Pumps for abrasive jet cutting
This is the last of four articles intended to help a prospective buyer evaluate the wide range of abrasive jet machinery on the market. The first article covers the abrasive jet process itself in comparison with other cutting processes to help the prospective buyer understand the pros and cons of using the process. The second discusses software, and the third covers table construction and accessory hardware. This final article discusses the pumps that provide the high-pressure water for the jet.
The Pressure, Maintenance, and Productivity Relationship
In an ordinary drilling or sawing operation, the metal removal rate depends on the mechanical forces on the tool and how fast the tool is moving. Yet, an operator cannot increase the metal removal rate continuously. Tools wear out more quickly at higher speeds, and replacing failed tools can become very costly.
The preferred cutting speed for a tool is the speed suitable for the material being cut that results in optimum tool life. Indiscriminately doubling the speed or chip load causes early failure and increased costs.
In abrasive jet cutting, pressure-the measurement of the tool's mechanical loading-affects more than just the cutting nozzle. The entire system is wetted by high pressure-all the way back to and including the pump. Productivity considerations encourage running the pump at very high pressure (See Table 1).
Raising the pressure from 40,000 PSI to 80,000 PSI increases the cutting rate by more than three times. Yet, there are very few cutting systems that regularly operate above 55,000 PSI, because of the very high maintenance costs caused by early failures at higher pressures.
Maximum Cutting Speed for in. aluminum with 0.013 in. orifice, 0.030 in. Mixing Tube, and 0.8 lb/min abrasive
Just as in mechanical cutting, a balance must be struck between productivity and maintenance costs in abrasive jet cutting. Because of the significant productivity increase achieved with high pressure, most jet cutting systems incur higher maintenance costs than conventional equipment. The highest maintenance cost comes from abrasive mixing tube wear caused by the abrasive. Pump maintenance is a close second. New owners expect nozzle wear, but often are surprised to find that the pump is a high-maintenance item. The remainder of this article discusses pumps and the relationship between pressure and maintenance cost.
Table 2 shows the speed at which water flows from a nozzle at various upstream pressures as calculated by using Bernoulli's equation. The table also can be used as a rough guide to determine how fast water would have to be moved by an impeller to develop desirable pump pressure.
A speed of about 385 ft. per sec. would be required to develop even a modest pressure of 1,000 PSI. This would equate to an RPM of 14,700 in a 6-in.-dia. impeller pump. Because of the impractically high RPM required of a high-pressure impeller pump and the associated frictional losses, all high-pressure pumps are positive displacement pumps.
The relationship between pressure and water speed
Among the positive displacement pumps are gear pumps, vane pumps, screw pumps, and plunger pumps. The first three rely upon close tolerance gaps to limit leakage from the high-pressure to the low-pressure regions. When pumping a viscous liquid, like oil, pressures up to 10,000 PSI are possible. However, for water, even small clearances allow significant leakage, and pressures above about 1,000 PSI are impractical for all but plunger pumps in which a positive seal is present.
High-Pressure Plunger Pumps
All water pumps for pressures above 10,000 PSI are plunger pumps. A solid plunger is pushed into a closed chamber, raising the pressure and expelling the pumped fluid through an outlet check valve. Then, the direction of the plunger motion is reversed, and low-pressure fluid fills the chamber through an inlet check valve. The continuously reciprocating plunger provides the pumping action. See Figure 1. Two popular drive types for moving the plunger are currently available. The crank pump moves the plunger with a crank similar to the one in an automobile engine. The intensifier pump drives the plunger with a hydraulic cylinder that usually is operated with oil.
Before examining the two drive types, consider what happens in the pumping chamber illustrated in Figure 1. At high pressures, the pumped liquid is compressible. At 55,000 PSI, water is about 12 percent compressible. That means that the plunger must move enough to fill 12 percent of the chamber volume before the pressure reaches 55,000 PSI. At that point the outlet check valve can open against the pressure already in the output line. Then, at the end of the stroke, when the plunger reverses and the outlet check valve closes, any water trapped in the cylinder continues to expand and push on the plunger until the plunger has moved far enough to drop the pressure on the inlet check valve. The energy put into the plunger motion by this expanding trapped water can be recovered or not depending upon the drive type.
In the crank pump this expansion energy is recovered in the same way that it is recovered from the expanding hot gasses in an internal combustion engine. It goes back into the kinetic energy of the rotating components.
In the intensifier pump, the energy is dumped into the oil of the hydraulic circuit, causing heating. For this reason, intensifiers operate at an efficiency of about 70 percent, whereas crank drive pumps operate at 95 percent and higher efficiencies. The heat dumped into the oil of an intensifier drive must then be removed-usually by an oil-to-water heat exchanger. This requires extra water for cooling purposes and significantly higher electric costs to pay for the wasted energy.
Other differences between the two pump types arise from the relative operating speeds of the plunger. Crank plunger speeds are about 30 IPS while intensifier plunger speeds are usually about 6 IPS. For comparable output flows, the intensifier plungers, cylinders, and check valves must be larger and therefore more expensive than the corresponding crank drive parts. Also, a crank is much less costly and complex than a hydraulic system. Both the initial costs and part maintenance costs are lower for the crank drive pump.
Why Two Drive Types?
Why are there two types of pump drives in the marketplace? Here, a little history is helpful. Through the 70s and 80s, crank drive pumps held almost the entire market for pressures 20,000 PSI and below, because of their low-cost reliable operation. Intensifier pumps were used for 30,000 PSI and greater for the reasons that follow. Early pumps were plagued by three problems that favored the slow operations of intensifiers at the higher pressures-metal fatigue, check valve wear, and seal life.
Metal fatigue is the failure of metals due to repeated loading and unloading, causing the initiation and propagation of cracks. Component life depends on the materials used, the stress levels in operation, and the number of load cycles applied. Steels have a stress level below which they will never fail no matter how many load cycles are applied. Usually, a stress level just below that which causes failure at 10,000,000 cycles will never cause failure. An intensifier achieves 10,000,000 cycles in about 3,000 hours and a crank drive pump in about 300 hours. Both can be designed for infinite fatigue life at pressures below 55,000 PSI using modern materials and stress control techniques. Fatigue is no longer an issue limiting crank pumps.
Check valve wear is another problem solved by modern technology. Metal seats wear by adhesive wear, when particles transfer from one surface to another. Wear life depends upon the number of open and close cycles and the operating pressure. Modern ceramics have the strength necessary for check valve components, and adhesive wear between metal and ceramics is so low that check valve life is no longer a concern on crank drive pumps.
Seal life is what currently limits crank drive pumps to pressures of about 55,000 PSI, and it is advances in this area that have allowed the pressure range of crank drive pumps to rise. Dynamic seal life is dependent almost entirely on the total length and surface finish of the plungers. When pumping a gallon of water, the dual plungers in an intensifier might travel about 200 in. each, whereas the three plungers in a crank drive would travel about 1,000 in. each. If each pump has the same plunger-seal technology, the intensifier seal would last five times as long. Differences between 300 hours and 1,500 hours could be seen.
Other Comparison Points
In addition to these formerly dominating issues, there are other points of comparison. Because of the low plunger speed, the intensifier delivers one or two large discharges per second, whereas the crank pump delivers 30 small discharges per second. The pressure output of the crank pump is very smooth, and the system does not require an accumulator to smooth the pressure output. No marks are left on the part because of pressure ripple with a crank drive pump, nor is there any large high pressure vessel that can cause a safety concern as with an intensifier pump. (At least two major manufacturers have had accumulators blow apart at the users' plants with flying parts.)
Even with the accumulator, the pressure dips about 2,000 to 5,000 PSI at each shift of the intensifier. For comparable cutting quality, the intensifier must run at a pressure 2,000 to 5,000 PSI greater than the crank drive to maintain quality at the dip.
The two pumps have nearly comparable pressure control. The intensifier output pressure is controlled by stroke variation and the hydraulic pump flow. Varying the RPM of the electric motor through a variable speed drive controls the crank drive output pressure.
The intensifier-accumulator combination responds quickly to load changes and can be used to run independent nozzles turned on and off at random, while maintaining a constant pressure level. The crank drive can run multiple nozzles, but for quick response, dump valves must be opened as the nozzles are shut.
The crank operating at about 600 RPM generates far less noise than the hydraulic system of an intensifier. Quiet intensifier pumps are possible only by providing costly sound control measures.
When service is called for, there are many mechanics who understand and can work on crank drive pumps because of the simplicity and close similarity with automobile and other internal combustion engines. Technicians familiar with hydraulic pumps, valves, filters, pressure controls, and heat exchangers are rare and unlikely to be found in the ordinary machine shop.
Making the Right Choice
The main reason for putting up with the low-efficiency, noise, cost, and pressure ripple effect of an intensifier system is to have the increased seal life that goes with low plunger speeds. The intensifier is the pump of choice for a 24-hour operation that runs for weeks without any chance of maintenance. If once-a-month maintenance is possible, the crank drive pump is preferable. At a cost of $0.13 per kW-hr., the electric power cost savings alone pay for a seal rebuild kit each 300 operating hours.
A Word of Caution
It is possible to put an excessively large electric motor on either type of pump. However, the power drawn from the motor depends only on the nozzle size and operating pressure. Two nozzles that operate at the same pressure and produce the same amount of water flow (depends on nozzle size) draw the same amount of power from the pump. A 50 HP-motor gives no benefit whatsoever in running a nozzle sized for drawing 20 or 25 HP. In fact, in some locations, the electric power bill is based in part on either the current drawn or the potential load, rather than on the energy consumed only. (The power company has to recover its distribution cost.) In such locations, the user must pay extra for the unused capacity of the larger motor. Make sure that the pump motor is sized correctly for the nozzle load that you will be running.
All high-pressure pumps are plunger pumps. Slow moving hydraulic drive pumps can have a longer service interval, but are more expensive to operate. A feature comparison table is presented below.