How to Run WMA in Cold Weather
By: Andy Welch, AQUABlack® Project Manager, Maxam Equipment Inc.
With the evolution of water based Warm Mix Asphalt (WMA), and its growing popularity, new challenges face the asphalt producer and their crews. WMA now allows for lower temperature mix production, thus reducing heat loss in cooler weather. This feature of WMA has opened the door for an extended paving season and began to change the old rule of “40°F and rising” for paving conditions. Cold weather asphalt is a reality now.
Asphalt plant production crews now face a whole new set of operating conditions not seen before. Previously asphalt plants would shut down when the first snow hits. With WMA, lower plant operating temperatures can affect many aspects of the asphalt plant performance. Asphalt heating, baghouse operation, drum and dryer performance, fuel heating, material flow and stockpiling just to name a few. Here we will discuss specifically cold weather operation of a water based WMA system and how to prevent needless down time at your asphalt plant when using water as an ingredient in your mix.
Even though the amount of water used in warm mix asphalt is very low, around a quart per ton of mix, you are still using a product that can freeze. Don’t let this frighten you, when it comes to the mix itself, because once injected into the liquid a/c, the water quickly converts to steam and only “air bubbles or steam” is left in the WMA. So once the WMA is produced, there is no chance for freezing in the mix. The cold weather issue with WMA is the handling, pumping, metering and storage of the water prior to injecting it into the process.
There are several warm mix asphalt plant equipment manufacturers in the industry today. The MAXAM AQUABlack® system is designed for cold weather operating conditions, but most warm mix asphalt plant equipment manufacturers leave it up to you to deal with. If your system is not designed for cold weather, the three aspects of the system to contend with are the pump and metering system, the water conveying lines, and the foaming/mixing apparatus.
To begin with, you need a water supply source. It is always best if this is a filtered and clean source to prevent contaminants from fouling the valves and orifices throughout the system. The source, if coming from a tap or valve should be disconnected at the end of each day, and the supply hose should be drained. Freezing in the supply valve can damage the valve seats and actually cause the valve body to rupture. A properly engineered metering system used in WMA production should have a water supply surge tank. This allows for the pump to pull from a constant head for most accurate control. A surge tank can be dealt with in a variety of ways. Larger tanks can be insulated with blanket type insulation and the water can be kept above freezing temperatures by use of a submersion type heater. Also keep in mind that the supply and feed line also must be heated or shut off and drained after each use. Use of a small tank for just metering head will allow for the tank to be mounted in an enclosure that can be easily heated using a space type heater. The meters and valves used for WMA control all contain some type of moving part(s), that when frozen will not operate and create a down time condition. Most water meters and valves contain “Buna” type seals and will not survive thawing by heating them with a torch to free them up. You most likely will destroy them.
The exposed piping and hoses might end up being one of the biggest possible blockage points you will face. The solution is to drain them or blow them out using compressed air to remove the free water. The AQUABlack® system comes equipped with an automatic “Air Purge” that, when the system is shut down, automatically purges the line all the way to the foaming chamber. Air purging using plant compressed air is the best way to insure that the water is removed from low spots that may not get fully emptied out by a simple brevity type drain out.
Most warm mix asphalt foaming chambers or devices are heated by some method; usually hot oil to prevent plugging. Leaving the hot oil flow on to the chamber should prevent any freezing at this point. If no type of heating is provided with your system, cleaning out the water after cold weather use becomes an absolute must. Some alternative heating methods, such as electric heat tape or cable could be used. Installation of a pressure gauge or remote transducer with a readout in the water line just before the chamber can be used to indicate a blockage in the water supply system due to freezing. Having the ability to monitor the pressure is a great aid in diagnosing a problem or blockage.
Long term shut downs can be handled a couple of different ways. One would be a complete drainage of water from the entire system. This option may take some time, as you will want to make sure that all the water has been removed. Water for example, trapped between two valves and not drained, will freeze, expand and possible damage both valves. The second method is probably the simplest and safest. Just perform a simple gravity drain not worrying about removing all the water and then add RV antifreeze to the tank and pump it into the system until you see some flush out at the foaming chamber (follow the RV antifreeze manufacturers recommendations). For startup the next time, flush the system out, replace the particulate filter, hook up you supply line, fill the tank with water and start your Spring cool weather WMA production. Following the above steps is not difficult to follow or perform, but they are critical for successful cool weather warm mix asphalt product. With an extended paving season, more tonnage will make for a warmer Winter.