Selection and maintenance of spray nozzles for pretreatment applications

SELECTION AND MAINTENANCE OF SPRAY NOZZLES FOR PRETREATMENT APPLICATIONS by David Foley Sprayjng

Systems

Co., Wheaton,

111

If your spray application involves cleaning and coating of metal parts and equipment. your spray nozzle requirements are probably complex--requiring one type of norzle foglow-pressure rinsing, another for high-pressure cleaning, and others for coating, cooling, 01 drying. The purpose of thi\ article is to review the various types of spray nozzles and how to &elect the proper nozzle for your particular application. Proper spray nozzle selection is important for a number of reasons. First. the type 01 spray nozzle you u\e impacts product quality. A surface that has not been properly cleaned or coated i\ more susceptible to corrosion and finishing flaws. Second, spray nozzles vary in the amount of water and chemical usage. Selecting the proper nozzle can ensure that you aren’t overusing these expensive resources. Third, specifying the right nozzle in the right corrosive-resistant material keeps nozzles performing at their peak. which means longerlasting nozzle!,. And, finally. a number of spray nozzles on the market today have design elements that speed up installation and removal, which can significantly reduce maintenance downtime. Depending on your application and the number of nozzle changeouts, specifying a “quick-change” spray nor& or a spray norrle with a self-aligning adjustment can be a real maintenance advantage.

SELECTING nozzle

There life

Clip-On

CHEMICALLY

are a number of options and reduced maintenance

available downtime.

RESISTANT to nozzle

SPRAY specifiers

NOZZLES to provide

long-lasting

Nozzles

Clip-on nozzles feature a stainless steel clamp that snaps a plastic nozzle body onto an existing spray header (see Fig. I). The outer cap is then used to secure a wide selection of nozzle tips. The clip-on nozzles offer easy, no-tool nozzle assembly for fast installation and removal. Plus. spray direction and alignment are easily adjusted by hand. In additmn to added convenience. clip-on nozzles feature polypropylene construction to withstand high temperatures and to resist corrosion. Many manufacturers use clip-on nozzles to phosphatire. hot seal, and rinse metal parts iis they pas\ through pretreatment tunnels.

Quick-Change

Nozzles

Plastic quick-change nozzles arc another option for spraying phosphates. acids. solvents, or caustics. Designed especially for use with liquids that can corrode metal nozzles. plastic quick-change nozzles are molded of chemically coupled, glass-reinforced polypropylene, which combines strength and durability with excellent chemical resistance (see Fig. 2). Convenient, no-tool spray tip removal and installation reduces nozzle maintenance downtime over conventional threaded nozzles.

280

Fig. I. Clip-on spray nozzles installation and maintenance.

snap inio place

quickly

and retain

their

memo?

to speed

High-Impact Nozzles The trend toward reducing dangerous and polluting chemicals during the cleaning or degreasing stages prior to conversion coating means that more of the cleaning action has to come from the spray impact provided by the nozzles. For the best cleaning, it’s important to specify a flat spray nozzle, which concentrates the width of the spray pattern in a small area-typically from I5 to M-to produce high impact on the surface to be cleaned. In other applications, a solid stream pattern with a 0” spray angle is best for focusing cleaning in difficult-to-reach areas of metal parts.

Fig. 2. Quick-change .sproy nozzle.s, constructc~d of chemicaland cormsion-resistant polypropylene, dramatically reduce maintenance downtime, us the spray tip installation and removal are tool .free.

281

Nozzles for Conversion Coating Once the parts have gone through pretreatment cleaning, they must be sprayed with a coating to create a suitable surface for finishing. The coverage must be even and complete. Otherwise, the surface will not accept the paint and will be more prone to corrosion. The spray nozzle you select depends on the type of conversion coating system used, whether it’s iron phosphating, zinc phosphating, or chrome conversion coating for aluminum. Flat spray nozzles (see Fig, 3) are often used for iron phosphating and chrome conversion coating systems, whereas hollow cone spray nozzles-with their lower impact and greater contact-are preferred for zinc phosphating systems.

Nozzles for Rinsing Removing the residue of the cleaning and coating chemicals is the objective in the rinse stage of the pretreatment system. Flat spray nozzles are usually preferred, as the impact assists in cleaning; however, if deionized water is used in the rinsing stage, wide angle detlected flat spray nozzles are sometimes used rather than a standard elliptical orifice flat spray nozzle in order to conserve liquid. Although the vanes create the potential for clogging, full cone spmy nozzles can be used only if a clean, filtered rinsing solution is used.

Nozzles for Bath Mixing To enhance the chemical reaction between the coating and metal substrate, iron and zinc phosphates are often applied in a supersaturated state. The chemical baths need to be agitated to prevent sludge from settling within the bath. Improved circulation helps regulate liquid temperature and the chemical balance of the bath. For this purpose, a tank mixing eductor can be used to amplify the capacity of the pump by a ratio of 5 to 1. improving circulation and reducing settling within the bath. It’s important that the parts moving through the pretreatment system do not dry out. If the parts are not kept wet, “drying” patterns result, which can ruin the final finish. Most pretreatment lines have nozzles installed between stages to keep parts wet if the line stops for any reason. Hydraulic atomizing nozzles and air atomizing nozzles can both be used effectively for this application.

Nozzles to Prevent Carryover The carryover of chemicals from one stage to another must be avoided. Carryover occurs from the overspraying of cleaning or coating chemicals. The best way to avoid carryover is 282

Fig. 4. Nozzle

erosion

to create an air or water curtain between pretreatment stages. Air nozzles create an air curtain, which prevents mist from one spray stage from entering another; a water curtain can also be created with the use of a deflected flat spray pattern nozzle.

Blow-off Nozzles for Drying Spray nozzles are often used as the parts enter the oven at the nozzles are installed in a manifold the use of Ryton-constructed spray

SPRAY

to blow off the final rinse water and to speed drying time end of the pretreatment system. For this application, air setup. Because temperatures up to 90°C are not unusual, nozzles is often required.

NOZZLE

MAINTENANCE

Nozzle maintenance, although often overlooked, is a critical component of any metal pretreatment system. In order to keep the system running at maximum efficiency to produce the highest-quality finished product, nozzle maintenance must become a regular part of system operation. Improperly maintained nozzles decrease spray impact and distort spray patterns, reducing your cleaning efficiency. As a result, you will spend more time, and use more chemicals to accomplish your cleaning tasks. Learning to identify, solve, and prevent spray nozzles’ performance problems in your parts washer can help cut costly spray liquid and energy waste, assure better washer performance, and reduce the chance of equipment damage.

What

Causes Poor Nozzle Performance? Erosion occurs as high velocity liquid wears away the metal surfaces of the nozzle orifice and the deflector surface (see Fig. 4). The pressure decreases, and the spray pattern becomes irregular. The rate of erosion varies according to the hardness of water, the spraying pressure, the chemicals being used, and the amount of use. In addition, particles that find their way into your system can cause serious erosion problems, Accidental damage is most often the result of unavoidable nozzle abuse during operation and maintenance (see Fig. 5). Although the orifice is recessed and is not easily damaged, the deflector surface, which deflects the solid stream from the orifice to produce a flat spray, can be damaged, reducing spray impact. After the natural wear of erosion, damage is the most frequent source of nozzle performance problems. Corrosion occurs when chemicals in your cleaning solution attack and break down the nozzle material (see Fig. 6). The physical effects of corrosion on the nozzle are similar to those of erosion described above, except the entire nozzle is damaged, not just the orifice and the deflector surface. The adverse effect on your spray pattern is much the same as in erosion, Caking is the buildup of chemical and mineral deposits on the inside or on the outer 283

Fig.

7. Caking.

edges of the orifice (see Fig, 7). The amount of mineral buildup. and the rate at which it occurs, depend on the hardness of the water and chemicals used. This dried layer can partially block the orifice or passage. The result is a distorted spray pattern. which can reduce the impact per square inch. High temperature is another problem, Nozzle materials may soften or break down when spraying liquids at elevated temperatures or when located in extreme temperature environments (see Fig. 8). Clogging, when unwanted solid particles block the nozzle orifice. restricting flow and disturbing the spray pattern, is probably one of the most common and annoying nozzle problems (see Fig. 9). Detect&

Nozzle

Problems drop. If your pumping system is operating at its capacity. the spraying pressure will decrease as the nozzle orifice enlarges. These lower spraying pressures deliver lower spray velocities and thus leas impact. But if the pump is running below its rated flow rate capacity. as the nozzle wears. the tlow through the pump will increase, wasting chemicals. Figures IO and I I compare a good spray tip to a worn spray tip. Initially. nozzle performance problems, such as the effects of dirty or damaged nozzles, are virtually impossible to detect. You may not be able to detect distortions in the spray pattern until a very late stage of deterioration. Increasedpressurr

284

Fig. 8. Nozzle

damage

caused by high temperature.

A single nozzle can be spraying as much as 30% over its rated capacity, and yet visually, the spray pattern can look nearly identical to that of a new nozzle. So the use of pressure gauges becomes critically important. The position of the pressure gauge and its accuracy are very critical when comparing identical systems. Gauges should be placed as close to the spraying headers as possible in order to determine flow rates, The actual reading on a pressure gauge should be used as a reference point when the nozzles are new. Now, early detection of nozzle wear is obtainable as the pressure fluctuates from this reference point. Spray pattern deterioration. Orifice damage or caking produces obvious changes in the uniformity of the spray pattern; however, when the orifice is wearing gradually, changes in the pattern may not be detected by visual inspection until the pressure has dropped substantially. Spy? impact changes. Nozzles with worn orifices will operate at lower pressures, thus

Fig. IO. Comparison (right).

of .spra.v patterns

Fig. Il. (right).

of flow’ distribution

Comparison

from

from

a good nozzle

(left) and one with a worn

a good nozzle (left)

and one with a worn

tip

tip

285

Table

I. Summary

Flat spray

Solid

of Spray

stream

Hollow

for Various

Applications

Couting 40-6~7~ well as 40-W” phosphate hollow

cone

Rinsinfi Flat spray Flat spray

(deflected)

Flat spray

(deflected)

Full

Requirements

40-65~ tlat spray nozzles ue typically installed with a slight offset to prevent disturbing the spray pattern of adjacent nozzles. Overlap is recommended to ensure complete coverage. IS-SO’ spray angles are used where a tightly directed llat ~pmy u,ith maximum impact is required. Deflected pattern\ also used at moderateI> low pressures in cemain situations when: elhptlcal orifice nozzles have it tendency to create foam with specific chemicals. 0’ spray angles may be wed to focus cleanmg solution on difficult to reach areas of metill parts.

(deflected)

Comer.rion Flat \pray

Nozzle

flat spray nozzles are often wed for iron phosphate systems as for aluminum coating processcu. hollow cone spray angles are widely used, especially for zinc systems. The lower impact and greater contact area of the cone pattern produces a superior coating.

40-95’ nozzles are installed in overlapping patterns and also in area specific patterns to rinse off chemicals from earher stages. 100-1 IS’ spray angle low,-impact no.ules are used for rtnvmg metal surfaces with a mmimum number of noules. These nozzles are particularly useful for conserving deionized water, which is often used for final rinsing. IS-SO’ nozzles are occasionally used when a small area requres a strong flow of rinsing liquid. Although they are not often used in pretreatment systems because the vanes create the potential for clogging, 50-l 15” full cones are occasionall! wed for rinsing when clean. filtered rinsing solution IF used.

cone

A@tUtkg Venturi/diffuser

Tank mixing eductors improvmg circulation

Wetring Fine spray

Hydraulic atomiring nozzles and air effectively for wetting applications. Although they are not often used in vanes make them prone to clogging, used for wetting when clean, filtered

Full cone

Temperatures

the impact operation.

Eliminating remedy 286

on the

the Causes

Because

nozzle

is by proper

of the pump by a ratio of 5: I, within the bath.

atomizing

nozzles

can both be used

pretreatment systems because the 50-l IS0 full cones are occaGonally rinsmg solution is used.

Norzles mstalled between stages provide an effectwe au cunnm that prevents mist from one spray stage from entering another. Water curtain\ UC sometime% set up using detlectcd tlat spray noulcs.

Flat spray (deflected)

lowering cleaning

amplify the capacity and reducing settling

surface

of Poor

performance

nozzle

may require

use of Ryton

to be cleaned,

which

material.

will

reduce

the quality

of your

Nozzle Performance problems are so difficult to detect, the most effective selection and a regular program of nozzle maintenance and

replacement. The following from recurring.

guidelines

can help you solve nozzle problems

and prevent

them

Prriodiculiy inspect your nozzles. A maintenance program, including regular cleaning and inspection for physical damage, is the key to good nozzle performance. Regular cleaning is one of the easiest ways to ensure that your spray nozzle properly meters and atomizes your cleaning solution. Regular cleaning helps prevent caking problems. In any cleaning procedure, avoid accidentally damaging the orifice or the deflector surface. Use plastic and wooden prods or plastic bristle brushes, rather than hard metal prods. Replace nozzles as soon as there is a problem. Replace nozzlrs regular/~. Based on your experience with your operations, you will be able to anticipate nozzle problems. A program of regular replacement offers another advantage as well. With nozzles known to be in good condition, problems with other system components will be easier to identify. .Ypec~fi wear-resi,stantnozzle materi&. Hard-surface materials will resist wear and last longer. Stainless steel can offer twice the abrasion resistance of brass nozzles. Hardened stainless steel--up to fifteen times more abrasion resistant than brass-will provide excellent service in nearly all high pressure applications that are used. Select nozzles to match pressure/@ow requirements. To protect other system components, be sure to select nozzles that meet the pressure and flow capacity requirements of the system. Nozzles that develop higher pressures than your system design allows, can cause damage to the pump and other components.

SUMMARY Spray nozzle performance will have a significant effect on your system’s operations and your operating costs. Taking steps to spot, solve, and prevent nozzle performance problems will be a worthwhile investment. Proper nozzle performance can reduce operating costs, reduce maintenance costs, and improve cleaning quality. The major factors in replacing nozzles are the cost of wasted water, chemicals, electricity, and the deteriorating quality of the product, resulting in rework and customer service problems. If nozzle replacement is found to be excessive, one can then be better equipped to judge the need to switch to different nozzle materials if there are excessive abrasive solids in the water that need to be removed. If your nozzles are wearing out rapidly just imagine the damage that is being done to the rest of the system. Proper spray nozzle maintenance procedures, and periodic replacement, will help your pretreatment system run at peak efficiency, producing a higher-quality finished product, and add to the profitability of your operation.

The Technology

of Anodizing

Aluminum,

Third Edition

$210.00 NEWEDITION This valuable book has been complerely reedited and considers significant new de& opments in anodizing technology. The expanded volume will satisfy the anodiier who requires more detailed technology. After an introduction, the reader is presented with practical application of the new technology, and the nxure of the industry with capital investment appraisal, budgeting, and cost control. An excellent summary of anodizing technologies in clear language encompassing the advice of experienced technicians. by A. W: Brace

410 pages

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287