APP-03Application
Tank Level & Inventory: Continuous Level, Overfill Protection and Volume
Knowing what is in a tank is a measurement problem, a safety problem and an accounting problem at once. Continuous level gives inventory; an independent high-high (LSHH) switch protects against overfill; temperature and pressure make the figures meaningful; and the control system ties them to pumps, valves and stock records.
| Tag | Service |
|---|---|
| LT-301 | Continuous level |
| PT-302 | Vapour-space pressure |
| LSHH-303 | Independent high-high level |
| TT-304 | Product temperature |
| FT-305 | Transfer flow |
01Measurement map
Measurement points on a typical storage tank
Each instrument sits where it is typically installed. Select one to see why it is measured, the typical technology and how its signal reaches the control system.
- Process line
- Electrical signal
- Instrument (ISA tag)
- Selected signal path
- 1PLC / SCADA
- 2Storage tank
- 3Inlet valve
- 4Transfer pump
- 5N₂ blanketing
- 6Receipt
- 7To process / dispatch
Continuous level
- Why it matters
- Continuous level is the inventory. It drives transfer planning, stops the pump before the tank runs dry and gives the early high-level alarm. The method is chosen for the product: vapour, foam, turbulence, build-up and density changes each rule some technologies in or out.
- Parameter
- Level, converted to volume through the tank table
- Typical technology
- Non-contact radar — largely unaffected by vapour and temperature changes
- Guided-wave radar — suited to foam, turbulent surfaces and low-dielectric products
- Hydrostatic or DP level — robust where density is known and stable
- Integration
- 4–20 mA / HART or a digital protocol to the PLC / SCADA. The control system applies the tank table to convert level to volume, raises high and low alarms, and stops the transfer pump on low-low level.
- Why it matters
- Continuous level is the inventory. It drives transfer planning, stops the pump before the tank runs dry and gives the early high-level alarm. The method is chosen for the product: vapour, foam, turbulence, build-up and density changes each rule some technologies in or out.
- Parameter
- Level, converted to volume through the tank table
- Typical technology
- Non-contact radar — largely unaffected by vapour and temperature changes
- Guided-wave radar — suited to foam, turbulent surfaces and low-dielectric products
- Hydrostatic or DP level — robust where density is known and stable
- Integration
- 4–20 mA / HART or a digital protocol to the PLC / SCADA. The control system applies the tank table to convert level to volume, raises high and low alarms, and stops the transfer pump on low-low level.
- Why it matters
- Where a tank is nitrogen blanketed, a small positive pressure keeps air and moisture out of the vapour space. Pressure monitoring confirms the blanketing regulator is holding and warns of over- or under-pressure before the breather valves act. On pressurised tanks it is also needed to compensate hydrostatic level measurement.
- Parameter
- Tank pressure, mbar(g)
- Typical technology
- Low-range gauge pressure transmitter
- Blanketing regulator and pressure/vacuum breather valve as the mechanical layer
- Integration
- 4–20 mA to the PLC with high and low alarms; on hydrostatic installations the pressure is used in the level calculation.
- Why it matters
- The independent high-high (LSHH) switch is the last layer before overfill. It must not share a sensor, cable or logic path with the continuous level it backs up — otherwise a single failure disables both. On trip it closes the inlet valve or stops the transfer into the tank.
- Parameter
- Point level — high-high trip
- Typical technology
- Vibrating-fork level switch
- Float or displacer switch
- Separate radar or capacitance instrument configured as a switch
- Integration
- Hard-wired, fail-safe contact into interlock logic that is independent of level control, closing the inlet valve; the trip is also reported to SCADA. The degree of independence and the test interval follow the site's risk assessment.
- Why it matters
- Volume changes with temperature. Correcting the gross volume to a reference temperature gives a standard volume that can be compared across tanks, receipts and dispatches. In large or heated tanks, a multi-point averaging element also shows stratification.
- Parameter
- Product temperature, °C — spot or averaged
- Typical technology
- RTD in a thermowell near the tank bottom
- Multi-point averaging temperature element for larger tanks
- Temperature transmitter with 4–20 mA / HART
- Integration
- Analog input to the PLC or tank-gauging system for volume correction; high-temperature alarm on heated tanks.
- Why it matters
- Metered transfers cross-check the level-based inventory. A difference between the volume moved and the change in tank level points to measurement error, a leak or an unaccounted transfer.
- Parameter
- Flow rate and totalised volume
- Typical technology
- Electromagnetic flowmeter for conductive liquids
- Coriolis flowmeter for mass flow and density
- Positive-displacement or turbine meters for clean hydrocarbons
- Integration
- 4–20 mA with pulse output, or a digital protocol for totals and diagnostics; reconciled with tank levels in SCADA.
02Engineering context
Why measurement matters
Continuous level is the basis of inventory, transfer planning and pump protection. The right technology depends on the product and the tank — foam, vapour, agitation, build-up, density changes and internal structures affect some methods far more than others.
Overfill is one of the most consequential tank failures. Good practice is an overfill protection layer that is independent of the level measurement used for control: a separate high-high sensor, acting through its own logic, to stop the inflow.
Level becomes volume only through the tank's calibration table. For products whose volume changes with temperature, a temperature measurement allows correction to a reference temperature, so stock figures agree with what is received and dispatched.
Operating conditions
- Product
- From water to hydrocarbons, chemicals and viscous products; foam, vapour and build-up all influence the level method.
- Tank
- Atmospheric or pressurised, fixed or floating roof, with or without agitators, heating coils and internal structures.
- Vapour space
- Nitrogen blanketing or vapour recovery where the product requires it — adding pressure measurement and control.
- Temperature
- Ambient to heated storage; large tanks can stratify, which may call for multi-point temperature measurement.
- Area classification
- Often hazardous for flammable products, which governs instrument certification and wiring practice.
- Access
- Existing roof and shell nozzles and stilling wells decide which methods can be installed without modifying the tank.
03Measurement parameters
What is measured, where, and why
The parameters typically measured in this application, with the location, the reason and the technologies commonly used. The right selection always depends on the process conditions.
| Tag | Parameter | Location | Why it matters | Typical technology |
|---|---|---|---|---|
| LT | Continuous level | Roof nozzle or stilling well | Inventory, transfer planning and pump protection | Non-contact radar, guided-wave radar, hydrostatic / DP |
| LSHH | High-high level | Shell or roof, independent of the continuous level | Overfill protection | Vibrating fork, float, or a separate instrument used as a switch |
| LSLL | Low-low level | Near the outlet | Pump dry-run protection | Point level switch or a continuous-level alarm |
| TT | Product temperature | Lower shell, or multi-point through the liquid | Volume correction and heating control | RTD in a thermowell, multi-point averaging element |
| PT | Vapour-space pressure | Roof | Blanketing control and hydrostatic compensation | Low-range pressure transmitter |
| FT | Transfer flow | Inlet or outlet line | Reconciliation and transfer control | Electromagnetic, Coriolis, positive-displacement or turbine |
LTContinuous level
- Location
- Roof nozzle or stilling well
- Why it matters
- Inventory, transfer planning and pump protection
- Typical technology
- Non-contact radar, guided-wave radar, hydrostatic / DP
LSHHHigh-high level
- Location
- Shell or roof, independent of the continuous level
- Why it matters
- Overfill protection
- Typical technology
- Vibrating fork, float, or a separate instrument used as a switch
LSLLLow-low level
- Location
- Near the outlet
- Why it matters
- Pump dry-run protection
- Typical technology
- Point level switch or a continuous-level alarm
TTProduct temperature
- Location
- Lower shell, or multi-point through the liquid
- Why it matters
- Volume correction and heating control
- Typical technology
- RTD in a thermowell, multi-point averaging element
PTVapour-space pressure
- Location
- Roof
- Why it matters
- Blanketing control and hydrostatic compensation
- Typical technology
- Low-range pressure transmitter
FTTransfer flow
- Location
- Inlet or outlet line
- Why it matters
- Reconciliation and transfer control
- Typical technology
- Electromagnetic, Coriolis, positive-displacement or turbine
04Field → Control → Supervision
From the field to the control room
The same application seen as layers of one system — and the Spaaronn capability that engineers each layer.
Field
Measure and analyse at the process
Instrumentation
Continuous level, independent point level, temperature, pressure and flow — each chosen for the product, the tank and the available nozzles.
- Level method selection
- Independent high-high (LSHH) switch
- Installation, calibration and functional checks
Control
Decide and act on the signals
Automation & monitoring
Pump and valve interlocks, transfer sequences and alarms, with overfill protection kept on its own independent path.
- Inlet-valve and pump interlocks
- High and low alarms
- Transfer sequences
Supervision
See, record and report across the plant
Integration
Levels converted to volumes and brought into SCADA for inventory, reconciliation against metered transfers, and reporting.
- Tank tables and volume correction
- Inventory and reconciliation views
- Alarm history and reports
05Project sequence
Typical project sequence
How this application is typically taken from requirement to a supported, running system.
- 01
Characterise product and tank
Product properties, tank construction, internals, nozzles and the area classification.
- 02
Select the level method
The continuous level technology that suits the product, and where it can be installed.
- 03
Specify the high-high switch
The independent high-high (LSHH) switch, separate from level control, and its interface to the site's protection system.
- 04
Engineer interlocks and inventory
Pump and valve interlocks, tank tables, volume correction and SCADA inventory views.
- 05
Commission and verify
Loop checks, level verification, functional checks of the independent high-high (LSHH) switch coordinated with the site's safety requirements, and a maintenance plan.
Next steps
Where to go next
- 01IndustryOil & GasMeasurement, analysis and control for production, processing and storage facilities.
- 02ResourceLevel Measurement Methods for TanksHydrostatic, DP, radar, ultrasonic, guided-wave and point-level methods compared.
- 03SolutionAMC & Lifecycle SupportPreventive maintenance, breakdown support, calibration support, troubleshooting and upgrades for installed systems.
Engineering enquiry
Measuring level and inventory in your tanks?
Share the product, the tank construction and nozzles, and how inventory is tracked today. We can help select the level method, the independent high-high (LSHH) switch and the integration.