A look at common problems with spray nozzles, and how to minimize them.
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Quick-disconnect spray tips eliminate misalignment by providing automatic, positive tip re-alignment.
Comparison of spray patterns from good (left) and bad nozzles. A clogged plastic spray nozzle causes a missing spray pattern in the center of the pattern.
Finishing applications can employ up to hundreds of spray nozzles that, like any other equipment, need routine maintenance in order to properly do their job. Anyone who powder-coats or e-coats is rinsing and finishing, and in the case of powder coating, there is a pretreatment stage. A consistent spray pattern and complete and uniform spray coverage is very important to the quality of your finished product.
The type of nozzle you choose can be a primary factor affecting both maintenance requirements and overall finishing performance. Threaded steel, stainless, or brass nozzles are not only subject to corrosion, they are often aimed in a single permanent direction. This obviously decreases their application flexibility.
Modern polypropylene nozzles, on the other hand, can include such enhancements as quick disconnects and the ability to be easily realigned to produce different spray patterns for different products at different times. In most cases, there are notable cost savings as well.
Frequent inspection of nozzle orifices for clogging is probably the biggest maintenance factor that affects the consistency of spray patterns in any industrial application. Nozzle wear is another important issue that needs to be consistently monitored. Polypropylene and other plastic nozzles (PVDF, PVC, and other materials) that carry abrasive materials will wear, but their service life can be extended with the proper care.
Visual inspection can point to malfunctioning nozzles. An example would be uneven or missing spray patterns where the consistency of the spray (flat fan, full cone or hollow cone) can visibly be seen. In flat-fan nozzles, poor performance will most likely be from a change in spray angle. In hollow-cone nozzles, it is common to see a missing pattern in the center or no pattern at all. In full-cone nozzles, the nozzle often becomes completely plugged.
Problems with nozzle spray patterns can result from several sources. The most common causes of poor spray patterns are worn nozzles, damaged nozzle orifices, excessive leaking, and poor nozzle alignment.
If it’s obvious to the eye that the spray pattern has become uneven or incomplete, then there’s a good chance that the nozzle orifice is worn. A worn nozzle may also be the cause of an unwanted increase in the flow rate, which could be accompanied by a drop in system pressure and poor spray pattern performance further from the pump. In extreme situations, you could even see your electrical costs rising.
The fix for a worn nozzle is replacement. To prevent excessive nozzle wear from becoming a problem in your application, maintain regular checks on fluid temperature. It may be too hot for plastic spray nozzles. Maximum temperature for polypropylene nozzles is 175°; PVDF nozzles can handle temperatures to 300°.
Other possible ways to reduce nozzle wear include reducing spraying pressure to reduce the liquid velocity and minimizing the abrasive particle content in the spray stream. If these remedies fail to bring nozzle wear to acceptable levels, consider a different type of nozzle material.
An abused and damaged orifice can also result in an irregular spray pattern that can affect product quality and/or waste materials. Orifice damage often results from improper cleaning of the nozzle. The only fix is replacement of the nozzle.
Users should never use a metallic object to clean nozzles. Fiber brushes, or even a toothbrush or toothpick, are more suitable for plastic nozzles. Nozzles should always be removed for cleaning.
Some dripping or leaking is normal, and is acceptable in some applications. If dripping seems excessive, check for incorrect connections between the nozzle and the pipe fitting or manifold. This could result from, for example, use of an incorrect size during nozzle replacement.
If you’re using clip-on style plastic nozzles, make sure all components are snapped into place securely. Worn O-rings occasionally cause leaking. Drilling an incorrect size hole will cause substantial leakage as well.
Many quality and efficiency problems are due to nozzles being aimed incorrectly after they have been removed for cleaning or other maintenance. Few people take the time inside a coating compartment or parts washer to assure that the nozzle is aligned exactly the way it should be, especially if it is in a hard-to-reach area.
Newer quick-disconnect spray nozzles can eliminate misalignment. Positive re-alignment of the tip is automatic, because such tips are simply twisted into the body until the tip locks into place.