Abstract

The hydrogen-side oil return control box is a key component of the dual-circuit seal oil system in hydrogen-cooled turbine generators. Its liquid level is regulated by float-operated ball valves, whose reliable operation is essential for maintaining hydrogen purity, preventing oil migration, and ensuring system safety. Conventional inspection and calibration of these valves rely on manual procedures that are labor-intensive, time-consuming, and susceptible to measurement errors.

To address these limitations, an automatic detection and calibration device was developed based on the existing maintenance workflow. The system automatically performs oil filling, gas pressurization, and liquid level calibration for the hydrogen-side oil return control box. A Programmable Industrial Controller (PIC) coordinates the operation of oil pumps and control valves according to predefined testing sequences, while flow and pressure sensors continuously monitor system conditions. By analyzing the collected data, the device accurately determines the opening and closing liquid levels of the float-operated ball valves.

Field validation demonstrated that the proposed device provides higher measurement accuracy than conventional magnetic flap level gauges while significantly reducing manual intervention. The system enables precise calibration of valve operating levels, shortens maintenance time, and offers a practical replacement for traditional manual inspection methods.

1. Introduction

Large-scale turbine generators commonly employ a water-hydrogen-hydrogen cooling system because of its superior cooling efficiency. In hydrogen-cooled generators, three key operational objectives are preventing hydrogen leakage, maintaining hydrogen purity, and avoiding oil contamination within the generator. The hydrogen-side oil return control box plays a vital role in achieving these objectives as an integral component of the dual-circuit seal oil system. It functions as an oil reservoir, stabilizes the oil level, maintains continuous oil supply, and prevents pressurized hydrogen from escaping the generator.

Liquid level regulation inside the control box is achieved through float-operated oil makeup and oil drain valves. Failure or improper adjustment of either valve may lead to abnormal oil exchange between the hydrogen-side and air-side seal oil systems, resulting in reduced hydrogen purity, excessive oil carryover, or hydrogen leakage. Consequently, during scheduled major overhauls, the operating performance of both float-operated valves must be verified, and their opening and closing liquid levels recalibrated whenever necessary.

At present, these inspections are performed manually. Owing to the elevated installation position of the hydrogen-side oil return control box and the limited working space, maintenance personnel must perform multiple oil filling, gas charging, and drainage operations while visually observing liquid level gauges. The entire procedure is labor-intensive, time-consuming, and prone to measurement errors.

Several studies have attempted to simplify this process. Liu et al. proposed a method utilizing blind flanges and oil-filling branch pipelines, together with flow sensors, to determine the operating state of the float valves. Although inspection steps were reduced, the procedure still depended on manual operation and manual reading of liquid levels. Lu et al. employed data acquisition modules and pressure-regulating valves during factory commissioning to record pressure and flow parameters. However, their approach could not determine the opening and closing liquid levels of the float-operated valves.

To overcome these limitations, this study presents an automatic detection and calibration device developed from the existing maintenance procedure. The proposed system automatically performs oil filling and gas pressurization of the hydrogen-side oil return control box while accurately determining the opening and closing liquid levels of the float-operated oil makeup and drain valves. The system operates independently of the plant Distributed Control System (DCS) level sensors, thereby reducing manual workload, shortening maintenance duration, minimizing outage risks, and improving the operational safety and reliability of hydrogen-cooled generators.

2. Maintenance of Float-Operated Ball Valves in the Hydrogen-Side Oil Return Control Box

2.1 Operating Principle of the Hydrogen-Side Oil Return Control Box

The hydrogen-side oil return control box serves as the primary oil reservoir for the hydrogen-side seal oil circuit in the turbine generator. It incorporates automatic oil makeup and oil drainage mechanisms that maintain a stable oil level during operation.

As illustrated in Figure 1, the upper section of the control box is connected to the generator defoaming chambers and communicates directly with the generator interior. A vent line equalizes the hydrogen pressure inside the control box with that inside the generator, allowing seal oil to return smoothly from the defoaming chambers.

Inside the control box are two independent float-operated mechanisms: an oil drain valve and an oil makeup valve. When the liquid level rises above the preset limit, the drain float rises and actuates the oil drain valve through a connecting rod, allowing hydrogen pressure inside the tank to discharge excess oil into the air-side oil tank. Conversely, when the liquid level falls below the preset value, the makeup float descends and opens the oil makeup valve, permitting seal oil from the air-side pump to replenish the tank.

Adjustment nuts installed on the float linkage rods allow independent calibration of the opening and closing levels of both valves. To ensure operational reliability, the control box is also equipped with manual override valves ("Force Open" and "Force Close") for both the oil makeup and oil drain circuits, enabling manual intervention if a float valve becomes stuck or malfunctions.

2.2 Conventional Manual Inspection Procedure

Malfunction or incorrect calibration of the float-operated oil makeup and drain valves may cause simultaneous oil drainage and replenishment, resulting in excessive oil circulation between the hydrogen-side and air-side systems. Such conditions can reduce hydrogen purity, increase oil ingress into the generator, and potentially lead to hydrogen leakage.

Therefore, both float-operated valves must be inspected and calibrated during scheduled major maintenance. The primary objective is to verify the closing level of the oil drain valve and the opening level of the oil makeup valve under simulated operating hydrogen pressure.

The conventional inspection procedure consists of the following steps:

1. Install temporary oil inlet and gas inlet pipelines at the upper flange connections of the oil return control box.
2. Disconnect the downstream pipelines of the oil drain and oil makeup valves, install blind flanges and ball valves, and prepare the test circuit for observing valve operation.
3. Close the oil makeup override valve to disable the automatic function of the oil makeup valve, allowing the tank to be filled to a high liquid level for drain valve testing.
4. Fill the control box with oil through the upper inlet while monitoring the downstream line of the oil drain valve for leakage.
5. Continue filling until oil first appears downstream of the oil drain valve, and record the corresponding liquid level as the valve opening level.
6. Add a measured quantity of oil, close the oil inlet valve, and pressurize the tank with nitrogen to simulate the normal operating hydrogen pressure.
7. Open the downstream ball valve of the oil drain valve and observe the moment when oil flow stops, indicating closure of the drain valve. Record the corresponding liquid level.
8. Release the oil makeup valve override so that the valve resumes automatic operation.
9. Open the downstream ball valve of the oil makeup valve and confirm that no oil leakage occurs while the valve remains closed.
10. Open the manual oil drain valve to lower the liquid level. When oil first appears downstream of the oil makeup valve, record the corresponding liquid level as the valve opening level.
11. Drain the remaining oil, vent the nitrogen, adjust the float linkage nuts according to the measured results, and repeat the inspection until all calibration requirements are satisfied.

Although this procedure is widely used in practice, it requires repeated manual filling, draining, pressurization, and visual observation, making it inefficient and highly dependent on operator experience.

3. Operating Principle of the Automatic Float Valve Detection Device

Because maintenance of the hydrogen-side oil return control box involves repeated oil filling, gas charging, drainage, and liquid level observation, field technicians are required to perform considerable manual work in a confined operating environment. Frequent oil discharge and nitrogen venting also create environmental contamination and increase maintenance complexity.

Furthermore, although liquid level signals can be transmitted to the Distributed Control System (DCS), calibration still depends on operators visually reading magnetic flap level gauges, which inevitably introduces reading errors and reduces calibration accuracy.

To overcome these shortcomings, an automatic detection and calibration device was developed based on the established manual maintenance procedure.

The proposed system automatically executes the complete inspection sequence by sequentially filling the hydrogen-side oil return control box with oil and pressurizing it with gas according to a predefined testing program. A Programmable Industrial Controller (PIC) coordinates the operation of oil pumps, solenoid valves, and control valves throughout the process.

Dedicated oil filling and gas charging pipelines are equipped with electrically actuated control valves for flow regulation and circuit isolation. Separate detection pipelines are installed downstream of both the oil drain valve and the oil makeup valve. Flow sensors mounted on these detection lines monitor oil movement in real time, allowing the controller to determine whether each float-operated valve is open or closed.

Pressure sensors continuously monitor the internal pressure of the control box to ensure that testing conditions accurately simulate actual generator operating pressure. Based on synchronized pressure, flow, and liquid level data, the system automatically calculates the opening and closing liquid levels of both float-operated valves with high precision.

Upon completion of the test sequence, the device automatically recovers the test oil, eliminating manual drainage operations and reducing environmental contamination. Compared with conventional manual inspection, the automated system significantly improves testing efficiency, enhances calibration accuracy, reduces labor intensity, and provides a safer and more reliable method for maintaining hydrogen-side oil return control boxes in hydrogen-cooled turbine generators.

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