MORE ON WHAT IS CAVITATION? (cont.) Jacques Chaurette p. eng., Fluide Design Inc. February 2003

Transcription

1 MORE ON WHAT I CAVITATION? (cont.) Jacques Chaurette p. eng., Fluide Design Inc. February 003 Net Positie uction Head Required (N.P..H.R.) The N.P..H. required proides us with the leel of head in terms of feet of water absolute required at the pump suction flange. When that leel of head is insufficient the capacity and head of the pump will drop and caitation will occur. How do the pump manufacturers measure N.P..H. required? The pump manufacturers measure the N.P..H. required in a test rig similar to that shown in Figure 6. The system is run in a closed loop where flow, total head and power consumed are measured. In order to proide a low N.P..H., a acuum pump is used to lower the pressure in the suction tank that will proide a low head at the pump suction. The pressure in the suction tank is lowered until a drop of 3% (see figure 7) of the total head is measured. When that occurs Figure 6 Test rig used to measure N.P..H. required the N.P..H. is (courtesy of the Hydraulic Institute calculated and recorded as the N.P..H. required for that operating point. The experiment is repeated for many operating points. Heating coils are also used to increase the water temperature thereby increasing the apor pressure and further lowering the N.P..H. as needed.

2 More on what is caitation

3 More on what is caitation 3 Figure 7 Measuring the drop in head to define the N.P..H. required at that operating point. Guideline for the leel of N.P..H. aailable As stated, a total head drop of 3% is the criteria for setting the leel of N.P..H. required. ince this results in a performance drop the user should ensure that there is a higher N.P..H. aailable. The recommendation that you will find in the literature is to hae 5 ft of water absolute or a 15% margin aboe the N.P..H. required whicheer is greatest. How can the N.P..H. aailable be increased and caitation aoided? Table 1 gies the major components of N.P..H. aailable and how they affect the leel of N.P..H. aailable. The N.P..H. aailable depends on: Effect on N.P..H. aailable. 1. The friction loss in the pump suction The higher the friction loss, the lower line. the N.P..H. aailable.. The height of the suction tank fluid The lower the height of the fluid surface with respect to the pump surface, the lower the N.P..H. suction. aailable. 3. The pressure in the suction tank. This cannot be changed for atmospheric tanks. For tanks that are pressuried, the lower the pressure, the lower the N.P..H. aailable. 4. The atmospheric pressure. This cannot be changed and depends on the eleation aboe sea leel. The lower the atmospheric pressure, the lower the N.P..H. aailable. 5. Fluid temperature. An increase in fluid temperature increases the apor pressure of the fluid which decreases the N.P..H. aailable. Table 1. How to affect the N.P..H. aailable.

4 More on what is caitation 4 How can you measure N.P..H. aailable? It is easier to measure N.P..H. aailable than to calculate it, here s how it s done. Figure 8 Location of the pressure gauges to measure N.P..H. aailable. By definition, the NPH aailable is the specific energy or head at the pump suction in terms of feet of fluid absolute, minus the apor pressure of the fluid. According to equation [4], the head at point is: The alue of H is : E H + g [4] H p ( psig) G.31 + G [10] G Where the eleation difference G compensates for the height of the pressure gauge aboe the pump centerline. By substituting the alue of H from equation [10] into equation [4} we obtain: E p ( psig) G G [11] G g ince N.P..H. is in feet of fluid absolute, the alue of the atmospheric pressure head must be added to E and the apor pressure of the liquid (H a ) is subtracted to get the N.P..H. aailable.

5 More on what is caitation 5 NPH aail ( ft fluid absol. ) E + H H [1] A a By substituting equation [11] into [1] we obtain: N. P.. H. aail( ft fluid absol.).31 p G ( psig ) G + G [13] + + H A H a g Or if the atmospheric and apor pressures are aailable in terms of psi, then: NPH aail ( ft fluid p ( psi) ( p p ) G A a absol. ) [14] G G g G What is the difference between the N.P..H.A. of equation [13] s the N.P..H.A. of equation [8] which is restated here below: 1. P.. H. aail. ( ft fluid absol. ) ( H F1 + HEQ 1 ) + + ( + H1) 1 N + HA H g a [8] Equation [8] corresponds to the NPH aailable that is calculated according to the amount of specific energy or head aailable at the pump suction, which depends on the static head in the reseroir minus the friction head in the suction line. Howeer equation [13] is the NPH aailable based on a pressure measurement at the suction of the pump or point. The head aailable at point is generated by the static head less the friction head loss in the suction line; their contribution to pressure at point is included in the pressure measurement. Why is the elocity head at the pump suction required in equation [13], but not in equation [8]? In equation [8] the elocity energy was considered but cancels out during the deelopment of the equation so that it does not appear in equation [8].