Disaster... tube explosion!

[KPBadger]

Just managed to resolve the curve issue in the previous posting and machine doing perspex cutting jobs as usual. with perfect curves.

was on for maybe half an housr at %30 power and noticed power diminish rapidly to zero over the space of 2 minutes or so,
about to stop the job when bang, rattle of broken glass being pumped through laser tube, and water p*ssing onto floor.

so run to switch off mains and get rags to deal with water amid much swearing.

doesnt appear to have been any cooling problem - water about 20 deg C pumping as normal. the inner cooling coil cracked and the gas fill ,nipple' of the glass outer tube blown off.

any ideas what could have caused this ?



fortunately i have spare tube with which to complete a couple of jobs but the pain of aligning it is something i could have done without this week of all weeks!


cheers KPB

[zax15uk]

Ouch, sorry to hear that.

It does sound like a temperature related failure. Could there have been an air bubble trapped near the anode or at the end of the tube?

Once the inner cavity is fractured the power quickly deminishes before the tube fails, but I am slightly surprised the outer tube broke so quickly too.

Hope you get the replacement tube in and aligned quickly. Good news, you shouldn't need much adjustment as the tube angle won't change.

Zax.

[KPBadger]

Zax,

tube replaced no problem and thankfully power supply etc is fine. back up and running with little down-time.
if i hadnt had a spare tube in i would have been stuffed.

when i installed the new tube i was careful to check for bubbles and sure enough there was a 5mm dia bubble between the metal anode and the inner glass tube. was difficult to remove it! ( rotating the tube and pulsing the cooling water) but it did look like that kind of bubble would be sufficient to cause a thermal fracture since the heat would be transferring pretty much straight to the glass at that point.


hopefully there will not be a repeat performance.


cheers
kpb

[zax15uk]

I would suggest re-checking it after a day or two. The pump and tubing side walls often hold small bubbles which detach and move around the system - collecting at that location. Once fully removed, providing you have a sealed system (or no way for air to re-enter) you should be ok.

Zax.

[nobacoutinho]

what kind of water are you using? destilled or another water?

[lasersafe1]

I didn't know this could happen. I am nearing completion of a modification to my M40 laser where I have used a CPU pump and heat exchanger. Flow rate is about 1/2 gallon per minute. The heat exchanger is installed in my exhaust fan port, so it will have no problems with heat removal. I only have one nagging question remaining. I hooked it all up and then realized that my flow is backwards through the tube with respect to the original flow which was from the rear mirror toward the output coupler. I can't see why it would make a difference, but does anyone else have a reason it might matter?

[KPBadger]

Nobacoutinho :-

i just use ordinary tap water - i know i 'should' really used distilled or purified water but the tap water in this part of the world (Scotland) is quite 'soft' - it contains very little minerals. It does contain some chlorine though which is probably not good for the chiller. but i just let the tap water stand overnight before filling - thus much of the chlorine escapes. i havent noticed any corrosion or deposits in the water system.

i had considered using a weak mix of car radiator coolant to inhibit corrosion & algae . But there doesnt seem to be any problem. It would just mean i have gallons of antifreeze to deal with when i change the water.




Lasersafe1 : -

There must be some valid reason for the direction of flow, or else it wouldnt be specified. Perhaps the internal shape of the glassware is designed so that coolant flows into the various chambers correctly. Maybe so that the water arriving at the business end is pre-warmed? im guessing here.

surely its no problem to switch the inlet/outlet hoses on your set-up ?


regards
KPB

[lasersafe1]

Well this is interesting (or not), I just scanned the internet and found that most documents I could find were showing water flow in the direction of output coupler to high reflector. This is the opposite of the way my tube was originally plumbed. The reasoning on one of these documents was to put the coolest water right on the output coupler, which is the item that must actually pass some optical power. A high reflective mirror on the back side of the cavity will typically not generate much heat.

[zax15uk]

The most important thing is to keep any temperature differentials at a minimum.

If you have turbulance in the coolant flow it will create hot spots and crack the glass. Too much flow can be as bad as insufficient.

Another comment, after using the laser (especailly at >25% power) keep the coolant flow going for 10-15 minutes to dissipate heat.

Zax.

[lasersafe1]

Another thing worth mentioning to anyone else considering a move to a closed loop cooling system like the CPU cooler. It is vital that the system contain a reservoir section and cap with a small vent hole to allow the system to equalize to atmospheric pressure. If the system were completely sealed like is typically found in the CPU cooler, the pressure will build as the fluid heats up. This could cause the tube explosion.

[fogassa]

If it´s a chinese made tube then it´s normal to happen.I´ve seing some tubes gone like that for no reason. I´had a problem with my tube, it broken the inlet water chanel that cools down the back mirror, I was lucky that the magic gas didn´t scape. so I glue it back with epoxy and it still running.not a good finish cutting acrylic but cuts wood very well. I´m looking for a better quality laser tube thou.

[DaveDoesIT]

Originally Posted by KPBadger i just use ordinary tap water - i know i 'should' really used distilled or purified water but the tap water in this part of the world (Scotland) is quite 'soft'

Hi,

It's not just soft water it is the stuff in it. My guess is the pump was cavitating slightly and generated some air bubbles, these collected in the tube at a narrowing spot or a sharp bend. As more bubbles joined in it became one big air bubble. The pump would not be delivering enough pressure to push the bubble through so flow stopped and the pump went into full cavitation, pumping nothing.

That's why you bleed the air out of brakes systems and car radiators when you refill them. The air bubble acts like a soft spring and the fluid can't get through.

This another reason why it is a good idea to have a flow switch right at the outlet of the tube. If water is not coming out it turns off the laser.

Dave