Solimar Offers Tips for Dry Bulk Systems

Feb. 1, 1999
Keith Solimar presented tips to improve the flow of dry products in loading and unloading. He pointed out: "All products are not created equal." As a

Keith Solimar presented tips to improve the flow of dry products in loading and unloading. He pointed out: "All products are not created equal." As a result, specific techniques are required for various materials.

Free flowing products such as plastic pellets move easily through the tank equipment. "However, some materials such as cement and flour are not naturally free flowing," Solimar said. "Worse yet, they become compacted as the trailer moves down the highway."

To make the compacted product flowable, it must be loosened with aeration. In this process, air is introduced in the bottom of a container through a permeable medium, such as textile fabric, sintered metal, stainless steel wire mesh, porous stone, or porous plastic. When air is introduced into the product, it will always seek its way to the top just like blowing into water through a straw.

When air is forced through compacted products, it will gradually expand the products. In the case of cement, the expansion is about 40%. Finally, air will burst out on top and the product is now fully aerated. This happens when the passing air has sufficient velocity to lift and separate the individual particles so that they more or less lose physical contact with each other and the product starts behaving like a liquid or fluid.

"Historically, the aeration medium in dry bulkers has been fabric," he said. "It is hard to beat fabric for low cost and as a dispersion medium of the air into the product. Today, using fabric with flour, starch, and resins causes some concern related to cleanliness and contamination. Several hours are required to dry the fabric after tank cleaning.

Condensation inside the trailer can cause problems, like hardening of the product and mildew in the fabric. The advantage of fabric is that it is flexible, so it is somewhat self-cleaning.

"The substitutes for fabric are sintered metals and woven stainless wire cloth. They work well when new. With condensation, rigidity, and repeated use, they become clogged with product and may have to be cleaned with acid."

Aerators, a rubber disc that lets air into the tank, act as a check valve to prevent air and product from backing up into the air supply line. They do not disperse air into the product as evenly as fabric does. Aerators act more as a circular nozzle that blows air into the product.

Pneumatic conveying phases are primarily a function of tank pressure. Dense phase conveying is used for food products over short distances when high transfer rates are desired. Because of the low velocity, dense phase conveying causes minimal particle breakup and degradation.

The other type of conveying, dilute phase, makes it easier to convey the product over longer distances. This phase requires a much higher air velocity.

"If we have a conveying line with a certain product in the bottom and we gradually increase the line velocity, we will eventually reach a point when the particles will be picked up and carried in the air stream, just like sand or snow is picked up by the wind," Solimar said.

"This is called particle pickup velocity and has to be exceeded to convey a product in the dilute phase. The advantage with the dilute phase is that the operating pressure canbe easily controlled with minimal risk of plugged lines. Additionally, product can be conveyed over great distances.

"Conversely, if the velocity in the dilute phases is decreased, at some point the particles will fall out of the airstream and collect on the bottom of the pipe. This is called saltation velocity. This is how sand dunes and snow drifts are formed."

If the velocity is decreased further, the product will collect and partially fill the pipe in heaps of nodes similar to sand in shallow water. The heaps may also drift along in the pipe like wandering sand dunes or snow drifts.

As the upper open portion of the pipe narrows, the velocity increases. This will cause the air to pick up more product off the top of the nodes. In this way, the flow is self-regulating and plugging is unlikely to occur. This phase could also be called an unstable phase since the condition inside the pipe keeps changing.

Solimar noted factors that affect unloading rates: tank pressure, line size, length of unloading line, unloading line with incline, air volume and line velocity, and clean-out cycle. A big challenge is that despite the most appropriate equipment on the tank trailers, the carrier has no control over shipper facilities.

Steps to correct line restrictions include: -Do not unhook the hose.

-Close the product valve.

-Close the aeration valve.

-Open the pressure control valve completely. Sometimes the pressure control air will find its way through the line and blow the line clean.

If the line has not cleared in a couple of minutes, close the pressure control valve momentarily until the pressure-relief valve on the air source opens. Then open the valve quickly. Repeat at least a dozen times. These pressure fluctuations may clear the line.

If the line remains plugged:

-Stop the air source.

-Open the blow-down inlet to depressurize the tank completely.

-Close the blow-down line.

-Leave all product valves and aeration valves closed, but open the pressure-control device.

-Start the air source again.

This will put full pressure on the product line. When the blower pressure-relief valve opens, open the product valve on the hopper with the least product in it. The line pressure will fall quickly. When this happens, the air in the plugged line expands quickly and rushes into the depressurized tank. It may only clear a foot or so of the plug in the line each time. So, it has to be repeated many times until the line gets unplugged. Sometimes it has to be repeated up to a hundred times. In that case, the tank gradually becomes pressurized again and steps one through four may be repeated. If it is not a dusty product like cement or flour, the blow down valve can remain open.