Industrial Utility Efficiency

Evaluating Operational Costs of Sandblasting Operations


Introduction

As you walk past the “sandblasting cabinet” back in the corner of the plant running alone and without the need for monitoring, does the thought of operational costs enter your mind? When it does, are you happy knowing the cabinet is automatic and does not need a full-time operator? Then, did you say to yourself, I wonder how much that abrasive media costs? How long does it last? Is this a more cost competitive alternative? Is there something that might last longer?

These are all great questions and should be evaluated as to their overall impact on your operating costs. What was missed, however, are the two most obvious questions related to operating cost. What type and size nozzles are on this unit and how much compressed air do they use?

Today, most cabinet type units use compressed air to flow the abrasive media (i.e. sand). There was a time in the past when many of these used “slinger type” blasting with paddles that “slung” the media against the object. Today many, if not most of these have been modified (some manual, some automatic) out to compressed air – your most expensive utility.

Before going too far on this, let’s realize that sandblasting is a very complicated topic and often the nozzle choice, operating pressure, and type of media are very critical to the end result of the surface being “blasted”. When this is the case and the blasting is critical to quality and productivity, we have discovered in plant air system reviews that the total package is usually very thorough in thought, well applied, and operating at a reasonable, if not optimum cost.

Sandblasting jobs can go from very small (glass etching) to very large (shipyards). There are many types of blasting media from dry ice to ground up corn cobs, to aluminum oxide to steel shot or grit. They all have a place but the thrust of this article is not on precision sandblasting, but routine sandblasting in plants as part of the production air and ongoing maintenance programs.

 

Win-Win

“Nozzle material selection depends on the abrasive chosen, how often you blast, the size of the job, and the rigors of the job site.”

— Scott van Ormer, Air Power USA

 

 

Industrial Cabinet Type Abrasive Blasting

Many sandblasting jobs may be only consume 1 to 2,000 hours per year, but there may also be multiple sandblasting cabinets used to prepare parts, de-burr parts, break down a surface, etc. These cabinets may be part of production on the direct product, in a part of the assembly process, or may operate to clean out and resurface molds such as those used in a glass bottling plant.

 

Chart 1.  Abrasive Consumption per Hour and Air Flow in Cubic Feet per Minute 

 

We see these cabinet blasters in plants all the time and when asking operators, “What type of nozzles do you have”? They answer, “I don’t know”. When we ask how long they run between nozzle changes, this is usually not a time of record. Sometimes we ask, “How do you know when to change nozzles?” Their answer, “Whenever the nozzles break” or “When it doesn’t clean well anymore”. In a recent audit, this situation came into discussing a blasting box that had twelve, 3/16” ceramic nozzles running at 60 psig. The calculated flow through this process would be 12 nozzles at 30 scfm each moving about 171 lbs/hour of media (see Chart 1 above). The media cost here is not really related to the lbs/hr because most of it is recycled, but the cost of compressed air is directly related to the operating cost. When new, with no leaks, this blaster should use about 360 scfm of compressed air. We installed a flowmeter on the 4” line and found it was actually using about 910 scfm.

To put this in perspective – a \$.06/kWh power rate on an average single-stage, rotary screw, lubricant-cooled compressor (4 cfm input horsepower at 100 psig) operating 8,000 hours per year will have an electrical energy cost of \$100/scfm/year. Under these conditions, when new, this unit will run utilizing compressed air that costs \$3,600/nozzle/year to produce but now it is costing \$9,100/nozzle/year in electrical energy to produce the same air to do the same job. How does this happen?

Remember those two questions at the beginning of the article? What type and size nozzles are you running? We knew the size of twelve, 3/16” diameter nozzles but the type was ceramic! Do we say congratulations or offer condolences? It will depend on the process.

 

Win-Win

“As in any production process using an expensive utility — compressed air — all steps should be considered to optimize this type of operations and measure the flow to know where you are and when to change.”

— Scott van Ormer, Air Power USA

 

What are the Best Nozzle Material Choices?

 

Nozzle material selection depends on the abrasive chosen, how often you blast, the size of the job, and the rigors of the job site. Here are general application guidelines for various materials.

Ceramic nozzles offer good service life at a lower price than other materials offered. They are a good choice in low usage applications where price is a primary factor and nozzle life is less important.

Tungsten carbide nozzles offer long life and economy when rough handling can’t be avoided and mineral and coal slag abrasives are used. All tungsten carbide nozzles are not equal.

Silicon carbide composite nozzles offer service life and durability very near tungsten carbide, but these nozzles are only about one-third the weight of tungsten carbide nozzles. Silicon carbide composite nozzles are an excellent choice when operators are on the job for long periods and prefer a lightweight nozzle.

Boron carbide nozzles provide longest life with optimum air and abrasive use. Boron carbide is ideal for aggressive abrasives such as aluminum oxide and selected mineral aggregates. Boron carbide will typically outwear tungsten carbide by five to ten times and silicon carbide by two to three times when aggressive abrasives are used. They are more susceptible to damage from outside forces.

The chart below represents relative service life of four specific nozzles running and three different types of media. There are two very important things to remember regarding nozzle wear:

  • At the same pressure as the nozzle wears, it will automatically take and use more air
  • At the same pressure as the nozzle wears, the blast pattern breaks up with higher concentration at the center (often a quality problem) and lower concentration on the outside (a coverage problem) which will often require more time (and more compressed air) to do the job.

 

Service Life Comparisons

 

In previous blaster cabinet case studies, two nozzles were broken off, all were worn and several holders were leaking. Because there are so many variables, it is sometimes hard to assign a value to the projected effect of proper nozzle type and material and proper monitoring of the process.

 

An Example Referring to Chart 1

Using aluminum oxide media and a 3/16” ceramic nozzle, we have a rated life that runs from 30 scfm at 60 psig to 90 scfm at 60 psig (5/16”) over four hours. Using a straight line average, we would have increased flow 60 more cfm in 4 hours or 15 scfm per hour. The cost to run the increased air demand due to wear on this blaster using the compressed air energy cost shown is \$100/scfm/year = \$1,500/year increase in electrical energy cost per nozzle due to wear.

On the other hand, if running a boron carbide nozzle which lasts 1,000 hours for the same job, we would go from 30 to 90 scfm in 1,000 hours. The average flow increase would be .06/scfm/hour per nozzles or a cost of \$6/year increase in electrical energy cost.

Total compressed air electrical energy operating cost at \$100/scfm/year

Current air usage when changing ceramic nozzle every 4 hours:

3,000 scfm base flow + 1,500 scfm = 4,500/scfm/nozzle = \$4,500/year/nozzles

Total estimated energy cost of 12 nozzles = \$54,000/year

 

Total nozzle cost of 2,000 ceramic nozzles per year at \$11.00 each = \$22,000/year

 

 

Proposed usage when changing boron carbide nozzles every 1,000 hours per nozzle:

3,000 + 6 = 3,006/scfm/year/nozzle = \$3,006/year/nozzles

Total electrical energy cost of 12 nozzles = \$36,072/year

 

Net annual electrical energy cost savings total per cabinet using boron carbide nozzles = \$17,928/year

 

Total nozzle cost of 8 boron carbide per year at \$152 each = \$1,216/year

 

Savings in nozzle cost = \$20,784

 

Total project savings = \$38,712/year

 

 

Comments

  • This is a very conservative estimate because the wearing out relationship to air use is not a straight line relationship. The open area of the nozzle goes up with the square of the radius.
  • In all probability, the nozzles are not changed every 4 hours as far as the ceramic type is concerned. Compressed air usage will continue on up until the feed and distribution lines become the limiting factor – the air pressure to the nozzles falls and the work done is scrap. At this point we are using significantly more air with poor results and increased time on job and scrap.
  • Alumina oxide is a very common blaster media we find in these applications.
  • When you operate, you run all the same nozzles, same pressure for all parts or objects. Perhaps it would be appropriate if you find better results with fewer nozzles – lower pressure, etc. each set optimized to the specific job.

 

Summary

As in any production process using an expensive utility – compressed air – all steps should be considered to optimize this type of operations and measure the flow to know where you are and when to change. That old, outdated blasting cabinet will just keep on running using more and more air unless it is managed.

 

For more information contact Scott van Ormer, Air Power USA, tel: 740-862-4112, www.airpowerusainc.com.

 

To read more Metal Industries articles, visit www.airbestpractices.com/industries/metals.