Problem: The water pump cannot achieve the rated flow rate or has low efficiency
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Background information: A long-distance pipeline with an outer diameter of 355 mm and a pipe volume of approximately 96 cubic meters per kilometer, with a length of around 170 kilometers. Water from the river is pumped into the pipelines using centrifugal water pumps to facilitate water circulation and pumping; simultaneously, pig balls are used to clean the pipelines and to check for any deformation in them ； The hydraulic diagram shows that the highest point along the flow path is 158 m higher than the water inlet point, and the friction loss along the flow path was estimated to be approximately 2.1 MPa during operation. Process flow of the water transfer pump: There are two circuits, one in use and one as a backup. Each circuit operates in series through a centrifugal feed pump located on a floating boat by the river (hereinafter referred to as 1# feed pump, with a rated flow rate of 320 cubic meters per hour, a rated head of 60 meters, a required net positive suction head of 5 meters, a motor power of 90 kW, and a voltage of 380 V) and a centrifugal transfer pump on land (hereinafter referred to as 1# transfer pump, with a rated flow rate of 320 cubic meters per hour, a rated head of 400 meters, a required net positive suction head of 5.7 meters, a motor power of 500 kW, a rated motor current of 35.3 A, and a voltage of 10 KV). The pump on land is located about 20 meters higher than the pump on the floating boat. Pump startup procedure: Initial shaft rotation and air release are carried out in accordance with standard procedures. First, start pump #1 for water supply; when the outlet valve is closed, the current value is 30A. Open the outlet valve fully. The inlet gate valve of pump #1 for water transfer is normally open, and once the inlet pressure stabilizes, start this pump. When the outlet valve is closed, the current remains stable at 28A. Slowly adjust the opening degree of the outlet valve while monitoring the changes in current; the current stabilizes at 35A when the outlet valve is only opened to about 10%. Due to concerns about excessive current, further increasing the opening degree is not possible. At this point, the inlet pressure is 0.65 MPa, the outlet pressure is 4.9 MPa, and the pressure at the exit point is 3.8 MPa. The electrical control panel indicates that the motor output power of this pump is 540 Kw. By reversing-calculating the pump flow rate for that period using the average flow velocity of the pig between two points in the pipeline (about 96 cubic meters of water per kilometer), it was found that the pump’s flow rate was approximately 260 cubic meters per hour, which is below the rated flow rate of 320 cubic meters per hour. Based on this flow rate, the efficiency of this pump is only about 55% (efficiency = ρgQH/N), which is quite different from the 75% promised in the company’s technical specifications ； It is preliminarily determined that the low efficiency of the pumps produced by this manufacturer, along with the use of motors that are too small, is the cause of this problem. Additional note: The same issue exists with the other backup water pump as well. The backup feed water pump (hereinafter referred to as 1# feed water pump, with a rated flow rate of 420 cubic meters per hour, a rated head of 80 meters, requiring a net positive suction head of 5 meters, and an electric motor power of 110 kW) and the backup transfer water pump (with a rated flow rate of 320 cubic meters per hour, a rated head of 400 meters, requiring a net positive suction head of 5.7 meters, an electric motor power of 500 kW, a rated motor current of 35.3 A, and a voltage of 10 kV) ； Vibration and temperature measurements using a portable vibration meter and an infrared thermometer are functioning normally ； The water pump is a three-stage BB4 type pump. Question: Regarding this issue, the manufacturer does not agree with our analysis; they claim that the outlet of their pump is 150 mm in diameter, while our pipes have a diameter of 355 mm (with transition fittings of different sizes), and therefore it is very likely that the rated flow rate will not be achieved. We consider this argument to be completely unfounded. Regarding this issue, I would appreciate it if experts could help analyze it; please give me some guidance. The common reasons for excessive power consumption of water pumps are as follows (please help analyze which possibility is most likely, thank you): (1) The pump speed is too high ； (2) The specific gravity of the liquid being transported exceeds the originally designed value ； (3) High viscosity of the liquid being transported ； (4) The pump impeller is too large in size ； (5) The resistance of the piping system exceeds or falls short of the predetermined value ； (6) The water pump has too low head pressure and too high flow rate; (7) There is friction between the rotor components and the stator components ； (8) Bearing wear or damage ； (9) Excessive pressure of the mechanical seal on the sealing seat ； (10) Breakage of the outlet pipeline ； (11) Bent water pump shaft ； (12) Uneven thermal expansion of different components within the pump ； (13) The dynamometer is inaccurate ； (14) The pump balancing device is not functioning.
The motor selected for pump #1 is too small; the pipeline is large and thus the resistance is low, so the flow rate should be higher under the same operating current conditions
“For a certain long-distance pipeline (with an outer diameter of 355 mm and a pipe volume of approximately 96 cubic meters per kilometer), the average flow velocity between two points in the pipeline was determined using cleaning balls; this velocity was around 96 cubic meters of water per kilometer. By using this value to calculate the pump’s flow rate, it was found that the pump’s flow rate was approximately 260 cubic meters per hour. Is such a calculation method accurate?
Explanation of flow rate calculation: Since there are no flow meters on site, based on previous operation data, the ship’s 1# feed water pump is capable of achieving its rated flow rate; the rated current is approximately 120A. Therefore, we used the formula ρgQH=1.732U×I×cosφ×η to derive Q1=Q2×H2×I1×η1/(H1×I2×η2). Point 1 was used as the reference point, while point 2 served as the rated condition point. It was assumed that η1=η2 (in fact, generally η1
LZ’s requirements are quite high – to measure the efficiency of the centrifugal pump on-site! In fact, to conduct measurements (which are essentially performance tests), corresponding measuring instruments are required, and it is difficult to obtain all of these on-site. The values calculated in this way are only for reference and cannot be used to determine the performance of the pump. Aren’t centrifugal pumps tested for performance before leaving the factory? Was LZ there to witness it at that time, and did the efficiency listed in the test report meet the requirements? If it cannot be achieved at that time, acceptance can be refused and the manufacturer can be asked to make corrections. The errors in the tests being conducted now are quite high; if that doesn’t work, it’s better to hire a third party to carry out the tests. Additionally, in order to win bids, pump manufacturers usually inflate the efficiency of the pumps artificially during the bidding process; it is very difficult for BB4 pumps to achieve an efficiency of 75%. :lol
Similar cases have occurred before; you can try it again. Motors usually come equipped with overload protection, so have someone monitor the motor’s current levels. Start the pump and the outlet; continue to increase the outlet opening before the motor shuts down. As the pressure decreases and the outlet flow increases, the current will drop once a certain level of opening is reached. It’s just experience-based advice, but it’s worth giving it a try. Alternatively, you can do some calculations
Factors to consider: Due to various reasons, including on-site and manufacturing factors, the actual operating efficiency of the pump is low, the actual operating power is high, resulting in the selection of a motor that is too small.
There were no witness performance tests (with such tests, issues could have been identified; this is an area where our management is weak). The test data provided by the manufacturer in the accompanying documents show efficiency levels that are generally consistent with those specified in the technical agreement, suggesting a high likelihood of data manipulation.
Reply to Floor 6: We previously tried increasing the opening degree of the outlet valve, raising the current to 36.2A. Since there is no monitoring (interlock protection) for the motor stator and resistance temperature, we did not dare to increase it any further; subsequently, the motor current was kept within the rated value of 35.3A.