LPG Gas Vapourisation and Regulators
LPG Gas Vapourisation
Propane has a boiling point of -42°C. The vapourisation of LPGas is directly related to ambient air temperatures, because the heat required to vapourise the liquid is absorbed from the air surrounding the tank or cylinder.
Heat is absorbed through the vessel shell and into the liquid. This is known as the “wetted area”. Some heat is absorbed through the vapour, but not very much, so it will be seen that the volume of liquid in the tank also plays an important part in the capacity of a vessel to vapourise to a gas. This simply means that the fuller the tank the more gas can be vapourised for a given temperature.
When the heat is absorbed from the air the air temperature drops. As air temperature falls, so does its capacity to hold any moisture in suspension. If it falls far enough, the moisture will drop out as a condensate and this temperature is known as the “dew point”. At any other time we would call this condensate rain. This condensate will lie on the tank surface and any heat it contains will also be absorbed through the tank wall. If the temperature of this water is allowed to drop to 0°C then it will freeze. If the process continues long enough a wall of ice will build up on the tank, and will act as an insulator, stopping heat transfer and therefore slowing or even stopping vapourisation.
If a tank ices up regularly it simply means that the vessel is too small for the vapourisation load placed on it (the customer has possibly given you incorrect thermal loads or misjudged site expansion re increased consumption – a common error) and steps must be taken to overcome the problem. A vapourisation table, by vessel size, is located at the bottom of this page.
Sometimes simply keeping the vessel as full as possible might cure the problem although it may not be practical. More cylinders or a larger or a second tank may be deployed to cope with the load.
However, in some instances these relatively simple cures will not suffice. There may be no more room for another vessel, or the cost of another tank cannot be justified, or the additional storage will attract onerous fire protection penalties that may be unable to be met due to water restrictions.
In this instance the only option is to supply some artificial means of aiding vapourisation and these units, for obvious reasons, are called vapourisers. There are various types on the market, direct-fixed, water-bath, and electric. All of them operate on the same basic principle i.e. liquid LPGas from the tank is fed into the vapouriser, is heated to above its boiling point, and the vapour is fed back to the tank ready for use in the system.
All of them have good and bad points. A direct fired vapouriser is reasonably inexpensive but is classified as a source of ignition and so it must be installed at least 10m from the tank. Elgas policy is NOT to use these units as they have an unsatisfactory safety history. The electric unit can be installed right beside the tank, but they require substantial 3 phase power to operate, are quite expensive to purchase and can cause condensates (oily residues) to form inside the vapouriser reducing the unit‟s efficiency.
The water-bath unit obviously requires hot water and unless your client has waste hot water available, a suitable hot-water boiler must be installed, again outside the 10m zone and the hot water piped to the vapouriser. Hot water bath units are the preferred Elgas option.
It should be remembered that all the vapourisers are classified as pressure vessels except fin vapourisers, and must be maintained in the same way as any other tank. Because of the costs involved, the use of a vapouriser should not be embarked on lightly, until all other options have been explored and a quotation obtained from your Elgas State Technical Representative. Remember, all liquid installations must be cleared with an Elgas Technical Representative before progressing with any work.
It should be remembered that constant freezing and thawing on the tank is not just an inconvenience for the customer in terms of lack of vapourisation but could have safety implications. If you notice a customer‟s vessel constantly covered in ice then please inform your Elgas State Technical Representative, as the vessel may require changing and upgrading to a larger size or have a vapouriser fitted.
LPG Gas Regulators
Having vapourised the gas, it must be supplied to the clients' equipment at a pressure suitable for their needs. To do this, we use regulators. A regulator is a spring loaded device which takes gas in at one pressure and reduces it to a constant lower pressure. Remember, a regulator will not increase pressure, only reduce it. There are “high pressure” and “low pressure” regulators available, and each has their uses.
We use high pressure regulators on a two-stage system, in which we reduce the tank pressure, which may be 800-900kPa. At this pressure we can force the gas down reduced size piping towards the customer‟s equipment. The use of high pressure piping can lead to quite substantial savings in reticulation costs. For instance, at 140kPa, a 20mm copper line can carry 3181MJ/hr over 20 metres, whilst at 2.75kPa the same line will only supply 196 MJ/hr. To supply anywhere hear the same amount of gas at 2.75kPa, a 50 mm line would be necessary. The difference in material cost alone would be significant.
When we get close to the equipment we reduce the gas pressure to the second stage, that is 2.75kPa, which is the pressure on which most LPGas equipment runs.
Over-pressure protection devices are required where the operating pressure at the inlet to a regulator exceeds 14kPa. (Refer to the section detailed “Pressure Protection” within AS5601 for details).
Overpressure shut-off, or OPSO regulators are a means of meeting the OPP requirements. These regulators are not certified to UL144 Type II and have a second smaller regulator piggy-backed onto them. The small regulator senses downstream pressure from the larger one, and if it exceeds a set safe limit, will slam shut a valve to close on the inlet of the main regulator and close off gas supply. To reinstate supply the small regulator must be manually reset. If the fault in the main regulator has not been rectified the gas will shut off again immediately.
Note: It is Elgas policy that, unless specifically approved by your Elgas State Technical Personnel, Elgas will NOT supply the Regulator (and its associated pipework) to the customer. Elgas can offer technical advice, but this part of the installation should always be handled directly between the customer and a qualified, licensed and insured gas fitter.
Important Design Points: Any regulator having an internal relief or discharge vent MUST have that vent piped away to a safe, external location. UL 144 Regulators having Series II or Full Bore Internal Relief Valving as required by AS5601 are the ONLY regulators that are approved for use with LPGas unless other means of OPP are utilised. Do not undersize a regulator as this will inevitably cause the customer operational problems. Domestic installations (except BBQ‟s) must now be fitted with a 2 stage regulator, normally supplied as a single combined unit.
- As a simple rule of thumb, when using vessels of say 2.75 or 5.1kL capacities, simply extrapolate between the two nearest size vessels but biasing your calculations on the conservative side (consult your State Technical Representative for advice)
- Always check with your State Technical Representative that the above vapourisation rates are correct for the particular vessel you have designated
- For sites that require a high vapourisation rate but it is not cost effective to install larger and / or multiple vessels, consider using a fin vapouriser
Vessels above 3 tonnes or over 7.5kL will be custom designed by Elgas to suit customer needs. Figures provided are only rough estimates based on previous designs