Building Services Engineering: Gas Installation and Pipe Sizing

When we design and install gas systems in Ireland we are dealing with a fuel that delivers reliable energy for heating, cooking and industrial processes, yet also carries real risks if we get the details wrong. Our job as mechanical engineers is to understand the behaviour of the gas itself, the legal rules that govern every installation, the way pressure and flow interact inside pipes, and the practical safety measures that protect people and property. This article walks you through each of these areas in clear steps so that you can size pipes correctly, select the right equipment and produce installations that are both efficient and fully compliant.

1. Gas Properties and Thermodynamics

European standard EN 437 places gases into three families according to a single number called the Wobbe index. This index tells us how much thermal energy a burner will receive when gas flows through a fixed orifice under a fixed pressure. Ireland works almost exclusively with the second and third families.

• The first family covers old manufactured town gas. You will almost never encounter it on modern projects.
• The second family is natural gas, which is rich in methane. Ireland uses Group H gas within this family.
• The third family is liquefied petroleum gas, usually propane or a propane-butane mix.

We measure the energy locked inside gas in two different ways, and the distinction is important for both billing and design. Gross calorific value (sometimes called higher heating value) includes every joule released when the gas burns, including the latent heat that is tied up in the water vapour produced during combustion. Gas network billing and the volumetric calculations we use for pipe sizing both rely on this gross figure. Net calorific value (lower heating value) deliberately leaves out that latent heat. Appliance data plates that carry CE or UKCA marking always state the heat input on a net basis.

Because our pipe-sizing calculations need the gross figure, we convert the net value shown on the appliance. For natural gas we multiply the net value by approximately 1.11. For propane we multiply by approximately 1.08.

Consider a practical example. A non-condensing boiler sends the water vapour straight out of the flue, so the latent heat is lost. Its gross efficiency typically sits around 80 %. A modern condensing boiler keeps the return water temperature below 55 °C. At that temperature the water vapour condenses inside the heat exchanger and the latent heat is recovered. Gross efficiency then rises to about 90 %. You still pay the supplier for the same number of gross kilowatt-hours, yet the building extracts far more useful heat from each unit of gas.

The model below splits the gross energy three ways: the heat that stays in the building, the latent heat that leaves as vapour, and the sensible heat that goes up the flue. Move the return water temperature through 55 °C and watch the latent share change hands.

Relative density (also called specific gravity) compares the mass of a cubic metre of gas with the mass of a cubic metre of air. Air has a molar mass of roughly 28.97 g/mol. Natural gas has a molar mass near 16.04 g/mol, so its specific gravity is about 0.6. Because it is lighter than air it rises, and we must provide high-level ventilation so that any leak can escape safely. Propane has a molar mass near 44.1 g/mol, giving a specific gravity of about 1.52. It is heavier than air and will pool in trenches, floor voids and drains. That single difference dictates where we place ventilation openings and gas detectors.

A leak does not disperse evenly through a room. It climbs to the ceiling or it sinks to the floor, and it stays there. Move the detector in the model below and see how easily a correctly specified head can be mounted in the wrong layer.

The Wobbe index itself is defined by the simple relationship

where $CV$ is the calorific value and $d$ is the relative density. Two gases that share the same Wobbe index will deliver identical thermal power to a burner without any change of nozzle size or inlet pressure. This is why the index controls appliance interchangeability across Europe.

The reason sits in the flow through the injector. Volume flow through a fixed orifice varies with the square root of the pressure divided by the density, and the heat carried is that volume times the calorific value. Put the two together and the heat input works out as the Wobbe index times the square root of the inlet pressure, times a constant for the jet. The calorific value on its own tells you nothing about what the burner will do.

To convert a boiler’s heat input into the volumetric flow rate needed for pipe sizing we use the local calorific values. For natural gas the gross calorific value is approximately 10.76 kWh per cubic metre, so

gives the required flow in cubic metres per hour. For propane the corresponding figures are 13.9 kWh per kilogram on a mass basis or 26.2 kWh per cubic metre on a volume basis.

All of these numbers refer to the Irish reference conditions of 15 °C and 1013.25 mbar. When the actual pressure or temperature at the site differs we correct the volume using the ideal-gas law

This correction ensures that the pipe we size will still deliver the correct mass of gas under the real operating conditions.

2. The Irish Legal Framework

Every gas installation in Ireland sits inside a clear legal framework. Technical Guidance Document J of the Building Regulations deals with combustion air supply, flue design and the safe storage of fuel. Technical Guidance Document B addresses fire compartmentation. It tells us how to protect gas risers, how to sleeve pipes that pass through fire-resisting walls and floors, and how to maintain the integrity of fire compartments.

The Commission for Regulation of Utilities runs the Registered Gas Installer scheme. Only an RGI is permitted to carry out Designated Gas Works on domestic systems and, increasingly, on commercial systems. Working outside this scheme is illegal and leaves the building owner without a valid Declaration of Conformance.

Statutory Instrument 201 of 1990 applies as soon as LPG storage exceeds 70 kg or 160 litres. It sets minimum separation distances from buildings, boundaries and ignition sources, and it requires Fire Authority licensing once the quantity becomes large.

Statutory Instrument 209 of 2015 implements the European Seveso III (COMAH) rules. Lower-tier LPG sites begin at 50 tonnes and upper-tier sites at 200 tonnes. These thresholds trigger detailed safety reports and emergency planning.

Part 8 of the General Application Regulations 2007 (the ATEX rules) requires us to classify any area where gas may be present into hazardous Zones 0, 1 or 2 and to prepare a formal Explosion Protection Document for meter rooms, plant rooms and LPG compounds. The classification decides the type of electrical equipment and the ventilation rates we must provide.

3. System Architecture and Pressure Tiers

Gas travels through a clear sequence of components. It leaves the high-pressure transmission network, enters the local distribution main, then travels along the service pipe to the primary emergency control valve. From there it passes through the regulator and meter, reaches the point of delivery, continues along the installation pipework and finally arrives at the appliance.

We work with three standard pressure tiers:

• Low pressure is 75 mbar or less. The meter outlet for natural gas is normally set at 21 mbar.
• Medium pressure runs from 75 mbar up to 2 bar.
• Intermediate pressure covers the range from 2 bar to 7 bar.

The model below draws that journey as a staircase on a logarithmic scale, for natural gas and for a two-stage LPG supply. Notice how much of the fall happens at one component, and how little pressure is left for everything after the meter.

Irish Standard I.S. 820 governs non-domestic installations up to defined pressure limits: 5 bar for commercial buildings or buildings with public access, and 0.5 bar for industrial premises. Once those limits are exceeded the design must follow the more rigorous requirements of I.S. EN 15001.

Meter housing for heavy commercial loads follows strict rules set by Gas Networks Ireland. The meter room must sit on the ground floor, its doors must open directly to outside air and be fitted with panic hardware, the walls and floor must provide at least 60 minutes of fire resistance, permanent high- and low-level ventilation that cannot be closed must be provided, and every electrical fitting inside the room must be ATEX certified.

4. Pipe Sizing Mechanics

When we calculate the load we treat heating boilers and combined heat and power plant as 100 % simultaneous. On a cold winter morning every boiler will fire together, so diversity factors do not apply. Kitchen equipment and multi-occupancy dwellings do allow diversity, yet we still add a 10 to 20 % margin so that future expansion does not force us to replace the pipework.

For low-pressure systems we use Pole’s formula, which treats the gas as incompressible:

In this expression $Q$ is the volumetric flow in cubic metres per hour, $c$ is Pole’s constant (0.0071 when we work in metric units), $\Delta P$ is the allowable pressure drop in millibar, $d$ is the internal diameter of the pipe in millimetres, $S$ is the specific gravity of the gas, and $L_{\text{eq}}$ is the equivalent length that includes the actual pipe length plus an allowance for every bend, tee and valve.

I.S. 820 sets two clear design limits. On a natural-gas low-pressure system the pressure drop from the meter outlet to the appliance isolation valve must not exceed 1.0 mbar. This ensures that the burner still receives at least 20 mbar. On an LPG low-pressure stage the drop from the second-stage regulator to the appliance must stay inside 2.5 mbar. Velocity is also limited: 15 m/s for unfiltered gas and 20 m/s for filtered gas downstream of the meter. Higher velocities create noise and can erode the internal surface of the pipe over time.

Here is the whole calculation worked through on a single load. A 300 kW boiler plant on natural gas needs 300 divided by 10.76, which is 27.9 m³/h. The measured run from the meter to the plant room is 25 m, and the bends, tees and valves add 20 %, so the equivalent length is 30 m. Turning Pole’s formula round to give the diameter,

which returns 48.8 mm. The next standard size up is 54 mm copper at 51.6 mm bore. That pipe drops 0.76 mbar and runs at 3.7 m/s, so it passes both limits with room to spare. The size below it, 42 mm copper at 39.6 mm bore, drops about 2.9 mbar. The burner would see 18 mbar instead of 20, and the plant would not make its output on the morning it matters.

5. LPG Specifics: Storage and Vaporization

Liquid propane expands by a factor of roughly 250 when it turns into gas. Its boiling point of -42 °C means it continues to vaporise reliably even in the coldest Irish winters. We never fill a storage tank beyond 85 % of its water capacity so that the liquid has room to expand if the ambient temperature rises.

The basic clearance rule is 3 metres from any building, boundary, drain or ignition source. When a solid fire wall at least 2 metres high and offering 30 minutes of fire resistance is provided on one side, that clearance may be reduced to 1.5 metres on the protected face.

Vaporisation capacity is limited by the amount of heat the tank can absorb from the surrounding air. If the gas demand exceeds that heat transfer rate the liquid cools, frost forms on the tank surface and the pressure collapses. A 150 kW boiler needs approximately 10.8 kg of propane every hour. At 0 °C and with the tank only 25 % full, a 1 200 litre vessel can supply only about 8.5 kg/h. The boilers will lock out. In that situation we must either specify a larger 2 000 litre tank (which can yield about 14 kg/h under the same conditions) or install an active liquid vaporiser.

The heat has to cross the shell wherever liquid touches it, so the wetted area is the real heat exchanger. Run the level down and that area shrinks, which is why the problem arrives near the end of a cold week rather than at the start of it.

Two-stage pressure regulation is the normal arrangement for LPG. The first-stage regulator, mounted at the tank, reduces the tank pressure of 4 to 10 bar down to a medium pressure of typically 1.0 to 1.5 bar. The second-stage regulator, located at the building, then drops that medium pressure to the final low-pressure level of 37 mbar that the appliances require. From the second-stage regulator onwards the allowable pressure drop is strictly limited to 2.5 mbar.

6. Safety Systems and Ventilation

In commercial kitchens the European standard EN 16282 requires a positive interlock between the gas supply and the ventilation system. A solenoid valve in the gas line may open only when both the extract fan and the make-up air fan are proved to be running, either by differential pressure switches or by current monitors. If airflow stops, the gas supply is shut off automatically.

We position gas detectors according to the density of the gas we are protecting against. Natural-gas detectors are placed high because the gas rises. LPG detectors sit between 150 mm and 300 mm above the finished floor because the gas sinks. Carbon-monoxide detectors that comply with EN 50291 are mounted at breathing height or on the ceiling, at least 300 mm clear of walls and between 1 m and 3 m horizontally from the appliance.

Plant rooms need permanent high-level and low-level louvres that cannot be closed. Vertical risers that carry gas pipes must be ventilated to outside air so that any leak cannot accumulate into an explosive mixture inside a confined void. All exposed gas pipework is painted ochre or yellow, and every emergency control valve carries a clear, durable sign that explains how to isolate the supply.

7. Testing, Purging and Commissioning

We always carry out the three pressure tests in a fixed sequence. The let-by test first proves that the main emergency control valve is completely tight and does not allow any pressure to build up on the downstream side. Next the strength test raises the pressure to between 1.5 and 2.5 times the maximum operating pressure using air or nitrogen. This test confirms the structural integrity of the pipework before any of it is concealed. Finally the tightness test is performed at the normal operating pressure (for example 21 mbar for natural gas). Stabilisation times and test hold periods increase with the volume of the installation.

Purging must never leave a combustible mixture inside the pipe. We calculate the physical internal volume of every pipe and meter, then multiply by a dispersion factor of 1.5 to 2.0 to obtain the required purge volume. Gas is then introduced so that it pushes the air out. The displaced mixture is either flared or vented safely outdoors until instruments confirm that the concentration has passed right through the flammable range and reached at least 90 % gas by volume.

When the installation is complete the handover package must contain the RGI Declaration of Conformance, the certificates that record the exact gauges used and the hold times achieved during testing and purging, and a complete valve schedule that identifies every isolation point.

By taking the time to understand each of these principles and by applying them carefully on every project, you will produce gas installations that are safe for the occupants, efficient in their use of energy, and fully compliant with Irish regulations and standards.