Unfastening the very tightly screwed PCP Cylinder

[kevinweiho]

The issues of detonation & dieseling should primarily be in spring air pistols, where the air is compressed rapidly. It's not clear to me that high pressure air like used in PCP pistols is necessarily a problem with petroleum lubes, but I've never dug into it to be certain.

Here's an interesting read from the "Godfather" of airguns:

https://www.pyramydair.com/blog/2007/05 ... an-part-1/

[Gwhite]

I never fill my PCP cylinders fast enough for them to get beyond slightly warm, so that hadn't occurred to me.

That's still not the whole story, because they do make petroleum based air compressor oil, which can run very hot. The Material Safety Data Sheet for one brand says:

Can be ignited at elevated temperatures but is not considered “flammable”.

So, (as usual) the devil is in the details...

[kevinweiho]

So, (as usual) the devil is in the details...

Murphy's Fifth Law: If anything simply cannot go wrong, it will anyway...

[Leon]

This is what I use....

Tupp.JPG (22.34 KiB) Viewed 3593 times

[divingin]

Dieseling is caused by the temp rise from the compression of the air in the chamber as the piston moves forward. It heats up enough to spontaneously ignite volatile hydrocarbons.

If anything, a PCP gun should get colder as it is shot due to cooling as the pressurized air is released.

[McMadCow]

What about the rise in pressure when attaching a full cylinder? The pressure in the valve goes from atmosphere to 200 bar in a fraction of a second. I'd think that would also be risky for any petroleum based lubricants in there or am I missing something?

[BobGee]

Dieseling is caused by the temp rise from the compression of the air in the chamber as the piston moves forward. It heats up enough to spontaneously ignite volatile hydrocarbons.

If anything, a PCP gun should get colder as it is shot due to cooling as the pressurized air is released.

I think you are right. A springer would heat the air during rapid compression as the spring is released but pcp pistols and the likes of the IZH46 do not compress the air in the pistol in the same way or at the same speed.

Bob

[nmondal]

Team - this time I tightly screwed up the large PCP cylinder Tank - at filling in range.
It is filled to 210 bar ( lucky me ) - it can take upto 300 bar.
Now I can not open it to fill the gun cylinders.

My Walther cylinder can take a bit more I think.
Any tips - how to open the valve w/o destroying myself in the process / or the cylinders?

[Rover]

"how to open the valve w/o destroying myself in the process / or the cylinders?"

Righty tighty, lefty loosy.

[David M]

Its the seal that does the work, you do not have to close the valve very tight.
Most tank valves have a built in shear drive.(part 8 in dia)
If its locked up tight, using force may break the shear.
You can remove the handle to turn valve stem (part 23) to release the air.
If that does not work then just crack the over pressure valve (part 12)
and allow the bottle to empty over a couple of days.

[spektr]

Team - this time I tightly screwed up the large PCP cylinder Tank - at filling in range.
It is filled to 210 bar ( lucky me ) - it can take upto 300 bar.
Now I can not open it to fill the gun cylinders.

Man I have never seen it so difficult to do things.
Take a moment to find a rubber or silicone jar opening pad.i have never seen these valves closed so tight you cant open them with a jar pad. These valves all close finger tight. Piling a ton of torque on them does nothing besides add drama. The cylinder to gun is also only finger tight, it also responds badly to being really torqued down. Think for a minute, divers are able to open and close these valves with wet hands. They require very little force to operate......

Lastly, when you take your tank in for its annual, tell the guy that GodIlla was cranking on the valve and possibly the valve for hurt.......

[nmondal]

Thank you all!
After I tried putting pressure on 3 different days - the valve opened today!
Lotta things to learn. Thanks specifically for the idea that I do not have to close the cylinder tight!
No one told me that - before.

[nmondal]

I was wondering if olive oil should be ok for PCP Cylinder joints?

[Rover]

I believe it's a traditional favorite for the lubrication of your "joint".

[brent375hh]

I was wondering if olive oil should be ok for PCP Cylinder joints?

All cooking oils get yummy over time.
Super Lube synthetic grease, of the non silicone variety, is a very good lube. Powdered molybdenum added to it is a plus.

[othello22]

I use a nonskid place mat for a pistol mat/rug. It does a great job of protecting the gun from bench ware and when the cylinder is tight it give you a good grip. When going to Camp Perry back in 2000 I was looking for an all weather pistol mat, I run across these at Camping World. I have them with all the club pistols and they have been used many times to unjam cylinders.

[divingin]

I never fill my PCP cylinders fast enough for them to get beyond slightly warm, so that hadn't occurred to me.

That's still not the whole story, because they do make petroleum based air compressor oil, which can run very hot.

The cylinder temp lags behind the actual air-in-the-cylinder temp. For one, it has to conduct the heat from the interior to the exterior, and for the second, there's a lot more mass to heat in the physical cylinder compared to the air inside. Not that I think that's a big problem for anything other than the final temp after cooling...

Most dive shops use oil-less compressors, but not because of ignition problems (if that was the case, probably nobody would use oil lubricated compressors); it was to avoid pumping oil into the breathing gas should an oil seal start leaking.

Many shops set the cylinders being filled in a water bath while they're being filled. Primarily to to try to cool the charge (so pressure doesn't drop as it cools later), and partially to try and control shrapnel in the case of a failed cylinder (which apparently doesn't really work.)

As to McMadCows comment about attaching a cylinder and the immediate rise in pressure in the valve: isn't that what happens every time you fire the airgun?

[McMadCow]

As to McMadCows comment about attaching a cylinder and the immediate rise in pressure in the valve: isn't that what happens every time you fire the airgun?

That's a good question that I didn't think about either. Still don't fully understand why PCP guns don't seem to be susceptible to dieseling.

[GoodEnuf]

As to McMadCows comment about attaching a cylinder and the immediate rise in pressure in the valve: isn't that what happens every time you fire the airgun?

That's a good question that I didn't think about either. Still don't fully understand why PCP guns don't seem to be susceptible to dieseling.

As I understand the progression of events, the cylinder is screwed in, its air (@up to 200 bar) first goes through the pressure regulator. The output pressure of the regulator as controlled by a stack of disc springs is <90 bar and is fed through to the firing chamber at that pressure. The firing chamber is very low in volume and the passages to and through the regulator moderate the severity of pressure change impact. Until the pistol is cocked and fired, the air resides in that chamber. On firing, a transfer valve is knocked open against a calibrated spring, and valve releases the air to the rear of the pellet. That decreases the pressure in the firing chamber, allowing the transfer valve to close by spring action - and regulated pressure builds up again for the next shot. Hence, the air pressure does not change instantaneously, but over a (short) time period. Compare this to the nearly instantaneous pressure buildup of a spring piston action being fired. [A desired side effect of our precision PCP pistols is the reliable, consistent air release provided by the regulated pressure and release of the regulated air via the calibrated transfer valve. - Constant pellet velocity over a declining cylinder pressure as shots are taken.

Because the pressure builds up so quickly in a springer, localized temperatures exceed ignition temperatures and "boom", detonation and potential dieseling if flammable oils are present. (heat, oxygen, fuel). PCP's achieve energy transfer over a longer period of time as air passes through the transfer valve and builds up behind the pellet, ejecting it out the barrel. The air is already compressed before the firing sequence, and even though the pressures build up behind the pellet, the pressures cannot increase over what it already was in the firing chamber, and except for frictional losses net heat would not be built up at that point. (Oxygen, fuel (maybe), not enough heat energy to ignite).

Think of it this way: in a springer, the energy transfer is from your arm via mechanisms to a big spring; on firing, the energy in the spring is almost instantaneously transferred via the now compressed air to the pellet, with great inefficiency (and heat build up of the compressed air). In a PCP, the compression comes first at its compression stage, and the inefficiencies occur at the compressor, with its heat build up. The compressed air eventually ends up behind the pellet, without additional energy input (and heat build up).

Or so I have read.

JE

[Gwhite]

Everything that happens inside a PCP pistol involves releasing pressure, from the cylinder to the regulator, from the regulator to the firing valve, and from the valve to the pellet & out.

Each decrease should actually result in a slight cooling effect.