Specific plan for cutting water pump impellers

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It is very important to choose the right water pumps for air separation systems. The circulating water pumps, refrigeration pumps, and normal-temperature pumps in my unit were selected improperly; I hadn’t joined this company at that time. Now I am working on modifying these pumps in order to reduce energy consumption. A few days ago, there was a question on a forum regarding impeller cutting, so I’m sharing this report here – I hope everyone can offer their guidance. Feasibility Report on the Renovation of Circulating Water Pumps: The circulating water pumps used in the oxygen production plant are KQSN350-M9/433 type single-stage double-suction centrifugal pumps manufactured by Shanghai Kaiquan Pump Co., Ltd. There are a total of 8 such pumps, and they have been operating stably since installation. However, the selected pumps do not match the existing pipeline system; their designed head is too high, while the resistance in the circulation pipelines of the oxygen production plant is relatively low. As a result, the flow rate at the operating point of these pumps is too high. When the outlet valve is opened to about 30%, the operating current already approaches the rated current, leading to low operational efficiency, high energy consumption, and substantial electricity usage. Considering the economic and practical aspects of such a renovation, it is initially planned to modify the pumps by cutting their impellers, thereby changing their actual technical parameters and achieving energy savings and reduced consumption. The original pump’s technical parameters are as follows: flow rate Q = 1000 M3, head H = 60 m, rotational speed n = 1490 r/min, shaft power P = 208 Kw. The impeller diameter is 433 mm. The motor used is of type Y355M-4, with a power of 250 KW, a rated current of 443 A, a COSφ value of 0.90, and a mechanical efficiency of 95.2%. Now, it is considered to reduce the pump’s head to 50 m and 40 m; the calculations are as follows: 1. When the head is reduced to 50 m, based on the relationship H1/H = (D1/D)², the diameter after reduction becomes D1 = D × (H1/H)⁰·⁵ = 433 × (50/60)⁰·⁵. 5=395.27; in theory, the original impeller would need to have 433–395.27=37.73 mm removed. Although relevant sources suggest that it is best to use the head calculation formula when applying the cutting law, even so, the amount to be removed as calculated by this formula still requires adjustment. The nominal specific speed of the original pump is shown as 90., while the actual value is ns=3.65n(Q/2)0. 5/H0。 75 = 3.65 * 1490 * (1000 / 3600 / 2)^0.5 / 600. Thus, 75 = 94.01. For pumps with ns ranging from 90 to 150, the cutting correction factor is generally between 0.94 and 0.85; using 0.93 as the value, the actual reduction in diameter is 37.73 * 0.93 = 35.09. Hence, the diameter of the impeller after cutting is 433 – 35.09 = 397.91. According to the cutting law, we have: Q1/Q = D1/D; therefore, the flow rate after cutting is Q1 = Q * D1/D = 1000 * (397.91 / 433) = 918.96 m³. Additionally, P1/P = (D1/D)^3; thus, the shaft power after cutting is P1 = P * (D1/D)^3 = 208 * (397.91 / 433)^3 = 161.42 Kw. Theoretically, the motor current decreases by: △I = △P / (30.5 * U * cosφ * η) = (208 – 161.42) / 1.732 * 0.38 * 0.90 * 0.952 = 82.59 A. Theoretical energy savings per hour after cutting are: 208 – 161.42 = 46.58 kWh. Annual energy savings amount to: 24 * 365 * 46.58 kWh = 408,040.8 kWh. II. If the head is reduced to 40 m, then according to H1/H = (D1/D)^2, the diameter after cutting is D1 = D * (H1/H)^0.5 = 433 * (40/60)^0.5. 5 = 353.54; theoretically, the original impeller requires a cutting amount of 433 – 353.54 = 68.46 mm. The typical cutting correction factor is between 0.94 and 0.85; using 0.93, the actual cutting amount becomes 68.46 * 0.93 = 63.67 mm. Thus, the diameter of the impeller after cutting is 433 – 63.67 = 379.33 mm. Based on the cutting formula, we can conclude that: Q1/Q = D1/D; therefore, the flow rate after cutting is Q1 = Q * D1/D = 1000 * (379.33/433) = 876.06 m³. Additionally, P1/P = (D1/D)³; hence, the shaft power after cutting is P1 = P * (D1/D)³ = 208 * (379.33/433)³ = 139.96 kW. Theoretically, the motor current decreases by △I = △P / (30.5 * U * cosφ * η) = (208 – 139.96) / 1.732 * 0.38 * 0.90 * 0.952 = 120.64 A. Theoretical energy savings per hour after cutting are 208 – 139.96 = 68.04 kWh. Annual energy savings amount to 24 * 365 * 68.04 kWh = 596,030.4 kWh. 3. For pumps with a specific speed of around 90, the ratio of the maximum allowable cutting amount to the outer diameter of the original impeller should not exceed 0.17–0.18. In both of the cutting methods discussed above, this limit is not exceeded. 2. The flow rate decreases after cutting; it is 918.69 M3 after the first type of cutting, and 876.06 M3 after the second type. The water consumption for one air separation unit on site is approximately 1800 M3–1950 M3. Moreover, the opening degrees of the return valves in the cooling tower system are not high. If the flow rate is low after cutting, it can be adjusted by using the valves on site to meet the requirements.