Pneumatic Relief Valves

[maddox]

No Holger, but what Helmut says is simpel.
The cheapest PRDs useable for our purposes has next to no flow.
So if you use ,by -accident- an 200 bar argon bottle, the PRD will vent, but will not protect your system from overpresurizing.

Now this is not a problem for us RW CO2 users, normaly...As the overpressurazation in a CO2 bottle will fall very fast when the valve is opened.
But Helmut has a point with flowrates.. and even the most minimal will do for us.As the valve will bleed from moment one, and CO2 can be up to 195 bar in the bottle ,but after opening of the bottlevalve this wil drop very fast.

[m2xt]

I will try and answer the above questions on the FRA pneumatic rules as best I can. Firstly, thank you for raising these points as I am keen that the pneumatic rules stand up to scrutiny by builders and if not, then we amend the rules accordingly.

Sorry but quite a long post:

The purpose of the PRD is to ensure that pneumatic components are not operated outside of their safe working pressure. There are 4 main ways in which excessive pressure can occur:
1. Over adjustment of gas regulator
2. Overfill of gas bottle
3. Failure of gas regulating device
4. Pressure rise caused by increased temperature

So using Helmut€™s 2 bar circuit as an example, as long as all components in the 2 bar circuit are rated sufficiently so that they are protected by the PRD on the circuit feeding regulator then you do not need an additional PRD.

Example:

HP to LP regulator set for 10 bar. All components in this LP circuit rated to at least 10 bar, PRD set for 11 bar. If you now have an another regulator taking this 10 bar regulated pressure down to a 2 bar circuit but all components in this 2 bar circuit are rated to at least 10 bar, then you do not need an additional PRD. If they were only rated to 8 bar for example, then you would require another PRD.

The PRD often supplied with regulators are generally unsuitable but not always. My experience of them is that they tend to be rather crude and often will not work well when used with a set pressure within 10% of the working pressure. If they do operate, they frequently do not seat again when the pressure returns to normal. Also, you will need to get the PRD certified because I am not aware of regulator companies supplying PRD test certificates with their regulators. So in summary, if the PRD is suitably certified or at least carries the relief pressure engraved / stamped on the PRD body, then it should be acceptable.

Kane, Pneumatic circuits operating at less than 3.5 bar do not require a PRD as you stated. This mainly relates to Rex€™s Robot Challenge type machines where a reservoir is charged using a foot pump or similar. If the circuit derives this working pressure from a higher-pressure circuit, then you may require a PRD to cover over-pressure conditions as described above.

Mario, the testing of a PRD will only be undertaken in exceptional circumstances. We are placing trust in the Roboteer to behave responsibly. However, should an incident occur which suggests that a PRD may have been altered or the performance of the robot suggests that not all is quite right, then the event organiser reserves the right to have the PRD tested. It is unlikely that the event organiser will be able to accurately test the PRD at the event but we may need to have access to a simple test rig that permits a quick check. I would suggest that until the need arises, we leave the testing aspect to one side but I will give it further thought.

Mentorn are rather relaxed about pneumatics as long as the Co2 bottle is fine and there is a dump valve, then they appear quite satisfied. The FRA needs to take a more active route to ensure safety standards are maintained. Lets assume that at RW€™s, someone gets hurt and upon investigation it transpires that the builder has compromised the Mentorn rules. What do you think Mentorn will do, personally I would expect them to hold the builder accountable. Let€™s avoid this situation by taking all reasonable precautions to ensure a safe system.

Helmut, the requirement for a certified PRD is for trace ability. I would accept a certificate of conformity that carries a batch number which corresponds with the PRD. I would also accept a PRD stamped with the pressure setting as long as there was some indication as to the standard used for the testing (such as DIN 3320 or BS6759). This should not result in a more expensive product, as any commercial PRD should carry this information anyway.

As for the rules specifying the nominal flowrate of the PRD, this is covered by the statement that the PRD must have a flowrate capacity that exceeds the maximum flowrate that can be expected under over-pressure conditions. This places the onus on the builder to engineer the system correctly. For the HP side, an over-pressure condition is most likely to be caused by over-temperature. The pressure rise will be gradual and a small PRD like ¼€ BSP size will be more than adequate. For LP circuits, an over-pressure is most likely to be caused by a regulator failure, regulators typically have a relatively low flow capability and once again a ¼€ BSP PRD is likely to be adequate. However if you have multiple regulators or a rather special regulator with a high flow capability, then you need to verify that the PRD can handle the potential flowrate.

Holger, the PRD has to accommodate the flowrate that occurs under fault conditions. So in the example you quoted, a FP system with buffers is not a problem. To cause the pressure of the FP system to rise above 1000 psi is going to be temperature (ignoring over-filling). From memory, 1000 psi is reached on a CO2 system at 31 DegC and as soon as the PRD starts to relieve, the CO2 starts to cool and the pressure momentarily falls. During the summer at an outside event, one robot with 4kg of CO2 had his 1000 psi relief valve €˜popping€™ quite frequently. I also experienced this, once I opened the gas bottle, the relief valve partially opened allowing some gas to escape, this continued until the first flip and then all was fine.

Hope that clears things up but if not, say so.

Regards

Paul Cooper
FRA Pneumatics Advisor

[jeremy_cuss]

I agree with everything that Paul has said. A CO2 gas bottle at 53barg will give a maximum flow of about 4500 litre/min of free gas through a Kenmac regulator with a Cv of 0.5. Assuming that this gas is at 16barg, a small relief valve with a 1/4BSP connection should offer a capacity in excess of 10000 litre/min of free gas at 17.6barg (depending on the make of relief valve).

Jeremy

[blazerbotics]

sorry to bring up an old thread (better then starting a new one tho). is this required on featherweights? for example G2 uses a 600g bottle, its then regulated dowm to 10bar. So in theory it needs a 1000psi PRD between the bottle and reg, and 10bar PRD on the LP side, is this correct?

If so its alot of money to put a 1000psi PRD into the system, as theyre about £50 its alot of money. I have no problem fitting a PRD on the LP side, and to be honest thats where ive seen most faults in systems.

Is there, or could there, be any exception for feathers, for example; that they have to have only a PRD on the working pressure (ie 10/16bar for LP systems and a 1000psi PRD for full pressure systems). I know CO2 is dangerous what ever weight were working with, but its alot of money for most featherweight builders.

The pipe we use between the reg and bottle is hydraulic copper piping, which is more then up to the job. All of the fittings are hydraulic aswell. The regulators have written on the Max WP 2000psi we know that the pressure will never get anywhere near that (as the disc should blow on the tank) so i cant see that danger, as long as its been built well and correct components have been used.

[stu]

Alan, in feather weigths - a burst disc on the bottle is acceptable. Jonno checked with Paul Cooper (FRA Pneuamtics bloke) and the Burst disc on the 600g co2 bottles is fine to act as a High pressure relief valve.

But in all honesty im not sure about a low pressure relive valve. I feel there should be a small one on the low pressure side as well - but i dont know, have to ask Paul on that one.

But yeah - you wont have to spend £50 on a HP relief valve, you already have one so to speak.

Mr Stu

[blazerbotics]

just to bring this thread back again! but after the confusion at Brighton over the 1000psi PRD on feathers. things need to be made clear for all feather builders.

My understanding of the rules were as above, in stus post. But then at brighton we were told that G3 couldnt run without a 1000psi PRD. There was then some confusion over the burst discs and the set pressures. as the code on the disc is PN250, people were assuming it was set to 250bar but the bottles are only tested to 205bar. we were then told that we were allowed to run g3, my understanding was due to somebody saying that the disc was set to about 1000psi.

I know the subject was raised at the FRA meeting, and it was stated that feathers MUST have a 1000psi PRD. If this is the case, then its going to cause a few arguments. I have already spoken to a couple of builders who are building and one of them has scrapped there plans to build theres due to the added costs. I have put my plans on hold for Delta as i can afford to buy a PRD for it. As i already have to buy one for my middle.

As i said above, i have no problem in fitting a LP PRD but i think a 1000psi PRD is abit over the top for a feather (see my above post for thoughts).

Could we please have a definate answer, and make sure people are aware of it.

[m2xt]

Alan,

Rule 8.2 states the maximum pressure within the pneumatics system shall not exceed 1000psi. Rule 8.8 further defines the maximum pressure being determined by the PRD.

At the FRA meeting I stated two exceptions to a 1000psi PRD requirement, for sytems having a max pressure below 50 psi and for compressor driven systems with a maximum output pressure lower than the lowest pressure rated component.

I certainly do not agree it is over the top for feathers, if its good for heavies its equally as good for feathers as the consequencies are just as serious. Remember, its function is not only safety but also to ensure a level playing field and avoid the need for test points.

If a builder cannot afford the safety devices then they should reconsider if CO2 powered pneumatics is for them.

If you wish to propose that the 1000psi relief valve is removed as a requirement for feathers, you know the procedure. I am an advisor, it is for the voting FRA members to decide one way or another and rightly so.

Paul
FRA Technical Team

[m2xt]

On a personal note:

Technobots currently do not sell the 1000psi PRD for 2 reasons, one is to avoid any suggestions of conflicts of interest and two more importantly is because demand has fallen to such a level as not to warrant the manufacture of another batch.

Paul

[blazerbotics]

where can we get them from?

In regards to:

Remember, its function is not only safety but also to ensure a level playing field and avoid the need for test points.

how can it effect a LP system, and as i said above, i have no problem putting a LP PRD in. also a test point would be alot cheaper to put in then a PRD.