Adjustable pressure relief valve?

[ayeckley]

In Team Update #5, rule <R72> was updated to mandate the use of an additional relief valve when solenoid valves with "maximum pressure ratings" less than 125psi(g) are used. Interesting that they didn't specify a relieving capacity (presumably they intend that it should be greater than the compressor max flow rate).

The bigger issue at-hand however is: where to find reasonably priced, properly sized, adjustable pressure relief valves? SMC doesn't appear to have anything other than residual pressure relief valves (which aren't the type of relief valves the GDC is referring to). I didn't find anything made by Festo either. Parker has some marginal candidates in their instrumentation product line, but they are still a bit too large and expensive. So far the best option looks like something from Swagelok, but they will still be relatively expensive.

Anyone found a better option yet?

[Peter Matteson]

McMaster and Grainger both cary a variety of pressure relief valves that would work.

I agree with yiou capacity would make sense to have on there but the kit PRV will still need to be in place, so they are probably counting on that to keep the system safe and the team to size the additional relief valve based on protecting their lower pressure rated solenoid valves.

[ayeckley]

Originally Posted by Peter Matteson

McMaster and Grainger both cary a variety of pressure relief valves that would work.

I believe you are correct. McMaster-Carr 8088K14 seems to be the best fit for this application. They seem to be intended for water as the working fluid, but usually air is acceptable as well (a call to McMaster should verify this). Unfortunately, the miniature nylon relief valves are only rated to 10psi and are not truly "adjustable".

It's not clear (to me, at least) from the rule change if a backpressure regulator such as 99045K11 would be legal or not.

[Peter Matteson]

Originally Posted by ayeckley

I believe you are correct. McMaster-Carr 8088K14 seems to be the best fit for this application. They seem to be intended for water as the working fluid, but usually air is acceptable as well (a call to McMaster should verify this). Unfortunately, the miniature nylon relief valves are only rated to 10psi and are not truly "adjustable".

It's not clear (to me, at least) from the rule change if a backpressure regulator such as 99045K11 would be legal or not.

I was thinking more along the lines of http://www.grainger.com/Grainger/items/6D915?Pid=search

It's smaller and lighter than what you were looking at.

I'm not sure about the back pressure regulator's legality or if you would want to use it. The p/n you list will constanly bleed air keeping your compressor running constantly. I would just use a second regulator and adjust the pressure down.

[ayeckley]

Originally Posted by Peter Matteson

I was thinking more along the lines of http://www.grainger.com/Grainger/items/6D915?Pid=search

It's smaller and lighter than what you were looking at.

Specific to that relief, I wonder if by "adjustable" they mean "discrete" settings. I've seen some that are adjustable only by means of changing the springs, and others where the springs establish the range of adjustment. The knurled part suggests there is a fine-tuning aspect, which is what most teams will need. Assuming it is, that part seems to be the best option so far.

Originally Posted by Peter Matteson

I'm not sure about the back pressure regulator's legality or if you would want to use it. The p/n you list will constanly bleed air keeping your compressor running constantly. I would just use a second regulator and adjust the pressure down.

That's not how I read the spec for that particular valve, although I agree that the typical backpressure relief valve is designed to operate with constant flow-through rather than shutoff. So, as-written it shouldn't bleed until the upstream side (the backpressure side) exceeds the set pressure (which would be somewhere between 60psi and the rated pressure for the solenoid valve we are trying to protect). I suspect that in reality this BPCV doesn't have shutoff capability, and that your statement effectively correct.

[Al Skierkiewicz]

I believe if you examine the effects of the rule change, based on a working pressure of 60 psi. A failure in the regulator could allow max high side pressure to enter the working side of the pneumatic system. If using parts rated at less than 125 psi then a relief valve (of a rating that will protect those parts) is needed as a safety to prevent catastrophic failures of those components.
Safety First everyone.

[ayeckley]

Originally Posted by Al Skierkiewicz

I believe if you examine the effects of the rule change, based on a working pressure of 60 psi. A failure in the regulator could allow max high side pressure to enter the working side of the pneumatic system. If using parts rated at less than 125 psi then a relief valve (of a rating that will protect those parts) is needed as a safety to prevent catastrophic failures of those components.
Safety First everyone.

Yep - I think we're all on board with the need for that valve to be there.

[dtengineering]

At least some of the pressure relief valves that have come with the compressor in previous year's KoP's have been adjustable.

Now the rules said that you weren't allowed to adjust them... but they were adjustable by turning the nut and head, then re-tightening them.

It should be fairly easy to demonstrate that they have been set to a safe working pressure (say 70psi) but temporarily taking the LP side regulator up over 60 for inspection, then dialing it back for competition.

Frankly, however, I'm not sure why this has suddenly become a big issue. (Yeah... because it's in the rules.) In the event of a catastrophic regulator failure (don't know as I've heard of one of those happening in an FRC robot before... although who knows what damage "the bump" will inflict this year) combined with a failure of the pressure sensor and/or Crio code to limit pressure to 110psi, the maximum pressure that the LP side would be exposed to is 125 psi. A valve that is rated for a safe working pressure of .7MPa (100psi) should be able to handle considerably more than that in an emergency situation.

I agree with the idea of safety first, but also in keeping our perception of safey hazards in persepective with the risks we take every day simply crossing the street. I agree that the second pressure relief valve makes the robot safer in theory, but am in need of convincing that it has anything but an infinitesimal value in practice. We'd be better off making sure that all teams and inspectors know what a pressure relief valve is and that they have one on their robot... I have seen at least one team make it to Galileo (yes, Galileo) without ANY pressure relief vale in their pneumatic system.

But hey... it's in the rules... so let's get on with it.

Jason

[robert.hatchett]

My initial response has been deleted... it has finally made sense to me...I guess due to the long day.

I still believe there is a typo/wrong wording in the update I think the intent behind this rule change is apparently this.........

"Normal" configuration of solenoids is on the "working pressure" side of the system (ie <= 60psi). No concern, right??

As many teams are augmenting pneumatic systems with potential energy schemes, there is the opportunity to back-drive the pneumatic system and create a system where (typo assumption here) pressures GREATER than 125/60psi can be created thereby the need for a low-side pressure relief.

Overpressuring systems should NEVER be taken lightly and can happen indadvertently (beware of the "it could never happen" philosophy).

[Homsar66]

Correct me if I'm wrong, but doesn't the regulator automatically vent pressure greater than what it is set at? So I'm confused as to the need for an additional burst disc.

[ebarker]

The Robot Rules specification clearly states that ALL components must be rated at 125 psi or higher. You have to provide documentation to the inspection station for non-KOP items to support the inspection process.

Many conventional directional valves, such as the KOP Festo and other manufacturers valves have maximum ratings a little below the FIRST specification of 125 psi. Therefore the valve you are buying to operate your robot will not pass inspection as the rule was originally written.

Plus there is the possibility of clever mechanisms creating an overpressure situation on the working pressure side of the system.

Therefore FIRST allow the use of a pressure relief valve on the working pressure side of the system that will protect both the directional valves and system designs that could create an overpressure situation. You could install a relief valve to release at 75 psi which would protect 100 psi rated valves and allow a 60 psi system to operate normally.

An example of this relief valve is the E292 here

[Al Skierkiewicz]

Everyone,
You are correct that a properly connected regulator, in good condition, in a properly working system should pose no problems. However, you need to inspect sometime with me to fully appreciate the need for some of these safety measures. I have seen teams who have had students disassemble pneumatic parts to see whats inside, and then reassembled with a few extra parts left over. I have seen systems where the pressure switch was on the low pressure side and the team wanted me to find them a new pressure relief valve since theirs kept venting. (they also wondered why their compressor ran most of the time. It only shut off when it reached protection temperature) I have seen teams with cracked knobs on their regulators from constant use and over tightening, regulators in backwards or improperly plumbed. Home made pneumatic parts, fittings and modified actuators. It is for these reasons there is a strict rule section on the use of pneumatics. I have witnessed catastrophic pneumatic failures and quite frankly it scared the **** out of me.

And that's just the pneumatics...

[ayeckley]

Originally Posted by ebarker

Many conventional directional valves, such as the KOP Festo and other manufacturers valves have maximum ratings a little below the FIRST specification of 125 psi. Therefore the valve you are buying to operate your robot will not pass inspection as the rule was originally written.

Frankly, I'm surprised that they went this long before addressing this exact discrepancy. We've always [metaphorically] crossed our fingers for luck during inspection, for fear that an inexperienced inspector may determine that a COTS solenoid valve is illegal while its exact equivalent from the KOP isn't. Following the letter of the law, that's exactly what *should* have happened (luckily it never did for us, and we certainly were never deceptive about it). Now, we've got much clearer guidance from FRC, and it is reasonably easy to implement.

[ayeckley]

Originally Posted by Homsar66

So I'm confused as to the need for an additional burst disc.

A burst disk is a completely different type of pressure relief device than what we're discussing. It's not really a valve. It's a membrane (usually metallic) that is designed to rupture at a precise pressure (although in practice there can be a large range of variability between discs) and allow the working fluid to pass, reducing the entrained pressure of the rest of the system. Once the disk bursts, it has to be replaced in order to seal the system again.

A relief valve will [theoretically] re-seat and re-seal once the pressure has been adequately relieved. In practice, there are usually some interesting dynamics that occur when a relief valve opens. If improperly selected, they can actually hammer themselves apart as a result. Often, they don't seal fully again once unseated. When you throw combustible cryogenic fluids into the mix, things can get even more exciting (not applicable to FRC).

[Gary Dillard]

So Al, how do we demonstrate to an inspector the setting of the pressure relief valve? If I take last year's relief valve, which is the same part number as this year's valve (Norgren 16-004-011), and set it lower (it is adjustable between 70 and 150 psig), what do I need to do to show that it is below 100 psi (the operating pressure of my solenoid valve)?