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