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Step 4 – Calculation of NPSHr

1. On completion of 5 iterations, open results_0.025_step1.csv.

2. The predicted userdef.head is 385442.36 Pa.

3. Similarly, open results_0.025_step5.csv and predicted userdef.head at step5 is 371753.34 Pa. The difference between the head at step 1 and step 5 is 3.55 % which is close to 3% head loss due to cavitation.

4. Compute using the relation , where is the Total Pressure at inlet for 3% head loss, is the Saturation vapor pressure and is the density.

5. In results_0.025_step5.csv, the variable userdef.in_pt is the predicted total pressure at inlet which is 36372.8 Pa. Hence, is calculated as shown below.

 

We got,
	= 36372.8 Pa
	= 3610 Pa
	= 998 kg/m3
	= 9.81 m/s2

 

6.9

 

The calculated net positive suction head required at the pump inlet is 3.35 m for 0.025 m³/s flow rate and 2500 rpm.