Gas Group A, B, C, or D Flow Controllers for Hazardous Locations

• Group A / B: ignite extremely easily, highest hazard
• Group C: intermediate
• Group D: lowest ignition severity^[i]

• acetylene: chemically unstable → special storage only
• hydrogen: containment difficulty + materials risk
• others: standard compressed or cryogenic liquefied gas practices

If a process involves methane, hydrogen, propane, acetylene, or ethylene, control is only part of the problem. One real constraint is whether the process has an area classification.

Most devices that measure gas stay outside the hazard zone. They push signals into panels, out of enclosures, inside purge systems. The system grows, complexity grows with it. Let’s take a different approach: put the controller where the gas is.

Alicat’s intrinsically safe mass flow controllers are engineered for Class 1 Division 1 and ATEX Zone 0, 1 environments, where explosive gas may be present continuously, and others are for Class 1 Division 2 and ATEX Zone 2 environments, hazlocs where explosive gas may be present sometimes. These are mass flow controllers, mass flow meters, electronic pressure controllers, and gauges made to operate in-zone, with integrated control, digital communications, and real-time process visibility.

Instability under pressure.

• Cannot be stored as a simple compressed gas
• Stored dissolved in acetone inside a porous cylinder matrix^[ii]
• Free acetylene becomes unstable and can decompose explosively without oxygen^[iii]

• Must not exceed ~15 psig in use systems
• High pressure alone can trigger decomposition, not just ignition^[iv]

• Requires flashback arrestors and check valves
• Backflow or pressure spikes can propagate an explosion upstream into cylinders

• Stored as compressed gas or cryogenic liquid
• Very small molecule, leaks through seals that hold other gases
• Requires high-integrity sealing and material selection

• Extremely low ignition energy, far below static discharge levels^[v]
• High-pressure storage increases leak-driven hazard, not just rupture risk

• Fast diffusion and low density lead to:
  1. Rapid mixing with air
  2. Wide dispersion zones
• Requires tight control at low flow rates

• Hydrogen embrittlement in metals, especially under pressure cycling

• Stored as compressed or liquefied gas
• Less demanding than hydrogen but still requires controlled containment

• Lower ignition energy than group d gases, requires tighter safety margins^[vi]
• Explosion characteristics more severe than propane or methane

• Reactive gas used in chemical processes
• Flow stability matters because:
• reaction rates depend directly on flow
• instability propagates into process variability

• May be stored as compressed gas or liquefied (LNG)
• Light gas, rises and accumulates in enclosed overhead spaces

• Conventional compressed gas behavior
• Main risk is accumulation, not pressure instability

• Requires stable low-flow control in blending and measurement
• High flow rates common in fuel systems

• Stored as a liquefied gas under moderate pressure
• Heavier than air, pools at low points

• Pressure tied to temperature due to phase equilibrium
• Tank heating increases pressure rapidly

• Phase change during withdrawal can affect:
• pressure stability
• regulator performance

hydrogen → design around leakage and ignition → fast response, leak-sensitive

ethylene → design around control stability → stable, precise dosing required

methane → design around volume and accumulation → steady bulk control

propane → design around phase and pooling → manage phase behavior

Class I Division 1 or Zone 0 mean continuous exposure. Equipment must operate without creating an ignition path.

Class I Division 2 or Zone 2 allow different assumptions. Hazard exists only under abnormal conditions. More installation flexibility.

Classification differences change decisions:

• where the controller sits
• whether you need purge or pressurization
• how many components your system requires
• how signals move

With multivariate instruments like Alicat’s laminar differential pressure products, one device inside the zone replaces multiple devices outside it.

• Class I Division 1, Class I Division 2 (ATEX and IECEx Zone 0, Zone 1, Zone 2)
• Integrated control valve, no external actuation loop
• More data: mass flow, volumetric flow, pressure, temperature in one device
• Turndown as good as 10,000 to 1 means a wider range of control with fewer devices
• Digital communications, display visible in-zone, means you can data log and troubleshoot with ease

No purge gas. No split architecture.

The result is simpler P&ID design: Fewer failure points, faster commissioning.