Background
A large integrated refining and petrochemical complex in Southeast Asia faced persistent issues with differential pressure transmitter reading drift on the reactor recycle/makeup hydrogen mixture flow measurement in their hydrocracker unit. This measurement point monitors process gas flow entering the high-pressure air cooler, directly linked to reactor temperature rise control and the emergency shutdown system (ESD).
Customer Challenge
Country/Region: Southeast Asia (large integrated refining/petrochemical project)
Application: Hydrocracker reactor recycle hydrogen measurement, hydrogen-rich gas (H₂ content > 80%), temperature approx. 180-220°C, pressure approx. 160 bar
Key Configuration Requirements: 500 mbar measuring cell, 316L process connection, diaphragm seal, 4-20mA HART output, SIL 2 certified
Delivery Request: 16 weeks (aligned with scheduled turnaround window)
The customer faced critical pain points:
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Continuous positive reading drift: 6-12 months after commissioning, DP transmitter output deviated gradually from actual flow, always in the same direction (positive drift). Operators were forced to perform frequent zero corrections.
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Abnormal diaphragm appearance: During inspections, diaphragms showed no visible corrosion but exhibited slight bulging deformation, with bubble traces found in the fill fluid.
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Root cause identified: Combining process conditions (hydrogen-rich, high temperature, high pressure) with failure modes (positive drift, bubbles, bulging), the root cause was determined to be hydrogen permeation—hydrogen ions penetrating the metal diaphragm and recombining into molecular hydrogen in the fill fluid, with bubble accumulation causing diaphragm deformation and measurement deviation.
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Excessive maintenance cost: This measurement point required transmitter replacement every 1-2 years—not only the direct cost but also the production loss from unscheduled downtime.
Our Solution
After process condition assessment and gold/rhodium coating selection analysis, the Deltabar S PMD75 differential pressure transmitter (Model: PMD75, 500 mbar cell, 316L process connection, gold/rhodium-coated diaphragm, 4-20mA HART output, SIL 2 certified, with integral 3-valve manifold) was recommended.
Solution Technical Highlights
Key Selection Justification
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Gold/rhodium-coated diaphragm: The core solution to hydrogen permeation. Gold's dense lattice structure effectively prevents hydrogen ions from penetrating the diaphragm into the fill fluid, fundamentally eliminating bubble accumulation and diaphragm deformation.
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Fully welded diaphragm seal system: Eliminates any potential leakage paths and maintains seal integrity.
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Platinum accuracy with SIL certification: ±0.035% reference accuracy meets DP resolution requirements for flow calculation; SIL 2 compatibility supports ESD loop design.
Customer Feedback
"Since upgrading to the gold/rhodium-coated diaphragm, this recycle hydrogen DP measurement point has been running stably for 18 months with no detectable reading drift. Previously we had to adjust zero every six months—now there's no such need. The cost savings from bulk procurement and improved reliability have accelerated our decision-making."
— Site Instrumentation Supervisor
Summary
For hydrogen-rich high-pressure applications such as hydrocracking, hydrotreating, and ammonia synthesis, the gold/rhodium-coated diaphragm is an engineering choice validated by materials science—it does not solve all problems, but it does solve the one that hydrogen permeation represents: a failure mechanism long underestimated.
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