[2026 Flow Measurement] Flow measurement methods at the pump station outlet under the condition of \"pressure present and pipe not full\"
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This post was last edited by The one on 2026-5-26 00:51. The outlet pipes of lift stations are usually filled with fluid and have high flow velocities, making them suitable for installing electromagnetic flowmeters to measure flow rates. In actual projects, due to limitations in on-site civil engineering and installation conditions, many lift pump stations experience the following situation, resulting in their outlet pipes being in a \"pressurized and non-full pipe\" state: ▶ There is a long horizontal section at the outlet, similar to the position indicated as ③ in the figure below ； ▶Waterfall, similar to the location in Figure ④ below. If the outlet pipe of the lift pump station is under pressure and not at full capacity, it is not possible to use electromagnetic flowmeters for accurate flow measurement, nor can common flow measurement methods be employed. “The issue of flow measurement under pressure and in partially filled pipes is a difficult and problematic aspect that arises in the operation of many lift pump stations.
1. Problem analysis: Currently, electromagnetic flowmeters are commonly used to measure the discharge flow rate at lift pump stations; ultrasonic Doppler flowmeters and ultrasonic cross-correlation flowmeters may also be employed. These flowmeters are all velocity-area type flowmeters. The core of it is to simultaneously measure the average flow velocity and the cross-sectional area of the flow passage. Q = A × V_avg; in the case of a partially filled pipe, the measurement of the cross-sectional area through which flow occurs is converted into a measurement of height.
▶In the fully filled pipe condition, when the flow cross-section is set to a fixed value, it is sufficient to measure the average flow velocity at that cross-section, for example using an electromagnetic flowmeter. ▶In the case of \"no pressure and non-full pipe,\" it is necessary to simultaneously measure the height of the flow cross-section and the average flow velocity. The height of the flow cross-section can be determined using pressure sensors and air-ultrasonic level gauges, for example, in the flow measurement of gravity-fed drainage networks. ▶In the case of \"pressurized and partially filled pipes,\" it is also necessary to simultaneously measure the area of the flow cross-section and the average flow velocity. In addition to the difficulties in measuring flow velocity when the pipe is not full, measuring the height of the flow cross-section also presents significant challenges: ♦ Pressure sensors are affected by the air pressure inside the pipe (positive or negative pressure), which prevents them from accurately determining the liquid level ； ♦Under pressure in the pipeline, it is usually not possible to install an air-ultrasonic level gauge on the pipeline ； Or the air ultrasonic level gauge cannot withstand the high pressure inside the pipeline.
Therefore, improving the measurement of outlet flow at pump stations under \"pressurized and partially filled pipe\" conditions presents two challenges: ▶ How to accurately measure the flow velocity under \"pressurized and partially filled pipe\" conditions； ▶How to accurately measure the liquid level under conditions of \"pressure and non-full pipes\".
2. Solutions 2.1 Flow velocity measurement under \"pressurized and partially filled pipe\" conditions: An insert-type ultrasonic cross-correlation flow meter can be used to perform a full-scanner of the flow cross-section； Under the conditions of \"pressure presence and non-full pipe,\" the measurement error of the average flow velocity at the cross-section is as follows: ▶ When the flow velocity is < 1 m/s, the error is < 0.5% of the measured value + 5 mm/s ； ▶At flow rates > 1 m/s, < 1% of the measured value.
2.2 Level measurement under \"pressurized and partially filled pipe\" conditions – Level measurement under \"pressurized and partially filled pipe\" conditions is one of the challenges involved. When on-site installation conditions are limited, the following can be considered: ▶ Install pressure sensors, with a requirement that their measurement accuracy reach 0.25 % FSO ； ▶Take measures to counteract the effect of pressure in the pipeline on level measurement.
2.3 Sensor Installation Method The plug-in cross-correlation flow velocity sensor and pressure sensor are installed at the lower side of the pipeline, as shown in the figure below.
3. Practical Cases 3.1 Project Background ▶ Outlet pipeline of a lift pump station； ▶DN600, steel pipe ； ▶The medium being measured is sewage, which contains many impurities and is not always at full pipe level ； ▶Existing electromagnetic flowmeters cannot perform stable measurements.
3.2 Measurement tasks ▶ Utilize high-precision flow measurement methods to monitor the operation of pump stations in real time； ▶Minimal installation effort, suitable for installation in complex conditions ； ▶Adapted to characteristics such as high impurities.
3.3 Solution ▶ Use an ultrasonic cross-correlation flow meter to meet the requirements for high-precision measurement； ▶Quantity: 1 set ； ▶Sensors: 1x CSM plug-in cross-correlation sensor, 1 set of pressure sensors ； ▶Installation method: Plug-in installation.