Abrasive waterjet cutting machines are considered to present the most innovative technology of formed cutting which allows for machining almost all materials. Water under pressure of 3 to 4 thousand bar is used to disperse the sand grains of garnet. The process of cutting is classified as abrasive blasting and is closest to the process of grinding. To obtain such high pressure of water two types of pumps are being used. The most popular pumps are those with a hydraulic pressure intensifier; other constructions are crankshaft pumps that use solutions similar to piston compressors.

Manufacturers of waterjet cutting machines are trying to demonstrate that their solution is the best one from the user's point of view. The potential benefits are affected by many factors, which often causes misunderstanding of parameters and the possibilities and limitations of individual solutions. In this paper, the substantial comparison of pressure intensifier and crankshaft pumps will be made and the impact of their design on the cutting quality and operating costs will be presented. For the comparison, the data available on the official websites of selected manufacturers of both types pumps have been used.

Capacity and power

When comparing the parameters of pumps, you should pay attention to operating conditions under which capacity and pressure parameters are given. It happens that some manufacturers provide the pump capacity in relation to the water supply at low pressure, while others provide the pump output at maximum pressure. Due to the compressibility of water (which begins to be of significant importance at such high pressure), these differences may reach up to 20%. For example, a popular crankshaft pump with a power of 22kW has the capacity of 3.8l/min in the supply but at the output the capacity is maximum of 3.2l/min. It should also be noted that the powers of individual types of pumps can vary considerably. But we can not consider the cost of operation only on the basis of the power consumed from the energy network. The speed of cutting in a waterjet machine is highly dependent on the amount of energy supplied to the head, which is directly related to the pressure and the capacity of the pump. The increase in pressure results in increasing the cutting speed, which causes that higher energy costs are compensated by a shorter duration of cutting a particular detail, which gives a comparable cost of electricity for pumps with different powers based on cutting a particular detail. However, cutting at higher pressure gives some specific savings in relation to the consumption of garnet. It is just the consumption of sand that generates the highest costs and often represents more than half of all the labour costs of waterjet machines, so it is important to minimize its consumption based on one detail. Not without significance is the fact that we save time with higher cutting speeds.

Efficiency of pumps

The output capacity of a sample pump with 37kW (50HP) power and with an intensifier at maximum 4150bar pressure is 4.16l/min, which gives the power of about 29kW to the nozzle, (pressure * capacity / constant 600), so the efficiency is 78%. Whereas the capacity of a sample 22kW (30HP) crank pump at a maximum pressure 3450bar is 3.2l/min, which gives the power of about 18kW to the nozzle, so the efficiency is 82%. However, in this case you should consider further losses in the inverter feeding the crank pump motor on the level of 5%, which gives 77% of efficiency. When the pump motor is powered by the frequency converter (inverter), the problem of the high level of current harmonics appears and which results from the impulse operation of the rectifier in the inverter, which can lead to excessive losses in cables and interference to other devices connected to the same power grid.

Wear of sealing

The wear of sealing is quite an important factor in generating costs at pressures of one thousand bars. Each stroke of the piston generates wearing away of sealing materials which periodically must be replaced. Due to the direct drive by an electric motor, the frequency of strokes in crankshaft pumps is dozens of times per second, which sets high standards for sealing and water quality. Using water treatment system becomes necessary, which allows to achieve acceptable time between replacement of seals. In pumps with pressure intensifiers the operating cycle is about two seconds, which greatly increases the life of seals and, in most cases, allows to eliminate water treatment systems which are expensive in maintaining.

Cooling

Due to processing significant powers in pumps to operate waterjet machines it is necessary to use a cooling system. An example crankshaft pump is cooled in the open system, which means that apart from the water that is consumed for cutting, the significantly greater amount of water is consumed for cooling and then discharged to the sewer. Pumps with an intensifier generally have a closed cooling circuit in which the oil is cooled by a heat exchanger with a fan. In this case, no cooling medium is consumed. Additionally, the use of the heat exchanger in winter allows the use of leaving energy for reheating premises.

Pressure adjustment

In this particular crank pump the manufacturer declares the adjustment range of capacity from 1.5 to 3.2 liters per minute, which gives the possibility to adjust the pressure for a typical orifice in the range from 1600 to 3450bar. This adjustment is possible with a built-in frequency converter. Note that the energy efficiency of such power decreases with decreasing engine speed due to the slip phenomenon occurring in induction motors. The adjustment of pressure is essential particularly for piercing brittle materials such as glass or stone. These materials often require a reduction of pressure to 500 bar at piercing, which in the case of crank pumps is impossible. Some solution is to use a special head that uses only part of the energy supplied by the pump. In solutions with an intensifier the pressure adjustment takes place through the oil pump with controlled pressure and variable capacity. The most modern pumps with an intensifier allow the adjustment of pressure in the range from 500 bar. The motor rotates at a constant speed and the control is obtained by adjusting the ring displacement of the oil pump by changing the load on the motor shaft. With this solution, the engine can still operate in the optimum area of its characteristics and the adjustment range can be very broad.

Reaction rate

Machines for water jet cutting, due to their characteristics of operation, require frequent switching the jet on and off to move to the next part. Both systems perform this task in a similar way. Using compressed air the valve at the cutting head is opened and closed. However, the pumps themselves behave differently now. A pump with an intensifier, thanks to low motion speeds, has the ability to stop immediately and switch to an inactive mode in which the oil pump, due to the lack of movement, reduces the engine load to an idle mode. Due to the large inertia of rotating elements speeding, crankshaft pumps are not able to stop immediately. The problem has been solved using a pressure relief valve which is not allowed to break the system after closing the head and a sudden pressure increase. Such a solution does not provide optimum performance because the pump cannot immediately stop absorbing energy during movements between the elements which are cut out.

Other costs

One of the most important differences between crankshaft pumps and pumps with an intensifier of pressure is the frequency of pistons strokes in high pressure cylinders. In pumps with an intensifier this frequency is about 0.5Hz, i.e.1 cycle every 2 seconds, while in crankshaft pumps driven directly from the engine the frequency is several times higher. Each single stroke generates unit wear of seals, so the higher frequency of strokes, the faster they wear. The quality of water has a big impact on the wear, as well. In the case of crankshaft pumps it leads to the need for additional water treatment equipment, which generates higher costs. Pumps with an intensifier are not so sensitive to water quality and in most cases the application of treatment is unnecessary.

Summary

Currently, these are pressure intensifier pumps which strongly dominate among the manufacturers of machines for waterjet cutting. This is because they felt that opportunities for development of pumps with a crank drive ended with 3800bar pressures and 30kW powers. Pumps with an intensifier are constantly modified and developed and the best example is the gradual introduction of the 6000bar standard to waterjet equipment. Pumps with an intensifier are produced with power ratings from 18 to 200kW. The most important advantage of crankshaft pumps are substantially lower production costs allowing their use in machines intended for semi-professional companies where they will be used occasionally and where labour and operating costs are not significant. If a user requires the ability to compete in the market or the process of waterjet cutting will be used extensively, he should definitely invest in a waterjet machine with a pressure intensifier pump.

Crankshaft pumps:



Advantages:
- quieter operation
- smaller sizes
- smaller amount of oil
- cheaper to produce

Disadvantages:
- high operation costs
- lower pressures and lower capacity
- open cooling system
- limited possibility to adjust the pressure
- losses of energy in the inverter
- need for water treatment

Pressure intensifier pumps:

Advantages:
- lower operation costs
- greater seal life
- wide-range pressure adjustment
- closed cooling system
- no need for water treatment
- higher pressures and greater efficiency
- no losses in the inverter

Disadvantages:
- louder operation
- bigger sizes
- higher amount of oil
- more expensive to produce