Overview: Understanding Propane Gas Systems

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Smoking Guru
Original poster
OTBS Member
Aug 27, 2008
A look at the propane gas system, from tank to burner, and everything in between...

1) LPG cylinder and contents: holds the high pressure contents, being liquified petroleum gas. The contents expand and contract with temperature changes, thereby resulting in changes in pressure. Extremely high temperatures result in excessive pressures due to liquid expansion and the higher vapor pressure of the hotter LPG. In extremely cold temperaures, the liquid contracts using less cylinder space, and also results is a lower vapor pressure from the LPG. Vapor pressures of the LPG vary depending on liquid temperature, and can range from approx 208-212 psi @ 100* F, to at or near 0 psi @ -40* F. The colder the temperature and/or the higher the gas volume demand, the lower the potential pressure will be in the cylinder. LPG boils to maintain pressure inside the cylinder as gas is released and pressure is decreased. The less liquid contained in the cylinder, or the lower the liquid temperature, the less vapor generation potential. The LPG gas cylinder pressure may increase with agitation or shaking, regardless of the liquid level.

2) OPD & excess flow valve on the tank: the OPD (overfill protection device) prevents overfilling of the cylinder so there is always a vapor space for the liquified gas to expand into with rises in ambient temperatures, which translate into high liquid temps, causing expansion of the liquid, thereby needing more space inside the tank. Without the vapor space, a burst disc (safety device) would rupture or the cylinder itself would suffer a catostrophic failure possibly resulting in an explosion with a fire. The excess flow valve is a stopper contained inside the cylinder valve body, and in the event a sudden pressure drop and/or sudden release of high volumes of gas, a properly functioning excess flow valve will close and stop all gas flow.

3) Tank connection, gas regulator: The connection obviously provides a (hopefully) leak-proof high-pressure coupling between the gas regulator and cylinder of LPG. Upon opening of the cylinder valve, the regulator takes the high pressure of the propane gas (supply pressure) and reduces it to a useable pressure by the gas appliance. This aids in achieving a more evenly controlled flow of gas through the burner control valve, especially during temperature changes or over longer periods of time requiring large quantities of gas usage. As the liquid level in the cylinder becomes low, the ability of the liquified gas to maintain it's threshold of pressue will be reduced. Also, if temperatures of the liquid become cooler or warmer due to ambient temperature changes, this will cause the internal cylinder pressure to rise with higher temps, and drop with cooler temps. The regulator compensates for these internal cylinder pressure changes for the most part.

***Note: I have had many years of experience with welding/cutting gases (propane/oxygen, acetylene/oxygen, argon), and extensive experience with 2,200-4,500 psi Grade D (standard) and E (diving) breathing air cylinders, high pressure breathing air purification systems, high pressure compressors and high pressure air and gas regulators, LPG fired grills/smokers. Gas regulators have one inherent fault: when the cylinder (supply) pressure decreases to what is below the threshold of it's designed inlet pressure, the regulated outlet pressure will increase. And, the more the inlet pressure continues to drop, the higher the resulting outlet pressure will be. On non-adjustable pressure regulators, this can cause equipment to malfunction. In the case of gas smokers and grills, it may cause excessive gas flows to the burner. With adjustable regulators, such as many found with turkey fryer burners, this can be compensated for by simpoly dialing the adjusted pressure down as needed.

4) Hose, burner control valve, gas orifice, venturi, plenum, burner: the hose supplies the burner control valve with the usable pressure of gas. The control valve adjusts the actual flow of the gas through the gas orifice. The gas orifice is typically a plug in a gas tube which has a small hole for the gas to escape under low pressure flow. The orifice hole-size is dependent on the manufacturer's specifications for proper air/fuel mixing based on the designed btu input of the burner. The gas flowing through the orifice goes directly into the into the venturi. The venturi is the point where air enters into the flow of the gas stream and mixes with the flammable gas to form an acceptable range of gas to air ratio for proper burning. The plenum is the final point where the fuel gas and air finish mixing, and in some cases, the velocity of the gas/air mix is slowed down to properly exit out the burner orifice. The number and size of the burner orifice holes is again dependent on manufacturer's specifications for the designed burner btu input of gas.

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