Natural gas compression using oilless air compressor

[Thaddeusw]

Hello,

First time poster so go easy on me ;-).

I was wondering if an oilless air compressor can be safely used to compress natural gas to around 60-100 PSI. My reason for doing so is I would like to use the very low pressure utility gas (.5 PSI)at my shop to run a cutting/welding torch. GTEC makes them but the price simply scares me. So I got to thinking.....

The compressor I was thinking of using is a GAST 4H model at 1/2HP (

http://www.gastmfg.com/piston.html).

The compressor is all aluminum except for the valves and rings, which are stainless steel. I would like a reservoir to hold the compressed gas before it was fed to a regulator, maybe a 1 or 2 gallon tank. All tubing would be copper with brass fittings. The only thing I would need is a pressure switch to cycle the compressor and a pressure gage to monitor the tank pressure. It seems strait forward in operation just feed the natural gas strait into the compressors intake port and pump it to the reservoir/torch. I know it can be very dangerous but I really want to have as much gas when I want it and not burn a hole in my pocket.

My first concern is that the motor isn't explosion proof. In the event of a leak, when the motor cycles on, the centrifugal start switch would create an arc that could ignite an explosive mixture. I will have to see if I can obtain a belt drive model or possibly change the motor to an explosion proof model.

My second concern is the heating of the gas due to compression. Should I cool the gas out of the compressor with a tube and fin type heat exchanger?

And last is my concern of any possible incompatibility of natural gas with certain materials to be used, such as tubing, fittings, valves, gauges, etc.

Thanks!

 

[Montemayor]

Thad:

You’ve caught me at a point where I’ve issued 2 - 600 HP Neuman & Esser, 3-stage, natural gas reciprocating compressors for purchase and presently am expediting their fabrication and packaging. I’ve applied and operated more and bigger ones than these in the past. Your application doesn’t even approach mine in size, but the essential basic requirements are the same.

That’s why I feel compelled to advise you on several serious hazards related to your application:

1. From what I can see, the little Gast machine can do the job you want, but requires that you know that it is both a compressor and a vacuum “pump” – i.e., it can draw a very strong vacuum if allowed to do so. That is a feature of any positive displacement machine and the reason that the suction pressure must be controlled at a positive value if the possibility of sucking in air through a leak in the system presents a hazard. In your application, sucking in air and allowing it to mix with natural gas with subsequent high discharge temperatures in the compressor represents a definite hazard in the event static electricity or a spark appears in the system because you may generate a combustible mixture. Don’t get me wrong. Natural gas by itself is pretty benign – especially at the low domestic supply pressures. However, you don’t want to mix it with air until you are ready to combust it in your torch. Industrially, we have no problem controlling the suction pressure at a positive level; however, on such a small machine and at such a small suction pressure – 13 inches of water column (or less) – you will have a very tough and expensive time finding a control scheme to ensure that the suction pressure is maintained above atmospheric. What is usually done is that a portion of the discharge gas is fed back to the suction to ensure that it is kept at a pre-set value. You may also use a Stop-start type of control based on reaching a max. pressure in your discharge tank and that is cheaper, but the possibility of forgetting to open the suction block valve before starting the compressor presents a hazard.

2. I don’t see anything wrong with the material of construction, but there is no information on the material used in the piston rings. I have to presume that they use some version of reinforced Teflon composition, but we can’t be certain. This would mean that ring wear is inevitable and will require careful monitoring to prevent ring failure and excessive temperatures.

3. Presumably the Gas only uses one single-stage of compression. This is going to result in what I can only guess at this time as an excessive discharge temperature – above 350 ^oF. Your Teflon rings can’t take this level of temperature without breaking down.
4. Regardless of whether you use a V-belt drive or not, you will need to have an explosion-proof motor for this application.
5. Because of the relatively high compression ratio you are proposing in one stage of compression, you will have to use some sort of an after-cooler. A fin-fan unit (such as those used for domestic A/C refrigeration condensers would do).
6. Stick to carbon steel, copper, brass, cast iron for your piping and valves and you shouldn’t have any problems.

A lubricated compressor for this application would also do well if furnished with an oil separator downstream of your aftercooler and prior to entering your discharge tank.

Basically, I believe you will have a lot of problems, technical and economical, putting together and justifying this application. I don’t know why you insist on using natural gas when you could use a propane torch as well, but that is your decision. I believe you will burn a hole in your pocket if you go this route – assuming that you instrument and design it in a safe, operable manner. The size of the application is just too small to be economical and the compression ratio you are proposing for what is a very cheaply made, dentist's air compressor will give you a lot of grief.

 

[sms]

"I was wondering if an oilless air compressor can be safely used to compress natural gas to around 60-100 PSI. My reason for doing so is I would like to use the very low pressure utility gas (.5 PSI)at my shop to run a cutting/welding torch. GTEC makes them but the price simply scares me."

1) The answer is no! An air compressor is designed to compress AIR, not natural gas. You cannot safely use an air compressor in natural gas service

2)If the price of an actual gas compressor is much higher than an air compress, what might that tell you?

-The future's so bright I gotta wear shades!

 

[dcasto]

The only differnce between air and natural gas is that k (Cp/Cv) and because of that the natural discharge temperature will be lower than for air and the horsepower lower, so on a molecular level, no problem.

Mechanically, natural gas will flow with less pressure drop through valves and piping than air. A piston doesn't know whats on the other otherside, how do it know, why do it care, how can a car engine work. compress air, compress explosive mixture. No problem here.

No we get to the heart, Mortemayor is correct. About the only thing I can add is look at the connects and surrounds and don't have any leaks, put the unit somewhere where it gets lots of air circulation. An explosion proof or TEFC in 1/2 HP would be warrented, at that size they typically have sufficient air cooling to move air around, your choice. Propane is wonderfull.

 

[zdas04]

While I agree with the essence of the above posts, there are a couple of details that I need to address.

Montmayor,
As you well know, natural gas must be between the LEL and UEL (5-15% methane in air) for a fire. That requires that you ingest 6 times as much air as you have methane in your system. The autoignition temperature of methane is 1,300F, so the heat of compression would melt many of the components of the compressor before you could possibly have an internal fire. I've been looking at this problem for 15 years and I can't find an single case of a compressor fire that can be traced back to vacuum operations.

Dcasto,
There is another difference, the natural gas is much dryer than air and the seals in an air compressor will tend to dry out in natural gas service. This can lead to leakage out of the compressor and creating an explosive atmosphere in the near vicinity of the non-explosion proof motor and other electrical components.

David

 

[dcasto]

very good zdas. I forgot that part about seals. Larger units have lubricating systems and on the smaller ones were end up with at least some manual way to lubricate seals. I've run lots of vacuum gas compressors and we've never had a fire / explosion. I've seen over 10% O2 during some times there were big packing leaks.

 

[Thaddeusw]

Thanks for the information guys. I love a good challenge but if it poses any danger I might just skip this one. The main reason for using natural gas is because it offers a good clean cut. I also dont have to worry about gas cylinder storage or rental. I could use propane but again it defeats my idea of a compact and near infinite supply whenever I need it. I mainly cut with a torch some brazing too, welding has to be done with acetylene. I leave the welding to the MIG. Maybe its time to get a plasma cutter.

 

[Thaddeusw]

"1) The answer is no! An air compressor is designed to compress AIR, not natural gas. You cannot safely use an air compressor in natural gas service

2)If the price of an actual gas compressor is much higher than an air compress, what might that tell you?"

1) A piston compressor is a piston compressor, the materials and design might differ but its the same basic principal. I thought of an oilless compressor because it seemed neutral in the sense that it was made of non sparking aluminum and had no oil that could react with the gas. Maybe an oil lubed compressor with 2 stages with metal piston rings would be a better start.

2) It costs more because of supply and demand. Demand is low so the price is higher. If everyone needed a NG compressor they would be cheap.

 

[iainuts]

Hi Thaddeusw, I think there's one more danger you need to be aware of in using this kind of machine. Piston rings such as the "lap joint" kind that exist in the Gast machine will have some small amount of leakage. In an air compressor that leakage simply goes back to atmosphere, but a typical flammable machine will have to collect that gas and either route it back to the suction or route it to a vent stack. The most common set up is to install various levels of protection such that you route piston ring leakage to suction and then provide a distance piece which is vented to a stack. I've also seen crankcases that are purged with nitrogen as an additional level of protection. In the case of the Gast machine, there is no collection port, the leaking gas will go directly into the atmosphere, so there will always be the potential of an explosive atmosphere around this kind of machine. That problem is probably the show stopper unfortunately. Even when the machine is turned off, if there is gas pressure on the suction, the rings will continue to leak.

Note the NFPA code in the US (also NEC code) govern flammable installations, so you may also want to review those codes as they are legally binding.

 

[sms]

jainuts beat me to it.

"A piston compressor is a piston compressor" Pardon my language but that is BS. There are hundreds of variations of piston compressors, all with specific design and compression service requirements.

Air compressor design is a bit sloppy if you will, because if air leaks to atmosphere, who cares? A little hot air in the vicinity is no big deal.

A gas compresser is specifically designed to contain the gas or redirect leakage to a safe place under either running conditions or shutdown conditions.

Gas compressors are more expensive because they are designed to handle gas safely. There is plenty of demand for NG compressors (how do you think it gets to your shop?), but since they are more expensive to build safely, there is more liability involved with them, and so they are more expensive.

Sorry, the rest of these guys have been perhaps a bit kinder, but I think the whole idea is very dangerous. Just out of curiosity did you run this idea by the folks at GAST. I wonder what they would think of it? I suspect their reaction would be about the same as mine.



-The future's so bright I gotta wear shades!

 

[Thaddeusw]

sms,

I don't mean to be rude but I wanted actual in depth information on why it is a good/bad idea. You offered no relevant information to the topic whatsoever. Sorry to be so blunt but everyone else was of great help and really gave me a clear picture of what I was up against.

Thank you all!

 

[dcasto]

Don't tell all the farm houses in Kansas that rely on these simple compressors to heat their houses and run the pumps that water the corn and wheat that goes on your table.

 

[Warpspeed]

This all has some considerable interest to me, as I am in the process of doing something fairly similar myself at the moment.

My system uses a swash plate automotive air conditioning compressor, because the drive shaft is particularly well sealed against potential gas leakage, and the compressor displacement at 8 cid per rev is ideal for me. This is a wet compressor, so it spits a small volume of oil, but a simple oil separator solves that problem.

A am using it to raise the gas pressure to feed a forced air natural gas burner used in a small metal melting furnace. There is no high pressure storage volume in my system. The compressor feeds the burner directly at around 2 to 50psi. Burner gas pressure (mainly for lighting) is controlled with a needle valve that bypasses compressor discharge gas back to the compressor inlet. This needle valve is located at the bottom of the oil separator, so it doubles as the oil return path.

I had not considered the problem of negative suction pressure and air leakage, but it is a very interesting observation.

In my case the compressor, burner, and furnace will always be attended while running. I use a magnehelic pressure gauge (0-10 inches of water)to indicate entering gas pressure, and it is a pretty fair indication that all is well on the gas supply side. There is also a pressure gauge on the discharge side to aid setting the needle valve.

For unattended operation, something a bit more sophisticated (and safe) probably needs to be done, especially if a high pressure storage tank is part of the system.

How about a pressure activated switch to shut off the compressor ? These are commonly used in washing machines to close the water solenoids when the tub is full. These are usually adjustable in the "several inches of water" pressure range, and may be worth investigating.

One question I have is the suitability of the refrigeration oil in the compressor. Is there any potential compatibility problem between natural gas and the crankcase oil ? Should I be using any special oil for this ?

 

[Lhenrique]

Thaddeusw

At what pressure your local NCG supplier company deliver in your shop? Is it not enough to fullfill a vessel and attend all your torch points?

Luiz
Brazil

 

[dcasto]

Automobile AC have a pressure shut off switch/solenoid that engauges the clutch on the compressor, that cycling you hear on the system when its running. The clutch also cuts out the compressor when it hits its making RPM (the AC shuts off when you pass in a lower gear for example). Those 12 volt sytem might be incoporated. Wonderful use of 'ol noggin.

 

[Warpspeed]

Luiz, in my case the the domestic natural gas supply pressure is only 4 inches of water or 0.14psi.

That is fine for a large open naturally aspirated gas burner working into ambient air pressure. But it creates a very large lazy silent blue flame, and is extremely intolerant of any ambient back pressure. Even a strong gust of wind will blow out the flame.

If you want an angry roaring hight temperature flame concentrated enough to melt steel, that requires a much higher gas supply pressure.

The other potential problem may be the limited available gas supply volume. In my case I can just get away with what I have. But If I required a much larger furnace, I would be in trouble. I would then have to pump up a large storage vessel sufficiently slowly over time, in order to have a significantly higher gas flow available over a short period.

 

[iainuts]

I wonder if small, hermetically sealed compressors used for flammable refrigerants are readily available and who makes them. I did a quick search on the net and didn't find much, but it seems there are hermetically sealed compressors used in the refrigeration industry that might be suitable and relatively inexpensive. I'm not in that industry though, so I'm not aware of what companies make such a thing.

 

[Warpspeed]

One of those hermetically sealed compressors would burn out the motor rather quickly in probably ten to thirty minutes.

The reason being, that in the normal refrigeration and air conditioning application, the incoming suction gas is always at a low temperature. The incoming suction gas always flows first around the motor windings in these compressors. If used for compressing ambient air or natural gas, these sealed units will burn out the motor within a very short time.

Unfortunately I must confess to discovering this for myself the hard way. Belt driven refrigeration compressors seem to be ideal for natural gas, but not if an internal motor relies on cold entering gas for cooling.

 

[vzeos]

Thaddeusw

You may be right in assuming that an air compressor may be capable of compressing natural gas. But I would not try it. I’m not familiar with the compressor you propose but I don’t have a problem using an oilless compressor. I installed a centrifugal line compressor using dry seals (Crane seals) on a natural gas transmission system about 16 years ago and still have no problems with it. In fact, there are distinct advantages in not having to deal with seal oil equipment.

I think you will have a problem in obtaining the fire/building permits to compress and store a high pressure flammable gas in your shop. It should be understood that a gas leak in a compressor unit having a 2 gallon gas receiver holding natural gas at 60 to 100 psi could fill your shop with a flammable mixture in a matter of minuets, whether the leak be caused by internal corrosion or by an external impact. Also, if you install a natural gas compressor that is not tested and approved for that service by a nationally recognized testing laboratory (i.e. UL or AGA) you will expose yourself to liability issues and perhaps void your insurance coverage.

I think you would be much better off installing a Hermetic Gas Booster intended for natural gas service, see attached link.

http://riveon.eclipsenet.com/parkstreet/public/ViewFile.aspx?aid=269054

 

[TPL]

I was wondering if an oilless air compressor can be safely used to compress natural gas to around 60-100 PSI. My reason for doing so is I would like to use the very low pressure utility gas (.5 PSI)at my shop to run a cutting/welding torch

Why not just use bottled gas (as in a proper cutting/welding kit) for this application?