Problems with flow meter measurement
Thread Content
The site is a hotel, with a cooling water circulation system in the basement. 3 pumps, with each pump having a capacity of 500 M3/H, a rated pressure of 0.4 MPa, and a head of 48 meters. The hotel is 100 meters high; the water is circulated and not used directly, and only one pump operates at a time. The pipe diameter is DN450, and the flow rate measured by the flow meter is very low, at 150 M3/H; values of 30 and 60 M3/H also occur. When 1 pump is operating properly, the pipeline pressure gauge shows 1.2 MPa, and the flow rate measured on the pipe at the top floor is the same as that measured in the basement. The data obtained by using 2 pumps was 378 M3/H; the same was true for the basement and the rooftop. With 2 pumps in use, it was evident that the water flow rate increased and the volume of water flowing increased as well. It seems like the water level inside his pipes isn’t at full capacity; there’s no need to worry about the basement, as it’s definitely full. The customer said that the flow rate of 1 pump should be 500 M3/H, but the actual measurement was too low. I would like to ask: Is the head capacity of his pump sufficient for a 100-meter tall building? The operating pressure of the pump is 0.4 MPa, while the pressure in the pipes reaches 1.2 MPa. One pump can deliver a flow rate of 500 M3/H, but even with two pumps the flow rate does not reach 500 M3/H. Is the problem with the pump?
Whether the head is sufficient depends on the pressure at the pump inlet; if there is no pressure at the pump’s suction side, then the head will definitely not be sufficient. If there is a high pressure at the pump’s suction side, for example, if the pipeline pressure reaches 1.2 MPa, it means that the pump inlet is under pressure, and in that case the pump’s head will be fine. As for measuring flow rate, if a orifice plate flow meter is used and it is installed vertically, it’s definitely a design issue that causes measurement errors, resulting in a flow rate that is lower than the actual value
Based on your description, the 3 pumps are in series. In series, head is increased; in parallel, flow rate is increased. The head required, as calculated based on the piping, is 120 meters; therefore, using three pumps in series gives 48*3=144 meters, which is sufficient. Generally, the actual value is slightly lower than this, as there are losses associated with both series and parallel connections. With only one or two pumps in operation, according to the pump’s performance curve, the outlet pressure increases while the flow rate decreases; as a result, the pressure shown on the gauge is high and the flow rate is low. The pump operates at its minimum flow rate (usually 7–15% of the pump’s rated flow rate). With two pumps in operation, the rated flow rate of 500 m3/h cannot be achieved under these conditions, but the head will be higher than that of 48+48. It is also important to pay attention to the allowable installation height of the pump; if the bottom of the pool is taken as the 0-meter reference level, the allowable installation height should be at least -5 meters.
There is no pool; it is a closed-loop pipeline. It seems that the water returning is not sufficient to meet the water supply demand
The diameter of 450 is too large: lol
I think your problem is related to air blockage; when only one pump is in operation, the pressure isn’t sufficient to push the air away, but when two pumps are used, the increased flow rate helps to resolve the issue. Check if there is an exhaust valve at the high point of the pipeline to see if there is air present.
Make sure the water supply to your basement is sufficient to keep the pipes full?
Based on your description, the pump outlet pressure is 1.2 MPa, and the pressure before the pump is at least 0.8 MPa. There are no issues with the pump’s head or pressure, and the flow meter is functioning properly as well; the readings taken before and after are consistent. But the flow rate measured by the flow meter is very low, at 150 M3/H; values of 30 and 60 M3/H also appear. It is preliminarily judged that the issue lies before the pump
The flow rate and the head of a pump cannot both be at their maximum values simultaneously; when the head is at its maximum, the pumping flow rate should be at its minimum. It would be best to check the performance curves of these pumps