Nozzle Maintenance: Avoiding Big Problems on Small Components
September 1, 2007 By Pulp & Paper Canada
Spray nozzles may be among the smallest components in a mill, but they can make a huge impact on the quality and efficiency of operation. Spray nozzles are designed for long-lasting, trouble-free perf…
Spray nozzles may be among the smallest components in a mill, but they can make a huge impact on the quality and efficiency of operation. Spray nozzles are designed for long-lasting, trouble-free performance. However, like all precision components, spray nozzles do wear over time and, subsequently, cause spray performance to suffer and costs to rise.
Cost of neglect
Even slight wear can cost tens of thousands of dollars annually in increased operating expenses. Here are two examples:
EXAMPLE 1: Dryer Section shower – One shower with 70 nozzles spraying a release agent on dryer felt
Operating conditions: System sprays 5 gpm (19 l/min) of a 1:20 aqueous solution of chemical at 40 psi (2.76 bar)
Chemical consumption: .25 gpm (.95 l/min); $0.52 per gallon
Operation: Three shifts, five days per week = 120 hours per week
Annual operating cost: no nozzle wear: $57,179
15% nozzle wear = 15% increase in operating costs:
Annual operating cost with 15% wear (increase in nozzle capacity):$65,802
Note: Does not include water filtration and recovery costs. All costs are in CAD.
The cost to operate this one shower increases from $57,179 to $65,802. That’s $8,623 of unnecessary cost. If you have six showers in the dry end, your costs increase by $51,738 annually.
EXAMPLE 2: Knock-Off Shower – One shower with 72 nozzles in the dry end of a press section
Operating conditions: Nozzle flow rate: 2.34 gpm (8.8 l/min) at 350 psi (24.1 bar)
Operation: 60 hours per week, 48 weeks per year
Increased water consumption daily due to a 15% increase in capacity:137,117 gallons
Increased annual cost of water and electricity for pump operation$67,978
Note: Assumes water cost of $3.13 per 1000 gallons; electricity at $0.08 KWh. All costs are in CAD.
Again, this example is for one shower, so you can see how quickly costs can escalate.
Symptoms of spray nozzle wear
Detecting nozzle wear can be difficult, as many symptoms are not visible. Testing with specialized equipment may be necessary. So, what can be done?
* Visually examine spray patterns and watch for heavy edges and changes in spray angles and distribution.
* Monitor flow rate. An increase often means orifice surfaces are starting to deteriorate.
* Monitor spray pressure. Changes could indicate nozzle wear.
* Examine process results. Quality problems often stem from changes in drop size due to nozzle wear.
* Contact your nozzle manufacturer for specialized testing to determine wear rate and provide maintenance and replacement guidelines.
The two nozzle tips here show little visible difference. The tip on the left is new. The tip on the right is worn to the point that it sprays 30% over capacity. Visual inspection shows little evidence of wear.
An optical comparator can show evidence of internal wear. The tip on the left is new, while the tip on the right shows evidence of 30% wear. The best nozzle spray manufacturers will allow you to send sample nozzles back for this test at no charge.
Other common problems and prevention/troubleshooting tips
Wear is not the only enemy of optimal spray nozzle performance. Nozzles can also be plagued by plugging, corrosion, scale build-up, slime/odour and caking. These are common problems in pulp and papermaking, largely due to the chemicals and water supplies being used. Establishing and implementing a nozzle maintenance program is the most effective way to prevent problems.
Plugging is very common with use of reclaimed or whitewater. Nozzles, lines and showers can plug due to debris and long fibres in the water. To prevent plugging, use water clarification devices and strainers. Be sure to specify nozzles with adequate free passage. Installing automated self-cleaning showers can be a real boon.
Chemicals can be extremely corrosive. Make sure to specify nozzles in the appropriate materials: 316 or 304 stainless steel depending on chemical concentration. Avoid iron and mild steel.
To avoid scale, control the hardness level of the water using chemical additives as needed. Conduct maintenance on a regular basis.
Slime and odour
Implement an effective biocide program to control biological growth. Use chemical additives as needed. Eliminate areas with stagnant water and conduct frequent wash-ups.
Build-up can occur inside the nozzle or on the exterior. The best prevention is to conduct maintenance on a regular basis.
The bottom line
Pay attention to your nozzles. Implement proper cleaning procedures and avoid accidental damage. Be sure to use tools significantly softer than the nozzles, such as a toothbrush, toothpick or soft brush. Never clean the orifice with metal brushes or objects. Soak nozzles in mild solvent to loosen debris for easier removal with proper equipment.
The price tag associated with nozzle neglect can be extremely high and is easily avoidable. With regular maintenance and periodic replacement, your spray nozzles will reliably deliver the performance you need — keeping quality and production high and operating costs low.
Christy Hofherr has nearly 15 years of experience with spray technology and specializes in pulp/papermaking and steelmaking. She has presented papers at many technical conferences and published several papers in trade publications. She can be reached at Spraying Systems: 630-665-5000 or Christy.firstname.lastname@example.org.
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