Hydrogen, The Challenges in Measurement of Basic Process Parameters
This article focuses on understanding the challenges or limitations associated with measuring basic process parameters such as Pressure, Temperature and Flow of gaseous Hydrogen (H2) in generation plants. It does not intend to cover the standard requirements and operating principles but to highlight the specific care for the measurements in H2 services because a lighter gas (low molecular weight) is highly permeable.
Hydrogen, an introduction
Hydrogen is a colourless, odourless, and tasteless gas. Hydrogen (H2) is a weight (molecular weight of 2 g/mol) element (natural gas weight of 16 g/mol) but with a high speed of sound. Hydrogen only becomes liquid at very low temperatures; at ambient pressures, it liquifies at -253°C, which is only 20°C above absolute zero. [1]
Hydrogen has received particular attention as it has the advantage of reducing carbon footprint (low or nil). Many countries, including India, are seriously considering Hydrogen as an alternate energy source and pursuing the concept of a Hydrogen economy.
H2 may be generated exclusively from different processes like Natural gas steam reforming or oxidation, coal gasification (grey H2), biomass gasification (white H2), gasification (blue H2, if supported with Carbon capture), Electrolysis (green H2, if renewable power is used), and also as a by-product from processes like Chlor-Alkali and H2 recovery from Coke Oven Gas in steel plants.
Ref: ScienceDirect.com
In generation plants and distribution through pipeline systems, H2 is handled at moderate pressures of 30 ~ 150 barg. For H2 storage systems, the H2 gas is compressed at very high pressures (>350 barg), and for mobility applications, the H2 fuel cells are filled at 350 to 700 barg. [1] So, examining the challenges or limitations of measuring basic parameters like pressure, temperature, and flow in Hydrogen processes is imperative.
Pressure Measurements
The conventional pressure measuring instruments used in other process applications can be used, but seal diaphragms are to be used as H2 is a hazardous medium. However, Hydrogen permeation (diffusion of Hydrogen ions) through seal diaphragms into the fill fluid side is a cause of concern. Austenitic steel alloy SS316 material has higher Hydrogen permeation resistance than other high nickel alloys. A plated seal diaphragm is recommended for H2 applications above 1000 psi (68.9 bar) and above 350 deg.F (176 deg.C), though it is also preferable at lower temperature/pressure applications. [2]
Temperature Measurements
Hydrogen is usually generated and handled relatively at low/moderate temperatures only. So, RTD can be the temperature measurement method for H2 gas. For thermowell, SS316 material is suitable compared to other alloys with higher nickel composition and no concerns of Hydrogen permeation are reported (Embrittlement of steel at low temperatures is a documented phenomenon which might even lead to cracks, and hence, the material needs annealing to bring back. Its permeation resistance is impractical at the site; therefore, as part of SOP, the thermowell will be inspected during routine maintenance).
Flow Measurements
The low-density H2 gas and high sound velocity make conventional flow measurements like differential pressure, vortex, turbine, positive displacement, etc., impractical, especially at high-pressure services. Hence, mass flow measurement using a Coriolis type or Ultrasonic flow meter is recommended for H2 services and invariably for Custody transfer metering applications. Thermal mass flow meters are also available for low-pressure applications (<50 bar) but are not quite ordinary.
In addition to mass flow, the Coriolis meter can provide density, temperature, volumetric flow and possibly concentration. Coriolis can be used at very low temperatures (as low as – 40 deg.C) and very high operating pressure services. Coriolis meters are available for line sizes 0.5” to 12″ for operating pressures of 100 ~ 400 bar and for line sizes ½” ~ 1” for very high pressure (~1000 bar) applications. An accuracy of +/- 0.5% for monitoring/control applications is enough, whereas the recommended accuracy for custody transfer is +/- 0.25%. [3]
Ultrasonic-type flowmeter is also available for use at very low temperatures (as low as – 40 deg.C) and high operating pressure (as high as 400 bar) and can measure flow in both directions. Integral types are available for pipeline sizes 100 ~ 600 mm, and insertion type can be the choice for larger line sizes. The wetted part of the sensor body (usually SS material) is to be provided with epoxy encapsulation for H2 service. Minimum operating pressure and velocity must be verified when selecting the Ultrasonic flow meter. It is noted that Clamp-on Ultrasonic type flowmeters are also on offer. Still, like any other application, the factor of pipeline material of construction (with or without lining) shall be considered during calibration to confirm the accuracy and, hence, is not very common in use. At the same time, the 2-path type can be regarded as for flow measurements, min. A 4-path type is recommended for custody metering to achieve higher accuracy.
In general, there are yet to be large-scale ISO 17025-accredited hydrogen flow calibration facilities available; the flowmeters are typically calibrated on water, air or natural gas. [1]
Safety aspects
Hydrogen gas is classified under Group IIC, and considering auto-ignition temperature, it might fall under the T1 class. However, as per hazardous area classification done in projects engineered in TCE, the H2 generation through Electrolyzers in Green ammonia plants or Chlor-Alkali process plants is mainly classified as Zone-2 and T3 temperature classes. However, for field instrumentation, the minimum recommended temperature class is T4 (but the T6 class is readily available from many suppliers). However, Ex.d (explosion proof) is acceptable for Zone-2 areas. I (Intrinsic safety) is a standard recommended practice.
Summary
The above recommendations are based on understanding technical information available in the public domain from different suppliers and articles/study papers from experts. These details will provide the necessary guidance to Engineers. Still, application-specific design aspects (measuring range, operating conditions, line sizes, straight length constraints, etc.) shall be considered while selecting the instruments.
[1] (24) Basics of hydrogen flow measurement | LinkedIn
[3] Product catalogues/datasheets are available on the websites of different manufacturers like Emerson, ABB, Yokogawa, Honeywell, FCI (Fluid Components International), Fox Thermal, etc.
