How to choose the right pump speed (RPM):
Motor speed is effective in the following cases. Therefore, in order to choose the right pump speed, we must examine the effect of increasing and decreasing the speed on the performance of the pump.
500 < NS < 4200 Centrifugal or Radial Flow
4200 < NS < 9000 Mixed Flow
NS > 9000 Axial Flow
Influence of motor speed on pump performance:
1. Specific speed NS
NS = N Q0.5 / H 0.75
Due to the above figure and the specific speed formula, if we need low flow and high head in some conditions, the following conditions will occur.
a) Due to the fact that we have a high head and head is in the denominator of the specific speed formula, the specific speed decreases sharply and subsequently the pump efficiency decreases (Figure above). Therefore, there are three ways to prevent excessive specific speed reduction. (Note that the specific speed and suction specific speed are calculated at the maximum diameter of the pump impeller)
– Increasing the number of pump stages: As the number of pump stages increases, the head in the denominator is divided by the number of stages. Therefore, the specific speed increases.
– Increasing the pump speed: Due to the fact that the pump speed is in numerator, with the increase of the pump speed, the special speed increases. (Eg 3000 rpm)
– Another way is to consider the pump flow higher than the required flow and to consider a bypass line for the pump. For example, if we need 50 cubic meters per hour flow, we choose the pump flow with 70 cubic meters per hour and the amount of 20 cubic meters per hour is returned through the bypass line. With the increase pump flow, we were able to increase the specific speed .
b) If low head and high flow are required in certain conditions, then according to the formula, the specific speed will increase sharply. Therefore, the following two methods can be used to prevent excessive increase in specific speed and control it:
– Double suction pump: By selecting the double suction pump, flow is divided into two in numerator of the fraction and therefore the specific speed is reduced.
– Reduction of pump speed: According to the specific speed formula, by reducing the speed, the specific speed will also decrease. (for example 1500 rpm)
2. The effect of pump speed on suction specific speed, Nss
According to the suction specific speed formula, if we increase the speed, the suction specific speed as well as NPSHr will increase. Increasing these two causes cavitation in the pump. Therefore, we do not increase the pump speed as much as possible.
NSS= N Q0.5 / NPSHr 0.75
3. The effect of increasing the speed on Absorb Power
As the speed of the pump increases, the specific speed increases and, consequently, the efficiency increases. Given that increasing efficiency reduces the absorption power, so increasing the speed in this case is also positive.
W absorb = W/η
4. The effect of speed on the volume and value of goods
In the previous cases, we mentioned that the two most important ways for increasing the specific speed are to increase the speed and the number of stages. If we achieve it by increasing the speed, the number of impellers, parts and volume of goods will be significantly reduced and will reduce the cost of the device, which the manufacturers are willing to do.
Examples for selecting speed.
1.Specify pump speed for below condition:
Q = 200 m3/hr = 200 / 3600 = 0.055 m3/s
H = 1100 m
N = 1500 rpm
NS = N Q0.5 / H0.75 Ns = 1500 x 0.0550.5 / 1100 0.75 = 1.85 out of range 9 < Ns < 155
Therefore speed shall be increased to 3000:
Ns = 3000 x 0.0550.5 / 11000.75 = 3.7 out of range 9 < Ns < 155
Since pump shall have two or more stages: 4 stages
Ns = 3000 x 0.0550.5 / (1100/4) 0.75 = 10.47 Good
The best choice is Between Bearing type with 4 stages and speed 3000 rpm.
2. Specify pump speed for below condition:
Q = 1100 m3/hr = 1100 / 3600 = 0.3055 m3/s
H = 200 m
N = 3000 rpm
NS = N Q0.5 / H0.75 Ns = 3000 x 0.30550.5 / 200 0.75 = 31.178 Good
New assumption: N = 1500
NS = N Q0.5 / H0.75 Ns = 1500 x 0.30550.5 / 200 0.75 = 15.58 Good
The best choice is Over Hung type with speed 1500 rpm
- Specific speed shall be within range of 9 < Ns < 155.
- Suction specific speed shall be less than Nss < 213.
- Preferred speed for motors is 1500 RPM.