Operating Condition Assessment for Differential Pressure Monitoring in Hydropower Station Water Diversion Systems

First Distinguish Between “What Differential Pressure Is Measured” and “What Pressure Is Withstood”

Hydropower differential pressure sensors in hydropower station water diversion systems are commonly used to monitor differential pressure across trash racks, filter blockage levels, pressure changes upstream and downstream of valves, local resistance in diversion tunnels, and operating conditions in turbine inlet sections. The differential pressure value itself is usually not high, but the static pressure in the pipelines on both sides is often high. Therefore, selection should not be based solely on the differential pressure range.

For example, if the normal operating pressure of a water diversion pipeline section is 1.6MPa and the normal differential pressure across the filtering device is only 20kPa to 60kPa, then even if a differential pressure range of 0kPa to 100kPa is selected, it is still necessary to confirm the static pressure that the high- and low-pressure sides of the sensor can withstand, as well as its overload capacity in the event of accidental pressure on one side.

During startup and shutdown of the diversion system, guide vane adjustment, rapid valve opening and closing, and sudden load changes, short-term pressure fluctuations can easily occur. Particularly in long-distance pressure pipelines, water hammer pressure may be much higher than the pressure under stable operating conditions. If configuration is based only on daily operating data, the sensor diaphragm, sealing structure, and internal sensing elements may all be at risk of damage.

Therefore, project managers should evaluate the differential pressure range, system static pressure, transient peak pressure, medium sand content, installation location, and control requirements separately. The measuring range is used to ensure measurement resolution, while pressure resistance and overload parameters determine whether the instrument can remain intact and recoverable under abnormal operating conditions.

Range Configuration: Balancing Resolution, Stability, and Alarm Requirements

Reserve an Appropriate Margin Based on Normal Differential Pressure

The differential pressure range should not simply be determined based on the idea that “the larger, the safer.” When the range is too large, the actual differential pressure accounts for too small a proportion of the full range, making changes in the 4mA to 20mA output less apparent. This affects PLC acquisition resolution and local display performance, while blockage trends also become difficult to identify in advance.

A more practical approach is to keep the long-term normal operating differential pressure within 30% to 70% of the rated range and ensure that the expected maximum stable differential pressure does not exceed 80% of the rated range. For trash rack or filter screen monitoring points requiring identification of gradual blockage trends, the range matching can be appropriately improved.

Taking filter differential pressure monitoring as an example, if the normal differential pressure is 15kPa to 35kPa and the cleaning or blockage alarm point is 60kPa, priority should be given to evaluating a 0kPa to 100kPa range. If the maximum stable differential pressure may reach 90kPa, a 0kPa to 160kPa or 0kPa to 200kPa range may be selected to avoid long-term operation close to full scale.

For pipeline sections where reverse flow, maintenance switching, or pump backflow may occur, it should be confirmed whether reverse differential pressure will arise. Conventional unidirectional differential pressure transmitters may experience negative output, zero drift, or even exceed their limits when subjected to reverse pressure. A bidirectional range or products with bidirectional overload capability should be selected according to pipeline network operating conditions.

Overload Protection: Do Not Look Only at Rated Static Pressure

Focus on Verifying Three Types of Pressure Parameters

In the technical parameters of differential pressure sensors, the rated differential pressure range, maximum working static pressure, and single-side overload pressure are different concepts. The rated differential pressure range determines the output range; maximum working static pressure indicates the allowable upper limit when both the high- and low-pressure sides are subjected to pressure simultaneously; and single-side overload pressure reflects the capacity to withstand pressure when one side is pressurized while the other side is depressurized or blocked.

In hydropower station water diversion systems, insufficient venting of impulse lines, accidental closure of the low-pressure-side valve, incorrect isolation valve operating sequences, depressurization through drain outlets, and pressure tap blockage may all cause one-sided pressurization. At this time, the sensor is not subjected to normal differential pressure, but may be exposed to pressure close to the actual static pressure in the pipeline.

For example, the system working static pressure is 2.5MPa and the normal differential pressure range is 0kPa to 50kPa. Even if the selected product has a differential pressure range of only 100kPa, its single-side overload capacity should still be no less than the highest possible system static pressure. A safety factor should also be reserved based on water hammer calculation results, rather than simply using a product with a 0.1MPa pressure resistance rating.

For projects with high water head, significant pressure fluctuations, or frequent valve operation, it is recommended to evaluate the sensor's single-side overload capacity at 1.25 to 1.5 times the maximum possible pressure. If the on-site water hammer peak is difficult to determine accurately, a combined assessment should be made using pipeline design documentation, valve closing time, and historical operating records.

Key Verification Items in Engineering Configuration

First, verify the maximum static pressure when both the high- and low-pressure sides are pressurized simultaneously. Although the pressures in the pipelines on both sides are close, the absolute pressure must not exceed the sensor's allowable static pressure; otherwise, the service life of the isolation diaphragm and long-term zero stability will be affected.

Second, verify the single-side overload capacity. When the low-pressure side accidentally loses pressure, the high-pressure side may be directly subjected to the full pipeline pressure; when the high-pressure-side pressure tap is blocked, pressure changes on the low-pressure side may also cause an abnormal differential pressure direction. The selection documentation should clearly specify whether forward and reverse overload capacities are consistent.

Finally, verify transient withstand capability. For short-term impacts, it is not enough that the sensor simply remains undamaged; zero recovery after impact, repeatability changes, and output drift should also be considered. For critical monitoring points, buffer devices or dampers may be added, or a higher-grade isolation structure may be used.

Impact of Installation and Signal Configuration on Monitoring Results

Sensor Protection Should Be Designed Together with the Pressure Tapping System

Even the most suitable range and overload rating require proper pressure tapping and installation conditions. High- and low-pressure tapping points should be arranged in locations with relatively stable flow conditions, avoiding local vortices, areas directly adjacent to elbows, and obvious sediment accumulation zones to reduce differential pressure signal fluctuations caused by flow field disturbances.

If the water diversion medium contains sediment, floating debris, or fine particles, impulse lines should incorporate anti-blockage structures and valve manifolds that facilitate flushing. Excessively long impulse lines cause response delay, while excessively small pipe diameters are prone to blockage. In general, the pressure transmission distance should be minimized, and conditions of the impulse paths on both sides should be kept similar.

For outdoor, humid, or high temperature-difference installation environments, the sensor enclosure protection rating, cable sealing, and grounding method must not be overlooked. It is recommended to use 4mA to 20mA output as the primary control signal, combined with RS485, HART, or other communication methods for parameter setting, status diagnostics, and remote maintenance.

Alarm settings should not include only one high-high limit. In practical projects, three-level logic consisting of early warning, alarm, and interlock can be set according to operating experience. For example, prompt inspection when differential pressure reaches 50% of the rated range, issue a blockage alarm at 70%, and trigger protection or require manual confirmation at 85%. Specific thresholds should still be determined according to equipment process requirements.

Recommendations for Selection and Acceptance of Hydropower Differential Pressure Sensors

Include Data Conditions in the Technical Agreement

When procuring hydropower differential pressure sensors, it is recommended to clearly specify in the technical agreement whether the medium is raw water, circulating water, or sediment-laden water, and state the normal differential pressure, maximum differential pressure, pipeline design pressure, maximum operating static pressure, expected water hammer pressure, ambient temperature, installation method, and output signal. Avoid placing general orders merely for “one differential pressure transmitter.”

The accuracy class should be determined according to the control objective. For monitoring points used to assess filter blockage trends, stability, repeatability, and anti-interference capability are generally of greater concern. For measuring points used for performance testing, flow calculation, or critical protection logic, overall accuracy, temperature drift, long-term drift, and calibration traceability should receive further attention.

During the acceptance stage, zero-point checks, full-scale output verification, static pressure effect tests, and pressure holding checks should be conducted. For applications with high static pressure and small differential pressure, particular attention should be paid to whether the zero point shifts significantly after pressurization. For models equipped with local display and intelligent communication functions, it is also necessary to verify whether the range, units, damping time, and alarm parameters are consistent with the design.

Xi'an Shenghongchuang Instrumentation Co., Ltd. can provide differential pressure sensor selection support based on the differential pressure range, static pressure rating, overload risk, and installation conditions of hydropower station water diversion systems. Project parties may compile existing pipeline pressure data, operating fluctuation records, and installation drawings, allowing technical personnel to further confirm the range, pressure resistance rating, connection type, and signal configuration solution.