Friday, September 23, 2011

New Spin on Tanks and Silos

Today I was reading this article: California Plant Becomes Reality about a state of the art hot plant that was permitted and erected in Southern California. It has all the bells and whistles like a double drum, warm mix, and automated controls but I was particularly intrigued with two items. The first was their oil tanks as seen in the photo below:

Traditionally oil tanks are laid down horizontally.  I believe this was under the theory that if you place the heating elements on the bottom of the tank you can cover more surface area to keep the oil warm and consistent.  One of the plants I worked at also stood their tanks up vertically like the tanks shown above and when I asked why they did this I was pleasantly surprised.  One of the biggest reasons is space.  Many hot plant locations are in urban areas and they do not typically have space to lay out the tanks horizontally, as seems to be an issue with the plant above.  As you may have noticed, standing the tanks up vertically takes up less than 1/4 the space it would take to lay them down horizontally.  The other reason was an engineering reason.  Heat rises.  Each tank has a heating element on the floor of the tank and as the bottom of the tank is heated, the heat rises to heat the rest of the oil in the tank.  With good insulation, you can use less energy with a smaller heating element to heat the entire tank, granted it might take a little longer than it would with a horizontal oil tank.  In addition, many vertical tanks also have circulators that move the oil around in the tank to maintain a consistent temperature.  It's a pretty neat technique that can save the company money and space.

The other item that I thought was interesting was that this facility is storing their aggregates in silos instead of in stockpiles.  This setup is seen in the background of the photo above and in the photo below:

I've actually never seen a setup like this one and I was extremely excited when I saw it.  The grate above is a drive over depository for aggregates.  Bottom dump trucks, transporting the plant's aggregate supply from a nearby quarry, drive over this grate and dump their load through the grate (also called a grizzly).  The grates stop any over sized materials (if any) and the rest of the rock drops through to the conveyor coming out from underneath it.  This conveyor takes the rock to the top of one of the designated aggregate silos where it is deposited for future use at the hot plant.  The hot plant can meter the amount of rock from each aggregate silo using their automated controls and no loader is used at all!  What I love about this set up is that not only is it going to lead to lower operating costs but also that it is all enclosed.  California is quickly moving towards tougher and tougher dust regulations for hot plants and this setup will come close to eliminating dust at their hot plant.  In addition, (as if that wasn't enough!) having the rock enclosed in silos with this setup will also help out a ton with storm water requirements since you don't have aggregates which could have chemicals or minerals that can negatively affect runoff sitting in the rain.  AND not having wet material because of the rain means lower dryer costs!

As an operations person I'm really excited to see these types of developments but as a quality control person I'm also a little hesitant.  One of the reasons that many operations choose not to use aggregate silos is that they have a propensity to segregate material.  As I mentioned here when aggregates are dropped from a distance the coarse materials can roll to the outside and the fines stick in the middle.  This could lead to coarse materials on the outside edges of your silos and fine materials in the center which can affect your hot mix.  There are several ways to combat this though, such as using a gob hopper, so I think that aggregate silos still have a chance at being really great if they are set up and monitored properly.  My other curiosity is whether the inherent moisture from the aggregates in the silos will drip down to the bottom of the silo leaving the moisture content of your aggregates inconsistent and possibly affecting your oil content (especially with a drum plant).  However, with traditional moistures around 2-3% I'm not sure that it will make that big of a difference in the long run.  I just thought I'd let you all in on some of the possible negatives though since there is no such thing as a perfect setup.  Overall I think that this facility, belonging to Kelterite, is leading the way to what we're going to be seeing more and more of in the future.

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