October 1, 2009
Water spray systems play a vital function in aggregate operations. Learn how to keep their productivity high as the temperatures fall.
by Mark Kestner, Ph.D.
Water spray systems have long been used to control dust produced by quarry operations. They protect workers from exposure to silica containing dusts and prevent visible emissions that upset neighbors. But what happens when the temperatures drop below freezing and winter weather challenges operators that have to keep spray systems running?
Once the leaves begin to fall and the north winds begin to blow, it’s time to winterize your spray system. An early frost can catch the plant unaware and freeze up hose, pipe, and nozzles. If the pump freezes up and bursts, that’s real trouble. When this happens, the plant can’t operate until the system is thawed or repaired and that means lost production. If operators have to spend a couple of hours a day thawing out nozzles, the dollars can really add up. The busy quarry operation would be wise to plan ahead and install a system that will minimize downtime during the winter.
Here are some guidelines for operating spray systems in cold weather:
1. Install a thermometer to alert operators to freezing temperatures so they can take action before nozzles freeze up.
2. Inspect spray nozzles daily to make sure that they are not plugged or frozen.
3. Inspect the pump daily. Verify that the pump is heated and that the inlet water line is not frozen.
4. Minimize the number of spray nozzles in service. Use only essential spray nozzles.
5. Consolidate spray nozzles. Using a single nozzle instead of two or three nozzles at the same spray point reduces the number of nozzles to maintain and allows the use of larger orifice nozzles that are less likely to freeze.
6. Make sure that nozzles are properly targeted and do not spray steelwork or conveyor belting.
7. Make sure water lines run straight and true and that drain valves are located at all low points where water can collect.
8. Drain water lines and nozzles immediately after they are taken out of service and leave drain valves open overnight. Do not allow water to sit in hose lines and spray manifolds.
Many quarries have taken winter operation a step further and installed compressed air and anti-freeze purge systems to protect water lines and nozzles from freezing. These add a serious measure of protection that any state-of-the-art quarry can ill afford to be without.
Advanced wet suppression system designs consolidate all moving parts onto a skid-mounted pump module where they can be protected. No solenoid valves are located outside where they can freeze and burst. Most importantly, the pump module must be housed in a heated enclosure. Thermostatically controlled convection or forced-air heaters can also be mounted directly on the pump skid to keep critical components from freezing. For portable plants, the pump module, surge tank, or other accessory equipment can be housed in a heated trailer.
Hose or pipe is used to supply water to nozzles. Hose has the advantage of being a better insulator and won’t freeze as fast, but it cannot be thawed with a torch like pipe.
In a typical spray system layout, water is supplied to various treatment zones in the plant. For example, in a primary crushing plant, Zone 1 is typically a feed hopper where nozzles are used to control dust when trucks dump. Zone 2 is a primary crusher. Zone 3 is a scalping screen and so on. Each zone is independently controlled using a solenoid valve that is switched on or off from a remote panel in the control booth.
As a practical matter, high-pressure spray systems can be operated down into the 20-to 25-degree (Farenheit) range. Below these temperatures, nozzles will start to freeze even while they are spraying. To operate continuously in sub-freezing temperatures, water lines have to be heat-taped and insulated. Using an anti-freeze solution of a glycol or brine is another alternative, but both methods are expensive, and using anti-freeze agents may affect the chemical and physical chemistry of the stone. Brines are also corrosive and may require special materials of construction in a spray system. Corrosion inhibitors can be added to brines, but significantly increase cost.
Spray systems that feature compressed air and glycol purge systems are designed to get the water out of spray lines before they have a chance to freeze. When the operator switches any line supplying a spray zone from the ‘run’ to the ‘purge’ position, the water solenoid valve closes and the air solenoid valve for that line opens. The sequence and duration of the purge cycle is determined by the control logic of a proprietary software program. The switch for the zone that is actively purging blinks to give the operator an indication that the purge cycle is in progress.
Spray systems that include an anti-freeze protection system use glycol to coat the interior of spray lines and nozzles rather than fill the lines with glycol. This conserves anti-freeze and saves money. Once compressed air has blown the line out, the PLC shuts off the air and turns on a metering pump, which dispenses anywhere from a quart to a half-gallon of propylene glycol solution into the line. After 30 seconds or so, the controller shuts the metering pump down and then turns the compressed air back on to blow the glycol down the length of the line to the spray nozzle/nozzles. This coats the interior of the line and nozzles with glycol to prevent nozzle tips from freezing up and drain valves from cracking.
This technology can be adapted to just about any spray system to protect it against freezing weather. This could include equipping the system with a simple manual air purge, all the way up to installing an automated air and glycol purge system complete with thermostatically controlled drain valves.
Installing an automated purge system in a new or existing spray system can save hours of labor thawing or repairing water lines and nozzles and keep your plant productive regardless of cold weather conditions.
Mark Kestner, Ph.D., aka Dr. Dust, is an expert in the field of dust control technology and has been published extensively on this subject. Projects to control pollution and improve air quality in the industrialized world have taken him to more than 1,500 mines and power plants in all 50 states and seven countries. To contact Dr. Kestner call 800-237-3878 or visit his Web site at www.drdust.com.