Automatic Tank Dewatering System: Process, Control and Applications

Automatic tank dewatering removes settled water from hydrocarbon storage tanks while using interface measurement to reduce the risk of draining saleable product. This guide explains the control sequence, the measurements involved and where the Agar ID-200 fits.

How automatic tank dewatering works

Water settles beneath crude oil because of the density difference. A reliable system must identify the water-rich phase, open the drain path only when the correct phase is present, monitor the changing interface and close the valve before hydrocarbon reaches the outlet.

  1. Measure: continuously detect the oil-water or emulsion interface near the drain point.
  2. Validate: confirm that the measured phase satisfies the configured water condition.
  3. Drain: command the valve according to the control philosophy and site interlocks.
  4. Stop: close the valve as the transition or hydrocarbon phase approaches.
  5. Record: use the signal and valve status for alarms, trends and maintenance.

Why interface measurement matters

Time-based draining assumes that settling conditions stay constant. In practice, crude properties, temperature, emulsions, throughput and tank operation change. Direct interface measurement gives the control system evidence of the phase at the measurement point, helping operators reduce product carry-under and avoid unnecessary manual sampling.

Where the Agar ID-200 fits

The ID-200H Series is a two-wire, 4-20 mA loop-powered interface detector with integrated HART communication. Agar describes the probe for liquid-liquid and vapour-liquid separation processes, including emulsions and foam, with applications in tanks, decanters, dehydrators and coalescers.

The measurement should be integrated with the site control system, drain-valve arrangement, alarms and operating procedures. Final probe length, materials, process connection and pressure-temperature configuration must be confirmed for the application.

Engineering questions to resolve before installation

  • What phases and emulsion range must be detected?
  • Where is the representative drain-point measurement location?
  • What are the process temperature, pressure and materials requirements?
  • How will valve failure, loss of signal and abnormal phase changes be handled?
  • What commissioning samples or process observations will verify the control thresholds?