When configuring a waterblast system, it is necessary to select appropriate pressure and flow rates. Parameters such as material properties, pressure loss and standoff distance should be evaluated to optimize cleaning.
The properties of a material to be removed and the surface to which the material is attached determine the selection of the operating pressure and flow rate. Every material has a minimum energy impact at which it will begin to be fractured by a waterjet; this is known as the threshold. If the material is removed by cutting, such as a rubber lining or other thick nonbrittle deposits, the most efficient pressure is typically three times the threshold. If the material is brittle, thin or not well bonded to the surface, higher flow rates at pressures just above the threshold can be more effective.
Pump flow is divided among orifices in the cleaning head. As the quantity of orifices is increased, the orifice sizes must get smaller to maintain the flow rate. If the material deposit is thick and massive, the fewest largest orifices should be used. For thin deposits, or if just the top surface of a material needs to be evenly removed, then more, smaller orifices should be used.
Allowable pressure loss depends on several parameters. If a specific pressure is desired to remove a material, the pressure loss subtracted from the pressure at the pump must be equal to or greater than this pressure: Pressure at nozzle orifice = Pressure at pump - Pressure loss. Operating pressure has no effect on the amount of loss, but it affects the pressure at the nozzle orifice.
Pressure loss occurs through hoses, fittings and tooling. Loss is related to the inside diameter and length of the hose or lance being used and the flow rate passing through. The following equation is used to calculate pressure loss through a hose or lance: Pressure loss = (Flow / (53 x (ID of hose2.5 / Length of hose.5)))2.
The maximum power delivery to the nozzle occurs when pressure loss equals one-third of the pump pressure, and the maximum pulling force occurs when pressure loss equals one-half of the pump pressure. In no case should pressure loss be more than one-half of the pump pressure.
The distance a jet must travel through the air from the exit of an orifice to the surface being cleaned, or standoff distance, is dependent on the orifice size and can therefore be expressed as the equation: Standoff distance in nozzle diameters = Standoff distance / Nozzle diameter.
Typical power loss is shown in Figure 1. At a standoff distance of 400 nozzle diameters, the jet power will be 40 percent of what it would be with no standoff distance. Therefore, if you are operating at 10,000 psi at the pump, the impact at the surface is comparable to 4,000 psi.
Once the required pressure for material removal has been determined, the combination of pressure and flow can be calculated according to this curve. Increasing flow while maintaining the operating pressure would require an increase in the size of orifice being used, thus changing the ratio of standoff distance to nozzle diameter.
If it is known the material to be removed requires an equivalent impact of at least 5,000 psi at a standoff distance of 24 inches, the orifice size to achieve a relative impact of 50 percent can be determined by dividing the 24-inch standoff distance by a ratio of 250, or a .096-inch orifice. Another approach would be to increase the operating pressure; if it were increased to 15,000 psi, the power could deteriorate to 33 percent and still achieve the relative impact of 5,000 psi. This occurs at a ratio of 550, resulting in an orifice size of .044 inches.
Applying a methodical approach to the selection of system components -- including material properties, pressure loss and standoff distance -- can improve cleaning results.
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