Glass for inspection window that meets CSA C22.2 No. 30 1

[BigMac23]

I am currently designing an Explosion-Proof Enclosure for Use in Class I Hazardous Locations (C22.2 No. 30). This enclosure will have an inspection window in it. It will be Ø3.25in (Ø2.50in unsupported). I have tested 9mm thick tempered borosilicate glass which has failed.

Does anyone have have recommendations for thickness and/or type and condition of glass?

To be acceptable, the window has to withstand a 4J impact and then a pressure test of 300-500psi.

Thanks in advance for all comments and help.

 

[Cster]

Big Mac23,

There are a few options for you with your glass lens. I guess the first thing to ask is have you tried Soda Lime, or making the glass you have thicker? Where and how exactly is it cracking. Do you know if the cracking is a function of the glass or your method of retaining the glass lens in place (retaining ring, cement material, clips, etc.). Perhaps your method of securment is causing unduly stresses on the glass lens?

Please know there is no requirement in CSA C22.2 No. 30-M1986, that says that the Impact test has to be done in series with the Overpressure test of Sec. 6.6 on the same sample. If the standards you are talking about are IEC 60079-1:2003 or the Canadian E60079-1, then its a different story, but you did say No. 30. So, perhaps if the impacts were performed on the same sample that didn't pass the overpressure you may see more favorable results if separate samples are tested.

You didn't mention who is doing the testing or if you are doing the testing in house, but sometimes test laboratories will perform testing on one sample if you are not agreeable to providing them separate samples.

Another thing you may consider. The equivalent UL Standard for Explosion proof equipment for Class I, Division 1, UL 1203. It has similar over pressure test requirements, but they call it a Hydrostatic Pressure test. You'll see if you look at UL 1203, Sec. 19 that there are a lot of stipulations for the pressure to be used. Many times they will use 4 times the maximum reference pressure obtained from performing explosion tests, but there is an option where you can have an enclosure tested at only 2 or 3 times the max. explosion pressure depending on the pressure rise time seen from explosion testings. This option, however, will require 100% testing at 1.5 times the explosion pressure on all production models. In addition to a Thermal Shock Test, UL also performs an Impact Test on glass materials completing an explosion proof enclosure, UL 1203, Secs. 22.1 and 22.2. UL 1203 also does not require that the Ipact test be done in series with the hydrostatic pressure test. Also note, the test time is only 10 seconds.

For follow up services purposes, UL will require that the glass lens material be tested on an annual basis to verify that the lengs material being used has the same strength as originally certified.

Another option you may consider is lowering your explosion pressure. If you are supplying conduit seals directly heither to the enclosure or supplying a factory installed seal, you will get lower explosion pressures than if the conduit seals were located at the maximum 18 in. length. This is because the explosion tests would be performed with the ignition source, (spark plug), closer to the enclosure conduit entry location, (as little as threaded right at the wall depending on the marking on the seal length). This could lower the explosion pressures the enclosure would see from testing, and the hydrostatic pressure strength test pressure requirement, (which is a factor of explosion pressure), wouldn't be so great.

Hope this is going to help you.