Full Report
This article examines the issues concerning pressure sensors and value discrepancies, provides knowledge of pressure measurement and offers recommendations.
Analysis Summary
# Main Topic
Analysis of issues concerning pressure sensors, identification of value discrepancies arising from incorrect installations or environmental factors, and recommendations for accurate pressure measurement in process industries.
## Key Points
- Value discrepancies often arise when readings defy physical expectations (e.g., downstream pressure exceeding upstream pressure).
- Pressure is classified into static and dynamic components; installed sensors typically measure static pressure, unless placed directly in the streamline, where they incorporate dynamic pressure (total pressure).
- **Altitude Effects:** Transmitters located significantly below the tapping point introduce hydrostatic head pressure (liquid head) if the tubing fills with liquid (wet leg), leading to over-readings unless compensated. Conversely, transmitters elevated above vapor tapping points can fill with condensate, causing under-reading.
- **Momentum Effects (Dynamic Pressure):** Pressure tapping placed in front of high-velocity fluid streams (e.g., feed nozzle, reboiler outlet) measures dynamic pressure along with static pressure, resulting in artificially high readings. Installations at pipeline elbows are particularly prone to measuring velocity head.
- Discrepancies may indicate sensor malfunction, but frequently point to incorrect pressure measurement tape (tapping) installation.
## Threat Actors
- Not applicable. This report details operational hazards and measurement errors in industrial process control systems, not malicious threat actors or cyber campaigns.
## TTPs
- Not applicable. The "techniques" described relate to design and installation errors leading to faulty sensor readings:
- Incorrect elevation placement (above or below process tapping).
- Placement directly in the fluid stream, capturing dynamic pressure/momentum.
- Placement at pipeline elbows or within distillation column downcomers.
## Affected Systems
- Pressure gauges and sensors (Pressure Transmitters) used in process industries.
- Affected equipment examples include: Pumps, compressors, distillation columns, heat exchangers, compressor knockout drums, and separator vessels.
- Systems operating with liquids or condensing vapors (e.g., steam, heavier hydrocarbon vapors).
## Mitigations
- **Calibration/Compensation:** If transmitters must be placed below the tapping point in liquid service, they must be compensated or calibrated as a "wet leg."
- **Design Phase Review:** Ensure correct positioning during system design, specifically avoiding installation in distillation column downcomers.
- **Elevation Management:** For condensing vapor applications, ensure tubing is self-draining or that the sensor is positioned at the same elevation as the tapping point.
- **Avoide Momentum Capture:** Do not install tapping points directly in front of high-velocity nozzles (feed, outlet) or at pipeline elbows where dynamic pressure spikes occur.
- **Verification:** When discrepancies occur (e.g., downstream pressure higher than upstream), verify the physical sensor installation rather than immediately assuming sensor failure.
## Conclusion
Erroneous pressure readings in industrial processes are frequently caused by design flaws related to fluid physics (hydrostatic head and fluid momentum) rather than sensor failures. Adherence to strict installation standards, particularly regarding alignment with tapping points and managing fluid dynamics, is critical for maintaining accurate process data and reliable system operation.