Knowing a waterjet—from the cutting head to the pump—gives a fabricator an idea of how much it costs to run one
October 14, 2008
For the end user of an abrasive waterjet, sometimes the maintenance involved with the new machine is underestimated. The downtime due to maintenance can result in decreased return on investment. To remain profitable, an owner must factor in costs associated with maintenance.
Editor's Note: This article is adapted from Jeff Day's paper, "Maintenance Components for an Abrasive Waterjet," presented at the FABTECH® International & AWS Welding Show, Oct. 6-8, Las Vegas, ©2008 by the Fabricators & Manufacturers Association Intl., Society of Manufacturing Engineers, and the American Welding Society.
End users of abrasive waterjet cutting machines sometimes underestimate the maintenance that will be involved with new machines. Unexpected downtime because of maintenance can result in a longer return on the investment.
To remain profitable, owners should be able to factor in costs associated with maintenance. These maintenance concerns cover items such as the cutting head, abrasive-feeding apparatus, pneumatic valves, high-pressure delivery lines, and the pump.
Abrasive waterjet machines are susceptible to destroying themselves. Combine 50,000 pounds per square inch (PSI) of water pressure with abrasive, and any part of the system that comes in contact with either of these elements—or both—will need to be replaced at some point.
The typical cost of operating a waterjet, including the cost for the machine, the operator, and overhead, can run around $85 per hour (see Figure 1). If an operator is not careful about how maintenance is performed, that hourly cost can increase by 10 percent or more (see Figure 2) because of increased consumable usage and downtime to repair or replace items. Depending on market conditions, this increased cost can put a serious crimp in end users' profits.
This maintenance checklist can help fabricators understand their waterjet cutting machines better and learn where the best opportunities are to keep operating costs reasonable and downtime to a minimum.
If fabricators are concerned about water quality at their homes, they should be especially concerned about water quality for their waterjets at their shops. This is perhaps the biggest factor controlling consumable costs. Hard water can cut expected life of consumable items at least in half.
Before making a waterjet purchase, a fabricator should test incoming water. It's imperative that the water quality meets the waterjet manufacturer's specifications if it's to operate as designed. Most pump manufacturers want to see less than 100 parts per million (PPM) of total dissolved solids (TDS), a measurement that tracks the amount of inorganic solids dissolved in the water that contribute to hardness, such as magnesium and calcium. Less than 10 PPM of TDS also can create problems, as the water actually will leach material from any component that it touches.
If the water needs treatment, at a minimum the fabricating operation will have to invest in a water treatment system. A water softener, which can have a price tag of $3,000 to $6,000, costs only pennies per hour to run and is generally all that is required if the incoming water is less than 200 PPM TDS. Overflow from this type of system goes directly to the drain.
If the incoming water is 300 PPM of hardness or greater, a reverse osmosis system may be more cost-effective in the long run. A reverse osmosis system can cost $20,000 to $30,000, which includes a chiller, but it can deliver water with more than 300 PPM of hardness. It costs about $0.20 to $0.30 per hour to run such a system, and the overflow is drained into public sewers.
A water recycling system calls for a similar investment as the reverse osmosis system and produces a similar type of water. However, it's considered a more environmentally friendly system because water is not drained; it's constantly reused. It costs about $2 to $4 per hour to run, which covers filters and electricity. The labor and downtime associated with changing out the filters also should be considered.
Nozzles (see Figure 3) typically cost from $80 to $200 and have a life of 40 to 80 hours, depending on the tolerance and edge quality desired. This is a cost of about $1 to $5 per hour.If the overall consumable parts cost per hour of a waterjet is $15 to $20 per hour, this could be 5 percent to 33 percent of consumable cost per hour.
The abrasive moving through the nozzle is the culprit behind the component's wear and tear. The abrasive slowly wears away at the inner diameters of the abrasive nozzle; the orifice grows by approximately 0.001 inch per hour of cutting. This wear pattern is not always even, so one side of the nozzle may wear faster than the other.
Symptoms that alert a fabricator that it's time to change the nozzle are inside cuts that become larger and outside cuts that become smaller than intended. Also, operators may see poor cut quality in one or more directions.
Repair time takes 5 minutes to 10 minutes to switch out the nozzles.
The best way to avoid poor-quality cuts related to worn nozzles is to:
Older nozzles don't necessarily have to be tossed in the garbage. Save them for less critical work.
Clogging of the abrasive nozzle also can cause problems. Clogs can occur from too much abrasive or an abrasive mesh size that is too large for a particular orifice diameter, large contaminants in the abrasive, or the nozzle tip contacting the workpiece during piercing or cutting.
A clog is easy to detect. The waterjet stops. Water may back up in the abrasive feed tube all the way back to the minihopper until the machine is stopped. This entire feed line and the minihopper must then be emptied and dried out entirely.
Because of the speed at which the abrasive is moving, generally the nozzle is clogged to the point where it cannot be used anymore. Sometimes people have success at clearing clogged nozzles by turning them upside down and turning on the water at lower power. The time necessary to repair or replace components related to clogging can be anywhere from 5 minutes to 30 minutes.
The best way to avoid clogging is to:
Occasionally an abrasive nozzle breaks. The tungsten carbide material the nozzles are made from is very hard, but very brittle as well. Breaking usually occurs when the nozzle drags on the material surface, collides with a tipped-up part, collides with the edge of the sheet material, or simply is dropped during regular maintenance. Obviously, breaking can have a huge impact on operating cost, especially if it happens early in the nozzle's life while it is still good.
The best way to avoid breaking a nozzle is to:
It takes about 5 minutes to 10 minutes to replace the broken nozzle.
The mixing chamber is where the abrasive is sucked into the waterjet stream. The abrasive particles are accelerated up to approximately 2.5 times the speed of sound.
The mixing chamber is susceptible to normal wear and has about a 500-hour lifespan. It costs between $100 and $200 for a new mixing chamber, and takes about 5 minutes to 10 minutes to replace.
Premature replacement of the mixing chamber may be needed if the mixing chamber and the water nozzle are misaligned. This will cause the waterjet stream to cut into the mixing chamber.
Clogging can occur in the mixing chamber as well if the abrasive is not screened properly.
The water nozzle is where the water pumped in at 50,000 PSI is sped up to a waterjet stream traveling at 2,500 feet per second (FPS). Jewels—sapphires, rubies, or diamonds—are used to create the waterjet stream.
How is this done? There is a very small orifice in the jewel. The orifice diameter is anywhere from 0.005 in. to 0.020 in., depending on the application. When water is allowed to exit the small orifice, it comes out at a very high rate of speed. The larger the orifice, the more water comes out, but the speed at which the water exits is greatly reduced.
Sapphires and rubies have a life of 40 hours to 80 hours, and diamonds last up to 800 hours. Of course, diamonds are much more expensive than the sapphires or rubies—$300 to $500 versus $15 to $30—but when you take into account the lifespan with the consumable cost, the overall cost per hour is similar. Cost savings can occur when maintenance intervals are reduced and not as much downtime is needed replacing worn jewels.
The best way to avoid contaminating the jewels is to:
An indication of a worn jewel is poor edge quality after the abrasive nozzle has been replaced. It takes 5 minutes to 15 minutes to switch out the worn jewel. An ultrasonic cleaner is a handy tool to help clean the water nozzle.
The pneumatic on/off valve opens and closes to control water flow to the cutting head. When the valve is worn, water will drip from the cutting head.
It takes 15 minutes or 20 minutes to replace the valve, and the cost could run between $100 and $200, depending on the total number of items to be replaced.
The high-pressure lines and fittings experience normal wear and have to be replaced at regular intervals. A fabricator can expect to get about 2,000 hours out of high-pressure lines. They cost $15 to $30 per foot to replace, and the job takes 15 minutes to 30 minutes.
More expensive prethreaded high-pressure lines might make sense if a fabricator is replacing lines infrequently, once or twice a year. A certain amount of skill is required to thread and cone the high-pressure lines properly. If an operator is doing it infrequently, it might be more cost-effective to buy the prethreaded lines and save the time and aggravation.
The best way to avoid unnecessary damage to high-pressure lines is to:
For the sake of the machine operator, all high-pressure lines should be secured. If a line breaks and it is unsecured, the line could be a very dangerous object considering the pressure being pumped through it.
To get a true sense of how many hand valves, T's, elbows, and other connectors are present in a waterjet, a fabricator needs to trace the water path from the pump to the cutting head. These items typically last 1,000 to 2,000 hours and cost $50 to $200 to replace.
The final filter catches any debris in the water before it enters the cutting head. The filter may either need to be replaced or just cleaned, depending on design.
Diagnostics software is of great importance when maintaining the heart of the waterjet cutting machine. The software monitors oil, air and water pressures, temperatures, and filter life and tests nonreturn valves for leaks. It provides warnings before a failure so that maintenance can be planned.
In some instances, remote diagnostics software allows the waterjet manufacturer's service tech to monitor the pump's performance. The software also can be used to allow the fabricator's operator to monitor waterjet performance while off-site.
The pump itself comprises many parts: high-pressure seals, nonreturn valves, safety relief valves, high-pressure cylinder and plungers, and the pressure accumulator. Water, air, and hydraulic oil are also pulsing through the equipment.
The high-pressure seals cost about $30 to $50 per set and come in sets of four to eight. They last approximately 500 hours. It can take about 30 minutes to 1 hour to replace all of the high-pressure seals on a pump. Water quality and attention to cleanliness can have a dramatic impact on seal life.
The nonreturn valves keep high-pressure water from returning to the pump or fresh water from going back out the inlet line.
The safety relief valve is similar in concept to the pneumatic on/off valve for the cutting head. When the cutting head shuts off, pressure in lines is released through the safety relief valve.
Sensors on the pump ensure proper pressures, temperatures, and volumes for air, water, and hydraulic oil. Various filters for each require periodic replacement.
The high-pressure cylinder and plungers have a life of 5,000 hours. On an hourly basis, the replacement cost for these items works out to about $1 to $2 per hour.
The pressure accumulator has a 10,000-hour lifespan. The replacement cost is about $1 per hour.
Most pressure accumulators now have a leak-before-fail design. If the machine operator notices a leak from the accumulator, it's an indication that replacement is needed before catastrophic failure occurs. Some pumps use electronically phase-shifted cylinders to eliminate the need for pressure accumulators altogether and avoid this replacement cost.
Is the sludge that accumulates after the abrasive is used a hazardous waste? More than likely, it is not.
To be sure, a fabricator can hire an environmental testing lab to conduct a toxicity characteristic leaching procedure (TCLP), an Environmental Protection Agency test method for determining if waste is hazardous or nonhazardous for the purpose of disposal. Also, the waste hauler likely knows what limits are for dissolved metals and will take the responsibility upon itself to test abrasive once per year.
The same TCLP test can be performed to determine whether overflow water can be sent to the public sewer. The water company can tell fabricators what the acceptable levels are for dissolved metals in water entering the public sewage system.
The installation of a settling tank that is inline with the overflow to the sewer might make sense. This helps to settle out most of the suspended solids. Failure to install such a tank could result in a clogged sewer line and a costly repair bill from the municipality.