Cutting and Welding with Propane - Some Technical Considerations

This article is originally from the IWDC Chronicles, which is a newsletter for IWDC members.  View the original article and the rest of the newsletter here.

This article is by Rich Mansmann, VP of Gas Programs,

“An IWDC Member had a problem with sludge build-up on the torch surfaces when using propane in a high volume preheating application. I had experienced a similar issue in my past, and thought is might be worth talking about a couple of issues that might impact your business.

First, many of you have been around since the days Airco vigorously marketed MAPP Gas as a less costly alternative to acetylene in some applications. The rest of the industrial gas bunch followed with some form of propylene or propane/propylene/methyl acetylene mixture. And, after a while, many of you also discovered it might be possible to run a cutting table with commercial propane and oxygen alone.

All these fuel gases offered an alternative to acetylene, but as you probably experienced, it wasn’t a seamless transaction. Now I’m not going to launch a technical explanation on why any one fuel gas is better than another. But for the sake of keeping the story informative but short, the first thing that’s important to mention is that nominal oxygen consumption is higher with these gases than with acetylene. Now we accept that oxygen is significantly less expensive than acetylene or any fuel gas on a cubic foot basis, but as propylene is again being promoted as a replacement product, it’s only fair to mention that the user’s oxygen consumption will increase… no doubt a small cost factor, but still one that needs noting. And going the full pass from acetylene to propane will typically increase oxygen consumption by two to two and a half times. So while the lunch may have gotten slightly cheaper, it’s not free.

The real reason I’m writing this is to identify a potential problem with propane, before it happens to you. Almost all the propane we all sell is used as a fuel, but its consumption relative to its container size is small. You all know that propane is shipped as gas under its vapor pressure, and that vapor pressure varies considerably with temperature, and we’ve learned to control that pressure variance with a regulator. But in high flow situations, especially when using cylinders, commercial grade propane can do some weird things.

The propane that we buy is typically pulled off a natural gas stream. This is truly natural gas as its’ pulled out of the ground, not the stuff that’s been stripped and cleaned up and that’s piped in mains and delivered to your homes and businesses. This originating natural gas normally has a propane content of 1-10% depending on where the gas is taken out of the ground. The stripping and clean-up process is managed by a Mid-Streamer refiner. Mid-Streamer refiners take crude natural gas from a well, cleans it up by removing the moisture, nitrogen, carbon dioxide and sulfur (and maybe some helium, but that’s another story), then preforms a simple distillation to separate the remaining hydrocarbon components into marketable commodities. Depending on its initial composition, the cleaned up gas can be divided into three to five component streams:

• Methane, that’s provided to natural gas utilities as “natural gas”

• Ethane, that’s normally sold for ethylene production

• Propane, as a “bottled gas” alternative where natural gas pipelines don’t exist

• Butane as Iso-butane, also sold as heating fuels, and also marketed to enhance heating values of poor quality methane streams, as well as propellants, and as a feedstock to petrochemical production

• Heavier hydrocarbons (pentanes, hexanes) that are handled as flammable liquids and further purified as solvents and feedstocks for other processes

As we’re concerned about propane, that product is collected and transported from the Mid-Stream site either by pipeline, railcar or tanker, depending on the size of the operation. But here’s the sticky part… in our gas world, we’re mainly used to very high purity, dry products. Commercial propane just isn’t that pure nor clean from its starting point, and only gets exposed to additional contamination in its journey from the gas fields to a cylinder filling operation, because “it’s only as good as it has to be” to meet basic heating and fuel requirements. It’s typically somewhere between 95 and 98% propane. There’s other “stuff” in it, and that “stuff” can give us problems in some of our applications.

The biggest cause of problems comes from water. The producers and propane distributors are aware of it, but most times, it’s not a problem. Water comes from poorly purged pipelines, trailers and railcars, and many times, from the intermediate storage sites as well, particularly if it’s stored in salt domes or other naturally occurring underground geological formations. Water is slightly soluble in propane. But as the temperature drops, the water can drop out of solution and form a separate liquid water, or worse, and ice phase. Treating the propane is often a two-step process. An alcohol amine is used as a large scale remedy in the pipelines. It’s added to the propane stream to maintain water solubility in transport. And, during cold weather, the propane distribution companies treat the individual delivery vehicles with methanol or ethanol, much like we once added these products to gasoline to prevent winter ice-ups.

Again, in most applications, and especially in warm weather, an end-user never experiences any problems. The water stays in solution and reduces the actual heating value a tiny amount, that’s almost impossible to measure. But in heavy draw applications in cold weather, there can be problems. The problems are caused by exceeding the practical vaporization rate of the liquid propane into gas. This can be seen by the supply cylinders getting extremely cold, even to the point of having ice forming on the cylinder surface. The resulting rapid gas flow is colder than normal, and can cause the water and its solvent, typically some combination of the drying amine and the alcohol, to drop out of the gas stream at the heating tip and form sludge. The sludge typically is composed on the congealed drying amine, with whatever dust, dirt, and residue that’s in the cylinder… remember that all these drying chemicals also have a cleaning effect.

The problem is normally solved by reducing the propane flow rate out of the individual cylinder and through a larger capacity regulator. This can be done, where permitted, by the installation of a supply manifold of 2-3 100 pound propane cylinders, or eventually upgrading the customer requirements to small portable storage (250-500 pound) or 500-1,000 gallon tank. In any case, these options provide  a larger surface area for vaporization, and the amount of gas required is small compared to the potential heat absorption of the storage container. Likewise, a larger capacity regulator will allow higher flow without the significant pressure drop that contributes to condensing the volatile tank residue.”


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