Choosing a high-pressure water pump is another important decision to make. You will need to decide how large a pump you want to purchase, and whether you want to use a crankshaft or an intensifier pump .
A three-cylinder direct drive high-pressure water pump
If you are going to be machining thick metal and machining time is very important, then get a pump capable of putting a lot of power to the nozzle. How much power you use does not really affect how thick you can cut, but it does affect how long you will have to wait for your part to finish. If you are machining thin metal, soft materials, or if price and operating cost are important, then you may want to get a cheaper, lower power pump. Keep in mind that the bottom line is making money, and that may not always mean making parts as fast as possible at all costs.
How fast you can cut is determined by how much horsepower goes through the nozzle (and how concentrated it is) not the horsepower of the motor driving the pump. For example, you may have a 50 hp (37 kW) intensifier pump, but it is probably generating about 30 hp (23 kW) at the nozzle. The rest goes to heat in the hydraulic system. There is nothing wrong with this in terms of cutting ability, but the operating cost will be higher if the power generated is not doing useful work.
Before buying the pump, check the availability of power to your shop. Many shops do not have the electrical wiring for higher horsepower electric motors. It can be very expensive to bring in new high power wires to your shop. This may or may not be a limiting factor in your pump purchasing decision.
Ask for the actual horsepower at the nozzle, or actual separation cut cutting speed data. Use the Waterjet Web Reference Calculator provided in the software download are of this web site to guide you. Don’t rely on the horsepower of the electric motor as a determiner of cutting speed.
Pressure vs. Maintenance
You may be looking at a 55,000 to 60,000 psi (3800 to 4100 bar) pump, but is it economical to run at that pressure? You may find that it is most economical to run at 40,000j to 50,000 psi (2700 to 3800 bar) for maintenance reasons. High pressure seals, check valves, swivels, on/off valves, hose and other plumbing items all wear out faster the higher pressure you go.
At pressures higher than 55,000 (3800 bar) you have to consider additional problems due to metal fatigue. Drive your machine like a Honda, and it will last like a Honda—drive it like a race car, and it will last like a race car. Running at 60,000 PSI (4100 bar) is typically the top of the ceiling before the maintenance issues become too severe to be practical in nearly all applications because of metal fatigue.
Ask your salesperson to give you component life information at the pressure that you will be operating at. Remember that this pressure may be different than the rated pressure of the pump.
- How easy is the pump to maintain?
- How long does it take to rebuild the pump?
- What spare parts are involved?
- What do spares cost?
- How often do you need to change seals, and what do they cost?
If you plan to do any cutting of brittle material, then be sure to get a dual-pressure pump, or other dual-pressure technology (dual-pressure is sometimes implemented by shuttling part of the water directly to the drain, reducing the effective pressure at the nozzle). It is also nice to be able to reduce the pressure for marking, milling, or other artistic reasons.
For most machine shops, this option is not necessary, but for those machining glass, stone, composites, or other brittle materials, this is important. Low pressure modes are also useful for part marking, etching, and scribing for various purposes.
For maximum flexibility, make sure the controller will automatically change pressures for you. For example, you may want the controller to pierce at low pressure, then cut at high pressure, then etch at low pressure, and so on. If the controller does not do this automatically, you will be forced to do all low pressure work first, then turn up the pressure and finish the piece at high pressure. This can be time-consuming and tedious, especially if you are doing any kind of production runs.
If your waterjet system lets you quietly cut under water, then you will want a quiet pump to go along with it. Intensifier pumps are noisier than crankshaft pumps, but they can be placed in a nearby room if desired. Some also offer extra sound protection built into the cabinets. If your system will not let you cut underwater, then a loud pump may be fine because the nozzle will be much louder than your pump, and you will be wearing hearing protection anyway.
Deciding on a pump
One way to choose a pump is to determine what will cut your parts the fastest at the lowest cost, and still provide the features you need. In other words, what is the ratio between cost and benefit?
To answer this, you need to determine three things:
- Cost per hour to run (the “cost”)
- Cutting speed (the “benefit”)
- Other features that are important (“side benefits”)
Determining cost per hour
Here are some pump related costs to consider:
What does it cost to maintain? ($/hour)
- Consider cost of “consumable” spare parts:
- check valves
- other items that need regular replacing or repair
- Cost of downtime of machine during maintenance:
- Loss of use of machine
What does it cost to operate? ($/hour)
- Water for cutting
- Water for cooling
What does it cost to purchase? (monthly payment)
Add up all of the above to determine your overall cost to run the pump.
Determining cutting speed
A good way to determine the relative cutting speed that a given pump can produce is to look at the amount of power it can put to the nozzle. This is calculated as a function of the pressure the pump will run at (continuously), and the largest size jewel (orifice) that the pump can sustain at that pressure. To make that calculation, use the Waterjet Web Reference Calculator, which you can download from this web site. For even more accuracy, use the Waterjet Web Reference Calculator as follows:
- Pick a standard material and thickness, such as ½” (1.3 cm) aluminum that you will use as a baseline for comparing all the pumps in question.
- Find out what the maximum continuous duty pressure recommended by the manufacturers of the pumps you are evaluating, to get their advertised seal life.
- Find out the maximum size jewel they recommend for that pressure and maintenance interval.
- Download the Waterjet Web Reference Calculator from this web site.
- Enter in the value for pressure and jewel diameter.
- Enter in your “baseline material” (such as 0.5″ (13mm) aluminum).
- Leave the abrasive feed rate and mixing tube diameter alone. These are factors that affect cutting speed, but are independent of the pump.
- Write down the linear cutting speed for that pump.
- Repeat the above for each pump in question.
When finished, you should have information that will determine which pump cuts the fastest.
Other features that are important
Once you have the cost and cutting speed information, all you need to do is make sure the pump supports any other features you might need, such as:
- Support for dual pressure
- Comfortable sound levels
- Meets your space requirements
- Meets your electrical requirements (what your shop can supply)
- Consistent, even pressure delivery
- Quick starting
Do you leave the pump on all day, or start it up each time you make a part? Some pumps take a long time to warm up, making it impractical to turn it off between runs, causing more hours to accumulate on the pump without actually making parts.
- Capability of running multiple nozzles
- Capability of running multiple machines
- Compatibility with the rest of your machine
- Skill level required to maintain
This topic of crankshaft vs. intensifier pumps is a somewhat controversial subject with a lot of different opinions.
Crankshaft pumps are also referred to as “direct drive” pumps.
The basic difference between the two pumps is that crankshaft pumps use a crankshaft to move the plungers that pressurize the water, and intensifiers use hydraulics. Because there is no hydraulic system, crankshaft style pumps tend to be much more efficient, and thus put a higher percentage of horsepower to the nozzle. This lets a lower power crankshaft pump compete in cutting power (speed) with a higher power intensifier pump, and dominate in terms of operating cost. Note that cutting speed is also greatly a function of the control software, and how well it predicts, compensates, and optimizes for the jet behaviours around corners.
For example, generally speaking a 30 horsepower (23 Kw) crankshaft pump is about the same cutting speed as a 50 horsepower (37 Kw) intensifier, but the the intensifier is creating about 20 horsepower (15 Kw) of heat that is doing nothing but wasting gobs of electricity.
Intensifiers, however, have advantages in terms of high pressure seal life, and their ability to run multiple machines from a single pump. The extended seal life is an effect of running the plungers at a slow speed, while crankshaft pumps typically operate at faster speeds, and therefore wear faster. Intensifiers are good for running multiple machines from a single pump because they can maintain a relatively constant pressure output at one machine when a nozzle on another machine being turned on and off.
- The main advantage of the crank-shaft style pump is that is provides more cutting power per dollar, and is therefore cheaper to purchase and run.
- The main advantage of an Intensifier style pump is that the high pressure seals last a little bit longer.
With both systems, component life is an inverse function of operating pressure—the lower the pressure, the longer the life.
There is also a slight difference in the way both pumps are used. With direct drive pumps, the pump is typically turned on when a part path is started, then turned off when the path is complete. An intensifier pump is often turned on at the beginning of a shift, and then left on, regardless of whether or not the machine is cutting parts. When no flow is being drawn, the plunger is motionless. During this time, no significant wear is created. However, it does inflate the “pump hours” that one can expect between maintenance intervals, making the intensifier seem like it is significantly longer lasting than a direct drive pump, when in fact, they are closer than it seems in this regard.
Which should you buy? This is a tough question, and the answer depends a lot on your preferences, and a big factor is the answer to the question: What is more important, operating cost, or frequency of maintenance? If the answer is that you want the least frequent maintenance, then an intensifier might be right for you. If the answer is that you want the lowest operating cost, then a crankshaft pump is probably a better choice.
Pump horsepower, in waterjet marketing literature, almost always refers to the horsepower of the electric motor that drives the pump, and not the actual horsepower that makes it to the nozzle. For example, due to inefficiencies, a 50 hp (37 kW) intensifier pump typically puts out 30 hp (22 kW) at the nozzle, while a 20 hp (15 kW) crankshaft type pump typically puts 19 hp (14 kW) to the pump. For this reason, talking about pump horsepower is misleading. Instead, you should consider nozzle, or cutting horsepower.
Nozzle horsepower is how much cutting power is at the nozzle. The more you have the faster you cut. A great way to compute nozzle horsepower, is to use the Waterjet Web Reference Calculator.
In basic terms, there are three critical dimensions in a nozzle: jewel diameter, mixing tube diameter, and mixing tube length.
Jewel (orifice) Diameter
As you can see in the picture below, the jewel is where the high pressure exits the plumbing and enters the air in the nozzle. This jewel is sized so that it maintains pressure behind it, while allowing water to flow at extremely high velocity into the Venturi mixing chamber of the nozzle. The larger the diameter of the hole in the jewel, the more water it flows, and the bigger the pump you need to maintain the same pressure.
Mixing tube diameter
The inside diameter of the mixing tube determines how fast the mixing tube will wear out, how precise of a cut you can make, and how quickly you can cut.
Properties of a small diameter mixing tube:
- Slightly improved cutting rate
- Slightly decreased nozzle life
- Improved precision
- Smaller kerf width
Properties of a large diameter mixing tube:
- Slightly reduced cutting rate
- Slightly increased nozzle life
- Slight decrease in precision
- Larger kerf width
Mixing tube diameter directly relates to kerf width diameter.
Mixing tube Length
Mixing tube length effects the ability of the nozzle to focus. Typically, longer mixing tubes focus better than shorter ones, because of their longer length. This will give you slightly more precision due to reduced taper.
Importance of pressure
Generally speaking, the higher the pressure of the water, the faster the speed of cutting. However, pressure is only one of many factors to consider. Among the other factors are:
- Operating cost
Your operating costs are often much lower for lower power machines. This is simply because lower pressures and lower water flow rates translate directly into longer life of every component that touches the water. It also translates into fewer consumables, because machines that run at lower pressure wear mixing tubes and jewels slower, and typically consume less garnet.
Higher pressure means higher wear on components, and that means more frequent maintenance, such as replacing seals and rebuilding pumps. The costs of maintenance extend beyond the price for replacement parts, as you also have downtime when you aren’t cutting.
- Fatigue limits of all high pressure components
At pressures of 60,000 PSI (4,100 bar) and higher, metal fatigue becomes a serious issue with many components. Although pumps that can reach 100,000 PSI have been around for many years, nobody runs them at such pressures because of the extreme maintenance issues involved. For this reason, most manufacturers purposely limit their pumps to below 60,000 PSI (4,100 bar).
How quickly you cut material comes down to how much cutting power is exiting the nozzle. This is determined not only by pressure, but also by the size of the hole you are sending the water through (jewel size).
To illustrate this concept, have a look at a few nozzle combinations, at various pressures:
0.25 mm Jewel
0.30 mm Jewel
0.36 mm Jewel
0.41 mm Jewel
As the above chart shows, even at 100,000 PSI (6,900 bar), you are still cutting at 35.52 horsepower (26.5 kW), if you run a 0.010″ (0.25 mm) jewel. Compare that to a system pumping 50,000 PSI (3,400 bar) through a 0.016″ (0.41 mm) jewel, which even at half the pressure, is still cutting at nearly the same rate.
Of course, few people really run at 100,000 PSI, because that puts an extreme amount of wear on all the high pressure components! Nevertheless, it is an important illustration that pressure by itself is not very meaningful.
To make the example even more extreme, consider the case of 1,000,000 PSI (69,000 bar) behind a jewel that does not have a hole in the middle. In this case, you have a lot of pressure, but no water coming out at all, which means it doesn’t cut at all.
As a general rule of thumb, it is horsepower at the nozzle (cutting horsepower), not the power of the motor turning the pump (pump horsepower), or pressure that determines how quickly a given system can cut.
This is a generalization, though. The best way to answer questions about how the various factors effect cutting speed, is to use the Waterjet Web Reference Calculator. This calculator answers many questions regarding cutting speed in a variety of materials, pressures, nozzle, and pump configurations.