Waterjetting 18d - Abrasive considerations

It would be best if, before I ended this short session on abrasive, I mentioned some of the practical constraints that sometimes limit the options for choosing abrasive types. To give a simple example, we were, at one time, demonstrating the ability of a waterjet drill to penetrate limestone. In the demonstration that morning we had used garnet as the abrasive and had made a steady penetration down to about 70-ft but the contracting office on the project did not seem overly impressed. So, after lunch, I suggested that we switch to an aluminum oxide abrasive, since we knew it was more aggressive.

Unfortunately for the afternoon program we were using a DIAjet type of system, where the abrasive is added to the water under pressure just downstream of the nozzle, and upstream of the delivery nozzle. While that worked well with the garnet abrasive (which passed without significant damage through the swivel on the end of the drill) that was not the case with the aluminum oxide. This is a much sharper abrasive and less prone to damage in mixing. As a result once we had the rig back in operation we were immediately struck by the black color of the water coming out of the hole – as the aluminum oxide stripped the inner lining from the hose carrying it to the nozzle. We then watched as, in real time, the pressure gage on the driving pump slowly slid back from the 10 ksi initial pressure to about 2 ksi as the abrasive ate out the orifices of the nozzle. Needless to say, having pretty much destroyed the downstream equipment in about five minutes, the afternoon demonstration was a bit of a disaster.

I also remember the first time that we used steel shot to try and cut through some rock, without giving too much thought to encasing the cutting operation. Those small spheres retained a lot more energy than most particles, and we were dodging the equivalent of shotgun pellets which ricocheted around the lab as we raced to shut the system down.

Both abrasives are, in their place, very effective tools in cutting materials that might be more difficult or uneconomic to cut by other means, but the peculiarities of their nature require that special precautions be used when they are used to make sure that there are not unintended consequences.

Sometimes the choices are simply practical. When we were cutting the walls of the Omnimax theater under the Gateway Arch in St. Louis, where we had to cut straight down (within half-an-inch either way over 15-ft of cut depth) through dolomite and chert it took less than a day to realize that the cost of using garnet to achieve the 12 – 15-inch individual cut depths was going to drive us out of economic reality within a week. Changing to a blasting sand (which we bought by the ton) did not change the cutting performance by much, but had a remarkable effect on overall costs.

Figure 1. Effect of abrasive type, size and feed rate on the depth of cut and optimal cutting condition when cutting rock. (after Yazici*)

Abrasive type and abrasive size both effect the depth of cut, and thus the economics of a cutting operation. Yet it is not possible to draw absolute rules since the different abrasives have different relative cutting efficiencies in different materials. For example, in the above plot boiler slag was relatively ineffective in cutting rock. On the other hand, with the right type of slag and steel Faber and Oweinah** have reported that slag can cut steel more than three times as efficiently as garnet. (This is partly because the slag shatters on impact and the fragments go on to scour the uplifted edges of the cavities generated by the initial impact of the particle.)

And while the British Welding Institute use smaller particles to cut softer materials, they have found it critical to use larger particles to get viable performance as the target material gets harder. In cutting steel I had mentioned in an earlier post, that garnet becomes less effective at a particle size below 100 micron. Yet in cutting aluminum (which is softer) the particles can be smaller and yet still effective.

Figure 2. The effect of particle size when cutting aluminum using corundum particles (after Faber and Oweinah ibid)

Yet, as discussed at the beginning, the cost of the abrasive must not only be set off against the potential for improving the cutting rate, one has to also look and see if there is an increase in the operating cost of the system when a harder, and thus often more effective cutting abrasive is used. Zaring et al showed this with a plot that they published at the 6th American Waterjet Conference***.

Figure 3. Relative benefits and costs of changing abrasive type (after Zaring et al***)

All things are, however, relative, and in some small cutting operations we have found it more economic to sacrifice the nozzle over the cutting time required in order to achieve a cut that could not be effectively achieved any other way.

As with many things in the waterjet business, while there are general rules that can be laid down to guide operations, when it comes to specific cases then it is often worth running a small series of tests on the projected target material, using different abrasives, at varying size ranges and feed rates, before calculating (usually using a normalized cost in dollars or gms per area of cut) the most effective abrasive for a given operation.

*Yazici, Sina, Abrasive Jet Cutting and Drilling of Rock, Ph.D. Dissertation Mining Engineering, Univ. of Missouri- Rolla, Rolla, MO, 1989, 203 pp.
**Faber, K., Oweinah, H., "Influence of Process Parameters on Blasting Performance with the Abrasive Jet," paper 25, 10th International Symp Jet Cutting Technology, Amsterdam, Oct, 1990, pp. 365 - 384.
***Zaring, K., Erichsen, G., Burnham, C., "Procedure Optimization and Hardware Improvements in Abrasive Waterjet Cutting Systems," 6th American Water Jet Conf, Houston, TX, Aug, 1991, pp. 237 - 248.

Waterjetting 7d - High-pressure Waterjet cleaning over sandblasting paint

Over the years I have been caught up in “discussions” with several folk about how good high-pressure and ultra-high pressure waterjet streams were as a surface cleaning tool, in contrast with chemical and abrasive use in removing paint and other surface layers. One debate was about cleaning some particularly toxic chemicals from various surfaces. The point that often comes up in these discussions is that of “how clean is clean?” And in this particular case it was stated that the surface could never be completely cleaned. The rationale for that position was because the chemicals would enter into any cracks and flaws in the paint, and could therefore be retained either in the top coat, or the underlying primer. My answer to that was to take a small sample and clean the surface over the first quarter, raise the pressure and remove the top coat on the second quarter, raise the pressure further and remove the primer down to bare metal on the third quarter, and then, after adding a small amount of abrasive to the water, remove a thin surface coat of metal from the sample. It seemed to be a convincing demonstration, though I will come back to one problem in a later post, and for this post I will discuss taking the paint off.

It is now reasonably well known that high-pressure water can be cost effective as a way of removing paint, particularly from large structures such as bridges, and ship hulls, but it took a while for some of the benefits to become evident.


Figure 1. It was originally estimated that it would save some $1.75 Canadian per square foot to clean the Quebec Bridge with ultra-high pressure waterjets, rather than sandblasting. That increases to $4.50 per sq. ft. were hand tools the alternative (WJTA Jet News, March 2000)

There are 8-million square feet of surface in the bridge. As I noted at the end of the last post, the historic method for cleaning surfaces, and removing deteriorated paint has been to suspend abrasive particles in an air stream, and to use those particles to abrade and erode the paint from the surface. When the paint, rust and other coatings have been removed the job is often considered finished when the surface is restored to a nice shiny surface finish. There is, however, a snag, when one does this. The numbers that I was once given were on the order of: from the time that a railroad wagon was put into service, it would take 5 years before it would require stripping and repainting. After that first treatment, however, the paint would deteriorate more quickly and often within another 18-months the wagon would have to be taken back for repainting.

So why is this, and why does high/ultra-high pressure paint removal help extend the life of that second paint coating? I, and the industry, are deeply indebted to Dr. Lydia Frenzel who did a lot of the pioneering work in helping to define the benefits of the technology, and then spread the word about them. The problem begins as the surface begins to corrode, and I will continue to use the wagon as the example, though the result holds true for many surfaces. As the rust and damage continues to eat through the paint and into the underlying metal, that surface is not attacked evenly, but, instead small pockets of corrosion develop, where the metal is eaten away more in the middle or along the sides of the pocket.

By the time that the surface is ready to be painted it is no longer, therefore, smooth, but rather is pitted and covered in corrosion.


Figure 2. Exaggerated illustration of the condition of the surface, with the overlying corrosion shown in green.

When the surface is cleaned with an abrasive, typically driven using an air stream to sandblast the surface, the particles will impact and distort the surface. Thus while the majority of the corrosion will be removed by the impact and scouring action of the abrasive, some will not. Further the impact of the abrasive particles will bend over the weaker structures on the surface as well as peeling over some of the metal on the surface.


Figure 3. Electron microscope picture of a piece of metal on the edge of a pass by an abrasive laden stream, so that the action of the individual particles in cutting into and plowing the surface can be seen. Note that this peels over metal edges, for example at the arrows.

The peeling over of the surface, and the flattening of it give the shine that used to be the sign that the job had been effectively done. There are, however, two disadvantages to this. The first is that by distorting the surface, the bending over of the metal traps small pockets of corrosion within the surface layer of the metal.


Figure 4. Representation of the metal surface after it has been cleaned with abrasive. Note the folding over of metal to trap corrosion products. The abrasive particles are also not small enough to penetrate into the smallest tendrils of corrosion migrating into the metal, and these pockets (green) also are trapped.

With corrosion already embedded in the surface, before it is painted, that will develop immediately and thus the relatively short time before it undercuts the paint and causes it to fall off. There is also another reason for this. As air pressure is increased to speed up the cleaning, and give that “shinier” surface it smooths the surface and makes it more difficult to anchor the paint on the metal. This was shown by F.W. Neville (and is quoted in the book “Blast Cleaning and Allied Processes, by H.J. Plaster) with this table:


Figure 5. Relative paint pull strength as a function of the pressure of the air driving the sandblasting stream in pre-cleaning the surface of the old paint, prior to repainting.

As the table shows, the higher the air pressure then the smoother the surface, and the poorer the bond made with the paint.

Now consider what happens when a high-pressure jet cleans the surface. The water does not have the power to distort the metal, but rather does have the ability to penetrate all the cracks and pits on the surface, and flush them clean. As a result the surface is left rough (to give a good paint bond) and corrosion free.


Figure 6. Illustration of the relative condition in which a high-pressure waterjet will leave the surface.

One of the difficulties that early proponents such as Lydia had in getting the technique accepted, however, lay in the cleanliness of the surface. Because the metal had not been distorted back into a smooth upper surface, it does not reflect light in the “shiny” manner that an abrasive cleaned surface does. Thus to those trained to the latter, it did not appear clean. There had to be a considerable amount of demonstration, explanation and training before it was accepted that this “grey” surface was actually cleaner. And there are now standards, issued by the Steel Structure Painting Council, that recognize this.


Figure 7. A primer coated plate (left) that has been cleaned to white metal (right) using a high pressure waterjet.

Note that actual microphotos of abrasive and waterjet cleaned metal surfaces can be found in the paper by Howlett and Dupuy (Howlett & Dupuy, NACE Corrosion/92, paper No. 253; Mat. Perf, Jan. 1993, p. 38, the waterjet pressure was 30,000 psi).

Wind in the Rockies is expensive

Government policy can, once decided, be implemented by a combination of laws and financial incentives/disincentives among other means. Thus, for example, when trying to change the ways in which America gets power, the government can limit the amount of coal burned, both by direct fiat, and by making it too expensive (through the cost of permits). The former limit will be established through the caps on the production of GHG, assuming that there is no immediate vast investment in sequestration. There is a fair amount of debate over whether the financial incentive will be through a direct tax or the more indirect route of charging for allocations. Having heard Congressman Waxman’s aide at the EIA meeting, I believe the decision is long over, and that, if anything does come out of this Congress it will be the cap and trade model that will be used.

That having been said, since the nation is growing in numbers, even if per capita use is held constant (as it has sensibly been in California) any reduction in fossil fuel use will need to be replaced with an alternative. At present the most widely touted of these are wind and solar, and the high costs of solar mean that, for many utilities seeking change, wind has been the choice, but even this is, in some cases, proving to be too expensive an option.

Wind farms are becoming more obvious around the country, and their message is strengthened with a steady campaign of adverts. T. Boone Pickens is seen more as a savior than as a salesman. And the Secretary of the Interior currently is pointing out that there is sufficient wind available to replace all the coal-fired power stations in the country. But under all the hype, and after the cranes have come and gone, the change in energy source has to make economic sense. Wind farms will only be established where there is a credible likelihood of their making money for the investors that raise them. The current comments of the Secretary come as hearings get under way around the country.

Salazar said ocean winds along the East Coast can generate 1 million megawatts of power, roughly the equivalent of 3,000 medium-sized coal-fired power plants, or nearly five times the number of coal plants now operating in the United States, according to the Energy Department.

Salazar could not estimate how many windmills might be needed to generate 1 million megawatts of power, saying it would depend on their size and how far from the coast they were located.

Mark Rodgers, a spokesman for Cape Wind, which wants to build a wind farm off Cape Cod, Mass., estimates it would take hundreds of thousands of windmills. The average wind turbine today generates 2 to 5 megawatts per unit, he said.

This is the first of four hearings, and focused on the East Coast. But the potential states most likely to benefit can be judged from the DoI plan to create Regional Energy Permitting Centers.

To expedite production of renewable energy on public lands while protecting land, water, and wildlife, Secretary of the Interior Ken Salazar today pledged to create four Renewable Energy Coordination Offices, one each in California, Nevada, Wyoming, and Arizona, along with smaller renewable energy teams in New Mexico, Idaho, Utah, Colorado and Oregon.

You may note that these are all in the West.

While the numbers quoted are large, and the potential benefits of moving to renewable energy are continually being cited, the underlying realities of getting a good return on the investment, at a lower cost than the alternative, is being kept quiet. Unfortunately, as an article in USA Today notes, even with the best will in the world, those benefits don’t always happen. And, in the case of Durango, CO, the utility is moving back to coal, from wind.

For two years, the city of Durango, Colo., bought electricity for all its government buildings from wind farms. The City Council ended that program this year, reverting to electricity derived from coal-burning plants and saving the cash-strapped city about $45,000.

The Durango plan has its roots back in 2007 when the city made the move to wind energy.

Green power currently constitute(d) about 10% of city power purchases. The extra the city pays for green energy will add about $120,000 a year to its electrical bill officials said. But the extra cost will be offset by an energy audit aimed at cutting power consumption. With the wind power option, electric customers pay $1.25 per block of 100-kilowatt-hours in addition to their regular rate. The extra that consumers pay for their power funds investment by power producers in alternative-energy sources such as hydropower, solar and wind. But electricity from all sources flows on the same line. In 2006, Durango used almost 8.15 million kilowatt-hours of power at a cost of $779,000.

At the time, as you may notice, the city recognized that the renewable energy would cost more, but felt that it could make up the difference with improved efficiency. Which is the same sort of argument that we are now getting from the new Administration.

Although the above cite is recent, the change actually occurred last December.

City Manager Ron LeBlanc recommended the city stop buying power from renewable energy sources when it became necessary to cut the 2009 budget by more than $500,000. The city council approved the budget, including his recommendation, earlier this month.

The La Plata electric association charges 80 cents more per 100 kilowatt hours for electricity from solar and wind power. LeBlanc says that adds $45,000 to the city’s annual electric bill.

La Plata does not generate the power itself, but is passing along a program from Tri-State.

The LPEA Green Power program was initiated in 1998 when Tri-State – from which LPEA purchases its power – responded to requests by its member systems, to include a green power option as part of its available resources to end-use consumers. Because of this program, LPEA customers who request purchase of green power receive it at LPEA’s cost from Tri-State, $1.25 per 100 kilowatt-hour block per month. Purchasing one block of Green Power costs consumers less than $.05 per day. To date, LPEA is among the leading purchasers of Green Power in Tri-State’s 44-member cooperative system, supplying nearly 800,000 kilowatt hours of Green Power generation each month.

Tr-State had lowered their costs to 80 cents in January, 2008. . However the company has also carried out an Integrated Resource Plan (IRP) looking at future energy supply, based on anticipated need. That plan concluded:

Tri-State proposes to develop and own two new 700-megawatt supercritical coal-based units at the existing coal-based, 360-megawatt Holcomb Station in western Kansas. The efficient units would include best available control technology to minimize air emissions and activated carbon injection to minimize mercury emissions. The IRP clearly reinforces the need for the first 700-megawatt baseload resource in 2012, which is when Tri-State first unit at Holcomb Station could be online.

Since the first Holcomb unit cannot be brought on line until 2012, Tri-State is left with a significant deficiency in both capacity and energy during the interim period; and its options are somewhat limited. For modeling purposes in this IRP, it is assumed that Tri-State will install combustion turbines (CTs) as soon as possible and purchase energy from the market. However, there are many other options available to Tri-State.

These are the coal-fired power plants that then-Governor Sibelius vetoed three times, with part of her argument being that more than 80% of the energy they would produce would be exported. Which leaves one wondering where the power will come from for Durango, since I am presuming that the turbines are gas-driven, and thus too expensive.

(Um! Yes I know that Durango said that they are going back to coal, and have so moved, but Tri-State still has their supply problem, or will as soon as the economy picks back up without the new stations they were relying on.)