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Press Cylinder Safety


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I just received a good question from another reader of IFI in regards to cylinder safety issues and I thought it would be good to post it here:

"I know a couple of smiths that are using 6"+ diameter air cylinders in a hydraulic setup with a pump putting out 2500.
From my understanding the air cylinders have a thinner wall and are not made for that kind of pressure. There is a rumor that this setup is dangerous and the cylinder can fail in the form of exploding, cracking and hydraulic fluid shooting out, etc. Though I am having a difficult time finding out any information about the kind of work loads these two cylinders can endure.

Any suggestions or literature that I can read up on?"

Here was my response:

"I've never heard of such a thing. The ones I have the walls are over an inch thick, more than enough to handle that pressure. Maybe Batson's book has info about this issue, but I don't remember.

Just asked a guy at work and he said that those are AIR cylinders, NOT a hydraulic cylinder. So you're right about the air clyinder, but hydraulic cylinders are made for that pressure. The air cylinder is used for the air hammers, but should never be used on presses.

See, I learned something today, too."

Any other input and where literature can be found regarding this issue is appreciated, too.

Sure wish Grant was still around. I know we'd get a good, detailed response from him.

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We all miss Grant but sometimes a short answer works perfectly. No. Don't do it. Are right to the point and work fine for me. The physics behind those two answers are out there but what's the point.
I've seen plenty of HYD cilinders used as air cilinders but not the reverse, I'ts just common sense...... :)

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There are HUGE differences in everything from wall thickness of the containing cylinder to pressure rating on the seals throughout.

You also bring up a second safety issue and that is we are using these cylinders around red hot metal. When used in this type of work not just any hydraulic fluid will do,you need the type that is not highly flamable. One of the worst accidents I saw involved someone welding on a live hydraulic line on an extrusion press. The operator hit the lever to move the cylinder on the opposite side of the press where he couldn`t see the work being done. The welder heard the system kick in,broke the arc yet the weld was orange hot when the pressure hit it and it ignited like a flamethrower pushing the welder off the ladder he was standing on to do the job. He recieved 2nd and 3rd degree burns to his head,face and neck after his fiberglass hard hat welding sield melted. He was also paralyzed from the neck down after he hit the deckplates head first when he fell off the ladder.
The company drained and replaced all the hydraulic fluids ( replaced with high temp,marine grade) in their presses after that as well as implementing new and better safety procedures, like lock out/tag out, for working around equipment.
Too bad it took ruining a man`s life to make these changes.
Be safe,it may cost more in the short run but your family will be glad you did it.

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Let’s be logical here. PSI is PSI no matter if it is air or fluid. The fact that air can be compressed and fluid cannot is of no matter. Even a heavy duty tie rod pneumatic cylinder is only rated at 250 PSI maximum. That is a far cry from the 2500-3000 PSI of a working hydraulic cylinder like 10 times the pressure. NEVER USE A PNEUMATIC CYLINDER IN A HYDRAULIC APPLICATION.

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Well I will disagree with ciladog. The fact that air can be compressed and fluid cannot is of great matter in the case of FAILURE as compressed air tends to drive shrapnel and hydraulic fluid not so much. For a gedanken experiment think of welding tanks---knock the valvestem off a pressurized Oxy tank and you have a rocket. Fill one with water and pressurize it to the same pressure and knock the stem off and you have a spurt and that's it.

I had this dinned into me in a MatSci class where we studied the failure of several pressure vessels one of which came close to cancelling the Professors tenure on life.

Now if you are not considering failure mode; beans is beans.

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The problem stems from fools listing "hydraulic cylinders' on ebay that are either high pressure pneumatic (~250 psi) or low pressure hydraulic (~1000 psi).
It LOOKS like a hydraulic cylinder but buyer beware. Fools like me see an 8" cylinder for 100$ and say 'Right on- my new 80 ton PRESS cylinder!~!"

Lesson learned: if the thing has 1/4" tie rods- stay away. If there is no tag on the cylinder, as many do not, what size is the ram? Is it 5/8"? Stay away.

Fast forward-
I'm really leaning towards forking over the big bucks for the right cylinder for a new press versus trying to glean a nugget out of the manure pile. Tried to get cylinders rebuilt these days? Forget it, I've had two 7's and 8's that I left at the cylinder shops when each wanted close to 2$K to rebuild them. The $1400 for the 3000 psi 8X8 at surplus center sounds better every day.

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you could use two 5X8 cylinders and build a press like Randy's. Northern Tool sells them for under $300 each.


I got a 6x20 for the press I'm about to start building. Randy was kind enough to let me come over last weekend to check his out, and I decided I wanted mine to have more tonnage than the 35 I had planned on, so I just ordered a second 6x20 cylinder. If anyone is interested, they are surplus from http://www.baileynet.com. There are currently 4 left, but they don't show up in the list when you view their surplus cylinders. Just type in 116-493 into the search bar on the homepage though, and it will come right up. They are only $295, so it's $600 plus shipping and you'll end up with a 70 ton press with more than enough stroke for using drifts, punches, etc. If you decide to go with that big of a stroke, remember you'll have to top-mount your cylinders, if that matters to you.
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There are pnuematic cylinders and there are pnuematic/hydraulic cylinders. and then there are hydraulic cylinders that can also operate on air.
If one looks, the air/hydraulic cylinders are usually 250 psi air/500 psi hydraulic rated.
If one looks there are hydraulic cylinders rated at 1000PSI, 2000psi, 3000 psi, and 5000 psi.

By "if one looks" I mean looks at the nameplate/catalog etc.

Hydraulic fluids DO compress. NEVER EVER make the assumption, often incorrectly taught in classes the fluids do not compress. Water for instance compress about 1/2% per 1000 psi. Takes and stores energy just like air, just stops expanding back quicker.
BUT the container is often stretched and when it fails the fluid and the shattered parts can and do fly.

As a R&D guy working in the labs of a major hydraulic and pnuematics cylinder maker, and later 21 years experience in the labs of a valve and fittings maker, I have intentionally burst tested more items than most people use in a lifetime. A nice 2.5" steel tube of say 3/8" wall will usually let go at about 19,000 psi, and when that bad boy lets go, it better be in a test cell, since it sounds about like a half stick of dynamite, and often flings a human hand sized piece with force enough to chip a crater in the concrete roof. It would decapitate a human.

I have burst tested low rated air cylinders. The industry standard is 6 times rating, so for a 250 psi cylinder you would see at least 1500 psi before failure to pass. Now lets think about failure mode of a tierod cylinder. The rods stretch, the barrel swells like a squid, and when the barrel to head/cap seal fails, you get an explosive spray of fluid at say 2000psi., the seal is extruded and then just pump flow can be made. But that first spray is pump flow + the contracting barrel.

Also remember that a pinhole leak at pressure above about 2800 psi will easly cut or inject the fluid if you contact the exit from the pinhole. And you just injected the body with containmanent, ensuring a rip roaring infection. Seen it to many times to count.

A nice split or pinhole leak if it contacts an ingition source is a flame thrower, think oil furnace burner gun.

The supposed "Less flammable" hydraulic fluids are mostly of the Ethylene glycol variety and are mixed with water. Note that the container for the concentrate will usually be labeled Flammable as pure ethylene glycol is very flammable. I am sure that everyone using these fluids of course floows the makers requirements and checks the concentration of the fluid to water mix using a hand held refractometer or titration right? oh, wait NO ONE DOES. Let the water evaporate from the mix in a nice hot forge shop and sooner or later you have FLAMMABLE hydraulic fluid. Google Ford ambulance fires for a great example. Or perhaps Google RAF use of pure ethylene gylcol cooling in aircraft engines

In the very eary days hydraulic fluid was water. Now that is indeed not flammable. But it takes very special seals, EATS pumps even when correctly designed with the tank say 30 feet in the air and the pump on the ground to provide a flooded suction. Oh and the very low viscosity and lack or lubricity pretty much eats everything in the system.

You can go to "High Water Content Fluids that are typically 95% water and 5% oils for lubricity and corrosion resistance as well as giveing a vapor off to protect your tank, but these still need that pesky refractometer.

The truth is most forge press failure I have seen, and unfortunatly I have seen MANY, are in the fluid conductors. That means those hoses, pipes fittings and valves, not the cylinders.

When you build with a hose, you build with a finite life in the system. The hose will fail, not a question of if, but rather when and where. Tubeing is better, but those pesky 37 degree flares most use tend to crack right at the flare and the ferrule type fitting are really only worth using if you buy the very best double ferrule type like Swaglok. Ever spend $50 for a 1" tee?

If you are not compentent in the arcane art of high pressure you may or may not get lucky.

I am working up a press slowely, using a hydraulic cylinder and a high quality pump. Mine will most likely have the majority of the components on the other side of the shop wall from the forge shop. It will have only rated fluid conductors in the shop.

Ptree the guy who built many many low, medium, high and very high as well as one extreme pressure machines. Ptree who also examined, cleaned up and did the first aid on industrial forge shop accidents for many years. Ptree who has not only burst tested thousands and thousands of items, but designed and built and maintained that 33,000 psi machine for 20+ years. Ptree who designed, modified and oversaw the maintenance of an entire high pressure test department, that tested 100,000+ items a month for 20+ years, with some of the test machines routinely operating at 10,5000 psi. Ptree who examined EVERY single returned pressure fitting or valve returned for failure mode for 20+ years. Ptree who saw the effects of many incorrectly installed, modified, or incorrectly chosed and applied items for that same period, returned from all over the world.

Ptree who has suffered PTSD from some of the accidents he has worked, so please excuse if this rant seems a little strong.

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Ptree, thanks for the good info. It is always a pleasure to read your posts. I was wondering about 10,5000 though? Is the comma in the wrong place (105,000) or one too many zero's (10,500). The way the number is written is confusing.
Please if you dont mind can you share a little more about how you are designing your press.Thanks r smith

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r smith
That number should be 10,500 psi, sorry. My typing skills are pretty bad.
Currently the press is a pair of very larg cylinders and power units, one set at my shop, and one at my cohort's shop. I suspect the frame will end up a H frame, hard piped with tube inside of larger tube for a leak sheild. The valving will most likey be on the outside wall as will the power unit and most plumbing. I expect to run at about 1/2 rated cylinder pressure. I expect to also have a sheet metal wrap to deflect spray around the cylinder, and a rod seal sheet metaldiverter as well. Since the cylinders are 8", and rated for 3000 psi I should be able to get about 37 tons, and stay well below design. If I go to 2000 psi that would be 50 tons.
Since the power units we have are surplus and have van pumps that will push about 30 GPM, and that will take about 20Hp at 1500 psi or so, I will probably use a small gas engine to turn the pumps, another reason to have the pumps on the other side of the wall.

BUT, until some more of the needed surplus shows up, no movement. Just because we are building from surplus means violate the safety side, just have to be patient.

Besides high pressure hydraulic and hydro testing my lab was capable to 5000psi gas pressures, also very scarey. I worked at 1000 psi nitrogen and 500 psi Methane almost every day, but anything over 2000 psi gas was in the test cell.

I would guess that the lab had probably $50,000 in fittings and lines over the years to keep the right stuff on hand.
A typical 10,500 psi hydro test machine, SINGLE station would cost about $30,000 to plume today, exclusive of the automation and frame.
I built an 8 station machine, totally manual, capable to a max pressure of 8800 psi in about 1991, and the fittings for that machine was about $30,000.
Oddly the air driven pumps were very reasonable at $500 each then. The entire machine was $105,000 installed and was used to test our ASME "N" stamp or nuclear rated valves. Still in use as far as I know in Texas by the last buyers.

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Good information Ptree. Pressure hoses and fittings are not an area to cheap out on. !00psi air can also be very dangerous as well. I once popped open a 2" camlock hose connector on a frozen hose that I thought had 100 psi of water behind it. Unfortunately the water had not arrived yet and the hose and heavy fitting flew over 20 feet and it sounded like a cannon going off. I also saw the aftermath of a 2" camlock fitting breaking in half. The hose tore through a bunch of doubled up snowfence like it was not there tore a huge chunk out of a light post and luckily just grazed a guys head. If he had not had a snowmobile helmet on or it had hit him square he would have been dead.

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An interesting little story about failure caused by lack of knowledge and perhaps pure stupidity, and 100% true. I consulted on the case after the fact.

A liquid tank to hold nitrous oxide for a hospital was needed. The tank would hold the liquid nitrous and gas would be drawn off as needed for a medical application. The tank was in the 3 tons of liquid size. The company had no Nitrous tank, but did have a CO2 tank. They called the maker of the tank and were told "DON"T". They asked why, and the maker told them all the reasons why, which the caller mostly wrote out.
SOOOO... They sandblasted the old tank to clean it. This sandblasting destroyed the insulation. So they reinsulated. The blasting also ate the insulation on the heating/cooling package.
Now the way these systems work is the pressure rises and a refrig system comes on and cools the liquid dropping the pressure. When gas is demanded, the pressure drops so a heater comes on and that raises the pressure to keep the gas at the right pressure.

Well the electrician ALMOST got the wiring right, except he hooked the heater to the normally closed contacts. The "in my opinion crimnally negligant idiots" then did the hydro test not with water, or something safe like nitrogen, filled the tank with nitrous. As the pressure began to rise, the refrig system came on but could not match the heater. As the pressure continued to rise the manway cover in the bottom of the tank began to leak. The #$%@'s tightened the manway, a NEVER do. When the manway inverted and blew through the oval manway rim, the tank which was welded to a trailer as a temp mount became a liquid fueled rocket, with 3 tons of liquid fuel. It blew the floor boards out, and shot gravel, insulation and sheet metal all around until the welds failed. The tank then flew about 500' landing in an intersection of a road. No cars luckly.
The sad part is that one of the workmen had brought his young son along to work with him that Saturday morning. He was riding his bike about 50' away and a hunk of something hit him in the leg, severed an artery and he bled to death.

Had the manway not inverted, the tank probably would have ruptured as the relief vale was both undersized and solidly plugged with sand.

The moral of this sad story is when an expert in the art-science-technology tells you DON"T, LISTEN

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John, I have had some wild times with blown air hoses. One of the reasons I like to have SHORT hoses :)

Once at the valve shop, an engineer was given the power house to supervise as well as his other jobs. ZIP air compressor experience, buts lots of piping experience. Now he saw that the 4 200 Hp screw comp[ressors used very expensive fluid a PAO type, and he found a much cheaper fluid and the next change that went in. It was a Phosphate ester type. The blowby oils began to attack the rubber hoses all over that 40+ acre factory compound, turning the rubber hose liners to cooked spagetti strenght. We replaced several thousand feet of hose and about 100 lexan filter bowls that crazed cracked before I found out what had occurred.

After the compressors were returned to friendly to rubber PAO oils, and the hoses rep[laced etc, he was replaced. As a thank you for finding and correcting the problem I became the powerhouse Supervisor in addition to everything else on my plate, and you guessed it, NO PAY RAISE :)

It did later help me get the title Plant Engineer :)

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Thanks for everyones input! I've learned a lot. Safety can't be expressed enough. From cylinders, to fittings, to hoses, to operation. As far as hoses go, I highly recommend getting protective sleeves for all of your high pressure hoses. They run from around $3 per foot for the type that slide over to around $10 per foot for the kind that velcrow on over existing hoses. They are made in nylon, kevlar and other materials. Check with your local supplier/hydraulic shop so you can talk to someone to recommend what will work best in your situation. I attached mine with zip ties. What the sleeves do is protect the hoses from rubbing and brief contact with something hot. Due to this it helps the life of the hoses. Plus if a hose would develope a pin hole leak it will be contained in the sleeve and not spraying you or becoming a flame thrower. Well worth the minimal investment.

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Bryan, in pressure vessels there are ways to enter the vessel for cleaning/repairs etc. These are called a Manway, or a hand way, for much smaller versions that are used for inspection. Typically an oval, the smallest manways are about 6" x18" and have a heavy wall thru the pressure vessel. The oval, domed cover is inserted INSIDE the vessel and a gasket is placed between the cover the the manway lip, on the inside of the vessel. A threaded rod is screwed into the cover that goes thru an arch the spans the outside. You tighten the threaded rod with a nut on the outside enough to hold the cover in place, and then pressure on the dome makes the seal. If the manway leaks, tightening the manway only overstresses the dome, and with enough stress the dome inverts, the cover comes through and then your day is ruined. These are very common on boilers as well as other pressure vessels.

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