Compressor gets hot and blows line

[matthewdenny]

So when we enable the robot the compressor kicks on, the pressure builds in our high pressure tanks, gets to 65 psi, holds there for ~20 seconds, then the line coming out of the compressor blows out. The line and compressor are very hot when this happens. Any ideas?

[Nirvash]

How much air storage to do have? And does the compressor have a fan on it? The VIAIR really needs one if it is going to be running for any extended period of time.

[IndySam]

Compressors create heat when they compress air. In normal usage the heat is dissipated and doesn't cause a problem. If however the compressor is on for a long period of time, like in practice, there can be enough heat generated to soften the hose and create failure.

In good practice to connect the pressure relief valve and pressure switch directly to the compressor with brass fittings. This helps isolate the hose from the heat. The more metal between the hose and the compressor the better.

[JamesCH95]

How many tanks, of what size, are you trying to fill?

Originally Posted by IndySam

Compressors create heat when they compress air. In normal usage the heat is dissipated and doesn't cause a problem. If however the compressor is on for a long period of time, like in practice, there can be enough heat generated to soften the hose and create failure.

In good practice to connect the pressure relief valve and pressure switch directly to the compressor with brass fittings. This helps isolate the hose from the heat. The more metal between the hose and the compressor the better.

To that end, note the compressor in the lower left-hand corner of the robot: coupler, tee to relief valve, couple, tee to pressure switch, couple, tee to pressure gauge, then the quick-disconnect to a hose.

We also added the medium-sized muffin fan from the KOP blowing across the compressor and fittings, which helps a lot.

[Jarren Harkema]

We found that this happens as well. Mounting a 12V server fan pointed right at it keeps it as cool as a cucumber... or at least one that has been sitting on the counter. We had to run without the fan last night, and remembered just how hot it got.

[FrankJ]

Originally Posted by IndySam

In good practice to connect the pressure relief valve and pressure switch directly to the compressor with brass fittings. This helps isolate the hose from the heat. The more metal between the hose and the compressor the better.

Not only is it good practice, but the relief valve is required to be teed to the compressor with a metal fitting by rule.

[bachster]

Originally Posted by FrankJ

Not only is it good practice, but the relief valve is required to be teed to the compressor with a metal fitting by rule.

Technically, R87 states:
The relief valve must be attached directly to the compressor or attached by legal fittings connected to the compressor output port. If using an off-board compressor, an additional relief valve must be included on the ROBOT.

I agree a metal tee fitting is the best here. However, I think I had a RI or LRI last year state that tubing and plastic fittings would be allowed between the compressor and relief valve as these are considered "legal fittings." Of course, "legal fittings" are not explicitly defined anywhere I can tell.

R77 explicitly allows only:
A. Items available in the 2014 KOP,
...
E. Additional pneumatic tubing, with a maximum 0.160 in. inside diameter, functionally equivalent to that provided in the KOP,
F. Pressure transducers, pressure gauges, flow control valves, and connecting fittings,
...

There is also R75:
All pneumatic items must be COTS pneumatic devices rated by their manufacturers for working pressure of at least 125psi (with the exception of R77-D).

I don't read this combination of rules to mean that tubing would be a "legal fitting" to be used in between the compressor and relief valve (it's tubing, not a fitting). I did inspect a robot last year which had a flow control fitting (and tubing I think?) in between their compressor and relief valve. This is clearly an unsafe condition to me - if the flow control fitting was closed, it would prevent the relief valve from doing its job, instead allowing the compressor and fitting to reach higher than 120 psi. I think I had another team using their vent plug in between the two, which presents the same issue. I did require both teams to switch to the standard metal tee implementation (which they had to borrow - maybe part of the problem is these brass fittings are no longer included in the KOP).

To me, using normal push-connect fittings and tubing in between the compressor and relief valve is still potentially unsafe (and in my interpretation illegal), as any sort of blockage (floating thread wrap, melted tubing, etc) which could present itself in this path would prevent the system from venting at 120 psi. The heat from the compressor affecting the integrity of the tubing is another great point.

Al, would you say that the proper interpretation of R87 is that "legal fittings" are only metal tees? Or is the fall back to R8, for unsafe condition? Thanks!

Katie

[Al Skierkiewicz]

Katie,
Your statement is true. Legal fittings are more than brass. However,
R87
The relief valve must be attached directly to the compressor or attached by legal fittings connected to the compressor output port. If using an off-board compressor, an additional relief valve must be included on the ROBOT.

implies that best practice places it close to the compressor. Brass is still legal too.

[Al Skierkiewicz]

Does the tubing pull out of the compressor or does the line fail by producing a bubble and then blow out at the soft spot?
If it produces the bubble then the compressor is likely running continuous for several minutes (~10 or longer). It sounds like the high pressure relief valve is not calibrated. This is supplied uncalibrated and needs to be adjusted by the team to release at 125 psi. A compressor that runs for an extended period will blow out the line. It does this by heating the tubing to a temperature where it fails under pressure. To calibrate, loosen the locking ring on the valve, it is the lower hex hardware on the valve. Then power the robot and enable, short the pressure switch which should run the compressor continuously. With an accurate pressure gauge on the high pressure side, watch for the pressure to rise to 125 psi and listen and feel for air escaping the top of the valve. If it does not, turn the top counter clockwise until air is vented from the valve. If air is released at a lower pressure, then turn the top clockwise until 125 psi is reached when it vents. Tighten the locking ring without letting the top of the valve rotate. Remove the short across the pressure switch and see if the compressor shuts off when the pressure reaches ~120 psi.

If it simply pulls out of the fitting, one of two things are occurring. Your gauge is not accurate and you are exceeding 150 psi and failing at the fitting. Or the fitting could be bad and not securing the tubing. Once inserted you should not be able to pull out the tubing without pressing the release ring first.

[Mark McLeod]

Originally Posted by matthewdenny

... the pressure builds in our high pressure tanks, gets to 65 psi, holds there for ~20 seconds ...

That sounds like your main regulator is not plumbed correctly.
On the regulator there is an arrow on one of the ports. The high pressure tubing must go into the port that is at the tail end of the arrow, and only that port. The other three ports are kept at working pressure ~60psi.

[matthewdenny]

We are pressurizing 3 of the 41in^3 tanks.

The setup

VI Compressor-->Tanks (with pressure switch, and pressure gauge installed on the tanks -->Regulator (60 psi) ----Splits to two paths

Path A: ---> 30 psi regulator -->solenoids
Path B: ----> Solenoids

[matthewdenny]

Let me check the regulators, and get back to you.

[Kyler Hagler]

Like many have said, we always use lots of brass fittings at the beginning of the line from the compressor and put the required fittings on there as well, this helps because the brass dissipates the heat before it reaches the tubing.

[Nate Laverdure]

Reverse the polarity to the compressor?

Originally Posted by Al Skierkiewicz

The compressor runs in either direction. It is more efficient in the proper direction so the opposite polarity gets it a little hotter than normal.

Originally Posted by Don Rotolo

Temporarily disconnect the compressor and use a voltmeter for testing. reverse polarity may make the compressor supremely unhappy.

[Al Skierkiewicz]

Matthew,
Is this the same robot you are asking about in the Labview Forum? If so, there is something seriously wrong with the DSC or your code. The compressor should not come on until the robot is enabled. Neither should the RSL.