Calculation of the head of a screw pump
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I would like to ask: is the method for calculating the head of a screw pump the same as that used for ordinary centrifugal pumps, or are there different requirements?
A screw pump is a positive-displacement pump; its head is related to the pipeline resistance, and you just need to calculate that resistance
You’re wrong; the head of a positive displacement pump is completely independent of the pipeline characteristics. It only relates to the pump itself. The head of a screw pump is the pressure difference between the inlet and outlet divided by density and g.
The original poster is correct! A screw pump is a rotary type of pump, and its head is determined solely by its own characteristics. For this type of pump, those who have taken the registered chemical engineering exams several times have, in most cases (even 90% of them), made mistakes by using the calculations applicable to centrifugal pumps!
The screw pump is also a type of positive displacement pump; the outlet pressure determines the strength of the pump body, the requirements for sealing, and the power required from the prime mover. It depends on the back pressure in the outlet pipeline, that is, on the characteristics of the pipeline, and has nothing to do with the pump itself. Its theoretical flow rate depends only on the pump itself.
I agree with your view. We are choosing a pump.
The head provided by a screw pump is related to the pipeline resistance; the greater the resistance downstream, the higher the head generated. Flow rate is a property of the pump itself and is not greatly affected by back pressure. Generally, positive displacement pumps do not require high head – what is needed is merely the ability to withstand pressure
For positive displacement pumps, head is not mentioned; instead, outlet pressure is considered. The outlet pressure is determined by the back pressure – so how can it be related only to the pump’s own characteristics? The pump’s own characteristics determine whether it can withstand that back pressure
Go and check the Petrochemical Design Manual 3 to see how it explains it!
So you mean it’s related to the inherent properties, and thus the head is a constant value?
When selecting a pump, it is sufficient that the pressure resistance of the screw pump is at least equal to the required head.