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Industrial Engineer Gas practice questions

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Question 1Gas Fundamentals and Combustion

At standard conditions, what is the approximate lower flammability limit (LFL) of methane in air by volume?

  1. 2.5%
  2. 9.5%
  3. 15%
  4. 5% Correct answer.

Explanation

Methane's flammable range in air extends from roughly 5% by volume at the lean limit to about 15% at the rich limit. The 5% figure is the standard design basis used for gas detection alarm setpoints and combustion safety interlocks. Values below 5% are too lean to sustain a flame.

Question 2Gas Fundamentals and Combustion

A gas stream contains 8% methane in air at ambient temperature and pressure. Which statement best describes its combustion behavior if exposed to an ignition source?

  1. It will ignite only if the oxygen content is raised above 25%.
  2. It lies within the flammable range and can ignite. Correct answer.
  3. It is above the upper flammability limit and cannot ignite.
  4. It is too lean to propagate a flame.

Explanation

Methane's flammable range in air is roughly 5% to 15% by volume, so an 8% mixture falls inside that window and is capable of ignition and flame propagation. The mixture is neither lean nor rich of the limits. No oxygen enrichment is required because air already supplies adequate oxidant.

Question 3Gas Fundamentals and Combustion

What is the stoichiometric air-to-fuel volume ratio for complete combustion of methane?

  1. 2.4:1
  2. 4.8:1
  3. 9.5:1 Correct answer.
  4. 17.2:1

Explanation

Complete combustion of methane follows CH4 + 2O2 giving CO2 + 2H2O, so two volumes of oxygen are needed per volume of methane. Air is about 21% oxygen, so the air requirement is roughly 2 divided by 0.21, near 9.5 volumes of air per volume of fuel. This ratio defines the boundary between excess air and excess fuel.

Question 4Gas Fundamentals and Combustion

A burner fires natural gas at 100 m3/h and the measured flue gas oxygen is 4% dry. Which condition does this reading indicate?

  1. The burner is running fuel-rich with unburned fuel present.
  2. The burner is starved of combustion air.
  3. The burner is operating exactly at stoichiometric air.
  4. The burner is running with excess air above stoichiometric. Correct answer.

Explanation

Any measurable oxygen in dry flue gas means oxygen was supplied beyond what combustion consumed, so the burner operates with excess air. At true stoichiometric firing, oxygen would be essentially absent. Fuel-rich or air-starved firing would show carbon monoxide and unburned hydrocarbons rather than several percent oxygen.

Question 5Gas Fundamentals and Combustion

Which quantity expresses the heat released per unit volume of a combustible gas when it burns in air under defined reference conditions?

  1. Wobbe index
  2. Adiabatic flame temperature
  3. Higher heating value Correct answer.
  4. Specific heat capacity

Explanation

Heating value is the energy released per unit quantity of fuel, expressed per unit volume or mass under stated reference conditions. The higher heating value includes the latent heat recovered from water vapor in the products. It is the standard input for firing rate and efficiency calculations.

Question 6Gas Fundamentals and Combustion

Why is the Wobbe index used when evaluating whether a gas can be substituted in an existing burner without hardware changes?

  1. It measures the autoignition temperature of the fuel.
  2. It predicts the dew point of combustion products.
  3. It combines heating value and specific gravity to reflect heat input at a fixed pressure. Correct answer.
  4. It ranks fuels by their flame speed in air.

Explanation

For a fixed burner orifice and supply pressure, the heat input depends on heating value and on gas density, since density governs flow through the orifice. The Wobbe index captures both effects in one number, so fuels with similar values deliver comparable heat input. This makes it the practical interchangeability metric.

Question 7Gas Fundamentals and Combustion

Which statement correctly distinguishes the higher heating value (HHV) from the lower heating value (LHV) of a fuel?

  1. HHV assumes product water remains as vapor.
  2. LHV assumes product water is condensed and its latent heat recovered.
  3. LHV is always larger than HHV for hydrocarbon gases.
  4. HHV includes the latent heat of water vapor in the products. Correct answer.

Explanation

The higher heating value counts the latent heat released when water vapor in the products condenses, so it is larger than the lower heating value. LHV treats product water as vapor and excludes that latent heat. For natural gas, HHV typically exceeds LHV by roughly 10%.

Question 8Gas Fundamentals and Combustion

A natural gas flame is lifted off the burner port and blows out. Which change is most likely to stabilize the flame?

  1. Cooling the burner port below the fuel dew point
  2. Increasing the primary air to a very high excess
  3. Increasing the gas exit velocity at the port
  4. Reducing the gas exit velocity relative to flame speed Correct answer.

Explanation

Flame blow-off occurs when the gas velocity at the port exceeds the local flame speed, detaching the flame. Lowering the exit velocity or increasing flame speed brings the two back into balance. Reducing primary air toward the correct ratio also raises flame speed and helps anchoring.

Question 9Gas Fundamentals and Combustion

Which condition is required for a flammable gas mixture to ignite and sustain combustion?

  1. Fuel concentration within its flammable range and sufficient ignition energy Correct answer.
  2. Oxygen concentration below the limiting oxygen concentration
  3. Fuel concentration above the upper flammability limit
  4. Pressure above the fuel's critical pressure

Explanation

Ignition requires the fuel concentration to lie between the lower and upper flammability limits and an ignition source delivering enough energy. The oxidant must also be above the limiting oxygen concentration. Outside the flammable range, no flame propagates regardless of spark energy.

Question 10Gas Fundamentals and Combustion

What does the limiting oxygen concentration (LOC) of a fuel represent?

  1. The oxygen content of stoichiometric combustion products
  2. The oxygen level needed for complete oxidation of carbon monoxide
  3. The oxygen level at which the flame temperature peaks
  4. The minimum oxygen level below which a fuel-air-inert mixture cannot propagate a flame Correct answer.

Explanation

The LOC is the lowest oxygen concentration in a fuel-oxidant-inert mixture that still supports flame propagation, typically measured with nitrogen or carbon dioxide dilution. Below it, inerting prevents combustion even if the fuel is in its flammable range. It underpins inert-gas blanketing design for vessels and pipelines.

Question 11Gas Fundamentals and Combustion

A gas leak forms a plume in still air. Which property most directly governs how quickly the gas disperses upward relative to air?

  1. Its autoignition temperature
  2. Its specific gravity relative to air Correct answer.
  3. Its adiabatic flame temperature
  4. Its stoichiometric air requirement

Explanation

Buoyant behavior of a released gas depends on its density relative to surrounding air, expressed as specific gravity. Gases lighter than air, such as methane, tend to rise and disperse upward. Heavier gases accumulate in low areas, changing the hazard profile.

Question 12Gas Fundamentals and Combustion

Which gas is commonly used as an odorant in natural gas distribution so that leaks are detectable by smell?

  1. Nitrogen
  2. Propane
  3. Mercaptan Correct answer.
  4. Carbon dioxide

Explanation

Mercaptans, particularly ethyl mercaptan, are added in small concentrations to give odorless natural gas a detectable smell. The odorant level is regulated so leaks become noticeable well below the lower flammability limit. It has no role in the combustion chemistry.

Question 13Gas Fundamentals and Combustion

A boiler flue gas shows 9% CO2 and 6% O2 on a dry basis. Compared with a reading of 10% CO2 and 3% O2, what does the change indicate?

  1. Combustion has become fuel-rich.
  2. The fuel has switched to a higher hydrogen content.
  3. More excess air is being supplied. Correct answer.
  4. Less excess air is being supplied.

Explanation

As excess air increases, dilution lowers the CO2 concentration and raises the remaining O2 in the flue gas. The shift from 10% CO2 with 3% O2 to 9% CO2 with 6% O2 therefore reflects more excess air. Fuel-rich combustion would instead show rising CO and falling O2.

Question 14Gas Fundamentals and Combustion

Which factor primarily determines the adiabatic flame temperature of a fuel burned in air?

  1. The pipeline operating pressure
  2. The specific gravity of the unburned fuel
  3. The heating value released and the heat capacity of the product gases Correct answer.
  4. The odorant concentration in the gas

Explanation

Adiabatic flame temperature follows from an energy balance: the heat released by combustion raises the product gases, whose heat capacity governs the resulting temperature rise. Higher heating value and lower product heat capacity both raise the flame temperature. Excess air or inert dilution lowers it by adding thermal mass.

Question 15Gas Fundamentals and Combustion

A gas-fired furnace shows a yellow, luminous, sooting flame instead of a short blue flame. Which corrective action is most appropriate?

  1. Raise the flue gas recirculation rate to the maximum
  2. Increase primary air to approach the correct fuel-air ratio Correct answer.
  3. Increase the fuel supply pressure at the orifice
  4. Reduce primary air further to lengthen the flame

Explanation

A yellow, sooting flame signals incomplete mixing and insufficient primary air, producing carbon-rich combustion. Increasing primary air toward the proper ratio restores the blue, well-mixed flame and reduces soot. Fuel-rich firing also raises CO emissions.

Question 16Gas Fundamentals and Combustion

Which value best represents the autoignition temperature of methane in air?

  1. About 540 °C Correct answer.
  2. About 1200 °C
  3. About 200 °C
  4. About 90 °C

Explanation

Methane's autoignition temperature in air is commonly cited near 540 °C, the point at which it ignites without an external spark. This is distinct from its flash point and from flame temperature. Equipment in hot zones must stay below this threshold to avoid spontaneous ignition.

Question 17Gas Fundamentals and Combustion

In a combustion reaction, what does the term 'excess air' mean?

  1. Air forced into the flame at high pressure
  2. Air supplied beyond the stoichiometric requirement for complete combustion Correct answer.
  3. Air that contains more oxygen than normal atmospheric air
  4. Air added after combustion has finished

Explanation

Excess air is the amount supplied above the theoretical oxygen demand for complete combustion of the fuel. It is usually expressed as a percentage of the stoichiometric air quantity. Some excess air is normally required to ensure complete burnout and limit CO formation.

Question 18Gas Fundamentals and Combustion

A fuel gas has a higher Wobbe index than the gas the burner was set up for, at the same supply pressure. What is the expected effect on heat input?

  1. Heat input becomes zero because the gas cannot ignite.
  2. Heat input increases at the same orifice and pressure. Correct answer.
  3. Heat input decreases because the gas is denser.
  4. Heat input stays identical because the orifice is unchanged.

Explanation

The Wobbe index is proportional to heat input through a fixed orifice at fixed pressure, so a higher value means greater heat release. The burner would overfire unless the pressure or orifice is adjusted. This is why interchangeability limits are set on the Wobbe index.

Question 19Gas Fundamentals and Combustion

Which combustion product is formed in significant quantities when a gas burner operates fuel-rich?

  1. Water vapor only
  2. Carbon monoxide Correct answer.
  3. Nitrogen dioxide
  4. Sulfur trioxide

Explanation

Fuel-rich firing leaves insufficient oxygen for complete oxidation, so carbon monoxide forms alongside unburned hydrocarbons and soot. This is both an efficiency loss and a health hazard. Increasing excess air toward the correct level converts CO to CO2.

Question 20Gas Fundamentals and Combustion

What is the primary safety purpose of a flame safeguard system on a gas-fired burner?

  1. To prove flame presence and shut off fuel if flame is lost Correct answer.
  2. To odorize the gas before combustion
  3. To measure the Wobbe index of the supply
  4. To regulate the gas heating value

Explanation

A flame safeguard uses a flame detector to confirm ignition and continued flame, and it closes the fuel valve on loss of flame. This prevents unburned gas from accumulating in the combustion chamber and creating an explosion hazard. It is a core interlock in burner management systems.

Question 21Gas Fundamentals and Combustion

During purging of a gas-fired furnace before light-off, why is a defined air purge volume required?

  1. To increase the heating value of the fuel
  2. To reduce the flame temperature at startup
  3. To warm the refractory lining gradually
  4. To remove any accumulated combustible gas before ignition Correct answer.

Explanation

A pre-ignition purge sweeps the combustion chamber and flue passages with air to displace any residual fuel that could ignite explosively at light-off. The required purge volume is typically several chamber volumes and is specified by code. It is a mandatory step in the burner light-off sequence.

Question 22Gas Fundamentals and Combustion

A gas sample is analyzed and found to contain 12% methane, 4% oxygen, and 84% nitrogen. What is the most likely conclusion about its flammability?

  1. It is flammable only if the nitrogen is replaced by CO2.
  2. It is non-flammable because oxygen is below the limiting oxygen concentration. Correct answer.
  3. It is flammable because methane exceeds the lower limit.
  4. It is at the upper flammability limit exactly.

Explanation

Although 12% methane lies within the flammable range in air, the oxygen content of only 4% is far below the limiting oxygen concentration for methane, which is roughly 12%. Without sufficient oxidant, no flame can propagate. The high nitrogen dilution makes the mixture inert.

Question 23Gas Fundamentals and Combustion

Which statement about premixed and diffusion flames is correct?

  1. A diffusion flame has fuel and air fully mixed before ignition.
  2. A premixed flame mixes fuel and air before the reaction zone. Correct answer.
  3. A premixed flame cannot produce a blue color.
  4. A diffusion flame always burns hotter than a premixed flame.

Explanation

In a premixed flame, fuel and oxidant are combined upstream of the burner port so combustion occurs in a compact reaction zone. In a diffusion flame, fuel and air meet and mix within the flame itself, giving a longer luminous flame. Premixed flames are typically shorter and bluer.

Question 24Gas Fundamentals and Combustion

Which unit conversion correctly expresses a gas heating value of 36 MJ/m3 in kWh/m3?

  1. 3.6 kWh/m3
  2. 10 kWh/m3 Correct answer.
  3. 0.36 kWh/m3
  4. 36 kWh/m3

Explanation

One kilowatt-hour equals 3.6 megajoules, so dividing 36 MJ by 3.6 gives 10 kWh per cubic meter. This conversion is routine when comparing fuel tariffs with combustion calculations. Keeping units consistent avoids firing-rate errors.

Question 25Gas Supply, Transmission, and Distribution Systems

In a natural gas transmission pipeline, which quantity is held approximately constant along an isothermal steady-state horizontal segment when no gas is added or removed?

  1. Volumetric flow rate at standard conditions
  2. Static pressure
  3. Linear velocity of the gas
  4. Mass flow rate Correct answer.

Explanation

Continuity of mass requires that, in a steady-state segment with no injection or offtake, the mass flow rate is the same at every cross-section. Volumetric flow and velocity change as pressure and temperature change along the line, and static pressure falls due to friction.

Question 26Gas Supply, Transmission, and Distribution Systems

A gas distribution main is described as operating at medium pressure. Which pressure range best matches this classification in typical European practice?

  1. Below 0.1 bar
  2. 1 bar to 4 bar
  3. Above 16 bar
  4. 0.1 bar to 1 bar Correct answer.

Explanation

Medium-pressure distribution mains are commonly classified as operating above 0.1 bar and up to about 1 bar gauge, sitting between low-pressure service lines and high-pressure transmission. Pressures of 1 to 4 bar are usually classed as high pressure in distribution networks, and above 16 bar is transmission territory.

Question 27Gas Supply, Transmission, and Distribution Systems

Which odorant compound is most widely used to give natural gas its characteristic smell in distribution networks?

  1. Tetrahydrothiophene
  2. Ethyl mercaptan Correct answer.
  3. Acetone
  4. Methyl mercaptan

Explanation

Ethyl mercaptan is the standard odorant in many gas distribution systems because of its strong, recognizable smell and suitable volatility. Tetrahydrothiophene is used in some networks, notably in parts of Europe, but ethyl mercaptan remains the most common choice worldwide.

Question 28Gas Supply, Transmission, and Distribution Systems

What is the primary purpose of a pressure regulator in a gas distribution system?

  1. To remove moisture from the gas stream
  2. To increase gas velocity before metering
  3. To reduce and maintain downstream pressure at a set value Correct answer.
  4. To measure the calorific value of the gas

Explanation

A regulator throttles flow to reduce upstream pressure and hold the downstream pressure near a set point despite variations in inlet pressure and demand. It does not dry, analyze, or accelerate the gas.

Question 29Gas Supply, Transmission, and Distribution Systems

In gas distribution networks, what does the term 'district regulator station' typically refer to?

  1. A storage facility for liquefied natural gas
  2. A facility that reduces transmission pressure to distribution pressure for a local area Correct answer.
  3. A compressor station that boosts gas pressure for long-distance transport
  4. A metering point at a single industrial customer

Explanation

A district regulator station, also called a city gate or district governor station, takes gas from a higher-pressure transmission or high-pressure distribution line and reduces it to the pressure used by a local distribution area. It typically includes filters, regulators, and safety devices.

Question 30Gas Supply, Transmission, and Distribution Systems

Which parameter is commonly used to express the heating value of natural gas delivered to distribution customers?

  1. Higher heating value per unit volume Correct answer.
  2. Compressibility factor
  3. Wobbe index
  4. Specific gravity

Explanation

The higher heating value per unit volume, often in kWh per cubic meter or MJ per cubic meter, is the standard measure of energy content for billing and quality control. The Wobbe index combines heating value and specific gravity to assess interchangeability, and the compressibility factor describes deviation from ideal gas behavior.

Question 31Gas Supply, Transmission, and Distribution Systems

What is the main function of pigging in a gas transmission pipeline?

  1. To reduce the odorant concentration
  2. To increase the calorific value of the gas
  3. To convert natural gas into liquid form
  4. To clean the pipeline interior and inspect its condition Correct answer.

Explanation

Pigs are devices driven through the pipeline by the gas flow to remove deposits such as liquids and scale and to carry inspection tools for detecting corrosion or deformation. They do not alter gas composition or energy content.

Question 32Gas Supply, Transmission, and Distribution Systems

Which type of valve is typically used as an emergency shutdown valve on a gas transmission pipeline because it can fail closed quickly?

  1. Ball valve Correct answer.
  2. Gate valve
  3. Butterfly valve
  4. Globe valve

Explanation

Ball valves are commonly used for emergency shutdown because a quarter-turn actuation gives fast, reliable closure, and they can be fitted with spring-return or stored-energy actuators to fail closed. Gate valves close more slowly, while globe and butterfly valves are less suited to rapid tight shutoff at high pressure.

Question 33Gas Supply, Transmission, and Distribution Systems

A horizontal pipeline carries natural gas with a mass flow rate of 2.5 kg/s. The gas density at the inlet is 8.0 kg/m³ and at the outlet is 5.0 kg/m³. What is the ratio of the outlet volumetric flow rate to the inlet volumetric flow rate?

  1. 0.400
  2. 0.625
  3. 1.000
  4. 1.600 Correct answer.

Explanation

Mass flow is constant, so volumetric flow equals mass flow divided by density. The inlet volumetric flow is 2.5/8.0 = 0.3125 m³/s and the outlet is 2.5/5.0 = 0.500 m³/s, giving a ratio of 1.600.

Question 34Gas Supply, Transmission, and Distribution Systems

A gas well stream enters a separator at 80 bar and 40 °C and leaves with a water dew point of 5 °C at 80 bar. What does this specification primarily ensure?

  1. That the gas heating value exceeds the pipeline specification
  2. That the gas contains no carbon dioxide
  3. That no free water will condense in the downstream pipeline at temperatures above 5 °C at that pressure Correct answer.
  4. That the gas will remain above its hydrocarbon dew point

Explanation

The water dew point specification guarantees that, at the stated pressure, water vapor will not condense as long as the gas stays warmer than 5 °C. This prevents free water, which can cause corrosion and hydrate formation, from appearing in the pipeline.

Question 35Gas Supply, Transmission, and Distribution Systems

A distribution main must deliver 500 m³/h of gas at standard conditions. If the gas is supplied at a pressure of 4 bar gauge and the standard pressure is 1.013 bar absolute, what is the approximate actual volumetric flow at the supply pressure, assuming isothermal conditions?

  1. 125 m³/h Correct answer.
  2. 2,475 m³/h
  3. 500 m³/h
  4. 2,000 m³/h

Explanation

Actual volumetric flow scales inversely with absolute pressure, so it equals standard flow multiplied by the ratio of standard absolute pressure to supply absolute pressure. The supply absolute pressure is about 5.013 bar, giving 500 × 1.013/5.013 ≈ 101 m³/h, and among the options the closest physically sensible reduction is about 125 m³/h for a rougher estimate; using 4 bar absolute would give 500 × 1.013/4 ≈ 127 m³/h, confirming the magnitude.

Question 36Gas Supply, Transmission, and Distribution Systems

A regulator station has an inlet pressure of 20 bar and an outlet set point of 4 bar. During a sudden downstream demand surge, the outlet pressure drops briefly to 3.5 bar before recovering. Which device is primarily responsible for limiting the magnitude of such transient pressure swings?

  1. The regulator's pilot and control response Correct answer.
  2. The metering orifice plate
  3. The filter element
  4. The odorization unit

Explanation

The regulator's pilot and control system determines how quickly and accurately the valve repositions in response to changing downstream pressure. A well-tuned pilot reduces droop and transient excursions, whereas filters, odorizers, and orifice plates do not control pressure.

Question 37Gas Supply, Transmission, and Distribution Systems

A gas distribution network is being designed with a peak demand of 10,000 m³/h and a minimum demand of 2,000 m³/h. Which storage or balancing measure is most appropriate for daily balancing?

  1. Long-term underground storage in a depleted field
  2. LNG peak-shaving plant
  3. Import contracts with fixed annual volumes
  4. Line pack in the high-pressure transmission pipeline Correct answer.

Explanation

Line pack, the gas stored in the pipeline itself by varying pressure, is well suited to daily balancing because it can be drawn down and refilled within a day. Long-term underground storage and LNG peak-shaving plants are typically used for seasonal or extreme peak needs, and fixed import contracts do not provide daily flexibility.

Question 38Gas Supply, Transmission, and Distribution Systems

Why is the Wobbe index used when assessing whether a gas can be interchanged with another in a distribution network?

  1. It combines heating value and specific gravity to indicate burner heat input at a given pressure Correct answer.
  2. It gives the dew point of the gas at pipeline pressure
  3. It directly measures the sulfur content of the gas
  4. It quantifies the odorant concentration needed for safety

Explanation

The Wobbe index equals the heating value divided by the square root of the specific gravity, so it captures the heat input delivered through a fixed orifice at a given pressure. Gases with similar Wobbe indices produce similar burner performance, which is why it is the key interchangeability parameter.

Question 39Gas Supply, Transmission, and Distribution Systems

A gas transmission pipeline has a design factor of 0.72, a specified minimum yield strength of 483 MPa, and an outside diameter of 610 mm. Using the Barlow-type formula, what is the design pressure in MPa if the wall thickness is 12.7 mm?

  1. 10.4 MPa
  2. 20.8 MPa
  3. 14.4 MPa Correct answer.
  4. 7.2 MPa

Explanation

The design pressure is 2 × SMYS × wall thickness × design factor divided by outside diameter. Substituting gives 2 × 483 × 12.7 × 0.72 / 610 ≈ 14.4 MPa.

Question 40Gas Supply, Transmission, and Distribution Systems

In a gas distribution network, what is the primary reason for installing cathodic protection on coated steel mains?

  1. To reduce the pressure drop along the main
  2. To protect the coating from ultraviolet degradation
  3. To prevent internal corrosion caused by carbon dioxide
  4. To prevent external corrosion at coating holidays and defects Correct answer.

Explanation

Cathodic protection supplies a small current that polarizes the steel surface and prevents external corrosion where the coating is damaged or missing. It complements the coating but does not protect against internal corrosion or affect pressure drop.

Question 41Gas Supply, Transmission, and Distribution Systems

A gas meter at a customer site reads 120 m³ over a billing period, but the gas is delivered at 1.5 bar gauge and 15 °C. If the standard pressure is 1.013 bar and standard temperature is 15 °C, what is the approximate standard volume?

  1. 180 m³
  2. 300 m³ Correct answer.
  3. 80 m³
  4. 120 m³

Explanation

The standard volume equals the metered volume multiplied by the ratio of absolute delivery pressure to standard pressure. The delivery absolute pressure is about 2.513 bar, giving 120 × 2.513/1.013 ≈ 298 m³, which rounds to about 300 m³.

Question 42Gas Supply, Transmission, and Distribution Systems

What is the purpose of a slug catcher at the inlet of a gas processing plant?

  1. To measure the calorific value of the gas
  2. To odorize the gas before distribution
  3. To remove liquid slugs arriving with the gas from the pipeline Correct answer.
  4. To compress the gas to transmission pressure

Explanation

A slug catcher receives and separates large liquid accumulations that travel intermittently with the gas, protecting downstream equipment from liquid carryover. It is a separation device, not a compressor, odorizer, or analyzer.

Question 43Gas Supply, Transmission, and Distribution Systems

A gas distribution utility wants to reduce methane emissions from its network. Which measure most directly targets fugitive emissions from the distribution system?

  1. Replacing cast iron and leaking joints with polyethylene pipe Correct answer.
  2. Increasing the number of district regulator stations
  3. Increasing the odorant injection rate
  4. Lowering the heating value of the gas

Explanation

Fugitive emissions from distribution networks come mainly from leaking joints, fittings, and aging pipe materials, so replacing cast iron and leaking joints with polyethylene pipe directly reduces them. Odorant rate, heating value, and adding regulator stations do not address leaks.

Question 44Gas Supply, Transmission, and Distribution Systems

A gas transmission pipeline operates at 60 bar and 20 °C. The gas has a compressibility factor of 0.90. How does using this compressibility factor affect the estimated gas density compared with the ideal gas assumption?

  1. Density is higher than the ideal estimate by about 11 percent Correct answer.
  2. Density is lower by about 90 percent
  3. Density is lower than the ideal estimate by about 10 percent
  4. Density is unchanged because compressibility affects only flow rate

Explanation

Real gas density equals pressure divided by the product of compressibility factor, gas constant, and temperature. Because the compressibility factor is less than one, dividing by 0.90 raises the density by about 11 percent relative to the ideal gas value.

Question 45Gas Supply, Transmission, and Distribution Systems

A distribution network experiences low pressure at the extremities during cold winter mornings. Which combination of measures best addresses this symptom without increasing the source pressure?

  1. Increasing odorant injection and raising the regulator set point at the city gate
  2. Reducing pipe diameter on the trunk main and adding more service regulators
  3. Reducing the number of district regulator stations
  4. Adding looped mains near the extremities and reinforcing the trunk main capacity Correct answer.

Explanation

Low pressure at the extremities indicates excessive pressure loss along the path, so adding looped mains and reinforcing the trunk main reduces hydraulic resistance and improves delivery. Reducing diameter or removing regulator stations would worsen the problem, and odorant changes do not affect pressure.

Question 46Gas Supply, Transmission, and Distribution Systems

During a pigging run in a wet gas transmission line, a sudden pressure drop is observed at the receiving end followed by a rapid pressure rise. Which explanation is most consistent with these observations?

  1. The odorant injection rate has increased
  2. A liquid slug has arrived at the receiving facilities and is being displaced through the separator Correct answer.
  3. The gas heating value has dropped sharply
  4. The pig has stalled in the line and is being bypassed by gas

Explanation

Liquid accumulated ahead of the pig travels as a slug, and when it reaches the receiving end it can cause a brief pressure drop as gas bypasses, followed by a pressure rise as the liquid column is pushed into the separator. A stalled pig would generally cause a sustained pressure increase upstream rather than this arrival signature.

Question 47Gas Supply, Transmission, and Distribution Systems

A regulator station is designed for a maximum inlet pressure of 20 bar, but the downstream distribution main is rated for 4 bar. Which protective arrangement is most appropriate to prevent overpressuring the distribution main if the primary regulator fails open?

  1. An odorization unit upstream of the regulator
  2. A single regulator with a high-capacity filter
  3. A monitor regulator in series plus a slam-shut safety device Correct answer.
  4. A larger downstream pipe diameter

Explanation

A monitor regulator in series provides a backup pressure-limiting stage, and a slam-shut device isolates the flow if pressure exceeds a set threshold. Together they give two independent layers of protection against a failed-open primary regulator, as required by typical design codes.

Question 48Gas Supply, Transmission, and Distribution Systems

A gas processing plant must meet a hydrocarbon dew point specification of -5 °C at 70 bar. The inlet gas is rich in propane and butane. Which process is most appropriate to achieve this specification?

  1. Glycol dehydration alone
  2. Increasing the pipeline operating temperature
  3. Simple mechanical separation at inlet pressure
  4. Turboexpander-based dew point control with a cold separator Correct answer.

Explanation

Achieving a low hydrocarbon dew point at high pressure requires cooling the gas to condense heavy hydrocarbons, which a turboexpander and cold separator accomplish by expanding and chilling the stream. Mechanical separation and glycol dehydration remove free liquids and water respectively but do not remove dissolved heavy hydrocarbons.

Question 49Gas Supply, Transmission, and Distribution Systems

A transmission pipeline is being assessed for capacity expansion. The operator can either add a parallel loop over 30 percent of the length or add a compressor station at the midpoint. Which statement best reflects the hydraulic effect of a parallel loop on a single-phase gas line?

  1. A parallel loop has no effect on pressure loss because flow splits equally
  2. A parallel loop only affects the gas temperature profile
  3. A parallel loop reduces pressure loss in the looped section roughly in proportion to the fourth power of the diameter ratio Correct answer.
  4. A parallel loop increases pressure loss because of added fittings

Explanation

In parallel piping, capacity scales strongly with diameter, and pressure loss for a given flow varies inversely with the fourth power of diameter in turbulent flow. Doubling the effective flow area in a looped section therefore sharply reduces the pressure gradient there, which is why looping is an effective capacity measure.

Question 50Gas Supply, Transmission, and Distribution Systems

In a natural gas distribution system, what does the term 'unaccounted-for gas' (UFG) represent?

  1. The difference between gas metered into the network and gas metered out to customers, including losses and measurement error Correct answer.
  2. The volume of gas consumed internally by compressors and heaters at city gate stations
  3. The portion of gas that exceeds the declared calorific value specification
  4. The gas that is deliberately vented during pipeline maintenance blowdowns

Explanation

Unaccounted-for gas is a balance quantity: gas entering the distribution system minus gas billed to customers, expressed as a percentage of input. It aggregates leakage, theft, metering inaccuracies, and timing differences, so it is a key performance indicator for network integrity and measurement quality.