Published 3/11/2011

Change the Way You Spray

With the right spray system for the application, and thorough monitoring, a company can minimize clogging and realize significant savings in the process.

Jon Barber, Director, Spraying Systems Company

Figure 1. Contaminants in the spray solution obstruct flow, and spray pattern distortion is a typical result.

Figure 2. T-strainers feature a removable bottom plug for complete withdrawal of the screen assembly during cleaning. On some models the bottom pipe plug can be replaced with a drain cock for quick-flush cleaning.

Figure 3. The suction scanner is a hollow stainless steel pipe with tubular nozzles evenly spaced along its length. The spiraling motion of the nozzles cleans 100 percent of the strainer screen.

Figure 4A and 4B. Spray patterns in maximum free passage nozzle designs should be closely evaluated. Even distribution is important. The lower illustrations represent poor distribution.

When used with a dedicated spray controller, automatic spray nozzles can use PWM flow control to reduce clogging.

Nozzles should be cleaned with a material that is softer than the orifice, such as a toothbrush or toothpick.

In automated systems, a nozzle with a sensor can provide instant detection of clogs. Modular automated spray systems help minimize clogging by making adjustments to compensate for changes in operating conditions.

Spray nozzles appear to be simple devices, but in service they function as highly precise instruments. A spray system that is not working optimally can very quickly cost an operation tens—even hundreds—of thousands of dollars annually. When nozzles become partially blocked, spraying efficiency is degraded and the process can be compromised.

The most frequent cause of clogging is the presence of contaminants in process water—a condition that is particularly common in systems that use recirculated water. Other factors that may cause clogging are particle agglomeration, scaling and bearding—the build-up of material on the inside and outer edges of the orifice. The negative effects of clogging on spray performance include decreased flow and disturbed spray patterns, as illustrated in Figure 1. When positive displacement pumps are in use, clogged spray nozzles will increase process pressure. Flow will be most acutely affected when centrifugal pumps are used.

Clogging Can Cause Expensive Complications

If the spray application directly affects product quality, nozzle clogging will inevitably lead to increased scrap. This can happen when coatings, release agents, lubricants or protective films are applied unevenly or with inconsistent coating thickness on the finished product.

If the spray is part of an intermediate cooling, cleaning, humidifying or moistening process, the specified performance will be compromised. Cross-batch contamination can also occur when spray patterns deteriorate and uneven cleaning results. Depending on the process, potential losses can easily reach tens of thousands of dollars in wasted product before a clogging problem is noticed and corrective action is initiated.

The potential for clogged spray systems leads to one or more of three possible process interventions: monitoring, maintenance and downtime. Systems that are plagued by clogged nozzles generally require close observation and documentation that performance continues to be on spec. When this labor-intensive process reveals a problem, maintenance is required and often results in downtime—the most expensive intervention of the three.

As an example, a plant running 24 hours a day, 5 days a week, 52 weeks a year has a labor rate of $15 an hour and a downtime rate of $100 per hour. One hour per shift is spent monitoring/maintaining the system, totaling $11,700 per year. The system is down for maintenance two hours per week, totaling $10,400 per year. In this case, the annual cost of clogging is $22,100.

Simple Solutions that Work for Many Systems

Proactive maintenance is the best way to prevent clogging problems from occurring, but even well maintained systems can experience problems from time to time. Many spray systems can be efficiently maintained in good operating condition by manually cleaning nozzles at regular intervals. But only materials that are much softer than the nozzle orifice surface should be used. Plastic bristle brushes and wooden or plastic probes are good. Wire brushes or pocket knives should never be used, as harder instruments such as these can easily damage the critical orifice shape and/or size and produce distorted spray patterns or excess flow.

When nozzles become clogged and manual cleaning with a non-damaging instrument doesn't solve the problem, the orifice can be soaked in a non-corrosive cleaning chemical to soften or dissolve the clogging substance.

Most applications can benefit from adding strainers, which are an easy and economical way to minimize clogging. Strainers, including spray nozzles with built-in strainers, trap larger particles and prevent debris from entering the spray nozzle orifice or vane. There are two types of popular and effective strainer types:

Integral strainers can often be ordered as part of the nozzle itself and are available in a variety of mesh sizes (down to 200 mesh) and materials (including brass, aluminum, nylon, stainless steel and polypropylene) for standard and quick-connect flat spray, full cone, hollow cone and fine spray nozzles.

T-strainers are widely used and available in many configurations. Most are designed to minimize pressure drop and provide simple, fast cleaning and maintenance (Figure 2). Self-cleaning versions, which allow the filtered liquid to pass through the strainer and liquid particles to be returned to the liquid supply, are another option. Strainers are available in a wide variety of materials—including FDA-compliant materials for potable water usage—for low-, medium- and high-pressure applications.

More Sophisticated Solutions for More Demanding Applications

Self-cleaning strainers offer a unique answer to effective filtration and clog prevention for process water, cooling tower, pre-filtration to water treatment and similar applications. These strainers consist of a series of automatic filters with a self-cleaning mechanism driven by an electric motor. A pre-screen protects the cleaning mechanism from large particles, after which the main screen removes debris down to 10-micron particle size.

As debris accumulates on the main screen, it creates a filter cake that causes a pressure differential across the screen. When a pre-set value is reached, the cleaning process is triggered. One strainer version uses a suction scanner that rotates in a spiral motion while vacuuming the filter cake from the screen and expelling it through the exhaust valve (Figure 3). Suction scanning is equally as effective at filtering two-dimensional scale and three-dimensional sand. Particle adhesive characteristics have no significant impact on the efficiency of this process, making it well-suited for greasy or oily water.

Maximum free passage (MFP) nozzle designs are another effective answer to clogging. These nozzles feature open flow passages to ensure that most contaminants can pass through. They are an excellent choice for applications requiring maximum liquid throughput such as aerating, dust control, chemical processing, food processing, pulp and paper, gas scrubbing/cooling and fire suppression/prevention, or any others with debris in the water or where recirculated liquid is being used.

However, performance of these nozzles can vary greatly. One simple test of the functionality is to drop a ball bearing of the free passage diameter size through the nozzle. It should pass through freely. Another attribute to evaluate carefully is spray pattern, keeping an eye out for very thick edges and light centers in full cone nozzles and heavy centers in hollow cone patterns (Figures 4A and 4B).

Self-cleaning nozzles are available in a number of configurations and operating styles. For example, nozzles with clean-out needles are ideal for intermittent spraying applications using liquid that can dry in the orifice between uses. The clean-out needle can be activated manually or automatically, depending on nozzle type, and slides through the liquid orifice to clear obstructions. Automatic spray nozzles can be equipped with a clean-out needle that cleans the liquid orifice after each spray cycle for maximum protection against clogging. Other self-cleaning nozzles retract a purge piston when line pressure is reduced to remove fibers and other solids from a clogged nozzle. Spray headers, with internal brushes that rotate and remove debris from nozzle orifices, are also widely used in applications where less than pristine water is used.

Heated spray nozzles and systems incorporating a fluid-filled heat jacket or electric band heater can ensure trouble-free atomization of hard-to-spray liquids such as waxes, adhesives, starches and syrups and can further minimize maintenance due to clogging. When spraying an extremely difficult solution such as chocolate, an automated system may be required. Heated automated systems use closed-loop temperature control to ensure liquid temperature all the way from the tank to the target. The pressure tank, liquid and air lines are usually jacketed or heated to ensure consistent flow rates and clog-free operation.

Anti-bearding spray setups reduce bearding and clogging by changing the air current around the spray tip to prevent drops from causing buildup by falling back onto the nozzle. In some cases, users can run their spray operations up to 20 times longer without shutting down.

Pulse width modulation (PWM), when combined with a spray controller, can be used with some electrically actuated spray nozzles to reduce clogging. By cycling the nozzles on and off up to 10,000 times per minute, low flow rates can be achieved using larger, clog-resistant spray tips that reduce maintenance downtime. For example, at a duty cycle of 50 percent, the nozzle sprays half the time and the flow will be 50 percent of the maximum flow rate at a given pressure for the nozzle.

Automated spray systems often allow the spray controller to automatically adjust spray performance to compensate for changes in operating conditions. This minimizes clogging in many applications. However, when spray nozzles do clog, it is detected immediately and operators are notified to minimize scrap and lost production time.

For quality control, a miniature sensor mounted near the orifice of a nozzle can optically detect a spray pattern emerging from the spray tip and send a “spray present” signal to the controller. Unconfirmed spray cycles—often the result of clogging—can activate alarms or stop production. Each spray cycle can be documented and logged by the spray controller.

In other types of automated operations, where a dedicated spray controller is not used (such as tank washing), monitoring devices can be added for clog detection and performance validation. For example, acoustic monitors can be used with all types of tanks and tank cleaning nozzles to “listen” for the normal sounds of tank cleaning. Mounted outside the tank, they detect weak sound waves that indicate a change in spray pressure and clogged or partially clogged nozzles.

In addition to clog prevention and detection, automated spray systems can accurately detect nozzle wear and monitor flow and other performance criteria with automatic shutdown for selected faults. They can also adjust flow rates and spray patterns for variable speed conveyors and varied product shapes. Added benefits include temperature control to ensure even application of viscous coatings and spray cycle verification in critical applications.

Spray System Optimization

Even with close attention given to nozzle clogging and its remedies, a clog-free nozzle that is spraying does not necessarily indicate a spray system at optimal performance. There are many other factors that can influence a system’s effectiveness.

To achieve optimal long-term, efficient performance, a spray system needs to be considered in its entirety, with a plan in place for monitoring, maintenance and evaluation. If you don’t already have a comprehensive spray system optimization program in place, contact the manufacturer of your spray nozzles for more information.