Improper Method of Adding Shock

My electrician handles the maintenance on his pool. He asks me questions about the pool, I ask him about electrical issues. We're all good.

He called me this past weekend somewhat in a panic about the piping at his equipment exploding after super chlorinating.

After finally getting the facts straight, he told me he added 2lbs of cal hypo shock directly into the pump housing and re-started the pump.

After starting the pump, he was leaning over the equipment and the explosion occurred. PVC pipe fragments were hurled 35'+. He sustained some facial cuts from the flying pieces and got a slight chemical burn from the shock. He was lucky.

Apparently he has done this in the past without an occurrence!

The chlorine gas, I suspect, built up in the pump and or piping and created this explosion.

Attached is the photo of the result.

I'm wondering if anyone has had a similar experience.

I explained the proper method to adding shock (cal hypo in this case) to the pool is to dissolve it in a CLEAN bucket of pool water in small amounts. Then walk the pool perimeter and add to the pool.

Some crumbs at the bottom must be dissolved completely.

In my 30 years in business I have not seen anything like this. I'm aware of a lethal cocktail when acid and CL are mixed. Note CLEAN BUCKET above.

The piping was repaired and the system is back to running normal.

PVC Explosion.jpg

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  • Concentrated Cal-Hypo in a small volume of water will saturate it somewhat similar to very concentrated bleach but high in calcium.  Like chlorinating liquid or bleach it can decompose (over time) into oxygen gas (accelerated by the presence of metal ions) and also outgas chlorine.  This is why one never adds water to Cal-Hypo but rather should add Cal-Hypo to larger volumes of water.

    However, it usually doesn't vigorously react unless acid is added to make the pH low or unless it mixes with organics such as Trichlor where the latter combination produces nitrogen trichloride that can be explosive.  It's possible that the regular movement of pH buffered water through the Cal-Hypo produced gas and it's also possible that the calcium mixed with bicarbonate in the water to plug up the pipe with calcium carbonate though I don't see evidence of blockage in the pipe itself.  It's more likely that calcium carbonate blockage would have occurred in the filter so maybe that happened and the back pressure burst the pipe instead of elsewhere.

    Did he mention that the filter pressure increased or was clogged and needing cleaning?  If not, then it sounds like the pool water combined with concentrated Cal-Hypo to produce an explosive gas, most likely nitrogen trichloride.

    • No mention of filter pressure.

      Shock had no place to go as it resided in the pump basket, built up gas and created the explosion.

      Pretty sure no tri-chlor in the skims either.

  • I use cal hypo like people drink water!! ( due to no cya).. You can MEVER ADD CAL HYPO TO THE SKIMMER BECAUSE IT IS A COAGULANT.. IF YOU ADD IT TO THE SKIMMER.. Go and watch the filter pressure.. IT RISES.
    ( I actually place it in my net and then walk around the pool and it will disolve quickly. Make sure you don't "sink your net at the bottom.. Or you will get white residue at the bottom of your pool)
    If you don't have or want to do the dissolve bucket method
  • It was quite an explosion. He was lucky.

    I have no idea what possessed to add the shock in that manner.

    He certainly won't do it again.

  • Hoooly S**T! I'd say your electrician friend was DAMN lucky.

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