Last week, we asked for help on tanks and were looking for answers. Several stepped up to the plate and offered their thoughts:
I think I have the answer you're looking for.
Peroxide tanks have a "floating manway" or "Emergency Relief Manway" - so in the case of bad juju (peroxide rapid decomposition which I believe is 1 - 10,000 in volume going to gas) there is a large relief and the tank doesn't explode.
The 1 to 10,000 is from memory, having worked in 2 new mills that had peroxide tanks installed. It might be "only" 1 to 1,000.
Have a roof that floats (easily). The first new mill I worked in had fiberglass roofs on tanks (in addition to overflows). These were installed on threaded rod with nuts threaded on about 6" up. It was quite spectacular when the sludge tank overflowed.
Sr. Applications Engineer
Kadant Black Clawson LLC
Setting up steam heat in any kind of tank requires thought and care because of issues like hot pockets. I don't know how many times we have reconfigured tanks to be properly and safely heated but I can say it's been quite a few.
When heating a tank, if you hear banging and thumping as the steam is injected, the design is faulty or your spargers have fallen off which is more than typical. Eruptions in a tank are typical when a sparger is either not installed properly or just fell off.
There is a relationship between the vertical location of the spargers in the tank (backpressure), how the spargers are located in the perimeter of the tank, the offset or distance that the sparger is pushed into the tank and of course the number of spargers (the more the better). From a controls perspective there must be a check valve in the steam line to prevent the fluid in the tanks from back filling the steam line. The check has to be located almost right before the sparger inside the tank.
There will be some that say the spargers never stay put and always fall off even when welded. If that is the case the design has to be changed and you need to look at the metallurgy welding rods you're using.
I don't think steam had anything to do with the explosion at De Ridder based on the following report in the media:
Gov. John Bel Edwards told the Louisiana delegation the explosion appeared to have stemmed from a welding operation, where sparks ignited flammable gas emitting from deteriorating wood chips inside the tank, U.S. Rep. Clay Higgins told KATC TV-3. That would make the incident similar to the 2008 CPA tank explosion in Tomahawk, Wisconsin, where three workers will killed and one injured.
The U.S. Chemical Safety Board investigated the Tomahawk incident. They determined sparks from a welding operation atop a storage tank ignited flammable gas, which had been emitting from anaerobic bacteria inside the tank. Feeding on the organic fiber waste inside the tank, "the bacteria likely produced hydrogen, a highly flammable gas," according to a safety bulletin released after the incident.
At the time of the latest incident, "PCA supervisors and workers were unaware of the risks of flammable gas production from anaerobic bacteria growth," according to the CSB. "PCA did not perform a hazard analysis or recognize fiber waste tanks as potentially hazardous."
A chip bin, maybe, or a batch digester sound like the culprits, the latter less likely to blow apart from an H2S or H2 explosion. There should be some industry 'scuttlebut' by now if you want more current info or someone is saying what the vessel was.
Your concern about avoiding harmful-sized steam bubbles in hot "soups", slurries, etc, normally is achieved by sparging the steam to avoid large "bubbles", which typically only can cause violent splashing if the bubble reaches the surface without collapsing due to steam cooling. This normally requires the heating contents to be near the boiling point. I can't readily think of what tanks contain close-to-boiling liquids with steam being added in pulp mills. The only vessels aggressively steam heated are batch digesters and they are protected from overpressure by liquid overfilling with ruprture discs and other overpressure relief valves. The incident at PCA DeRidder sounds like a Hot Work Permit screw-up to me.