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HKIE Professional Assessment - Gas Discipline study materials
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HKIE Professional Assessment - Gas Discipline practice questions

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Question 1Gas supply systems and network design

In a medium-pressure distribution main, the primary purpose of a district governor is to do which of the following?

  1. Reduce the upstream transmission pressure to a controlled lower pressure for downstream consumers Correct answer.
  2. Increase the gas pressure so that larger downstream flows can be delivered
  3. Measure the cumulative volume of gas passing through the main
  4. Remove hydrogen sulphide from the gas before it reaches customers

Explanation

A district governor is a pressure-regulating device that reduces the higher pressure of a transmission or medium-pressure main to a stable, controlled lower pressure suitable for the downstream distribution network or consumers. It acts as a pressure control element, not a metering or gas-treatment device.

Question 2Gas supply systems and network design

A gas distribution network is designed to operate at pressures not exceeding 7 bar. Under the Hong Kong Gas Safety Ordinance framework, this network is normally classified as which type?

  1. A low-pressure network
  2. A high-pressure network
  3. An intermediate-pressure network
  4. A medium-pressure network Correct answer.

Explanation

Distribution systems operating at pressures above low pressure but not exceeding the medium-pressure ceiling (commonly around 7 bar) are classed as medium-pressure networks. Low pressure typically refers to pressures of the order of a few tens of millibar, while high pressure refers to transmission-level pressures above the medium band.

Question 3Gas supply systems and network design

Which gas property most directly determines the sizing of a low-pressure distribution main for a given heat demand?

  1. The colour and odour of the gas
  2. The calorific value and relative density of the gas Correct answer.
  3. The sulphur content of the gas
  4. The moisture dew point of the gas

Explanation

Main sizing depends on the volume flow required to meet the heat demand, which is obtained by dividing heat demand by calorific value, and on the pressure loss, which depends on relative density. Sulphur, colour and odour do not govern hydraulic capacity.

Question 4Gas supply systems and network design

Natural gas supplied to consumers is odorised primarily to achieve which objective?

  1. To raise the calorific value of the gas to the declared figure
  2. To reduce the flame speed at burner ports
  3. To inhibit corrosion of the internal pipe surface
  4. To enable detection of unburnt gas leakage by smell before it reaches a dangerous concentration Correct answer.

Explanation

Odorant is added so that gas leakage can be detected by smell at concentrations well below the lower explosive limit. It does not change calorific value, inhibit corrosion or modify combustion characteristics.

Question 5Gas supply systems and network design

In polyethylene (PE) gas mains, the standard method of joining pipe lengths for buried distribution mains is which of the following?

  1. Butt fusion or electrofusion Correct answer.
  2. Riveted lap joints
  3. Lead-caulked bell and spigot joints
  4. Threaded screwed couplings

Explanation

PE pipe is joined by heat fusion, either butt fusion for larger diameters or electrofusion fittings for smaller and service connections, producing a joint as strong as the pipe. Threads, rivets and caulked lead joints are not used for PE gas mains.

Question 6Gas supply systems and network design

A gas main is to be laid in a trench crossing beneath a busy carriageway. Which provision is most appropriate for protecting the main?

  1. Laying the main directly on the road sub-base without bedding
  2. Reducing the wall thickness of the pipe to save cost
  3. Installing a sleeve or duct and increasing the depth of cover Correct answer.
  4. Omitting the marker tape to avoid interference with road works

Explanation

Where a main crosses beneath a carriageway, additional protection is normally provided by a sleeve or duct and by greater depth of cover to resist traffic loading and third-party damage. Proper bedding and marker tape are retained, not omitted.

Question 7Gas supply systems and network design

Which document type is used to record the as-installed route, depth and fittings of a newly completed gas main before it is commissioned?

  1. A material test certificate
  2. A gas safety data sheet
  3. An as-built record drawing Correct answer.
  4. A hazardous area classification drawing

Explanation

As-built record drawings capture the actual installed route, depth, offsets and fittings and are essential for future maintenance, excavation and network analysis. Safety data sheets describe gas hazards, test certificates prove material properties, and hazardous area drawings relate to electrical classification.

Question 8Gas supply systems and network design

Before a new low-pressure gas main is put into service, which test is performed to confirm its pressure integrity?

  1. A soil resistivity survey along the route
  2. A gas chromatography test of the gas composition
  3. A strength and tightness test using air or inert gas at specified test pressures Correct answer.
  4. A flow test with the gas burners operating at full rate

Explanation

New mains are pressure-tested, normally with air or inert gas, to prove strength and tightness before purging and commissioning with gas. Composition analysis, soil resistivity and burner flow tests do not establish the mechanical integrity of the main.

Question 9Gas supply systems and network design

A designer is sizing a medium-pressure steel main. Which relationship correctly describes the effect of increasing the pipe internal diameter while keeping all other parameters constant?

  1. Capacity falls because the surface area for friction increases
  2. Capacity rises only in direct proportion to diameter
  3. Capacity is unchanged because pressure, not diameter, governs flow
  4. Capacity rises strongly because pressure loss varies inversely with a high power of diameter Correct answer.

Explanation

In the standard flow equations for gas mains, pressure loss is inversely proportional to a high power of the internal diameter (commonly the fifth power in the low-pressure formula). A modest increase in diameter therefore produces a large increase in capacity.

Question 10Gas supply systems and network design

A low-pressure distribution main serves a group of domestic customers. If the connected heat demand doubles and the main diameter cannot be changed, what is the most direct consequence for the network?

  1. The relative density of the gas falls in proportion to demand
  2. The calorific value of the gas automatically adjusts to compensate
  3. The odorant concentration rises to maintain smell detection
  4. The pressure loss along the main increases, and terminal pressure may fall below the acceptable minimum Correct answer.

Explanation

With a fixed diameter, higher flow increases frictional pressure loss along the main, so the pressure available at the far end falls. If this falls below the minimum required at the meter or appliance, the design is inadequate and reinforcement is needed.

Question 11Gas supply systems and network design

When selecting the location of a district governor installation, which combination of factors is most relevant?

  1. The prevailing wind direction only
  2. The number of employees in the local office
  3. Accessibility for maintenance, safe separation distances and the pressure zones it serves Correct answer.
  4. Proximity to the nearest retail outlet and the colour of the kiosk

Explanation

Governor siting must consider safe separation from buildings and ignition sources, ventilation, access for maintenance and the boundary between pressure zones it controls. Aesthetic or unrelated commercial factors do not govern the location.

Question 12Gas supply systems and network design

A steel distribution main in a coastal urban area shows an impressed current cathodic protection system. Which measurement best indicates whether the protection criterion is being met?

  1. The odorant concentration at the extremity
  2. The gas flow rate through the main
  3. The calorific value of the gas at the city gate
  4. The pipe-to-soil potential measured against a reference electrode Correct answer.

Explanation

Cathodic protection effectiveness is assessed by pipe-to-soil potential measurements, compared against the accepted protection criterion. Flow, calorific value and odorant concentration are unrelated to electrochemical protection of the steel.

Question 13Gas supply systems and network design

During purging of a newly commissioned gas main, which practice is appropriate?

  1. Introducing gas at maximum pressure without any venting
  2. Sealing both ends and leaving the main full of air
  3. Controlled venting to a safe outdoor location with continuous monitoring and exclusion of ignition sources Correct answer.
  4. Purging directly into an occupied basement to save time

Explanation

Purging must displace air to a safe, well-ventilated outdoor point while monitoring gas concentration and excluding ignition sources, so that a flammable mixture is not released where people are present. Purging into an occupied space or leaving air in the main is dangerous.

Question 14Gas supply systems and network design

A polyethylene main is to be installed by trenchless methods beneath a river crossing. Which consideration is most important in the design?

  1. The number of valves in the downstream network
  2. The colour of the marker posts at each end
  3. The odour of the gas in the completed main
  4. The pull loads, bend radius and external pressure during and after installation Correct answer.

Explanation

Trenchless installation subjects the pipe to pulling or thrust forces and requires respect of the minimum bend radius, while the completed crossing may experience external water pressure and buoyancy. These mechanical conditions govern the design.

Question 15Gas supply systems and network design

A network analysis shows that a low-pressure main reaches the minimum permissible pressure at the extremity during peak winter demand. Which reinforcement option addresses the cause most directly?

  1. Reducing the number of pressure test points along the route
  2. Lowering the governor set pressure at the district station
  3. Increasing the odorant injection rate at the city gate
  4. Replacing a length of the main with a larger diameter pipe or adding a parallel main Correct answer.

Explanation

The problem is excessive pressure loss under peak flow, so the hydraulic fix is to reduce resistance by upsizing the main or providing a parallel path. Lowering the governor set point would worsen the extremity pressure, and odorant or test point changes do not affect hydraulics.

Question 16Gas supply systems and network design

In the design of a gas service pipe to a domestic premises, which factor governs the minimum cover depth in a private garden?

  1. The calorific value of the gas being supplied
  2. The pressure rating of the meter regulator
  3. The risk of third-party damage from gardening and excavation Correct answer.
  4. The diameter of the appliance injector

Explanation

Service pipe cover depth is set to protect the pipe from foreseeable third-party damage such as digging in gardens, in accordance with the relevant code of practice. Gas calorific value, regulator rating and injector size do not determine burial depth.

Question 17Gas supply systems and network design

A steel main is to carry gas with a measured dew point close to the ground temperature along the route. Which design measure is most appropriate?

  1. Reducing the wall thickness to lower the thermal mass
  2. Installing a condensate trap or drip point at a low point with provision for removal of liquid Correct answer.
  3. Removing all line valves to avoid liquid accumulation
  4. Increasing the gas velocity to carry liquid into the customer's meter

Explanation

If condensate can form, the design should provide collection points such as drip traps at low points, with means of removal, so that liquid does not obstruct flow or reach meters and regulators. Carrying liquid downstream is unacceptable and reducing wall thickness is unsafe.

Question 18Gas supply systems and network design

A district governor station serving a medium-pressure network is fitted with a slam-shut device. What is the function of this device?

  1. To isolate the supply automatically when the downstream pressure exceeds or falls below set limits Correct answer.
  2. To reduce the noise of gas flowing through the regulator
  3. To inject odorant into the downstream main
  4. To measure the volume of gas delivered to each district

Explanation

A slam-shut or over-pressure/under-pressure shut-off device closes the supply automatically when downstream pressure moves outside set limits, protecting the downstream network from over-pressure or loss of supply. It is a safety isolation device, not a meter, odoriser or silencer.

Question 19Gas supply systems and network design

Which factor most strongly influences the choice between steel and polyethylene for a new medium-pressure distribution main?

  1. The operating pressure and diameter relative to the pressure rating of each material Correct answer.
  2. The number of customers connected at the far end
  3. The calorific value of the gas to be transported
  4. The colour required by the local planning authority

Explanation

Material selection turns on the pressure rating and diameter capability of each material at the intended operating pressure, together with installation and corrosion considerations. Colour, customer count and calorific value do not determine whether steel or PE is technically suitable.

Question 20Gas supply systems and network design

A designer uses the general gas flow equation for a medium-pressure main. If the upstream absolute pressure is doubled while downstream absolute pressure and all other terms are held constant, what happens to the flow capacity?

  1. It remains exactly the same
  2. It decreases, because higher pressure increases friction
  3. It increases, because the pressure-squared difference driving the flow becomes larger Correct answer.
  4. It becomes zero because the flow becomes choked

Explanation

In the pressure-squared formulation of the gas flow equation, capacity depends on the difference between the squares of upstream and downstream absolute pressures. Raising upstream pressure increases this difference, so the flow capacity rises for the same pipe and downstream pressure.

Question 21Gas supply systems and network design

A network has two parallel low-pressure mains of different diameters connecting the same two nodes. Which statement best describes how the flow divides between them?

  1. Flow divides equally regardless of diameter
  2. All flow passes through the smaller main because of higher velocity
  3. Flow divides in proportion to each main's conductance, so the larger main carries the greater share Correct answer.
  4. Flow division depends only on the odorant concentration in each main

Explanation

Parallel paths share flow according to their hydraulic conductance for the same pressure drop, so a larger-diameter main carries proportionally more. The division is not equal unless the paths are hydraulically identical, and odorant has no influence.

Question 22Gas supply systems and network design

A medium-pressure main crosses a zone where the ground is known to be subject to mining subsidence. Which design provision best addresses the resulting ground movement?

  1. Using rigidly anchored flanged joints throughout to prevent any movement
  2. Reducing the burial depth to zero so the pipe can move freely
  3. Providing flexible joints and designing for differential settlement, with increased monitoring Correct answer.
  4. Increasing the operating pressure to stiffen the pipe

Explanation

Where ground movement is expected, the design should accommodate differential settlement through flexible joints or bends and appropriate support, together with monitoring. Rigid anchoring concentrates strain, and reducing cover or raising pressure does not provide the required flexibility.

Question 23Gas supply systems and network design

In a medium-pressure network, a section valve is installed on each side of a river crossing. What is the principal design purpose of this valve arrangement?

  1. To limit the inventory that can escape and to allow isolation for maintenance of the crossing section Correct answer.
  2. To eliminate the need for pressure testing of the crossing
  3. To reduce the number of customers connected to the network
  4. To increase the calorific value of the gas crossing the river

Explanation

Sectionalising valves around a river crossing allow the crossing to be isolated for maintenance or in an emergency, limiting the volume of gas that can escape and protecting the rest of the network. Valves do not change gas quality, customer numbers or testing requirements.

Question 24Gas supply systems and network design

A low-pressure network is being extended and the designer must ensure adequate pressure at the furthest meter. Which approach gives the most reliable verification of the design?

  1. Calculating cumulative pressure loss along each path from the governor to the extremity and comparing with the minimum required Correct answer.
  2. Checking only the pressure at the governor outlet
  3. Assuming the extremity pressure equals the governor set pressure
  4. Using the average pressure of the network as the extremity pressure

Explanation

The extremity pressure is obtained by subtracting the cumulative pressure loss along the actual flow path from the available governor outlet pressure, then comparing it with the minimum required at the meter. Assuming no loss or using averages ignores the hydraulic reality of the network.

Question 25Gas supply systems and network design

A steel main is being designed for a location where stray direct currents from a nearby rail system are present. Which combination of measures is most appropriate?

  1. Coating only the fittings and leaving the pipe bare
  2. Installing insulating joints, drainage of stray current and monitoring of pipe-to-soil potential Correct answer.
  3. Increasing the gas pressure and removing all insulating joints
  4. Relying solely on the odorant to warn of any corrosion

Explanation

Stray current corrosion is managed by electrical isolation of the main where appropriate, provision for current drainage, and ongoing monitoring of pipe-to-soil potentials. Raising pressure, partial coating or relying on odorant does not control stray current effects.

Question 26Gas supply systems and network design

A new medium-pressure polyethylene (PE) distribution main is being designed to operate at a gauge pressure of 4 bar. Which pipe specification requirement applies to the material grade and wall thickness selection for this duty?

  1. PE 100 compound only, with wall thickness selected solely from the nominal outside diameter table
  2. PE 80 compound only, with the wall thickness fixed by the nominal diameter and independent of pressure
  3. PE 80 or PE 100 compound with a standard dimension ratio chosen so the maximum allowable operating pressure is not exceeded at the design temperature Correct answer.
  4. Any PE 63 compound, provided the pipe is fully encased in a protective sleeve along the whole route

Explanation

For medium-pressure PE mains the design pressure must not exceed the maximum allowable operating pressure of the pipe, which is derived from the material grade (typically PE 80 or PE 100) and the standard dimension ratio (SDR) at the design temperature. The wall thickness is therefore a function of both diameter and pressure class, not a fixed value.

Question 27Gas supply systems and network design

A low-pressure gas main terminates at a riser serving a single domestic block. Which arrangement best ensures that the pipework can be isolated and purged safely during future maintenance without interrupting the supply to the adjacent properties?

  1. A tee-off with the branch capped and no valve or purge point
  2. A direct welded connection from the main to the riser with no valve
  3. A tee-off with an isolating valve on the branch and a purge point at the riser Correct answer.
  4. A service tee with a valve downstream of the meter regulator only

Explanation

An isolating valve on the branch allows the riser to be shut off from the live main, and a purge point at the riser allows controlled purging and re-commissioning. This arrangement keeps the rest of the low-pressure network live during maintenance.

Question 28Gas supply systems and network design

A designer is checking the capacity of a medium-pressure steel main using the general gas flow equation. If the absolute inlet pressure is held constant and the absolute outlet pressure is raised so that the term (p1² − p2²) halves, what happens to the flow rate, all other factors unchanged?

  1. It remains unchanged because the inlet pressure is constant
  2. It falls to 50% of the original value
  3. It falls to about 71% of the original value Correct answer.
  4. It rises to about 141% of the original value

Explanation

In the general flow equation the flow rate is proportional to the square root of (p1² − p2²). Halving that term multiplies the flow by the square root of one half, which is approximately 0.707, so the flow falls to about 71%.

Question 29Gas supply systems and network design

A district governor installation is being designed for a medium-pressure network feeding a low-pressure district. Which combination of features is required to protect the downstream low-pressure main if the governor fails open?

  1. An upstream filter and a downstream pressure gauge only
  2. A downstream non-return valve only
  3. A bypass line around the governor with manual control only
  4. An upstream slam-shut device plus a downstream relief or monitor arrangement set to limit the downstream pressure Correct answer.

Explanation

Protection against governor failure requires an independent safety device, typically an upstream slam-shut valve that closes on over-pressure, together with a downstream relief valve or a monitor regulator that limits the pressure reaching the low-pressure main. These devices act independently of the main governor.

Question 30Gas supply systems and network design

A low-pressure distribution network is supplied from two governor stations at opposite ends of a long spine main, with a branch feeding a housing estate at the mid-point. During peak demand the pressure at the estate falls below the minimum permissible. Which option best describes the most effective design response, assuming the governors and the spine main cannot be uprated?

  1. Raise the governor set-point at both stations to the maximum allowable low-pressure limit
  2. Reduce the number of customers connected to the estate to lower the demand
  3. Add a dedicated pressure-boosting or additional supply point near the estate to shorten the hydraulic path from source to load Correct answer.
  4. Increase the diameter of the estate branch only, leaving the spine main unchanged

Explanation

The pressure loss is dominated by the long hydraulic path along the spine main, so shortening that path with an additional supply point or local boosting gives the greatest improvement. Enlarging only the short estate branch addresses a minor part of the total pressure loss.

Question 31Gas properties, combustion and utilisation

Which property of natural gas is primarily responsible for its characteristic odour being added before distribution to consumers?

  1. Its wide flammability range
  2. Its lack of inherent smell Correct answer.
  3. Its high calorific value
  4. Its low density relative to air

Explanation

Pipeline natural gas is largely methane and is odourless in its natural state, so a sulphur-based odorant such as mercaptan is injected to make leaks detectable by smell. The other properties are genuine characteristics of methane but none of them explains the need to add odour.

Question 32Gas properties, combustion and utilisation

A gas has a relative density (specific gravity) of about 0.55 with respect to air. What is the expected behaviour of a released cloud in still air?

  1. It will sink and pool at low level
  2. It will remain neutrally suspended
  3. It will rise and disperse upward Correct answer.
  4. It will dissolve rapidly into the ground

Explanation

A gas lighter than air, with relative density below 1.0, tends to rise and dissipate upward when released in still conditions. Methane-rich natural gas behaves this way, which is why it is less prone to collecting in low-lying pits than LPG vapour.

Question 33Gas properties, combustion and utilisation

Which statement correctly describes the stoichiometric combustion of methane in air?

  1. Two volumes of methane require one volume of oxygen
  2. One volume of methane requires two volumes of oxygen Correct answer.
  3. One volume of methane requires one volume of oxygen
  4. One volume of methane requires three volumes of oxygen

Explanation

The balanced equation CH4 + 2O2 gives CO2 + 2H2O shows that one mole of methane consumes two moles of oxygen. Since gas volumes are proportional to moles at the same conditions, one volume of methane needs two volumes of oxygen.

Question 34Gas properties, combustion and utilisation

In gas utilisation, what does the Wobbe index allow an engineer to compare between two different fuel gases?

  1. Their flame temperatures at equal pressure
  2. Their dew points at pipeline pressure
  3. Their heat input through a fixed injector Correct answer.
  4. Their sulphur content before odorisation

Explanation

The Wobbe index combines calorific value with relative density so that gases with the same index deliver the same heat input through a given injector at the same pressure. It is the standard interchangeability measure for appliances.

Question 35Gas properties, combustion and utilisation

A burner is adjusted so that the primary aeration is increased while gas rate stays constant. What is the most likely effect on the flame?

  1. A longer, luminous yellow flame
  2. A flame that lifts off the burner ports
  3. A flame that produces more soot
  4. A shorter, sharper blue flame Correct answer.

Explanation

Increasing primary air moves combustion toward the stoichiometric mixture, producing a shorter, harder and bluer flame cone. Excessive aeration beyond the optimum can eventually cause lift, but a moderate increase sharpens and shortens the flame.

Question 36Gas properties, combustion and utilisation

Which product of incomplete combustion is the principal acute toxicity hazard from an unflued gas appliance in a poorly ventilated room?

  1. Carbon dioxide
  2. Nitrogen
  3. Carbon monoxide Correct answer.
  4. Water vapour

Explanation

Carbon monoxide binds strongly to haemoglobin and is the main acute toxic hazard when gas burns with insufficient air. Carbon dioxide and water vapour are normal complete-combustion products, and nitrogen is largely inert.

Question 37Gas properties, combustion and utilisation

The calorific value of a gas is quoted as gross (higher) rather than net (lower). What accounts for the difference between the two values?

  1. The energy needed to ignite the gas
  2. The heat lost through the appliance casing
  3. The heat carried away by excess air
  4. The latent heat of vaporisation of water formed in combustion Correct answer.

Explanation

Gross calorific value includes the latent heat released when the water vapour formed during combustion condenses, while net calorific value excludes it. The difference therefore equals the latent heat of the water produced.

Question 38Gas properties, combustion and utilisation

A gas appliance is rated at 24 kW gross input and operates at 80% gross efficiency. What is the useful heat output?

  1. 4.8 kW
  2. 19.2 kW Correct answer.
  3. 30.0 kW
  4. 24.0 kW

Explanation

Useful output equals input multiplied by efficiency, so 24 kW times 0.80 gives 19.2 kW. The remaining 4.8 kW is lost as flue and casing losses.

Question 39Gas properties, combustion and utilisation

Why is the lower flammability limit of a fuel gas important when designing ventilation for a plant room?

  1. It defines the maximum flame temperature
  2. It fixes the odorant injection rate
  3. It determines the gas's calorific value
  4. It sets the minimum gas concentration that can ignite Correct answer.

Explanation

The lower flammability limit is the minimum fuel concentration in air that will support ignition. Ventilation design must keep concentrations well below this value to prevent a flammable atmosphere forming.

Question 40Gas properties, combustion and utilisation

In a premix aerated burner, what is the primary function of the burner cap or port geometry?

  1. To store gas for peak demand
  2. To control flame shape and stability Correct answer.
  3. To cool the combustion products
  4. To measure the gas flow rate

Explanation

The ports and cap shape the emerging gas-air mixture and anchor the flame, controlling flame shape, height and stability. Port size and spacing are chosen to prevent light-back and lift.

Question 41Gas properties, combustion and utilisation

A flame lifts off the ports of a burner. Which combination of conditions is the most plausible cause?

  1. High humidity in the combustion air
  2. Insufficient primary air and low gas pressure
  3. Blocked ports and reduced air supply
  4. Excessive gas pressure and over-aeration Correct answer.

Explanation

Lift occurs when the mixture velocity at the ports exceeds the flame speed, which is promoted by excessive pressure and over-aeration. Correcting the pressure or reducing primary air restores a stable flame.

Question 42Gas properties, combustion and utilisation

Which statement best explains why carbon monoxide levels rise when a gas flame impinges on a cool surface?

  1. The surface absorbs the odorant
  2. Quenching cools the flame below the temperature needed to complete oxidation Correct answer.
  3. The surface increases the gas pressure
  4. The surface adds oxygen to the flame

Explanation

Flame impingement on a cool surface quenches the reaction zone, lowering temperature and preventing CO from being fully oxidised to CO2. This is why appliance design keeps flames clear of cool surfaces.

Question 43Gas properties, combustion and utilisation

Which of the following is a correct statement about the density of liquefied petroleum gas (LPG) vapour compared with air?

  1. It is buoyant only when warm
  2. It is lighter than air and rises
  3. It has the same density as air
  4. It is heavier than air and tends to collect at low level Correct answer.

Explanation

LPG vapour, being mainly propane and butane, is denser than air and collects in low-lying areas such as pits and basements. This is a key difference from natural gas and drives different ventilation and detection strategies.

Question 44Gas properties, combustion and utilisation

A natural gas appliance is converted to operate on LPG. Which change is normally required at the injector and why?

  1. A larger injector, because LPG is lighter than air
  2. A smaller injector, because LPG has a higher calorific value per unit volume Correct answer.
  3. No change, because the Wobbe index is identical
  4. A larger injector, because LPG has a lower calorific value per unit volume

Explanation

LPG delivers more heat per unit volume than natural gas, so a smaller injector is fitted to maintain the correct heat input at the same supply pressure. Conversion also usually involves adjusting aeration and possibly the governor spring.

Question 45Gas properties, combustion and utilisation

During combustion analysis of a condensing boiler, the flue gas shows a CO2 reading close to the stoichiometric maximum with a low excess-air figure. What does this indicate?

  1. The burner is over-aerated
  2. The gas supply is contaminated
  3. The burner is correctly set with efficient combustion Correct answer.
  4. The flue is blocked

Explanation

A CO2 reading near the maximum for the fuel, with only slight excess air, indicates combustion close to stoichiometric and good efficiency. Excessive air would dilute the CO2 and lower the reading.

Question 46Gas properties, combustion and utilisation

What is the main reason natural gas is compressed and transported as a gas rather than liquefied for typical town-gas distribution?

  1. Liquefied gas cannot be burned
  2. Liquefaction removes the calorific value
  3. Liquefaction requires very low temperature and is uneconomic for routine distribution Correct answer.
  4. Natural gas cannot be liquefied

Explanation

Methane liquefies only at around -162 degrees Celsius, so LNG is reserved for bulk storage and long-distance marine transport. Routine town distribution uses compressed gas because continuous liquefaction would be uneconomic.

Question 47Gas properties, combustion and utilisation

A gas fire produces a long, yellow, smoky flame. Which adjustment is most appropriate?

  1. Increase the primary air and check the injector size Correct answer.
  2. Remove the flame supervision device
  3. Increase the gas pressure only
  4. Reduce the primary air and increase the gas rate

Explanation

A long yellow smoky flame signals incomplete combustion from insufficient primary air, so increasing aeration and verifying the injector restores a clean blue flame. The injector must be correct so the gas rate matches the design.

Question 48Gas properties, combustion and utilisation

Which gas property is most directly relevant to sizing a gas pipe for a given heat load?

  1. Its odorant concentration
  2. Its calorific value and relative density Correct answer.
  3. Its dew point
  4. Its sulphur content

Explanation

Pipe sizing depends on the volumetric flow needed to deliver the heat load, which requires the calorific value, and on pressure loss, which depends on relative density. Together these determine the required pipe diameter.

Question 49Gas properties, combustion and utilisation

In terms of combustion chemistry, why does a blue flame indicate more complete combustion than a yellow luminous flame?

  1. Blue light is emitted by nitrogen
  2. The blue cone shows a hot, well-mixed preheated reaction zone Correct answer.
  3. Blue indicates the presence of water vapour only
  4. Yellow flames are always hotter

Explanation

The blue cone arises from a hot, well-mixed reaction zone where fuel and air are intimately combined, favouring complete oxidation. Yellow luminosity comes from incandescent carbon particles formed when the mixture is fuel-rich.

Question 50Gas properties, combustion and utilisation

A gas meter is to be checked for correct registration. Which relationship is used to convert a measured volume flow to a heat input?

  1. Temperature multiplied by volume
  2. Volume flow multiplied by calorific value Correct answer.
  3. Pressure multiplied by volume
  4. Volume flow divided by relative density

Explanation

Heat input is obtained by multiplying the volumetric flow rate by the calorific value of the gas, with corrections for temperature and pressure as needed. This is the standard basis for checking meter registration against appliance rating.