For operators of natural gas transmission pipelines, maintaining gas quality at interconnection and delivery points is both a commercial obligation and an operational necessity. Cricondentherm Hydrocarbon Dew Point (CHDP) is one of the most critical — and technically complex — gas quality parameters to monitor continuously.
This step-by-step guide walks through the complete process of implementing robust CHDP monitoring in a transmission pipeline, from measurement point selection through SCADA integration, alarming, and compliance reporting.
Step 1: Identify Your Measurement Points
Not every point on a pipeline needs CHDP monitoring — but the key measurement points are well-defined by operational and commercial requirements:
- Custody transfer (interconnect) points: Where gas changes ownership between a producer, shipper, and transmission operator. CHDP specification compliance at these points is legally binding under the tariff agreement. Every interconnect point with a process GC should have CHDP monitoring.
- Inlet to compressor stations: Liquid hydrocarbon ingestion is particularly damaging to centrifugal compressors. CHDP monitoring upstream of each compressor station provides early warning and automatic interlock capability.
- Outlet of separation facilities: Verify that condensate scrubbers and slug catchers are performing correctly — consistently low CHDP downstream confirms proper liquid removal.
- Pressure reducing/regulating stations: Pressure reduction can bring gas into the two-phase region (retrograde condensation) even if the inlet gas is above CHDP at high pressure. Monitor both upstream and downstream of significant pressure drops.
Step 2: Confirm GC Coverage and Data Quality
CHDP calculation requires detailed gas composition data from a process gas chromatograph (GC). Before implementing calculation-based CHDP monitoring, verify:
- Component coverage: The GC must analyze through at least C6+ (preferably C7+) to provide accurate CHDP. A GC analyzing only C1–C5 will systematically underestimate CHDP for rich gas streams.
- GC maintenance and calibration: CHDP accuracy is only as good as the GC accuracy. Verify calibration gas cylinder dates, check that split/unsplit C6+ fractions are correctly handled, and confirm the GC normalization is functioning (sum of components = 100% ± 0.5%).
- Data availability in SCADA: Individual component mole fractions must be available as SCADA tags, not just the calculated BTU or gravity values. Most process GCs output all components to SCADA/DCS; confirm these tags are available and correctly labeled.
Step 3: Deploy the Calculation Service
Select and deploy an EOS-based CHDP calculation service. Key requirements:
- Uses PR78 or SRK with validated BIPs for the C1–C12 component range
- Returns CHDP, water dew point, and full phase envelope (cricondenbar, critical point) in a single calculation call
- Provides sub-second response time for SCADA real-time integration
- Operates without internet connectivity (local Windows Service or on-premise server)
- Includes adequate documentation for SCADA integration and troubleshooting
DPCloud meets all of these requirements and supports both TCP (for legacy SCADA) and REST API (for modern systems) interfaces. The 30-day trial allows full integration testing before production commitment.
Step 4: Configure the Data Flow in SCADA
The data flow from GC to CHDP tag in SCADA follows this sequence:
- GC analysis cycle completes (every 3–15 minutes): New composition values written to SCADA tags.
- SCADA calculation trigger: A trigger (composition update event or cyclic timer) initiates the calculation call.
- Composition package assembled: SCADA reads the relevant component tags and assembles a composition vector for the calculation API.
- Calculation call made: TCP or HTTP POST to the DPCloud service with the composition data.
- Results received: CHDP, WDP, phase envelope parameters returned in under 150ms.
- SCADA tags updated: Calculated CHDP and WDP written to SCADA display tags and historian.
- Alarm evaluation: SCADA alarm logic evaluates CHDP against configured limits.
Step 5: Configure Alarms and Displays
Recommended Alarm Setpoints
| Alarm Level | Condition | Typical Action |
|---|---|---|
| Hi-Hi (Critical) | CHDP > contract specification limit (e.g., 0°C) | Notify gas control; initiate flow curtailment procedure; log compliance event |
| Hi (Warning) | CHDP > specification − 3°C (e.g., −3°C) | Notify operations; investigate composition source; increase monitoring frequency |
| Calculation Stale | No successful calculation for > 20 minutes | Notify instrumentation; investigate GC or calculation service |
| GC Data Quality | Component sum outside 99.5–100.5% | Flag CHDP as “unreliable”; initiate GC maintenance |
SCADA Display Recommendations
- Display CHDP with a color-coded status indicator (green/yellow/red relative to the specification limit)
- Show the timestamp of the most recent GC analysis cycle alongside the CHDP value
- Display the trend (last 24 hours of CHDP values) on the measurement point overview screen
- Include water dew point alongside CHDP on the gas quality display faceplate
Step 6: Establish Compliance Documentation
For custody transfer points, the calculated CHDP values stored in the historian form part of the compliance record. Establish:
- Daily/monthly CHDP reports: Automated reports showing maximum CHDP per period, time above specification, and GC data quality statistics. Most historians (PI, eDNA) support scheduled report generation.
- Exception documentation: Any CHDP specification exceedance should be documented with: time, duration, peak value, source GC composition data, and corrective actions taken. This documentation is required for tariff dispute resolution.
- Calibration records: Link GC calibration records to CHDP calculation records to demonstrate data traceability.
Step 7: Periodic Validation Against Chilled Mirror Measurements
Calculation-based CHDP monitoring should be periodically validated against direct measurement using a portable or installed chilled mirror hydrocarbon dew point analyzer. Recommended validation frequency:
- New system commissioning: Side-by-side comparison across the full range of expected gas compositions.
- Annual validation: At each custody transfer point, compare EOS-calculated CHDP against chilled mirror measurement at the same operating conditions.
- After significant composition changes: If the gas supply composition changes significantly (e.g., new gas field added to the commingled stream), re-validate CHDP accuracy.
Note that the two methods have different uncertainties — chilled mirror analyzers have their own calibration requirements and can be affected by contamination. A discrepancy between the two methods warrants investigation of both before concluding that one is wrong.
Conclusion
Continuous CHDP monitoring in transmission pipelines is achievable with modern calculation-based approaches that integrate directly with existing SCADA infrastructure. The seven steps above — from measurement point selection through compliance documentation — provide a complete framework for implementing a robust, auditable gas quality monitoring system.
For operators ready to implement or upgrade their CHDP monitoring, DPCloud offers a complete solution — EOS-based calculation, dual SCADA interfaces, and zero internet dependency — with a 30-day trial for integration validation before production deployment.