After the fix, the level display matches usable inventory, the marked maximum fill level leaves verified space below the overflow, and an independent high-level trip stops transfer before acid can escape. Treat instrument calibration, operating capacity, and shutdown protection as three separate settings; assigning 100% to the vessel’s physical capacity does not make that level safe for operation.
What should 100% level represent?
No universal fill percentage follows from the acid concentration alone. The correct value depends on tank geometry, overflow and vent elevations, measurement range, transfer stopping time, operating procedures, and the rules applicable at the installation. Confirm the governing basis with the site’s responsible engineer and regulatory authority before changing a credited protective function.
First identify what 100% means on the current display. It may represent physical tank capacity, the bottom of the overflow connection, the instrument’s upper calibrated range, or the approved maximum operating level. These are different elevations. If the display reaches full scale while the liquid can continue rising, operators lose quantitative indication during the most critical part of an overfill event.
A defensible arrangement keeps the measurable range above the normal maximum fill level and places that operating limit below the overflow. One proposed conservative spacing is at least 6 in (150 mm) below the bottom of the overflow, but that distance is a design input rather than a universal legislative requirement. Accept it only after checking the transfer rate and total shutdown delay.
Which physical elevations control the decision?
Measure elevations from one documented datum. Record the tank bottom or transmitter reference, maximum operating level, high alarm point, independent trip point, overflow inlet, vent connections, and the upper measurable limit. For a non-linear tank, convert elevation to volume with the approved strapping or calibration table rather than applying a linear percentage.
The free space must absorb material entering after the trip condition occurs. That continued inflow includes detection delay, logic execution, output actuation, valve or pump stopping time, and material already moving through the transfer system. Compare the resulting incoming volume with the available volume between the trip elevation and the first spill path. If available volume is smaller, move the trip lower, shorten the stopping sequence, reduce the transfer rate, or modify the system.
If the overflow elevation or tank capacity is uncertain, stop the calibration change and obtain verified drawings or direct dimensional measurements. A guessed datum can make a correctly adjusted transmitter report the wrong inventory.
What does the signal chain show before adjustment?
Look at the trend first. Compare the raw level measurement, converted engineering value, operator display, alarms, shutdown status, and an independent inventory reference through a fill or controlled level change. Tuning does not fix wiring: a stable but incorrect value points toward scaling, geometry, density compensation, reference pressure, installation, or signal mapping rather than controller tuning.
| Signal | Source or comparison | Wrong-value symptom |
|---|---|---|
| Raw level | Primary level instrument | Does not move with a confirmed inventory change, indicating a measurement, installation, or wiring problem |
| Engineering level | Controller or display scaling | Tracks movement but reports the wrong elevation or percentage, indicating range or conversion error |
| Contents | Tank calibration table | Elevation is correct but litres, tonnes, or percent full is wrong, indicating an incorrect geometry or density conversion |
| High-level trip input | Independent sensor or channel | Changes with the primary indication or shares its failure, calling independence into question |
| Final shutdown action | Transfer pump, valve, or unloading interlock | Trip input operates but inflow continues, indicating logic, output, actuator, or process-response failure |
If the raw signal is wrong, repair the measurement chain before respanning it. If raw level is correct but the display is wrong, correct scaling and unit conversion. If both are correct, continue to the operating-limit and trip checks.
Can the person controlling transfer see the safe limit?
The contents indication should be readable at the intake point. A direct mass indication in tonnes can support transfer planning; litres or percent full can also be used when the maximum fill level is clearly marked. Whatever unit is selected, the displayed limit must correspond to the documented operating elevation rather than an unexplained round percentage.
If the gauge is not visible to the unloading operator, use a written offloading procedure that defines available capacity, communications, responsibilities, supervision, and the command that stops transfer. When inventory is established by dipping rather than a continuous indication, provide written confirmation that adequate space exists before unloading begins.
A sight glass should not be the primary remedy for poor remote indication. Concentrated-acid service makes mechanical damage and loss of containment consequential. Where a sight glass remains in service, protect it against impact and provide a means to isolate it following damage.
Will the independent high-level trip stop inflow in time?
The high-level trip must act independently of the routine contents indication and stop incoming flow before the overflow point. Independence means a single measurement, scaling, logic, or output failure must not silently defeat both operator indication and automatic shutdown. Review the actual architecture and common utilities to identify shared failure paths.
Test from sensing element to final element. A forced controller bit or simulated display value proves only part of the chain. Raise or simulate the level at the independent device using the approved test method, observe the trip input, confirm the shutdown command, and verify that the pump stops or the isolation element reaches its safe state. Measure total response time and use the maximum credible transfer rate to calculate run-on volume:
Run-on volume = transfer rate × total shutdown time
Add any separately calculated line drainage or displaced volume that continues toward the tank. The available space between the trip point and overflow must exceed the resulting incoming volume under the approved design margin.
How should the recalibration be performed and verified?
- Freeze the current configuration and record the existing lower range, upper range, displayed units, alarm points, trip points, overflow elevation, and maximum operating level.
- Confirm the tank datum and geometry. Reconcile drawings, the tank calibration table, and a physical level reference before changing scaling.
- Select the display convention. If
100%represents physical capacity or the overflow elevation, mark the lower maximum fill limit clearly. If100%represents maximum operating level, retain measurable headroom above it so an abnormal rise remains visible. - Map the transmitter range into the controller and display once. Remove duplicated conversions that can apply scaling twice, and verify any elevation-to-volume table at multiple points.
- Set the operating limit from the approved free-space calculation. Position the independent trip low enough that measured shutdown run-on cannot reach the overflow.
- Perform a controlled test at low, intermediate, maximum operating, alarm, and trip points. Compare indicated elevation and contents with the independent reference, then verify alarms, transfer shutdown, operator visibility, and event recording.
- Update operating procedures, display markings, calibration records, proof-test instructions, and management-of-change documentation required by the site.
Acceptance requires monotonic indication across the working range, correct unit conversion, visible headroom above the maximum operating level, and a proven final shutdown action. Do not accept a display-only test as proof of overfill protection.
Frequently Asked Questions
How do I choose the 100% point on a sulfuric acid tank?
First decide whether 100% means physical capacity, overflow elevation, or maximum operating level. Keep the normal fill limit below the overflow and retain measurable range above that limit so an abnormal rise does not go off-scale.
How do I set the maximum fill level below the overflow?
Calculate the volume entering during the complete detection and shutdown delay, then compare it with the tank volume between the trip and overflow elevations. A proposed spacing of 6 in (150 mm) can be evaluated, but transfer rate, stopping time, geometry, and the site’s approved margin decide the final elevation.
How do I test an independent high-level trip?
Initiate the approved test at the independent sensing device and observe the input, logic, output, and final transfer-stopping element. Record the total response time and confirm that calculated run-on volume remains within the available free space.
When should I stop a tank level recalibration and escalate?
Stop when the overflow datum, tank calibration table, legal basis, instrument range, trip independence, or final-element response cannot be verified. Escalate through the tank or instrument manufacturer’s official support channel, the site process-safety authority, and the responsible regulatory authority before returning the altered protection to service.