Pump Specifications
Power	30 HP (22 kW)	50 HP (37 kW)	75 HP (56 kW)	100 HP (75 kW)	150 HP 112 kW)
Max. Continuous Output Pressure	60,000 psi (4137 bar)
Max. Output Flow - Gallons/min (Liters/min)	0.65 (2.46)	1.1 (4.16)	1.6 (6.06)	2.2 (8.33)	3.2 (12.1)

Pump Specifications
Power	30 HP (22 kW)	50 HP (37 kW)	75 HP (56 kW)	100 HP (75 kW)	150 HP 112 kW)
Max. Continuous Output Pressure	60,000 psi (4137 bar)
Max. Output Flow - Gallons/min (Liters/min)	0.65 (2.46)	1.1 (4.16)	1.6 (6.06)	2.2 (8.33)	3.2 (12.1)

Flow Rate (gpm) Through an Orifice
	Pressure (psi) x 1000
Orifice Diameter (inch)	20	25	30	35	40	45	50	55	60
0.003	0.03	0.03	0.03	0.04	0.04	0.04	0.04	0.04	0.05
0.004	0.05	0.05	0.06	0.06	0.07	0.07	0.08	0.08	0.08
0.005	0.07	0.08	0.09	0.10	0.10	0.11	0.12	0.12	0.13
0.006	0.11	0.12	0.13	0.14	0.15	0.16	0.17	0.18	0.18
0.007	0.15	0.16	0.18	0.19	0.20	0.22	0.23	0.24	0.25
0.008	0.19	0.21	0.23	0.25	0.27	0.28	0.30	0.31	0.33
0.009	0.24	0.27	0.29	0.32	0.34	0.36	0.38	0.40	0.41
0.010	0.30	0.33	0.36	0.39	0.42	0.44	0.47	0.49	0.51
0.011	0.36	0.40	0.44	0.47	0.51	0.54	0.57	0.59	0.62
0.012	0.43	0.48	0.52	0.56	0.60	0.64	0.67	0.71	0.73
0.013	0.50	0.56	0.61	0.66	0.71	0.75	0.79	0.83	0.86
0.014	0.58	0.65	0.71	0.77	0.82	0.87	0.92	0.96	1.00
0.015	0.66	0.74	0.81	0.88	0.94	1.00	1.05	1.10	1.14
0.016	0.76	0.85	0.93	1.00	1.07	1.11	1.19	1.25	1.30
0.017	0.85	0.95	1.05	1.13	1.21	1.28	1.35	1.41	1.47
0.018	0.96	1.07	1.17	1.27	1.35	1.43	1.51	1.59	1.65
0.019	1.07	1.19	1.31	1.41	1.51	1.60	1.68	1.77	1.84
0.020	1.18	1.32	1.45	1.56	1.67	1.77	1.87	1.96	2.03
0.021	1.30	1.46	1.59	1.72	1.84	1.95	2.06	2.16	2.24
0.022	1.43	1.60	1.75	1.89	2.02	2.14	2.26	2.37	2.46

Flow Rate (Liters/min.) Through an Orifice
	Pressure (Bar)
Orifice Diameter (mm)	1379	1724	2068	2413	2758	3103	3447	3792	4137
0.08	0.10	0.11	0.12	0.13	0.14	0.15	0.16	0.17	0.17
0.10	0.18	0.20	0.22	0.23	0.25	0.27	0.28	0.30	0.31
0.13	0.28	0.31	0.34	0.37	0.37	0.42	0.44	0.46	0.48
0.15	0.40	0.45	0.49	0.53	0.57	0.60	0.64	0.67	0.69
0.18	0.55	0.61	0.67	0.72	0.77	0.82	0.87	0.91	0.94
0.20	0.72	0.80	0.87	0.95	1.01	1.07	1.13	1.18	1.23
0.23	0.90	1.01	1.11	1.20	1.28	1.36	1.43	1.50	1.56
0.25	1.12	1.25	1.37	1.48	1.58	1.68	1.77	1.85	1.92
0.28	1.35	1.51	1.65	1.79	1.91	2.03	2.14	2.24	2.33
0.30	1.61	1.80	1.97	2.13	2.27	2.41	2.54	2.67	2.77
0.33	1.89	2.11	2.31	2.50	2.67	2.83	2.99	3.13	3.25
0.36	2.19	2.45	2.68	2.90	3.10	3.29	3.46	3.63	3.77
0.38	2.51	2.81	3.08	3.32	3.55	3.77	3.97	4.17	4.33
0.41	2.86	3.20	3.50	3.78	4.04	4.21	4.52	4.74	4.93
0.43	3.23	3.61	3.96	4.27	4.56	4.84	5.10	5.35	5.56
0.46	3.62	4.05	4.43	4.79	5.12	5.43	5.72	6.00	6.23
0.48	4.03	4.51	4.94	5.33	5.70	6.05	6.37	6.69	6.95
0.51	4.47	5.00	5.47	5.91	6.32	6.70	7.06	7.41	7.70
0.53	4.92	5.51	6.03	6.52	6.96	7.39	7.79	8.17	8.49
0.56	5.40	6.04	6.62	7.15	7.65	8.11	8.55	8.97	9.31

2.    You would look down under the “60” column (corresponding to the 60,000 psi pump) in the “Flow Rate” chart until you found a number equal to or less than 1.1.  In this case, you would end up at the cell with 1.00.

 

3.    You would then follow that row across to the left to see the maximum size orifice you could use for single-head cutting. In this case, the cell shows that a 0.014” would be the maximum recommended orifice for one head at 60,000 psi.

 

You would then follow that row across to the left to see the maximum size orifice you could use for single-head cutting. In this case, the cell shows that a 0.014” would be the maximum recommended orifice for one head at 60,000 psi. 

If you were in a tight spot where you only had a 0.015” orifice you might be able to use it by running the pump at 55,000 psi.

You would be at the 1.1 gpm limitation of the pump.  If there were any water leaks in your system between the pump and the cutting head, you would likely have a pump “over stroke” situation where the pump would try to cycle too fast attempting to create the required pressure.  With modern pumps, there is no harm done if this happens. The pump is simply shut down to protect itself from damage and an error message is displayed for the operator.

Example 2 – 50 HP Pump, 2 Cutting Heads, 60,000 PSI, 1.1 GPM water output

If you wanted to run 2 cutting heads, then you would take the 1.1 gpm number, divide it by 2 for a maximum of 0.55 gpm per head.  Look for the cell under 60 kpsi that has a number smaller than or equal to 0.55.

 

In this case, under the “60” column you would end up in the 0.51 cell, meaning that one 0.010” orifice would put out 0.51 gpm.  The maximum number of heads that you could run with the pump at 60 kpsi would be two (1.1 ÷ 2 = 0.55.  0.55 > 0.51 = OK).

 

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Orifice selection – The easy way

Since the pump design engineers have done most of the hard math already, most users need only refer to an “Orifice Selection Chart” similar to the following, which is usually supplied by the pump manufacturer.

 

Here you would quickly see that for the 50 HP pump, you could use either one 0.014” orifice or two 0.010” orifices.

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General pump selection guidelines

The next step in determining which pump is appropriate for your application is to determine the types of material you will be cutting and how many cutting heads you want to be able to run at one time. 

If you are cutting parts out of foam, wood, cardboard or other soft materials, then you would be dealing with a pure water application.  For pure water applications, a 30 HP pump is usually sufficient.  As you can see from the previous Orifice Selection chart, up to three cutting heads could be used with 0.006” orifices.  If more cutting heads were needed, then a 50 HP pump could handle up to five cutting heads with 0.006” orifices.

For abrasive applications, the 50 HP is the general starting point.  With this pump, you can run one head with a 0.014” orifice or two heads with 0.010” orifices.  The 0.010” orifice will perform exceptionally well with respect to speed and cut quality on thinner material (1/2” and under). 

The following graph shows cutting speeds for single head versus dual head cutting in ½” stainless steel.  As thickness get beyond 2” (50 mm), the absolute difference in cutting speeds between the nozzle combinations starts to decrease more dramatically.

Cutting with two heads with 0.010” orifices effectively doubles the cutting speed (2 x 5.5 ipm) versus cutting with one head with a 0.010” orifice.  Compared to cutting with one head with the 0.014” orifice, cutting with two heads will increase output by about 20 percent.  To put this concept into real world terms, it would be like if you started cutting on a job on Monday morning with two cutting heads, then you could get it to your customer by Thursday afternoon.  If you cut the job with one head, your customer wouldn’t get the part until Friday afternoon.

For someone looking to cut thicker materials on a consistent basis, then we would suggest allocating 50 HP per cutting head.  If you selected a 100 HP pump, you could run two heads with 0.014”.

 

If you are cutting very small parts in high volumes, you might want a very large cutting table where you could run four cutting heads and quadruple your production over a single-headed system.  In that case you would likely choose the 150 HP pump that can run four heads with 0.013” orifices.

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Abrasive nozzle selection

As a general rule, the diameter of the orifice for the abrasive nozzle should be approximately three times the water nozzle orifice. 

Some people would suggest using a smaller ratio, about 2.5 times.  Using a smaller ratio does produce faster cutting speeds.  The trade-off is increased nozzle wear and costs.  Part tolerance will suffer because of the increased speed of nozzle wear. 

Following is a quick reference guide for the most common orifices for abrasive waterjet cutting.  Typical abrasive amounts and water flow are also shown for easy reference.

 

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Abrasive amount and cutting speeds

In Chapter 1, “Cutting Characteristics,” under the section “Creation of the abrasive waterjet stream”, we discussed how the abrasive waterjet stream is created.  The cross-section of the cutting head is shown again here.

As abrasive is added to the waterjet stream, the abrasive particles are accelerated to near the speed of the waterjet, approximately 2200 miles per hour (almost three times the speed of sound).  This speed imparts momentum to the abrasive particles so that they can erode the material.  Adding more abrasive gives more energy to the process and erosion occurs faster.  Eventually a saturation point occurs where adding more abrasive robs speed and power from the waterjet stream and cutting speeds will start to decrease.  Each waterjet manufacturer goes through extensive testing with various orifice and nozzle combinations to find the optimum abrasive amount, balancing cost and cutting speed.

The following graph shows what happens to cutting speed for ½” stainless steel as abrasive is added.  Speeds are shown for the two most common orifice/abrasive nozzle combinations, 0.010” orifice with a 0.030” abrasive nozzle (orange line) and 0.014” orifice with a 0.040” abrasive nozzle (blue line). 

Starting with an abrasive flow of zero pounds per minute, there would be absolutely no penetration of the material except for maybe a very light etching of the top surface of the material.  At this point cutting speed is zero.  Cutting speed increases as more and more abrasive is added. For the larger nozzle combination, speed increases until around 1.5 pounds per minute.  At this point, cutting speed starts to decrease as too much kinetic energy is removed from the waterjet stream by the abrasive.  A similar thing happens with the smaller orifice/nozzle combination, but at slower speeds and lower abrasive amounts.

The optimum cost point may be slightly below what looks to be the apex of the speed curve.  The “Law of Diminishing Returns” becomes evident.  As the maximum speed is approached, each additional unit of abrasive that is added results in an ever smaller increase in speed.  In the case of the orange line, increasing the abrasive from 0.7 pounds per minute to 1.0 pounds per minute yields an increase in speed of only 0.2 inches per minute.  This 3.6% increase in speed results in a 4.7% increase in cost per inch.

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Speed and efficiency of nozzle combinations

We can see in the following graph that larger orifice/abrasive nozzle combinations will cut faster than smaller combinations.  With the larger nozzle combination, virtually all of the pump’s power is used, so it will cut faster.  The smaller orifice/nozzle cuts slower because less total power from the pump is used. 

 Smaller orifice/nozzle combinations are more efficient though in their use of water and abrasive.  The available power of the waterjet is concentrated into a smaller area, so more power is directed to the cut. 

In the picture above, the diameter of the larger waterjet stream on the right is 33% wider and the area of this diameter is 78% greater.  This makes the energy density lower than the smaller orifice/nozzle combination.  The result is that, as shown earlier and repeated in the graph below, cutting with two 0.010”/0.030” heads is more cost-effective and productive than cutting with one 0.014”/0.040” head in thinner materials (approximately ½” and thinner).

Nozzle wear

As the abrasive nozzle wears, the diameter of the abrasive waterjet stream increases.  The diameter increases by approximately 0.0001” per hour of cutting.  Power per square inch is reduced.  Therefore, the feed rate must be reduced in order to maintain the same edge finish, or the quality of the edge will deteriorate.

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Cutting speed calculators

Various waterjet feed rate calculators are available online.  With these calculators, you can play around with the various parameters to see how cutting speed and cost per inch are affected.  Below is an example of one such calculator.

 

With all of these calculators, it is more important to focus on cost per inch (or foot, or meter), rather than cost per hour.  Focusing on cost per foot takes into account the diminishing returns discussed in the previous section.

Another thing to keep in mind is that these calculators are only showing straight line cutting speeds.  Depending upon part geometry, piercing times, machine design and more, actual part cutting times can vary significantly from just taking the total linear inches of cutting in a part and dividing that number by the inches per minute shown in a calculator.  The calculators are useful though in at least getting an idea of speeds and costs.

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Machinability Index

 

The Machinability Index shown in most waterjet feed rate calculators defines the relative cutting rates of different materials.  Materials with higher numbers cut proportionately faster than lower numbers.  Mild steel has a baseline value of one.  Stainless steel at 0.9 would indicate that it cuts about 10% slower than mild steel to achieve similar edge quality and tolerance results.

 If you know the machinability indexes of two materials, you can estimate fairly easily a good cutting speed of one material from the other.  We know that the Machinability Index of stainless steel is 0.9 and below we see that Aluminum’s Machinability Index is 2.9.  You know that ½” stainless cuts at 5.5 ipm for the edge quality you want.  You want to know the speed for ½” Aluminum.  Divide the machinability of Aluminum by the machinability of stainless.  2.9 ÷ 0.9 = 3.2.  Multiply the cutting speed for Stainless by 3.2 for the cutting speed in the same thickness of Aluminum.  5.5 ipm x 3.2 = 17.6 ipm.  Therefore, 17.6 ipm would be a good place to start for ½” Aluminum.

Summary

In this chapter we looked at the three critical specifications of a high pressure pump: horsepower, pressure and water output.  We reviewed how to figure out what size orifice to use based upon these pump specifications and how many cutting heads are being used.  We covered the productivity increases and cost savings of using two smaller orifices versus one large orifice.  Additionally, abrasive nozzle diameter selection was discussed in terms of the ratio to the orifice being used.  Finally, we looked at waterjet feed rate calculators and how Machinability Indices can be used to extrapolate the cutting speed of one material from a different material.

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