What is LPG

LPG is a gas in normal conditions but is stored in cylinders under pressure as a liquid. LPG is no longer limited to conventional petroleum-based sources. Renewable alternatives like Bio LPG and rDME (Renewable dimethyl ether) are already being utilized and tested. These renewable options offer a greener and sustainable approach to LPG production.

There are two basic types of LPG, Propane and Butane. The difference in their properties means that each is particularly suited to specific uses.

The chemical composition of Propane is C3H8, and Butane is C4H10. The larger Butane molecule gives both gases differing properties:

  • Propane’s lower boiling point suits outdoor storage and is primarily used for central heating, cooking, and numerous commercial applications.
  • Butane, on the other hand, doesn’t work in colder conditions and is best used indoors. It’s perfect for powering indoor portable heaters or used in warmer months for outdoor camping and cooking.
  • Butane, having a larger molecule, when burnt in air dissipates more heat than propane.

LPG combustion produces fewer harmful emissions compared to other fuels. With its lower carbon content, LPG emits significantly fewer greenhouse gases, particulate matter, and sulphur dioxide, leading to cleaner air and reduced environmental impact.

LPG is compressed under pressure into portable cylinders or road tankers, which in turn fill static bulk tanks at point of use. When the gas is compressed, it easily changes to a liquid state, allowing a large amount of gas to be contained in a relatively small space. These storage and distribution methods enable efficient transport and minimize energy losses during the supply chain.

Likewise, the reverse process occurs when decompressed or allowed to escape the cylinder or pressure vessel. The liquid gas boils and quickly reverts to its gaseous state, so LPG is often called a vapour.

Gas offtake decreases the gas pressure within the pressurised container, this pressure is regenerated by liquid vaporisation. The vaporisation needs energy from heat, and this is provided by the ambient air. This gives energy to the vessel to maintain the pressure, but the heat exchange is not perfect, as the temperature of the liquid starts to decrease, consequently the pressure decreases.

Therefore, cylinders and bulk tanks have recommended offtake rates of vapour withdrawal to ensure that the ambient air is sufficient to heat the pressurised container, maintaining the required force to power the downstream gas appliances.

The larger the pressurised container, the greater the surface area in contact with the liquid LPG, and the more vapour can be generated.

When stored in a suitable pressurised tank or cylinder, LPG will remain liquid, occupying only a small space. Release some of the liquid and it will immediately boil and revert to its gaseous state, expanding as it does so. Liquid Propane expands to 250 times its liquid volume when reverting to gas. The reverse applies, as gaseous LPG will ‘shrink’ by 250 times when compressed and liquefaction takes place.

Inside the cylinder or tank the liquid is boiling, releasing vapour as it does so, just like boiling water releases steam, albeit at a much cooler temperature. Propane will do this right down to minus 42°C, which ensures that it will vaporise in all but the coldest climates and conditions.

Butane, however, will only vaporise readily from temperatures 0 – 2 °C, so when stored outdoors in the winter, appliances will need a propane supply. During warmer months, butane can also be utilized for outdoor activities like camping and cooking, providing a cleaner alternative to traditional fuel sources.

Commercial propane contains some butane, which occasionally remains as a liquid in containers during sustained cold weather conditions. Propane, having a lower boiling point, preferentially evaporates first. Only when the remaining butane liquid warms up will it start to produce pressure. This can sometimes be seen when an automatic changeover switches to reserve cylinder bank before all the contents has emptied normally in very cold weather conditions. However, once the cylinder has warmed, the residual butane will evaporate providing sufficient pressure to pass through the regulator to the appliances.

By choosing LPG as an alternative to traditional fossil fuels, individuals and businesses can play a part in mitigating climate change and promoting a greener and more sustainable future.

Commercial LPG Properties

Technical Typical Properties Of Commercial LPG Grades Commercial Propane Commercial Butane
Gas: Air ratio for combustion 1:24 1:30
Flame temperature in air max. *C 1930 1900
Flame Speed cm/sec 44 44
Relative Density of liquid at 15.6*C (Water at 0*C = 1.0) 0.51 0.58
Litre/tonne at 15.6*C 1975 1742
Relative Density of gas at 15.6*C (Air at 15.6*C = 1.0) 1.52 2.01
Volumes of gas (litres) per kg of liquid at 15.6*C 537 411
Ratio gas: liquid volume at 15.6*C 279 238
Boiling Point at 1 atm. *C -45 -2
Vapour Pressure-typical bar g at 0*C / 15.6*C 3.8 / 6.4 0.5 / 1.6
Limits of flammability (percentage of gas by vol. In gas-air mixture) Upper / Lower 10 / 2 9 / 1.8
  • LPG exists as a gas at normal atmospheric pressure & temperatures but may be liquefied by the application of moderate pressure. If the pressure is released the liquid will revert to vapour.
  • LPG as a liquid is colourless, and as a vapour cannot be seen.
  • Pure LPG has no distinctive smell, so for safety reasons a stenching agent is added during production to give a pungent, unpleasant smell and so aid detection.
  • LPG is non-toxic, but at very high concentrations in air, LPG vapour acts as an anesthetic and subsequently an asphyxiate by diluting or decreasing the available oxygen.
  • When LPG is mixed with air, a highly flammable mixture is produced. The flammability range is between 2% to 11% by volume of gas to air. Outside this range any mixture is either too weak or rich to potentially ignite.
  • One volume of liquid will produce approximately 250 times that volume of gas vapour.
  • LPG vapour is heavier than air. Any escape will find its way to the lowest level where it can remain and form a flammable mixture. Therefore, LPG vessels must be sited away from drains and appliances must not be sited in basements or cellars. Cylinders in boats and ships must be stored in purpose built sealed gas lockers.
  • LPG is lighter than water and floats on top of it in a similar way to oil and petrol. Therefore, LPG vessels must be sited away from drains and gullies.
  • The pressure LPG exerts on a vessel varies with ambient temperature. The higher the temperature of the liquid, the higher the vapour pressure. Conversely, the lower the temperature, the lower the pressure. This means LPG must be protected from heat sources and protective safety distances imposed on the siting and storage of LPG.
  • Commercial Propane has a vapour pressure of approximately 6bar (87psi) at 15°C
  • Commercial Butane has a vapour pressure of approximately 2bar (30psi) at 15°C
  • Because of these characteristics, Commercial Butane can be used indoors, and Commercial Propane must only be used outdoors.
  • When LPG is heated it expands very rapidly. To allow for expansion, LPG cylinders and tanks are only filled to a maximum of 87% of the total volume of the retaining vessel.
  • The boiling point is the temperature below which LPG will not vaporise to form a gas vapour. Boiling point of Commercial Propane is approximately –42°C. Boiling point of Commercial Butane is approximately – 2°C
  • Commercial Butane can be affected by cold weather, resulting in poor pressure, and should not be used outdoors in winter months. Commercial Propane is not adversely affected by cold weather in the UK and is an ideal fuel source for heating, cooking, and industrial applications.
  • LPG in both its liquefied and gaseous state has a very low viscosity and will flow easily like water, petrol etc. This means they will penetrate any breaks or weakness in the installation. Therefore, special jointing compounds must be used for LPG installations which are certified for use with the service conditions, such as Clessetite.
  • LPG is aggressive to certain non-metallic materials like natural rubber and many plastics; therefore, equipment and hoses must be suitable for LPG. Clesse use only the best rubber hoses from certified European manufacturers.
  • The Calorific Value of a fuel is described as “The amount of heat released when a known quantity of fuel is burned”. Commercial Propane = 95 MJ / m3. Commercial Butane = 121 MJ / m3. Natural Gas = 38 MJ / m3.
  • Because LPG appliances release more heat than Natural Gas, it is important that any gas appliances fueled by LPG are designed and manufactured for that purpose.

Fuel / Air Mix
Commercial Propane = 23:1
Commercial Butane = 30:1
Natural Gas = 9.6:1

It is important that appliances fuelled by LPG are provided with adequate ventilation and serviced regularly to ensure that they burn efficiently.

Flow Rates

Flow rate is indicated as kg/h (kilograms per hour) as required by EN standards. This equates to the mass of liquid used per hour in a gas cylinder or bulk tank and can be easily converted to kilowatts (kW) by multiplying by 13.9 or dividing the kW on the appliance data plate by 13.9 (these figures are for propane gas).
Clesse declares the flow rate at the lowest inlet pressure (worst case) to the regulator to ensure that consumers are covered in most climatic conditions. Be careful when comparing with other manufacturers or suppliers, who declare higher flow rates but omit the inlet pressure or outlet pressure drop to achieve this result.

Installation
Installation must be carried out in accordance with the instruction leaflet that accompanies the regulator. Where applicable, all installations, adjustments, and maintenance work must be carried out by persons who have the necessary skills to do so and are qualified according to the regulations in force for that particular installation.

Maintenance and Durability
Normally Clesse regulators do not require any maintenance. Functionality of the product and of the gas installation must be checked periodically, and according to any maintenance schedule or rules governed by the regulations in force for that particular installation. The recommended replacement period of a regulator by Clesse UK Limited is normally 10 years.

The contents of this User Guide, advice, and reference section are provided for guidance purposes only and. Although we have taken every precaution to ensure its accuracy, it should not be interpreted as representing any explicit or implicit guarantee covering the products or services described, or their use or applicability. We reserve the right to modify or improve the designs or the specifications of the products at any time and without notice.

Rubber Material and Gas Quality
All Clesse regulators are built to withstand the use of LPG (liquefied petroleum gas) in vapour phase, NG (natural gas), air, and nitrogen. In order to ensure correct operation and a long life expectancy, the LPG gases be sufficiently pure, and should contain no aggressive components. It is important that a bulkhead mounted regulator is fitted with short hoses the regulator inlet is above the cylinder valves to ensure any LPG condensate (vapour gas reliquefying in the hoses) returns into the cylinders and not the regulator. Always purchase your gas from reputable gas distributors.

Gas Types & Vaporisation Cylinder Offtake

LPG (Liquefied Petroleum Gas) contained in cylinders is supplied either as Butane or Propane. Commercial Butane contains approximately 80-90% Butane, whilst commercial Propane contains approximately 90% propane. The remainder will be made up of other hydrocarbons (Pentane, Hexane etc.) and the other LPG type (for example, Commercial Propane will contain an amount of Butane and vice versa).

Large quantities of flammable vapour can be produced from relatively small amounts of liquid LPG stored in cylinders and bulk tanks; this makes LPG an ideal portable fuel. Cylinders must always remain upright to ensure only vapour exits the cylinder valve before it enters the regulators.

At atmospheric pressure Butane boils at -2°C and Propane boils at -45°C

Contained in cylinders or bulk tanks, LPG remains a liquid under pressure, and this pressure depends on the type of LPG and the ambient temperature surrounding the vessel. The chart shows the vapour pressure within a gas cylinder at various ambient temperatures.

Vaporisation in the cylinder
In a cylinder, LPG is liquid at the bottom and vapour under pressure at the top. When there is an off take of gas, the gas volume is regenerated by boiling off the liquid part. This vaporisation cools down the liquid and heat is required to continue to boil the surrounding LPG, which is taken from the surrounding air or ambient temperature in contact with the cylinder.

Propane delivers high pressure at colder temperatures and is vaporised faster, so is used and stored outside. Butane is used mainly for indoor or summer use.

During off-take, first cylinder temperature decreases and then cylinder pressure decreases. When only a small amount of liquid remains in the cylinder, the pressure is significantly lower than when the cylinder was full. On the same principle, the larger the cylinder, the greater the surface area, the more vapour can be produced.

Maximum flow rate depends on:

  • Type of gas
  • Level of liquid in the cylinder
  • Ambient temperature
  • Usage time
  • Dimension and material of the cylinder
  • Number of cylinders

Cylinders can only supply a certain rate of vapour and must be sized to meet the heat input of the appliances. Often an overlooked part of an LPG installation is the correct sizing and quantity of cylinders. Taking this into account will ensure that the cylinder regulator delivers the correct pressure, or an Automatic Changeover uses the full contents of a cylinder before selecting the reserve cylinder.

At Clesse, the capacity we declare is normally “worst case scenario” to ensure the regulator or ACO operates in both very cold or hot climatic conditions and with low cylinder contents. This means in most cases, our regulators are “understated” on flow rate.

Off-take rates for cylinders are typically as indicated below and based on a continuous off take rate. Some installations will require more than one cylinder, so Automatic Changeovers are used.

When supplying a cooker (12kW), boiler (28kW) and fire (14kW), total load is 54kW. Therefore, 2x47kg cylinders combined will need to be used. An ACO will need to be used with total 4 cylinders (2 per side). Compact 800 @ 5kg/h supplying 70kW at 37mb outlet pressure would be ideal.

Cylinder size (kg) Offtake rate (kg/h) Offtake rate kW (kilowatt)
Butane 15 0.70 9.70
Propane 13 1.05 14.60
19 1.32 18.35
47 2.37 32.94

Note: 1 kg/h Gas Flow rate = 13.9kW = 47,500BTU/h

Stages of Regulation

In an LPG installation, there can be 1, 2. or 3 pressure regulation stages.

Single stage
The regulator or the automatic changeover is directly connected to the cylinder or tank and reduces the pressure from the vessel directly down to the appliance pressure.

First stage
The regulator or the automatic changeover is directly connected to the cylinder or tank and reduces the pressure from the vessel down to an intermediate pressure, for a second stage regulator application.

Second stage
The regulator reduces the pressure from the intermediate pressure down to the appliance pressure, or to a third stage intermediate pressure.

Third stage
The regulator reduces the pressure from the second stage regulator pressure down to the appliance pressure, normally associated with 3 stage bulk tank installations.

High or Low Pressure Regulators
For LPG installations the regulators families are generally defined as follows:

High pressure regulators
Regulators delivering an outlet pressure (fixed or variable) higher than 500mbar. These are defined as Single or First stage regulators, generally up to a maximum of 4 bar.

Low pressure regulators
Regulators delivering an outlet pressure (fixed, adjustable or variable) lower than 500mbar. These are defined as Single, Second, or Third stage.


Automatic Changeovers
Automatic changeover devices are used with 2 or more Cylinders.

The first cylinder or cylinder bank is called ‘service’ the second is called ‘reserve’. The Automatic Changeover firstly draws the gas from the “service” cylinder. When the ‘service’ cylinder is empty, or when its vaporising capacity is not sufficient (for instance, high flow rate during a long time, use of butane-propane mixture, low temperatures, low level in the cylinder, etc.) it automatically changes to and takes the main flow from the ‘reserve’ cylinder. An indicator on the device shows that the “service” cylinder is empty. By using an Automatic Changeover device, you can guarantee continuous flow of gas without fear of interruption of service to the appliance, maximizing the amount of gas useable in the cylinders.

There are 2 types of Automatic Changeover Device:

  • High Pressure versions delivering a first stage intermediate pressure of 1, 1.5, and 3 bar for higher capacity and specialist applications, for poultry and game rearing brooder heaters, or remote cylinder installations.
  • Low Pressure versions which are equipped with an integral second stage pressure regulator, providing a single stage outlet pressure at 37mb.

Inlet pressure
The inlet pressure can be expressed in minimum and maximum values. When supplied within the declared range of inlet pressure, the regulator can deliver a stable outlet pressure and flow rate declared on the data label. For LPG first or single stage regulators, the maximum inlet pressure is 16bar, to ensure the correct flow rates are achieved as defined in the EN standard.

Outlet Pressure
The outlet pressure can be:

Fixed: Outlet pressure is factory pre-set. The types of regulators with this function are sealed units with no possible chance of adjustment. Typical examples would be certain first stage intermediate pressure regulators delivering a fixed 0.75bar or 1bar, or single stage low pressure regulators delivering a fixed 37mb.

Adjustable: The outlet pressure is factory set but it can be readjusted internally by removing the cap on the regulator diaphragm lid. The cap can be hand screwed or opened with a cap removal tool. Compact 100 and Compact 800 automatic changeovers are adjustable via an allen key (4mm) underneath a push in grommet on the front of the regulator.

Variable: The outlet pressure can be set using external adjustment methods built onto the regulator, either as a Multi-turn T-Bar, a knob with hexagonal locking nut, or a single gradient turn 1-10 position head which is not lockable.

Examples include: 20-300mbar means a variable setting can be achieved, minimum setting 20mbar, maximum setting 300mbar.

0.75 (0.5 – 2) bar: The regulator has been pre-set at the factory to the un-bracketed figure but can be engineer adjusted between the bracketed figure, so long as there is a minimum inlet pressure of at least 0.5 bar above that required setting.

How an Automatic Changeover Works

Cylinders from one group ‘Service’ are used in preference to the other ‘Reserve’ cylinder group. When the ‘Service’ group is empty, an indicator turns red, showing that the ‘Service’ group of cylinders are empty or near to being empty. At this point, the Clesse changeover automatically starts to draw gas from the ‘Reserve’ group, ensuring uninterrupted gas supply to the user.

An order of gas can then be made whilst the “reserve” group keeps the supply running. When the new cylinders have been fitted, the driver will turn the indicator knob, making these the reserve whilst the existing cylinders run empty.

  • When first installed select which cylinder(s) you want to be in service.
  • When the indicator shows red continuously, it’s time to call your gas supplier.
  • When the empty cylinders are changed, the person who exchanges the cylinders will turn the indicator knob to the existing part-used cylinders, ready to indicate when they become empty; these then becomes the ‘Service’ group.
  • Always install the changeover regulator above the cylinders. This ensures that your Clesse regulator does not fill with condensed LPG product, giving longer service life.

Safety Devices

Pressure Relief Valve
A PRV is a safety device for the relief of excess pressure. Excess pressure can result from:

Thermal expansion of trapped gas after the regulator. LPG vapour has a large expansion coefficient and is affected by changes in temperature. When associated with an OPSO system, the PRV prevents nuisance tripping of the shut off.

Creeping lock up pressure due to dirt between the regulator seat and the seat pad. A PRV is the first line of safety for downstream appliances.

The type of PRV used in Clesse regulators are Limited Relief valves. This type of relief valve discharges a low flow (less than 10% of nominal regulator flow rate) and deals with excess pressures resulting from the situations above.

The relief valves will be either vented through small, concealed holes in the regulator, or in some cases will have an external vent hole that can be connected to a pipe to be vented away from the regulator position.

There is an additional PRV style on certain regulators that will be able to be positioned in multiple locations dependent on the installation, these are called Giro Vents.

Overpressure Shutoff (OPSO)
The OPSO safety device cuts off the gas flow in the event of abnormal overpressure, for example, an operating fault in a regulator (impurities on the valve seat or deterioration of a part), a defect in installation, or liquid LPG entering the regulator. OPSO safety devices can be fitted to high or low pressure regulators.

OPSO is the last line of defence to ensure gas pipework and appliances are not subjected to high pressures above and beyond manufacturer’s limits.

If an OPSO device continues to trip, DO NOT continue to use, and get a qualified gas installer to examine the gas installation to ascertain if any fault is causing the OPSO to activate.

The triggering pressure of OPSO safety devices can be adjustable or fixed, however, they always come factory pre-set at an appropriate pressure. Low pressure OPSO must operate below 150 mbar and are normally set at 100 mbar.

OPSOs are now mandatory for most installations. Clesse UK would advise checking with a Gas Safe LPG engineer to ensure your installation is in accordance with codes of practice set out by Liquid Gas UK.

Underpressure Shutoff (UPSO)
The UPSO device cuts off the gas flow in the event of an abnormal fall in pressure that can be caused by flow rate exceeding the capacity of the gas installation (pipework sizing, wrong regulator etc.), or the run out of gas and are normally associated with bulk tank supply.

The UPSO triggering pressure is not adjustable (except on large capacity BP2402FC). Regulators provided with an adjustable outlet pressure will see the triggering pressure automatically adapted to the outlet pressure setting.

Where applicable, all installations, adjustments, and maintenance work must be carried out by persons who have the necessary skills to do so and are qualified according to the regulations in force for that installation.

Always fit wall mounted regulators above level of the cylinders, with hoses falling gradually to the cylinder valve, so no condensed LPG can collect in ACO or hoses.

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