Testing of home-made pneumatic rams

I have a question. Rule 8.7.1 states Custom made components, or any parts operating above the suppliers max working pressure, must be independently tested and certified at 120% of the maximum system pressure available at that point.. I have a custom made ram which is only 1-way acting, and has holes drilled around the top so that when the ram is at full height the CO2 can escape (gives it a bit more speed when activating). Do I need to get this tested, and if I do, how can I do this if the ram will not hold pressure when activated due to the holes?

Any suggestions greatfully received...

Dave

I assume the CO2 exhausts through these vent holes when the piston has reached close to full stroke otherwise you will be losing power whilst stroking. Assuming this is the case and I am at a bit of a loss to appreciate why you have this set-up, anyway, blank off the holes or limit the travel of the piston to just below the holes using tie rods or similar.

It is not uncommon to have holes around the top of the cylinder to allow the top side of the piston to breath more easily but not the underside (that is usually achieved using a 3 port valve). The way you describe will waste valuable gas and reduce the final velocity and force of the piston.

Or am I missing something?

Paul
FRA Pneumatics advisor

One thing you might try... and Menthorn would look at... If the ram gets stuck somewhere along the way then it might not make it to the venting holes. So as a test fix the ram in such a position and put the pressure on. If it blows it blows otherwise.. a clean bill of health id say. As for exuasting the high pressure CO2.. make sure to use a falve vent (i.e. 3/2 valve). Just incase the piston doesnt reach the venting position.

Having the holes above the final position of the piston sounds like a better idea than having them below the final resting position. The reason we have them there at the moment is to let the air around the top of the piston move out quicker when I activate it. If I put the holes in the top then I suppose I can get it tested just like any other piston. We use a separate valve for exhausting the CO2.

Thanks for the advice.

Dave

Ive used the method you mention on a (vru old) featherweight.
It did have some advantages, such as reducing the vent time and allowing more flips per second, but the downsides were too numerous and I ditched it (low pressure setup)
I have suggested this before, and will again.
Andy Kane of Kan-opener, and FRA chairman, once offered to test any hydraulic rams if people wanted, if you e-mail him, this may apply to pnematic rams too. The holes are no problem, Im sure the ram can be positioned so they are not used.
Cheers
James

A further question. I have made a full pressure pneumatic ram, and through work I can get it hydraulically tested for free. I would like to take advantage of this offer if it is acceptable. If necessary i can hydrualically test it to 200Bar- is this satisfactory for a 70 bar ram, or must it be pressure tested using gas?

As I understand it, hydraulic cylinders are de-rated by 50% for pneumatic use, and pneumatics parts must be tested to 120% of their operating pressure - so you are OK in terms of static pressure, but when you use it with gas your cylinder will be moving much faster, and your cylinder will have to cope with the dynamic forces when it hits the end of its travel at high speed. Testing this is more difficult, perhaps someone experienced with high pressure pneumatics could help here.

Eddy i dont know myself but at a guess I would say it would be ok as in the rules Rule 8.1.2 states: Components origionally designed for hydraulics use will be de-rated by 50% for pneumatics use.

Maybe I am reading it wrong but surely if you test it hydraulicly to 200bar you can use it at 100 or am I wrong.

Regards
Ian

Eddy,

Its safer to hydraulically test the ram, as the stored energy is much less. This is the industry-standard test method for vessels, so I imagine that it would be fine for rams.

Jeremy

If you take an unmodified standard hydraulic ram, you can use it on pneumatics to a pressure level of 50% of the rams hydraulic rating or 1000 psi - which ever is the lower without requiring further test certification. It is permitted to mod the ram as long as those mods cannot affect the pressure integrity of the ram. Cushioning is a common mod undertaken to absorb some of the dynamic forces.

You can use gas or fluid to pressure test the ram, if using oil you will need to clean / flush the oil out afterwards.

Paul
FRA Technical Team

I made the ram myself at work from some hydraulic tube. It was built with theyre help and guidence (the company is Southern hydraulic and the make and repair hydraulic rams). So although it is not strictly a modified hydrualic ram, it is exactly the same thing thay theyd sell if you went to them and asked for a custom ram. The only difference are the seals, which are rated to a lower pressure (about 150Bar iirc) but are of a nice low friction. Under the rules is it acceptable If i hydraulically test it to about 150Bar? (seeing as it *is* a hydraulic ram, made custom at a place which builds hydraulic rams)

P.S Im aware of the dirrences between static and dynamic pressure testing. But this ram is pretty belt and braces :)

Okay, my first post on this forum, but here goes...

There is a huge difference between the energy stored in a compressed gas and a compressed liquid for the same pressure (compressed gas store orders of magnitude more energy than a liquid for a given pressure due to the substantial difference in compressibility and the whole P*dV work stuff). Is there any difference in safety guidance between gas vs. liquid systems? It would seem that gas systems would be more strictly controlled. If a gas system fails, the energy release and associated hazard of this failure would be much higher than for a liquid system.

Regards,
Mack

You are entirely correct, but it is quite difficult to actively discriminate between the two. It is best to take the view that they are both very very dangerous (or rather, have the potential to be if mistreated) and extreme care and caution must be taken which designing and building both systems. If either of the systems to fial, lets hope its in the arena. To give an idea of the magnitude of the mess of a system failure, I would point you towards whichever heat it was whan Spawn Again losts its End cap when running a Low pressure ram on Full Pressure. Certainly not something youd want to happen in your garage, but nothing the arena couldnt cope with.

I have a question based on what weve acheived above in the conversation: should I treat my purpose build ram as a hydraulic one when interpreting the rules? Or should i just be safe and pressure test it to 200Bar?

Eddy .....There is a BIG event on this weekend .....so I doubt that youll get an official type answer much before Tuesday..

You dont need to tell me Alan- I wish I was there! I was just posting so that its there ready for monday :)

Well ... In lieu of an accredited expert Ill throw in my twopenorth.

Id suggest that you get your company to treat it as they would any other hydraulic ram and certificate it to a working pressure of 150 bar ... It would then become a standard ram.

This should result in a ram that ( when its derated by 50%* ) should easily meet the FRA rules.

* If you take an unmodified standard hydraulic ram, you can use it on pneumatics to a pressure level of 50% of the rams hydraulic rating or 1000 psi - which ever is the lower without requiring further test certification

Im working on the assumption that Pauls hydraulic rating is refering to working pressure as opposed to test pressure.

Eddy, assuming the ram has been built using standard cylinder tube, rod and end caps that your employer would normally use then it sounds very much a standard ram to me. I would suggest that you ask your employer for a certificate of conformity for it to confirm this fact. Thank you Alan, that just about sums it up.

Mack, all pneumatic systems are considered as gas systems rather than liquid even though the CO2 is stored as a liquid and sometimes injected into rams as a liquid. This is because the CO2 liquid rapidly boils off as a gas when the pressure drops.

Paul
FRA Technical Team

Thank you paul. I will do that- its not quite built as a normal ram in that I have stronger, tie-roded end caps and low friction o-rings, but certainly it could pass as one :-)

Many thanks for your confirmation,

Eddy

Slightly off topic I know but I was just curious. Do I need a cert for my 600g bottle? I have pressure test certs for my ram, valve, buffer and my 2 prds but I dont have one for my 600g co2 bottle. Do I need one or does everyone just kinda say ah well its a 600g bottle we know thats safe.

Regards
Ian

Normaly the stamp and other markers applied by the firms working with them are enough, as a normal bottle is its own certificate.

Things handy to know and to look for on a bottle.

Datemark of last testing-some bottles have up to 4 datemarks up on them. Every 5 years a bottle needs retesting, max age of a CO2 cannister is 20 years.
With a very new bottle a Pi stamp.
A country related pressure testing mark is another stamp common.
A tarra and netto weight, sometimes a volume is stamped in.
Test pressure and work pressure.
Brandname, or ownername-there are some Maddox branded bottles around, as the firm that does the CO2 bottles for me marks them as being mine.