Industrial Engineer Gas cheat sheet

Stoichiometric combustion of methane: CH4 + 2O2 → CO2 + 2H2O; 1 m³ CH4 requires 2 m³ O2, i.e. about 9.52 m³ air at 0 °C, 101.325 kPa.

Higher heating value (HHV) includes latent heat of water vapour in products; lower heating value (LHV) excludes it. For natural gas HHV/LHV ratio is roughly 1.11.

Wobbe index W = HHV / √(relative density); appliances are rated by Wobbe index because it fixes heat input at a given burner pressure and orifice.

Air requirement rule of thumb: about 1.1 to 1.3 times stoichiometric air (10–20 % excess air) gives complete combustion with minimal excess-air loss in atmospheric burners.

Flammability limits of methane in air: approximately 5 % LEL to 15 % UEL by volume; below LEL the mixture is too lean, above UEL too rich, to ignite.

Complete combustion products are CO2 and H2O; CO forms from insufficient air, poor mixing, flame impingement, or chilling. Target CO in flue gas well under 100 ppm air-free for natural-draft appliances.

Dew point of flue gas rises with hydrogen content and excess air reduction; condensing appliances recover latent heat only when return water is below roughly 55 °C.

Gas density relative to air: natural gas ≈ 0.6 (rises and accumulates at high points), LPG vapour ≈ 1.5–2.0 (settles in low points, pits, basements).

Pressure tiers commonly used: transmission > 7 bar, intermediate 2–7 bar, medium 0.1–2 bar, low pressure below 0.1 bar (approx. 50 mbar); regulators step down between tiers.

Sizing uses the general flow equation; for low-pressure networks the simplified relation Q ∝ √(Δp / (L·S)) holds, where S is gas relative density and L equivalent length.

Polyethylene (PE) pipe is standard for buried distribution up to about 4–10 bar depending on SDR; steel is used for higher pressures and above-ground sections.

MOP (maximum operating pressure) must not exceed the design pressure of the weakest component; MAOP is set by the pipe and fitting rating, not the regulator setting alone.

District regulator stations include slam-shut, monitor regulator, relief valve, and filter; a monitor regulator limits downstream pressure if the active regulator fails.

Purging displaces air with gas (or gas with air) to below the flammable range; purge volume is typically 2–5 pipe volumes, verified by gas concentration measurement.

Cathodic protection and coating protect buried steel mains; PE is immune to corrosion but susceptible to point loading, rock impingement, and third-party damage.

Odorisation is required for public distribution; typical target is a distinctive odor at 1/5 of LEL, often achieved with THT at about 10–20 mg/m³.

Appliance categories by flue arrangement: Type A (no flue), Type B (open flue, room air), Type C (room-sealed, balanced flue); Type C is mandatory where the room cannot provide combustion air safely.

Rated heat input Q = gas rate × calorific value; burner pressure must match the data plate for the gas family declared (natural gas, LPG, or manufactured gas).

Flame supervision: thermocouples suit pilot burners; flame rectification (ionisation) is used on main burners and responds in under a few seconds.

Gas valves must be of the multiblock type with class A, B, or C closing; class A is required for appliances over 120 kW or where downstream leakage is critical.

Combustion performance targets: CO/CO2 ratio below 0.004 (0.008 acceptable on older appliances) and CO air-free below 100 ppm for natural gas.

Modulating and condensing boilers require minimum return-water temperature control or bypass to avoid flue-gas condensation in non-condensing sections and corrosion.

Appliance turndown ratio = maximum input / minimum input; high turndown improves part-load efficiency but can affect flame stability and CO at low fire.

Commercial catering and radiant appliances often need flame-failure and overheat devices plus forced ventilation interlock to prevent CO build-up.

Open-flue appliances need permanent ventilation: typical rule is 5 cm² free area per kW of rated input for high-level and low-level openings combined, unless the standard for the appliance states otherwise.

Room-sealed appliances take air directly from outside; the terminal must respect clearances from openings, windows, and boundaries to avoid recirculation of products.

Flue draught must be verified: a stable draught with no spillage at the draught diverter; spillage indicates blocked flue, inadequate air, or adverse pressure.

Flue sizing depends on appliance input, flue height, number of bends, and material; corrugated or flexible liners reduce effective area and require larger nominal size.

Chimney and flue materials must resist condensate where flue-gas temperature falls below dew point; stainless steel or plastic (PP/PPS) is used for condensing appliances.

Balanced flue terminals must not be installed where a mechanical extractor or air-conditioning unit can create reverse flow; minimum separation from extractor terminals is typically 300 mm.

Ventilation openings must be non-closable and measured as free area, not grille size; insect mesh and louvres reduce free area by 30–50 %.

CO alarm placement and testing belong to the safety regime; alarms do not substitute for correct combustion air and flue integrity.

IEC/EN 437 defines gas families and test gases; appliance approval is specific to the gas family and pressure declared on the data plate.

EN 1775 covers gas pipework in buildings; EN 15001 covers industrial installations; EN 12954 covers cathodic protection of buried metallic structures.

EN 1127-1 and ATEX directives govern hazardous-area classification and equipment selection for gas installations in explosive atmospheres.

Pressure equipment above certain volume/pressure thresholds falls under PED 2014/68/EU; small gas pipework is usually excluded but pressure vessels and large filters are not.

Gas detection and safety systems in plant rooms must follow EN 60079 for electrical equipment in zoned areas and EN 50194 for domestic detectors.

Commissioning documentation must include tightness test, purge record, combustion analysis, and as-built drawings; these are the compliance evidence in audits.

Gas Safety (Installation and Use) Regulations in the UK require competent persons, warning notices for unsafe appliances, and records of work; equivalent national regimes apply elsewhere.

Manufacturer's instructions are part of the compliance basis; deviations from them must be justified by engineering assessment and recorded.

Tightness test: pressurise with air or inert gas to the specified test pressure, stabilise, then measure pressure drop over a defined period; a drop exceeding the allowance indicates leakage.

Gas leak response sequence: eliminate ignition sources, isolate supply, ventilate, evacuate if concentration rises, and call the emergency service; never operate electrical switches in the affected area.

LEL and UEL define the flammable range; gas detection alarms are typically set at 10 % LEL (warning) and 20 % LEL (action/shutdown).

Hot work near gas systems requires a permit, gas-free certification, continuous monitoring, and fire watch for the duration plus a defined period after.

Confined-space entry into pits, culverts, or gas meter chambers requires atmosphere testing for oxygen, flammable gas, and toxic gas before entry and during work.

Emergency isolation valves must be accessible, labelled, and tested; the shut-off time for large valves should be verified against the process risk assessment.

Static electricity ignition is controlled by bonding and grounding during purging and by using conductive hoses; LPG transfer is particularly sensitive.

Incident investigation should establish the gas concentration, ignition source, and barrier failures; near-miss reporting is the leading indicator for gas safety performance.

Strength (pressure) test is performed at a higher pressure than the tightness test, typically 1.5 times design pressure for pipework, before commissioning; tightness test follows at operating pressure.

Pneumatic testing is preferred over hydrostatic for gas pipework because water residues cause corrosion and are hard to remove; test medium is air or nitrogen.

Combustion analysis records O2, CO2, CO, CO/CO2 ratio, and flue temperature; excess air is derived from O2 (roughly 5 % O2 corresponds to about 30 % excess air for natural gas).

Gas rate check: meter dial timing over a known volume confirms rated input; deviation beyond ±5 % of the data plate indicates orifice, pressure, or calorific value issues.

Functional testing covers flame supervision, safety shut-off valves, interlocks, and alarms; each device is proved to fail safe.

Purge verification uses a calibrated gas detector or combustibles meter at the purge point; the target is a stable reading above the upper flammable limit before ignition.

Commissioning records should include test pressures, durations, measured drops, instrument calibration certificates, and sign-off by the responsible engineer.

Non-destructive examination of welds: visual plus radiographic or ultrasonic for transmission and high-pressure lines; PE joints are examined visually and by pressure test.

Preventive maintenance intervals are set by risk: regulators and safety devices annually, combustion analysis annually, and filters per differential-pressure trend.

Common CO faults: blocked flue, inadequate combustion air, incorrect gas pressure, dirty burner ports, and flame impingement; always measure before adjusting.

Regulator creep or lock-up is diagnosed by observing downstream pressure with zero flow; rising pressure indicates seat leakage and requires servicing.

Flame rectification faults often trace to a dirty or oxidised flame sensor, poor earth, or reversed polarity; clean and verify earthing before replacing parts.

Reliability metrics: MTBF = operating time / number of failures; availability = MTBF / (MTBF + MTTR); use them to prioritise spares and maintenance strategy.

Condition monitoring of compressors and blowers uses vibration, oil analysis, and performance curves; deviation from the curve signals fouling or wear.

Root-cause analysis (5 Whys, fishbone) distinguishes random component failure from systemic issues such as incorrect gas family or water ingress.

Spare-parts strategy should cover safety-critical items (thermocouples, flame detectors, regulator diaphragms) and long-lead items identified by criticality analysis.

Annual gas consumption = rated input × annual equivalent full-load hours / boiler efficiency; part-load efficiency dominates real consumption.

Load duration curve and boiler sequencing: lead boiler sized for base load, lag boilers for peaks; condensing boilers should run at low return temperature for maximum gain.

Seasonal efficiency (SEDBUK-type) accounts for part-load operation, standby loss, and cycling; it is always lower than steady-state full-load efficiency.

Simple payback = capital cost / annual savings; NPV discounts future savings at the project discount rate; IRR is the rate at which NPV equals zero.

Life-cycle cost includes capital, energy, maintenance, and replacement; it is the correct basis for comparing boiler plant options.

Flue-gas heat recovery and economisers can raise efficiency by 3–6 percentage points when flue temperature exceeds about 150 °C.

Excess air optimisation: each 1 % reduction in excess air raises efficiency by roughly 0.02–0.05 percentage points for natural gas; balance against CO formation.

Load analysis uses degree-day data: consumption ∝ heating degree days when the building is heating-dominated; deviations indicate control or envelope issues.