Pneumatic Relief Valves

Hi!
I have a question about point 8.8. of the rules:

A certified pressure relief device [PRD] must be installed in each part of the pneumatics system where
a different operating pressure is used (operating pressures separated by a pressure regulating device).

That means, if i have three different working pressures for example:
i use full pressure for main cylinder, 2 bar for retracting main cylinder (for saving gas) and 7 bar for a short stroke pushing device on the backside of the robot (to push the robots frontside down during acceleration). In this case, i need 3 pressure relief valves according to the rules! Normally a 3 way pressure regulator have in fact a built in relief valve to prevent a higher pressure than adjusted in the secondary (working) line. Is this kind of valve legally valid as a relief valve ?
Obviously i´m talking only about the low pressure circuits.

BTW: a big thank you to all organizers and moderators for this great new forum!!

Helmut

The rules all work together... so:

8.8. A certified pressure relief device [PRD] must be installed in each part of the pneumatics system where a different operating pressure is used (operating pressures separated by a pressure regulating device).
8.8.1 PRD(s) must have a rating of 1000 psi (68 bar) or 110% of the pneumatic component with the lowest €˜maximum working pressure€™ rating protected by that particular PRD, whichever is the lower.
8.8.2 Pneumatic systems employing pressures less than 50 psi or systems employing air compressors that have a maximum output pressure lower than the pneumatics component with the lowest €˜maximum working pressure€™ do not require a PRD.

2 Bar would not require a PRD as you can run up to 50psi (3.5 Bar) without a PRD.

The PRD need only be rated at 110% of your components rating, not your actual working pressure.
In other words, regardless of your actual running pressure, the PRD is rated for your components to protect them.

As you have stated, the low pressure circuit is already protected by an integral PRD in the regulator, usually set to 200psi.

Basically, although you are running 3 different pressures, you are only using 2 different ratings of components. And therefore only require 2 PRD.

Kane Aston
http://www.makerobotics.comhttp://www.makerobotics.com/

Thanks!
as far i understand now, i need only one relief valve for the main low pressure circuit although i provide some subsystems from this point with additional regulators at diffferent pressures, if the rating of the used components is equal or higher than the main supply pressure. Is this correct ?

Helmut

In most cases the regulator,to provide low pressure in a system has its own presure relive valve.So most people using low pressure systems are safe on that acount.(if they didnt fiddle with it)
Combined with the commercialy available 1000psi PRD with a 1/4 bsp tread,most regulators can be modified with that, just replace the bottleside clock for this PRD.

Now for the pure Full pressure users this commercial PRD is not difficult to be fitted in the system,anywhere between the bottle and the first valve in the system is more than adequate.Nothing more that a T adaptor.

Another ,and more interesting question.How are the technicals going to test this PRDs?

Mario
my question wasn´t is it adequate? (designing, construction and repair of adequate hydraulic and pneumatic circuits is my occupation) rather the question is is the circuit according to the rules?

How are the technicals going to test this PRDs?
in theory this should be easy: a certified PRD have a lead seal and and is adjusted to the certified pressure. In practice, this point will be a little bit tricky.......

Helmut, that you dont have any trouble with making a safe system is already known to me, and that wasnt my point.
Just commenting that in most dual systems this PRD point is mostely adressed to, on the exception of customized things.

I myself have equiped several robots with custom PRDs,or these were completey selfbuild(copies in better material)or adapted bought ones, Non came with a lead seal.

Now my main Q, as in robot wars many people are pushing the limits of the stuff, and in some cases invent new things to do wath they want,How are the technicals going to check if all complies tot the FEA rules.

In Robot Wars I never had any pressure check on the PRDs, and these arent cheap components when bought.So why fit them, the take space, weight and money.. and nobody bothered?

Also, Im somewhat the main CO2 man in the Netherlands and Belgium,and I want to know... does anyone has an idea to check everything works like it should...Some of the roboteers can build good stuff, but this leaves open the possibility of mistakes or wrong messurements.

mario
i test and adjust my relief valves, pressure switches and my custom made stuff in the workshop normally with a enerpac hand pump (and AFAIK, you too), works up to 1500 bar with normal hlp 32. works easy, properly and safe, needs no heavy aditional equipment, but cleaning after test can be a problem on a event.
the annoying word in the rules is certified.
In austria and germany a certified PRD is 2 times more expensive than the equivalent normal valve (same brand and type, but pre-adjusted and sealed).
in all the rules (RW, BB, FRA) i´m still missing the nominal flow of the PRD at relief pressure. Theoretically I can use a certified NS 0.1 mm PRD, witch is useless in some cases of overpressure.

quote:

N.B. The PRD(s) dictate the maximum pressure available in that part of the pneumatics system. The
PRD(s) must have a flow rate capacity that exceeds the maximum flow rate that can be expected under
€˜over pressure€™ conditions.
This may cause problems. If I do understand this correctly, if Im using a FP setup with buffer tanks, I have to use PRD with a higher flow rate than the one between buffer tank and ram...?
This is close to impossible.

Holger

ups, haven´t read this remark in the rules...
holger, IMO you need only 1 PRV with sufficient flow rate in the FP circuit on the primary (supply) side, not between buffer and ram. Your huge Niedozka- Valve will do the job easily :)

quote:

huge*lol* Compared to the gas bottles outlet diameter - agreed. Hope this is how its meant to be.

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.

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

Paul, thank you for the detailed explanation. All points sounds very reasonable and useful.

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

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.

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

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.

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

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

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.

It shouldnt affect LP systems, but it will affect FP for obvious reasons. If someone knows of a supplier of 1000psi PRDs, then please let me know before I begin an assault on the RS website.

Alan,

The effects on regulated LP systems are less obvious than HP systems. Higher bottle pressures means higher delta pressure across the regulator and thus improved flowrate. Also as you know, as you use CO2 it chills, start off with a warm / hot bottle and this effect will be delayed. For reference and from memory at 31 Deg C, CO2 behaves in a different way with further temperature rise causing an exponential rise in pressure. This point is I believe at 1000psi. Dont worry, I have used and understand the arguments against the 1000psi relief valve.

As I said propose its removal if you wish.

Paul
FRA Technical Team

Gloor has some regulators for oxygen that have a 900psi relief valve.
I had a batch of those, and can probably reorder, but not for 2 ofm.
The last time I was rather unplesantly surprised by the pricetag for less than a 100 seperate ones. And normaly they come as a part of a repair/overhaul kit, that is about as expensive as a new regulator.
If enough people contact me, I can try again for a useable deal.

And, when using CO2 or any other storaged gas on pressures higher then 16 bar(max nominal pressure low pressure systems) I insist on a relief valve, set at the maximum allowed pressure of the system or at 1000 psi as maximum in the rules.

Now, according to some I do not know what Im talking about, so ignore the rambling.

No, but maybe people are being deceived by your poor grammar...
For those who are: you are dealing with a Belgian for whom English is not a native language nor has he had English courses for 8 years in a row like I (fellow Belgian) had. It does not have anything to do with his technical knowledge. He is however speaking not only out of school training, but also some 10 years of practical experience as well.
Mario is of the Technical Committee in the Dutch roboteers association DRG too. Like Paul Cooper he gives advice, he does not state laws. And is open for suggestions.

Im just trying to make things clear to everyone, not simply defending my lifemate. Right ?

CO2 behaves in a different way with further temperature rise causing an exponential rise in pressure. This point is I believe at 1000psi.

CO2 Dynamics Graph...... In support of Pauls statement.

http://www.warpig.com/paintball/technical/gasses/co2pv.gifhttp://www.warpig.com/paintball/tech...sses/co2pv.gif

sorry, did I miss something?
who said Mario doesnt know what hes talking about?

Thank you Alan, you provided that link some time ago but couldnt find it this time.

James, certainly not me!!

I think for those ppl that know Mario, Do know he knows what he is talking about

does he?

Maybe thats why my robot was on his coffee table in the living room.

:)

Im glad some people do.. considering I dont read half of the post anyway.. and when i do, I havent the foggiest what you lot are on about *giggles*

oh.. the joy of ignorance *giggles*

Ah Audrey, but that fades away in the blazing glory of learning... think of all the new exciting ways you can view the world once you learn the mechanics and formulas that make it tick. Or you can go tock. Or Cuckoo...

If Paul is no longer going to stock this item does anyone know if they are available elsewhere ? If not there seems little point in building a pneumatic system as you cant use it anyway.

Simon, working on it...In the worst case I can custom build one.And so can some other roboteers.
We just need to pool the resources, and find a way to certify the relief pressure.

PHEW !!! :)

Mario
Beware that regulator relief valves often do not carry a CE mark because in the regulator manufacturers small print, they state something like the relief valve is to protect the regulator and not the downstream devices. They may also quote a restriction on line size of 25mm. I have some concern over the validity of this means of bypassing the PED.

Since the PED came into force, all of the relief valve manufacturers Technobots has dealt with, now fully certify their safety relief valves with a CE mark. My understanding is that this is a requirement within the PED.

Simon etc. If sufficient interest exists, I will get another batch of 1000psi relief valves made. Consider that it takes nearly one year for Technobots to sell a single batch and that was with RW in full swing! I can get smaller batches made but the price goes up steeply.

Paul

What sort of numbers are you thinking of Paul? And how steep is steep? Could make the difference between me continuing the build or not.
Thanks

If you writing a list of people who need a 1000psi relief valve, stick me on it! :) If not, any ideas how mutch they cost EACH new? As I will need one before Debenham.

Regards
Dave moulds
Team Turbine

I would buy one too Paul

Anyone had any luck with a possible supplier ? Ive e-mailed a few but no responce to date.

i spoke to our local hydrualics supplier, they dont sell them ,but are looking for suppliers.