Common commercial and industrial practices use dimensioned versions which are of equal utility. Specific speed N s, is used to characterize turbomachinery speed. JSTOR ( July 2014) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. Preferred speed for motors is 1500 RPM.This article needs additional citations for verification.Suction specific speed shall be less than Nss Specific speed shall be within range of 9 The best choice is Over Hung type with speed 1500 rpm Comments : The best choice is Between Bearing type with 4 stages and speed 3000 rpm. Since pump shall have two or more stages: 4 stages Therefore speed shall be increased to 3000: 1.Specify pump speed for below condition: 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. 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. The effect of speed on the volume and value of goods Given that increasing efficiency reduces the absorption power, so increasing the speed in this case is also positive. The effect of increasing the speed on Absorb PowerĪs the speed of the pump increases, the specific speed increases and, consequently, the efficiency increases. Therefore, we do not increase the pump speed as much as possible. Increasing these two causes cavitation in the pump. The effect of pump speed on suction specific speed, NssĪccording to the suction specific speed formula, if we increase the speed, the suction specific speed as well as NPSHr will increase. – Reduction of pump speed: According to the specific speed formula, by reducing the speed, the specific speed will also decrease. – 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. Therefore, the following two methods can be used to prevent excessive increase in specific speed and control it: b) If low head and high flow are required in certain conditions, then according to the formula, the specific speed will increase sharply. With the increase pump flow, we were able to increase the specific speed. 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. – Another way is to consider the pump flow higher than the required flow and to consider a bypass line for the pump. – 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. – 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. (Note that the specific speed and suction specific speed are calculated at the maximum diameter of the pump impeller) Therefore, there are three ways to prevent excessive specific speed reduction. 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). Specific speed NSĭue 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. Influence of motor speed on pump performance: 1. 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. Motor speed is effective in the following cases. How to choose the right pump speed (RPM):
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