The location selection is critical for safety and operational efficiency. The primary criteria include:

• Proximity to Sampling Points: Located as close as possible to the process tap points to minimize sample transport lag time (Fast Loop length).
• Wind Direction: Positioned cross-wind or upwind of potential hazardous gas release sources (process vents, flares) to prevent gas accumulation inside the shelter intake.
• Hazardous Area Classification: Ideally located in a non-hazardous or Zone 2 area. Avoid Zone 0 or Zone 1 locations if possible to reduce explosion-proofing costs.
• Accessibility: Must allow easy access for maintenance personnel, cylinder replacement trucks, and emergency egress.
• Vibration & Noise: Situated away from heavy vibrating machinery (compressors) which can affect sensitive optical alignments in analyzers.

Analyzer shelters are designed to withstand harsh industrial environments:

• Material: Typically Stainless Steel (SS316 or SS304) or GRP (Glass Reinforced Plastic) to resist corrosion in chemical plants.
• Insulation: Walls and ceiling must be insulated (sandwich panel construction) to maintain internal temperature and reduce HVAC load. Fire-retardant foam (PUF or Rockwool) is used.
• Flooring: Antistatic, chemical-resistant vinyl or epoxy flooring to prevent static buildup and damage from reagent spills.
• Fire Rating: Often required to be A60 rated (60 minutes fire resistance) depending on the plant safety case.
• Doors: Fitted with panic bars (push-to-open) opening outwards, with self-closers to maintain internal pressure.

Electrical safety is managed through protection methods defined by IEC/NEC standards:

1. Pressurization (Ex p): The entire house is maintained at a positive pressure relative to the outside to prevent the ingress of flammable gases. This allows the use of General Purpose equipment inside.
2. Explosion Proof (Ex d): Junction boxes and equipment located on the outside of the shelter must be Ex d or Ex e rated.
3. Intrinsically Safe (Ex i): Signal cables entering the shelter are often protected by IS barriers.
4. Interlocks: If pressurization is lost, a Z-Purge or X-Purge system logic is triggered (Power disconnect for X-purge, Alarm for Z-purge).

To prevent Electromagnetic Interference (EMI) and ensure safety:

• Spacing: Power cables (110VAC/230VAC) must be separated from signal cables (4-20mA, communication) by a minimum distance (typically 300mm) or run in separate metal conduits/trays.
• Crossing: If they must cross, they should cross at right angles (90 degrees).
• IS vs. Non-IS: Intrinsically Safe (IS) cables must be segregated from Non-IS cables to prevent accidental energy transfer. Blue sheaths/trays identify IS cables.

Three distinct earthing systems are usually required:

1. Power Earth (PE): Protective earth for electrical safety (metallic enclosures, motors, HVAC). Connected to the main plant earth grid.
2. Instrument Earth (IE): A low-noise earth for shield grounding of instrument signals (4-20mA). It usually connects to the main earth grid at a single point to avoid ground loops.
3. Intrinsically Safe Earth (IS Earth): Specifically for IS barriers. It demands high integrity (often < 1 Ohm resistance).

• Location: Cylinders should generally be located outside the shelter in a shaded, ventilated rack to prevent asphyxiation hazards inside the enclosed house.
• Restraint: Cylinders must be chained or clamped to prevent falling (damage to valves).
• Manifolds: Automatic changeover manifolds are used for critical carrier gases to ensure continuous operation.
• Tubing: Tubing entering the house from cylinders must pass through bulkhead fittings.

• Illumination Level: Typically 300-500 Lux at working height (1 meter) to allow technicians to read gauges and perform maintenance on small components.
• Fixture Type: Fluorescent or LED fixtures. If the house is pressurized, standard industrial fixtures may be used. If not, Ex-rated fixtures are mandatory.
• Emergency Lighting: Battery-backed emergency lights (Ex-rated) located near exits and critical panels, providing illumination for at least 90 minutes during power failure.

This is a critical safety and design feature:

• Panic Egress: In case of fire or gas leak, personnel running toward the exit can push the door open without stopping to pull a handle.
• Internal Pressure: Analyzer houses are positively pressurized. Outward opening doors utilize the internal pressure to help keep the seal tight (though closers are used to overcome this for opening). More importantly, if there is an explosion/overpressure, the door acts as a relief path rather than being blown inward.

• Closed Drain (Recovery): Hydrocarbon samples are often returned to the process (Fast Loop return) to avoid waste.
• Atmospheric Vent/Drain: Small quantities of liquid sample are drained to a slop tank or chemical sewer.
• Seal Pots: Drain headers must have seal pots (p-traps) to prevent sewer gases or hydrocarbons from backing up into the analyzer house.
• Segregation: Stormwater drains (outside) must be separate from chemical drains (inside).

The AIB is the demarcation point between the Analyzer House and the Main Control Room (DCS).

• It collects all signals (Analogs, Digital Alarms, Solenoids).
• It acts as the barrier location for IS signals.
• It facilitates easier troubleshooting; technicians can measure signals at the AIB without opening the analyzer or the main DCS cabinets.

The SCS prepares the process sample so it is compatible with the analyzer hardware:

1. Pressure Reduction: Reducing process pressure to a safe level for the analyzer.
2. Temperature Control: Heating (vaporizing) or cooling the sample to the required phase.
3. Filtration: Removing particulates and contaminants.
4. Flow Control: Regulating the flow rate to the analyzer detector.

Lag Time is the time taken for a sample to travel from the process tap to the analyzer detector.

• Formula: Volume of line / Flow Rate.
• Target: Typically < 60 seconds for control loops.
• Minimization:
  • Fast Loop (Bypass): Using a high flow rate loop that brings fresh sample close to the analyzer, then returns it to process.
  • Line Sizing: Using smaller diameter tubing (e.g., 1/4" instead of 1/2") to reduce volume.
  • Location: Placing the analyzer shelter closer to the tap.

A Fast Loop transports a large volume of sample at high velocity from the process tap to the analyzer shelter and immediately back to the process (usually to a lower pressure point).

• Purpose: Ensures the sample at the analyzer house is "fresh" and representative of current process conditions.
• Mechanism: The analyzer takes a small "slip-stream" from this fast loop (e.g., fast loop is 10 L/min, analyzer takes 0.1 L/min).
• Component: Usually requires a differential pressure between the take-off and return points, or a pump.

• Insertion Probe: Extends into the center (middle 1/3) of the process pipe. It draws sample from the cleanest part of the stream, avoiding sludge on pipe walls. It often includes a primary filter.
• Sampling Valve: Usually just a root valve on the pipe wall. Not recommended for analytical sampling as it draws debris and boundary layer fluids which are not representative.

The sample must remain in a single phase (100% liquid or 100% gas) to be accurate.

• Fractionation: If a liquid sample partially vaporizes, the lighter components boil off first, changing the composition of the remaining liquid. The analyzer will read incorrect values.
• Condensation: If a gas sample cools below its dew point, heavy components drop out as liquid, altering the gas composition.

A Coalescer is used to remove liquid mists/aerosols from a gas stream.

• Mechanism: It forces small droplets to merge (coalesce) into larger drops on a filter element (usually borosilicate glass).
• Gravity: The larger drops fall by gravity to the bottom of the housing to be drained, while the dry gas exits the top.
• Use Case: Essential for protecting Gas Chromatographs and IR analyzers from liquid damage.

A Vaporizing Regulator is used when the process is liquid, but the analyzer requires a gas sample (e.g., GC analysis of LNG).

• Function: It drops the pressure and applies heat simultaneously.
• Physics: To counteract the Joule-Thomson cooling effect (temperature drop during gas expansion), the regulator is heated (electrically or steam) to flash the liquid instantly and completely into gas without fractionation.

A Membrane Filter (often bypass filter) uses a hydrophobic membrane.

• Function: It allows gas to pass through the membrane pores but blocks liquid water and particulates.
• Self-Cleaning: By running a bypass flow across the face of the membrane, it sweeps away the rejected liquid/dirt, keeping the membrane clean for longer periods.
• Application: Final protection immediately before the analyzer.

• Glass Tube: Used for general purpose, low pressure, non-hazardous fluids where visibility of the float and fluid condition (color, bubbles) is useful.
• Metal (Armored) Tube: Mandatory for high pressure, high temperature, or hazardous/toxic fluids where glass breakage would be catastrophic. Magnetic coupling is used to indicate flow on a scale.

DBB provides positive isolation for maintenance safety.

• Configuration: Two isolation valves in series with a bleed valve in between.
• Usage: When closing off the process line to change a filter or sensor, the technician closes both blocks and opens the bleed. If the first valve leaks, it bleeds to a safe location rather than pressurizing the line being worked on.

Heat tracing maintains the temperature of the sample during transport.

• Gas Samples: Prevents condensation (keeps temperature above the Dew Point).
• Liquid Samples: Maintains viscosity for flow and prevents freezing or waxing (pour point maintenance).
• Types:
  • Electric Tracing: Self-regulating cables (easier control).
  • Steam Tracing: Copper or SS tube carrying steam (reliable, high heat, but harder to control temp).

It is a manufactured product where the process tubes (and heat trace line) are wrapped in insulation and a weatherproof outer jacket (PVC/TPU) at the factory.

• Advantages: Consistent insulation quality, faster installation (no manual lagging), no gaps for water ingress, and better thermal efficiency than field-insulated lines.

• SS316: Standard for most hydrocarbons and water.
• Monel (400): Required for samples containing high concentrations of Hydrofluoric Acid (HF) or seawater/brine, where SS316 corrodes rapidly.
• Hastelloy (C-276): Used for extremely corrosive, high-temperature sour gas (high H2S), sulfuric acid, or high-chloride environments.
• Sulfinert/SilcoNert Coating: Used for trace sulfur analysis (ppm/ppb levels) to prevent sulfur adsorption onto the steel surface.

The line temperature must be maintained at least 10°C to 15°C above the Hydrocarbon Dew Point (or Water Dew Point, whichever is higher) of the sample gas at the operating pressure.

• This safety margin ensures that even if there are cold spots in the line, no liquid condensation occurs.

Sample lines should have a continuous slope from the tap to the analyzer (or analyzer to return).

• Minimum Slope: Typically 1:12 or 5-10 degrees.
• Purpose: To prevent liquid pockets (in gas lines) or gas pockets (in liquid lines) from trapping in low/high points, which causes erratic flow and measurement spikes.
• Design: Avoid "U" bends. If unavoidable, a drain valve must be installed at the low point.

NFPA 496 is the standard for Purged and Pressurized Enclosures.

• It defines the requirements to create a safe area inside an enclosure located in a hazardous area.
• Type X Purging: Reduces Class I, Div 1 (Zone 1) to Non-Hazardous. (Requires auto power disconnect on pressure loss).
• Type Z Purging: Reduces Class I, Div 2 (Zone 2) to Non-Hazardous. (Requires alarm only on pressure loss).
• Pressure Requirement: Minimum 25 Pa (0.1 inches of water column) positive pressure.

The intake stack is a critical safety component:

• Height: Must be drawn from a safe area, typically requiring a stack height of at least 3 to 10 meters (based on area classification study) to avoid heavy gases at ground level.
• Filtration: Must have sand traps and chemical filters if the ambient air contains corrosive gases.
• Gas Detection: The intake duct often contains a combustible and toxic gas detector to shut down the HVAC damper if outside air is contaminated.

While pressurization is static, airflow is required for cooling and ventilation.

• Standard: typically 6 to 10 ACH is common to dissipate heat generated by analyzers and equipment.
• Emergency: During a "Pre-start Purge" cycle, higher flow rates are needed to sweep the volume 5 times before power is applied.

• Heat Load: Analyzers (especially GCs with ovens) generate significant heat. If HVAC fails, temperatures rise rapidly (above 40°C), causing electronic drift or failure.
• Reliability: 2x100% units allow one to run while the other is on standby. They usually cycle (e.g., switch every week) to ensure equal wear.
• Maintenance: Allows repair of one unit without shutting down the analyzer house.

• Condensation: High humidity can cause condensation on cool metal surfaces or inside electronics, leading to short circuits.
• Static: Very low humidity increases the risk of static electricity discharge.
• Target: Typically maintained around 50% RH (±10%).

Placement depends on the density of the gas being monitored relative to air.

• Heavy Gases (Propane, Butane, most HC vapors): Mount low (approx. 300-500mm from floor).
• Light Gases (Hydrogen, Methane/Natural Gas): Mount high (near the ceiling/roof apex).
• General Rule: In an analyzer house with mixed streams, detectors are often placed at the HVAC return air grille (where air mixes) or both high and low.

• Breathing Zone: Since the primary risk is human toxicity, they are typically mounted in the "breathing zone," roughly 1.5 meters from the floor.
• Near Source: Also placed near specific leak sources (e.g., cylinder racks).
• Density: H2S is slightly heavier than air, but breathing zone mounting is the priority for personnel safety.

Required if large volumes of Nitrogen or Helium (inert gases) are used as carrier gases.

• Hazard: A Nitrogen leak can displace oxygen in the enclosed shelter, leading to asphyxiation without warning.
• Setpoint: Alarm typically set at 19.5% O2 (falling).
• Placement: Usually breathing height or lower (as Nitrogen is slightly lighter than air, but can fill the room).

Voting logic prevents false trips.

• 1ooN (One out of N): Any single detector alarm triggers the executive action. High safety, but high nuisance trip rate. Used for warning lights.
• 2ooN (Two out of N): Requires two separate detectors to confirm gas presence before tripping the main power/process. Higher reliability.
• Executive Action: Upon confirmed gas detection (High-High alarm), the system will isolate power to non-Ex equipment, shut off sample streams, and activate beacons/horns.

While plant standards vary, a common convention is:

• Red: Combustible Gas (LEL) Danger / Fire.
• Blue: Toxic Gas (H2S) Danger.
• Amber/Yellow: HVAC/Pressurization Fault or General Trouble.
• Green: Safe / System Healthy (less common on beacons, usually on panels).

• Catalytic Bead: Cheap, detects all combustibles (including Hydrogen). Susceptible to "poisoning" by silicones or sulfur. Requires Oxygen to work.
• Infrared (IR): More expensive, immune to poisoning, works in inert atmospheres (no O2 needed). Fails to detect Hydrogen (as H2 has no IR signature).
• Selection: Use Catalytic for H2 applications; use IR for general Hydrocarbons in high-maintenance areas.

• Point Detector: Detects gas at a specific location. Used inside analyzer houses.
• Open Path (Line of Sight): Shoots a beam across a distance (e.g., 50m). Detects gas clouds crossing the beam. Used for perimeter monitoring outside the analyzer house or process block.

The ESD (Emergency Shutdown) logic typically triggers:

1. Power Cut: Isolate all non-Ex rated power (general purpose outlets, lighting, PCs). Ex-rated emergency lighting remains on.
2. Sample Stop: Close the solenoid valves on incoming sample lines (block process fluid entering).
3. HVAC: Shutdown HVAC blower and close fire dampers to seal the room.
4. Annunciation: External and internal Horns/Strobes activate.

Most analyzers vent to atmosphere. If they share a common vent header that is undersized or obstructed:

• Pressure Effect: Back pressure increases the density of gas in the detector cell.
• Result: Since many detectors measure partial pressure, an increase in back pressure causes a falsely high reading.
• Solution: Use large diameter atmospheric vent headers, separate vents for high-flow streams, or atmospheric reference correction.

• Location: CEMS shelters are located at the base of the stack/chimney, often far from process units.
• Sample Line: Uses heated umbilical lines (often >120°C) to prevent acid dew point condensation (SO2, NOx).
• Calibration: Requires EPA/Environmental regulated automatic daily calibration checks.
• Hazard: often located in Safe Areas, unlike process shelters which are often in Zone 2.

1. Fast Loop Flow: Is the bypass flow rate sufficient? Check rotameter.
2. Filters: Is the primary filter at the tap plugged?
3. Line Length: Was the line routed unnecessarily long?
4. Pressure: Is the driving pressure (differential pressure) from the process adequate?

• Corrosion: Copper reacts with Acetylene (forming explosive acetylides) and Ammonia. It corrodes easily in sour environments.
• Adsorption: Active surface sites on copper can adsorb sample components.
• Standard: SS316 is the industry standard for durability and chemical compatibility.

Used when multiple streams feed one analyzer.

• Issue with Single Valves: If a valve leaks, stream A contaminates stream B (Cross-contamination), causing bad quality data.
• DBB SSV: A specialized manifold where the unselected streams are blocked and any leakage is vented to a bleed port, ensuring 100% isolation of the selected stream.

• Analyzers: To prevent hard shutdowns that damage operating systems or cool down ovens/detectors uncontrolled.
• PLC/AMADAS: To maintain data communication and monitoring.
• Gas Detection System: Critical for safety monitoring even during power loss.
• Emergency Lighting: For personnel safety.
• (Note: HVAC and Heaters are usually on Utility Power, not UPS, due to high load).

A system designed to collect liquid samples exhausted from analyzers and pump them back into the process.

• Reason: Environmental regulations often prohibit flaring or draining hydrocarbons to open sewers.
• Components: Collection vessel, level switches, and a pneumatic pump to push fluid back into the process header.

If a sample line or steam trace operates above 60°C (140°F), it must be insulated or caged where accessible to personnel to prevent burn injuries. Inside the house, hot lines entering the cabinet are often covered with perforated mesh guards.

If an analyzer vents flammable gas to the atmosphere, a flame arrestor prevents a flashback.

• Scenario: If lightning or a static spark ignites the gas at the vent tip, the flame could travel back down the pipe into the analyzer house/analyzer.
• Device: The arrestor (metal mesh/honeycomb) absorbs the heat and extinguishes the flame front.

• FAT (Factory Acceptance Test): Performed at the system integrator's yard. Checks plumbing, wiring, leaks, and preliminary calibration using cylinder gas. The house is fully powered up.
• SAT (Site Acceptance Test): Performed at the final plant location. Checks damage during transport, reconnection of external cables/pipes, and final loop checks with the DCS.

The P&ID (Piping and Instrumentation Diagram).

• It shows the tap point, fast loop routing, sample conditioning components, the analyzer tag, and the return/vent destination.
• It defines the interface boundaries (battery limits) between Piping and Instrumentation.

• Front: Minimum 1 meter (to open doors and stand in front).
• Rear: Minimum 0.6 to 0.8 meters (if rear access is required for plumbing/wiring).
• Height: Analyzers are usually mounted such that the display is at eye level (approx. 1.5m).