculation and local display at the tank: The tank hub calculates average temperature, observed density and volume; values such as level, ullage, level rate, up to 16 temperature points, free water level, and vapor and liquid pressure can be viewed on the integrated display.

·         Configurable alarm relays: Two solid-state relays configured for level, temperature and water level are operated through ten independent virtual relay functions; in addition, a separate SIL 3 relay dedicated to overfill prevention can be added.

·         Secure and flexible host connection: The system hub supplies data simultaneously to different host systems via Modbus TCP, serial Modbus RTU and an OPC UA server offering encryption, authentication and authorization. User-defined Modbus mapping reproduces the register structure of legacy systems one to one.

·         Scalable system capacity: Depending on configuration, the system hub serves up to 16, 48 or 64 tanks; it offers three Ethernet ports, a web-based configuration interface and USB/SD card support for logging diagnostic data.

·         Metrological sealing and write protection: The radar gauge and the hubs can be metrologically sealed; write protection at software and hardware level prevents unauthorized changes to custody transfer configurations.

·         Built for harsh site conditions: The polyurethane-coated cast aluminum housings of the radar gauge and tank hub provide IP 66/67 and NEMA 4X protection; the radar measuring head is protected against lightning, humidity and sulfurous or salty atmospheres and can be exchanged with the tank kept closed.

·         Installation and verification while in service: Most antenna types can be installed while the tank is in service. In liquefied gas applications, reference pins in the still pipe create echoes at fixed distances, allowing the measurement to be verified while the tank is in operation.

Technical Specifications

Because a tank gauging system consists of several subsystems, each value is labeled with the subsystem it belongs to. Every row applies only to that subsystem; final values should be confirmed at project stage according to the selected antenna, configuration and approval options.

The performance values of guided wave radar transmitters and general-purpose pressure and temperature transmitters are not the same as the radar level gauge values in this table; data for these devices is given separately on the relevant product pages. Custody transfer approval, hazardous-area certification and SIL level depend on the ordered configuration

Key Advantages

·         Reliable stock and loss control: Millimeter-level accuracy and temperature-corrected net volume calculation give tank-by-tank stock reconciliation and product loss analysis a dependable data basis.

·         Metrological confidence in custody transfer: Configurations with legal metrology type approval and sealable devices support quantity reconciliation between buyer and seller and audit processes.

·         Overfill safety through independent protection layers: Running inventory measurement and the high-high level alarm on separate electronics provides an additional, SIL certified protection layer against overfill risk.

·         Low installation cost: Bus-powered 2-wire cabling, reuse of existing cables, redundant measurement through a single opening and wireless transmission for remote tanks reduce site labor and material needs.

·         Uninterrupted operation: Because antenna installation, the addition of floating roof monitoring and modernization by emulation can all be done while tanks remain in service, the need for planned shutdowns is minimized.

·         Easy integration through open protocols: Support for Modbus, OPC UA, HART and WirelessHART, together with user-defined register mapping, makes it easier to feed tank data into the existing DCS and SCADA structure without changes.

·         High system availability: Redundancy that can be set up at system hub, field port and field device level keeps inventory data flowing if a single component fails.

·         Reduced maintenance workload: Non-contacting measurement with no moving parts, an antenna geometry that lets condensate drip off and a measuring head that needs no recalibration reduce maintenance interventions over the operating life.

Application Areas

Tank gauging systems are used at facilities where bulk liquids are stored and the quantity is critical for commercial or safety reasons. Onshore, the radar-based architecture is typical; on ships, electro-pneumatic measurement is the usual choice.

Refineries and Tank Terminals

Used for inventory, movement tracking and custody transfer measurement in feedstock, intermediate and finished product tanks. Data from large numbers of tanks is passed to the inventory software and DCS through a single system hub.

Crude Oil and Fuel Storage

Still-pipe measurement is preferred on floating roof crude oil tanks and on gasoline/product tanks with or without an internal floating roof. Suitable antenna options exist for everything from light products through heavy fuel oil to bitumen and asphalt.

Liquefied Gas (LPG/LNG) Storage

In pressurized or cryogenic tanks the radar signal is guided inside a still pipe, so an adequate echo is obtained even from a boiling surface. An optional pressure sensor provides vapor compensation; a double-block pressure seal, formed by a PTFE sealing element plus a fire-resistant ball valve, isolates the tank side.

Floating Roof Tanks

Roof tilt, buoyancy, a stuck roof and drain sump status are monitored continuously, giving early warning against the risk of overfilling and vapor release; monitoring can be retrofitted.

Marine: Ballast, Fuel and Draft Measurement

On all types of ships, ballast, fuel, miscellaneous liquid tanks and void spaces are measured electro-pneumatically; draft measurement and trim/list monitoring with an inclinometer can be included in the same system.

Modernization of Existing Tank Farms

At sites running servo or float gauges from various manufacturers, emulation allows tanks to be switched to radar measurement one at a time while the existing host and wiring are kept.

Design Criteria for a Tank Gauging System

A tank gauging system is not a single device selection but an architecture decision for the whole site. For a sound design we recommend clarifying the following at the start of the project:

Tank and Product Properties

·         Tank type: fixed roof, floating roof, internal floating roof, spherical/pressurized or cryogenic

·         Existing openings, nozzle and manhole diameters; whether a still pipe is present, its diameter and inner surface condition

·         Product type (light product, crude oil, heavy fuel oil, bitumen, LPG/LNG) and the temperature and pressure range inside the tank

·         Tank height and required measuring range; difficult conditions such as condensation, deposits or a boiling surface

Measurement and Inventory Requirements

·         Whether the measurement is for custody transfer or operational inventory; the legal metrology approval required

·         Number of points for average temperature, free water measurement and the need for pressure measurement for hybrid measurement (density/mass)

·         How up to date the tank capacity tables are, the tank geometry, the volume correction tables to be used and the reference temperature

·         Functions expected from the inventory software for reporting, movement tracking and loss control

Safety and Overfill Prevention

·         The SIL level required for the overfill prevention function as a result of the risk analysis

·         The required independence between inventory measurement and the overfill prevention layer (2-in-1 gauge or a separate device)

·         Type of interface to the emergency shutdown system: safety relay or 4–20 mA output

·         National overfill prevention approval requirements, the hazardous area classification and the required Ex protection method

Communication and System Integration

·         Number of tanks, their distribution across the site and the cable distances between the tanks and the control room

·         Existing gauges and protocols to be retained on site; need for emulation

·         Host side: DCS, SCADA, inventory software; preference for Modbus TCP, OPC UA or a serial connection, and network security requirements

·         Expected redundancy at system hub, field port and field device level; remote tanks that require wireless transmission

Marine Applications

·         Number of tanks and channels to be measured; preference for a scanning or continuous measuring arrangement

·         Need for pressurized tanks, draft points, and trim and list measurement

·         Interfaces to the integrated automation system (IAS) and independent displays; the classification society approval required

·         Cabinet layout, cabinet elevation relative to the tank entry and instrument air quality

When modernizing an existing tank farm, a site survey and an inventory of the protocols used by the old gauges and the host system make it easier to design the new architecture together with a phased migration plan.

Custody Transfer Approvals, Functional Safety and Standards

The main approvals, certificates and standard references available for the components of the tank gauging system are summarized below. Each approval depends on the ordered configuration.

Which approvals and certificates apply to a project should be assessed case by case together with the TLY Enerji engineering team, based on national regulations, hazardous area classification and the results of the risk analysis.

Tank Gauging and Inventory Solutions from TLY Enerji

TLY Enerji approaches tank gauging projects through site-wide inventory and safety objectives rather than a device list. Our engineering team assesses tank types, products, existing openings and still pipes, and custody transfer and overfill prevention requirements together, and designs the architecture of radar gauges, temperature and pressure measurement, tank hubs and system hubs. How up to date the capacity tables are, the condition of the still pipes and the independence of the overfill prevention layer are among the topics we address before selecting devices.

We provide support with phased modernization plans based on protocol analysis of the existing gauges and host system, data mapping for DCS and SCADA integration, redundancy and wireless transmission concepts and the preparation of project documentation. With commissioning and subsequent field support, our goal is for your tank farm to produce traceable, dependable inventory data from day one.

·         Site survey and analysis of the existing tank gauging infrastructure

·         System architecture, device and antenna selection

·         Design of the overfill prevention concept together with SIL requirements

·         Emulation and phased modernization planning

·         DCS/SCADA and inventory software integration

·         Project documentation, commissioning and field support

Frequently Asked Questions

What is a tank gauging system and how does it differ from a process level transmitter?

A tank gauging system is an integrated structure that combines level, average temperature, free water and pressure measurements tank by tank to produce volume and mass inventory. A process level transmitter usually sends a single measurement signal to a control loop, whereas a tank gauging system combines functions such as volume calculation from the capacity table, temperature correction, metrologically approved measurement for custody transfer and an independent overfill prevention layer.

How accurate is a radar tank gauging system?

The radar level gauge used in the system has an instrument accuracy of ±0.5 mm and a repeatability of 0.2 mm under reference conditions. The total uncertainty achieved in the field also depends on the tank structure, the mounting point, the condition of the still pipe and the quality of the temperature measurement. For custody transfer applications, the devices are selected together with OIML R85:2008 or national metrology type approvals and can be delivered with a calibration certificate.

How is net standard volume (NSV) calculated?

First, the measured level is converted into total observed volume (TOV) using the tank capacity table. Deducting the free water volume gives the gross observed volume, which is converted into gross standard volume at the reference temperature with a volume correction factor that depends on average temperature and density. Finally, the sediment and water allowance is subtracted to obtain the net standard volume. The calculations are based on the API volume correction tables and ASTM D4311.

How are SIL 2 and SIL 3 achieved for overfill prevention?

SIL 2 is achieved by connecting a SIL certified radar gauge to the emergency shutdown system through the 4–20 mA output or the safety relay on the tank hub. SIL 3 uses a 2-in-1 gauge with two independent radar electronics in one housing, together with the separate SIL 3 relay in the tank hub. This relay is normally energized; it opens at the alarm level or on a device fault and moves to the safe state.

Can existing servo or float gauges be modernized without replacing the whole system?

Yes. The tank hub replaces the old gauge by emulating the tank gauging protocols of different manufacturers; the existing host continues to see the new radar gauge as the old device. The site can therefore be upgraded gradually while the tanks remain in service and the existing wiring is kept. The system hub also communicates with existing field devices, enabling old inventory software to be replaced and host compatibility to be preserved through user-defined Modbus mapping.

How is tank gauging data transferred to a DCS or SCADA system?

The system hub can send tank data simultaneously to the DCS, SCADA and inventory software via Modbus TCP over Ethernet, OPC UA with encryption and authentication support, or serial Modbus RTU over RS-232/RS-485. User-defined Modbus mapping preserves the existing register structure. If required, a direct connection to the host system can also be made through the 4–20 mA/HART analog output of the tank hub.

How is roof tilt monitored on floating roof tanks?

Non-contacting radars placed at equal intervals on the tank top calculate the roof tilt by comparing their distances to reflector plates on the roof; with the liquid level as reference, buoyancy is monitored as well. Alternatively, guided wave radars can be mounted directly on the roof and run wirelessly on battery power. Automatic alarms are generated for tilt, buoyancy, a stuck roof and a blocked drain sump, and the solution can be retrofitted while the tank stays in operation.

Can wireless communication be used in tank gauging?

Yes. A wireless adapter connected to the tank hub transmits tank data such as level and temperature over a WirelessHART (IEC 62591) network to a gateway. The radio uses channel hopping in the 2.4 GHz band, and the update period can be set between 8 seconds and 60 minutes. This lowers installation cost for remote tanks where running cables is expensive and for measurements on floating roofs.

How are ballast and fuel tank levels measured on ships?

The common method on ships is electro-pneumatic measurement: air is blown through a pipe running from the measuring cabinet to the tank, and the hydrostatic pressure — and from it the liquid level — is calculated from the pressure needed for the air to escape. The system measures over a 0–32 m range, supports 24 channels per cabinet and connects to the ship automation system via RS-485 Modbus RTU. Pressure repeaters are used for pressurized tanks, and pipes taken from the hull bottom for draft measurement.

Can guided wave radar be used in a tank gauging system?

Yes. Multi-tank tank hub configurations also support guided wave radar level transmitters; these devices can be used, for example, in tanks that need liquid–liquid interface measurement and in tilt monitoring solutions mounted on floating roofs. On ships they can also be integrated into the system via analog channels. Product selection, probe types and performance values are covered in detail on the guided wave radar page.