Could you please help me calculate whether the effective net positive suction head and installation height of these two pumps meet the requirements?
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This post was last edited by Bapuerben on 2022-10-27 at 17:29. Our factory currently has three circulating cooling water pumps: two large pumps of model 500S22 and one small pump of model 350S26. Last month, when these pumps were disassembled, it was found that the impellers suffered from cavitation to varying degrees, with the situation being more severe in the small pump, where the corrosion damage reached half of the impeller’s thickness. The likely cause is insufficient water volume, as the liquid level in the tank has consistently been at -1.2 meters above sea level. I would like to ask everyone how to calculate the effective net positive suction head available for the pumps, and whether this operating water level can lead to cavitation. The specific pump parameters and installation drawings are provided in the attached files
The inlet elevation is only -2.105 m, so a NPSH of 5.5 meters is required; other loss factors haven’t even been taken into account yet. If the water inlet tank is at atmospheric pressure, the actual NPSH available for the equipment is less than 2.105 meters. Such a low NPSH is likely to cause damage to the equipment
With the current installation method, even when the tank is at full level, the pump cannot meet its design requirements; without taking into account factors such as water temperature and resistance losses in the pipes, the pump inlet elevation must be below -5.5m
Do impeller spraying; it gives good results and the technology is mature
Does that mean the pump’s installation height needs to be reduced by another 3.5 meters? How deep must it be? The installation height of pumps these days isn’t designed to be that extreme
Yes, that’s the case at my workplace too. There’s no other option but to use spraying as a way to combat cavitation
This post was last edited by SihaiYouxianTian on 2022-10-27 at 19:55. Calculated based on a cooling water temperature of 32°C, with the current installation height of the pump taken as reference level 0: for the large pump, the NPSHa value is 10.5 meters, giving an allowable installation height of 3.5 meters; for the small pump, the NPSHa value is 10.66 meters, resulting in an allowable installation height of 4.16 meters. It can be concluded that cavitation will not occur. There are generally no design issues with this type, but it is most likely an operational problem; common causes include too low a liquid level, the inlet valve being closed too tightly, or high water temperature.
The cavitation margin of this device is sufficient according to the rated parameters, although the value indicated on the nameplate seems a bit low. This cavitation is mostly caused by a low actual outlet head and operation at a high flow rate; check the operating pressure first
For pumps from 2015 and 2017, did you check whether there was cavitation in the impeller during previous repairs? Based on the NPSH parameter indicated on the pump’s nameplate, it should be possible to operate it at a depth of -1.2 meters for an extended period at a water temperature of 30 degrees Celsius. Check the operating conditions of the pump. Furthermore, the use of stainless steel for the impeller also enhances its cavitation resistance.
Thank you. Regarding the check of the inlet valve, it is indeed in the fully open position, and the water temperature refers to the circulating water temperature; in summer, it never exceeds 33 degrees. There are no issues with these two aspects. Since cavitation in the large pump is far less severe than that in the small pump, I suspect two possible causes: 1) Severe air leakage from the seal of the small pump, which leads to increased cavitation; 2) Sediment accumulation at the suction inlet valve, resulting in a reduced water flow rate. 3) The low outlet head forces the pump to operate at a higher flow rate. The outlet pressure gauge on the small pump shows only 0.19 MPa, while that on the large pump is between 0.2 and 0.21 MPa. We have several pumps operating in parallel
Yes, stainless steel impellers are more expensive. For the ordinary impellers requested in this purchase order, the operating conditions of the pump are not available; it’s necessary to check whether the pump is operating under conditions that deviate from its normal operating parameters