Spray nozzle maintenance
Spray nozzles are often small in size and usually a small component in a larger system. But size is no indication of their importance. Spray nozzles that aren't properly maintained can wreak havoc in any application. The costs associated with worn nozzles can be significant. Spraying extra water alone can cost tens of thousands of dollars per year.
Spray nozzles are often small in size and usually a small component in a larger system. But size is no indication of their importance. Spray nozzles that aren't properly maintained can wreak havoc in any application. The costs associated with worn nozzles can be significant.
Spraying extra water alone can cost tens of thousands of dollars per year. Add in the cost of additional chemicals and energy, scrap resulting from quality control problems, unscheduled production downtime, and additional labor and the total expense can amount to hundreds of thousands of dollars annually.
Causes of poor performance
Spray nozzles are precision components. They wear over time or can suffer damage during operation or cleaning. Here are the most common problems that result in sub-standard spray performance:?
Erosion/wear . Gradual removal of metal causes the nozzle orifice and internal flow passages to enlarge and/or become distorted. As a result, flow is usually increased, pressure may decrease, the spray pattern becomes irregular, and drops become larger.
Corrosion . Nozzle material may break down due to the chemical action of the sprayed material or environment. The effect is similar to that caused by erosion and wear, with possible additional damage to the outside surfaces of the nozzle.
High temperature . Certain liquids must be sprayed while at elevated temperatures or in high temperature environments. The nozzle may soften and break down unless special, high temperature-resistant materials are used.
Caking/bearding . Build-up of material on the inside, outer edges, or near the orifice can occur and is caused by liquid evaporation. A layer of dried solids remains and obstructs the orifice or internal flow passages.
Clogging . Unwanted solid particles can block the inside of the orifice. Flow is restricted and spray pattern uniformity disturbed.
Improper re-assembly . Some nozzles require careful re-assembly after cleaning so internal components, such as gaskets, O-rings, and valves are properly aligned. Leaking and inefficient spray performance can result from improper assembly.
Accidental damage . Damage to an orifice or nozzle can occur by scratching or being dropped during installation or operation.
QC/QA issues and increased scrap . Worn, clogged, or damaged nozzles will not perform per specification and could produce uneven coating, cooling, cleaning, humidifying, and drying (Fig. 1).
Increased maintenance time . Unscheduled spray system downtime or an increase in cleaning frequency is an indicator of nozzle wear.
Flow rate change . The flow rate of a nozzle will increase as the surfaces of the orifice and/or internal vane/core begin to deteriorate. In applications using positive displacement pumps, the spraying pressure decreases as the nozzle orifice enlarges. Even small changes in flow rate can have a negative impact on quality. Routine monitoring can reveal potential problems.
Deterioration of spray pattern quality . As orifice wear occurs in hollow-cone nozzles, spray pattern uniformity is destroyed, streaks develop, and the pattern becomes heavy or light in the circular ring of fluid. In full-cone nozzles, the pattern distribution typically deteriorates as more liquid flows into the center of the pattern. In flat-fan sprays, streaks and heavier flows are visible in the center of the pattern and the effective spray angle coverage decreases. In some instances, the spray pattern looks fine and collection of spray fluid is required to reveal wear (Fig. 2).
Spray drop size increase . Although difficult to detect visually, liquid flow increases or spraying pressure decreases as nozzles wear. The result is larger drops and less total liquid surface area. If a problem is suspected, arrange for drop size testing.
Lowered spray impact . Worn spray nozzles can operate at lower pressure, resulting in lower spray impact. However, in applications with centrifugal pumps, spray impact may actually increase because of increased flow through the nozzle. Theoretical impact can be calculated but special testing may also be required.
The cost and length of production downtime due to nozzle problems and maintenance
The cost of labor for the additional maintenance
The cost of spraying excess water and chemicals
The cost of increased energy usage
The cost of quality problems; increased scrap and/or process problems
In some applications, it may be possible to temporarily compensate for a worn orifice by decreasing spray nozzle pressure to deliver the original required flow rate. However, using lower pressures may compromise spray coverage and uniformity of the spray. The result may be larger drop sizes and lower impact. In the end, using lower pressures may be a false economy since this may cause quality problems.
If replacement intervals seem too short, change to a different nozzle. The additional short-term cost is usually quickly recouped through longer wear life and better performance.
Before making any decisions, implement a nozzle maintenance program and document the procedures. In a short time, it should be possible to determine the best nozzle maintenance and replacement strategy for achieving optimal performance.
If you have any questions about nozzle maintenance or performance call 630-665-5000. An online calculator is available at www.spray.com/save to help estimate the cost of poor nozzle performance. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, firstname.lastname@example.org .
Approximate abrasion resistance ratios
Spray nozzle material
Hardened Stainless Steel
Silicon Carbide (Nitride bonded)
Synthetic ruby or sapphire
Preventing and solving problems
A comprehensive nozzle maintenance program helps ensure optimal spray nozzle performance. The following checklist should become the foundation of a nozzle maintenance program. Consistent evaluation of the factors listed will help to detect wear early and enable appropriate action to be taken. The application will determine how often each factor should be checked. The frequency may range from the end of every shift, to every few months, or longer.
For centrifugal pumps : monitor flow meter readings to detect increases. Or, collect and measure the spray from the nozzle for a given period of time at a specific pressure. Compare these readings to the flow rates listed in the manufacturer's catalog or compare them to flow rate readings from new, unused nozzles.
For positive displacement pumps: monitor the liquid line pressure for decreases; the flow rate will remain constant.
Spray pressure (in nozzle manifold)
For centrifugal pumps: monitor for increases in liquid volume sprayed. The spraying pressure is likely to remain the same.
For positive displacement pumps : monitor pressure gauges for decreases in pressure and reduction in impact on sprayed surfaces. The liquid volume sprayed is likely to remain the same. Also, monitor for increases in pressure due to clogged nozzles.
Visually inspect the spray pattern for changes. Check the spray angle with a protractor. Measure the width of the spray pattern on the sprayed surface. If the nozzle orifice is wearing gradually, changes may not be detected until there is a significant increase in flow rate. If uniform spray coverage is critical, request special testing from the nozzle manufacturer.
Drop size increases cannot be visually detected in most applications. An increase in flow rate or decrease in spraying pressure will impact drop size.
Check uniformity of spray coverage of flat-spray nozzles mounted on a manifold. Spray patterns should be parallel to each other. Spray tips should be rotated 5 to 10 degrees from the manifold centerline.
Check for uneven coating, cooling, drying, cleaning, changes in temperature, dust content, and humidity.
If after implementing a spray nozzle maintenance program, it is suspected the nozzles aren't performing as well as they should, try replacing the nozzles more frequently. However, there are a few other options to consider before implementing that decision:
Change nozzle material.
Materials having harder surfaces generally provide longer operating life. To determine if a different material should be considered, consult the chart on the right.
Decrease spraying pressure.
Although it is not always possible, decreasing pressure, which will slow the liquid velocity through the orifice, may reduce the orifice wear/corrosion rate.
Reduce the quantity of abrasive particles or concentration of corrosive chemicals.
In some applications, it is possible to reduce the amount of abrasive particles in the feed liquid and/or change the size or shape of the particles to reduce wear. Using different concentrations and/or temperatures, depending on the specific chemicals involved, might also reduce a solution's corrosive activity.
Add line strainers or change to nozzles with built-in strainers.
Orifice deterioration and clogging is typically caused by solid dirt particles in the sprayed liquid and is particularly common in systems using continuous spray water recirculation. Strainers or nozzles with built-in strainers are recommended with a screen mesh size chosen to trap larger particles and prevent debris from entering the nozzle orifice or vane.
Improve cleaning procedures.
Cleaning should be done with bristle brushes and/or wooden and plastic probes. It is easy to damage the critical orifice shape or size and end up with distorted spray patterns and/or increased capacity. If faced with a stubborn clogging problem, soak the clogged orifice in a non-corrosive cleaning chemical to soften or dissolve the clogging substance.
Does it cost more to keep using problem nozzles or to replace them? In order to determine this, document the following:
Some symptoms are easy to detect while others require special testing. Watch for these clues:
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