Application of Pneumatic Control Valve in Nuclear Power Plant and Analysis of Common Problems

Application-of-Pneumatic-Control-Valve-in-Nuclear-Power-Plant-and-Analysis-of-Common-Problems

1. Pneumatic control valve and its working principle

1.1 Structure and type of pneumatic control valve

Pneumatic control valve consists of pneumatic actuator, valve body and accessories. Pneumatic actuators are divided into membrane type and piston type; The valve body can be divided into straight stroke and angle stroke according to its stroke. It can be divided into straight single seat valve, straight double seat valve, sleeve valve, angle valve, diaphragm valve, butterfly valve and ball valve according to its structure. It can be classified into linear type, equal percentage type, quick open type and parabolic type according to the flow characteristics of the valve core. It can be divided into two position type, open type, adjust type and cut off type according to the adjustment form. It can be divided into fault open type, fault close type and fault position keeping type according to the safety failure mode; Accessories include positioner, E/P electrical converter, filter pressure reducing valve, flow amplifier, handwheel mechanism, etc.

1.2 Working principle of pneumatic control valve

Pneumatic control valve is a variety of pneumatic control valves composed of pneumatic actuator and valve body. It uses dry and clean compressed air as the power source, air cylinder or membrane air chamber as the actuator, and with the aid of electrical valve converter/positioner, solenoid valve, air filter pressure reducing valve, limit switch and other auxiliary components, when the converter or positioner receives a 4mA~20mA weak current signal from the controller or control system, The air pressure signal of 20kPa~100kPa is output to make the pneumatic actuator balance the air pressure and spring force in the air chamber, so that the valve stem drives the valve core to move. The flow area of the valve core and valve seat is changed to adjust the valve opening. The accurate control of process parameters such as pressure, temperature, flow, liquid level, etc. is finally realized by changing the medium flow.

2. Commissioning of pneumatic control valve

Before using the pneumatic control valve, focus on the adjustment of spring preload, neutral point, E/P converter, positioner, etc. to meet the performance indicators such as valve stroke, valve displacement linearity, switching time, leakage, etc. Before commissioning, (1) the process pipeline must be strictly purged and qualified; (2) The hand wheel mechanism shall be in the "release" position; (3) Check whether the air supply pressure of the pressure reducing valve meets the air supply requirements.

3. Analysis of common problems during debugging

In the process of commissioning, in view of the problems of valve internal leakage, external leakage, unqualified opening and closing time, inadequate opening and closing, etc. that often occur during the commissioning process, the causes of the problems of the pneumatic control valve are mainly divided into control element failure, actuator failure, valve body failure.

3.1 Control element failure

In order to ensure that the pneumatic control valve meets the requirements of stability, accuracy and speed, and solve the problem of inconsistent opening of the controller and the valve caused by inaccurate valve position feedback measurement or pneumatic valve positioner failure, the pneumatic control valve must be adjusted and verified as a whole. The controller inputs the input signal into the electrical conversion/positioner steadily in the increasing and decreasing directions to measure whether the corresponding valve opening (stroke) at each point meets the control requirements. It is mainly divided into five key adjustment points: 0%, 25%, 50%, 75% and 100% opening for verification, and 0% opening and other opening are respectively set through corresponding adjustment mechanisms to make the opening of the manual operator or DCS system consistent with the opening of the on-site valve. Since the actual stroke value of the regulating valve cannot be determined, the starting point and the end point must be adjusted correctly, otherwise the valve may not be closed tightly, leading to internal leakage or insufficient adjustment accuracy. The electric/pneumatic valve positioner, as the valve control accessory, is essentially the combination instrument of the electric/pneumatic converter and the pneumatic positioner. It converts the electrical signal into the air pressure signal and then into the displacement of the actuator. It can make the valve reach the target opening according to the control requirements through the valve displacement feedback. According to the problems, it focuses on the inspection of the air chamber pipeline leakage, choke hole blockage, baffle position and finish, whether the solenoid coil is disconnected Whether the zero range adjustment is correct, whether the zero adjusting spring is deformed, whether the installation position of cam and feedback lever, and whether the positive and negative pole wiring are correct.

3.2 Actuator failure

The pneumatic actuator shall not only have sufficient thrust, but also have a small dead zone. It shall overcome various resistances such as the unbalanced force of the valve plug, the valve seat load required by the valve closure level, and the friction force of the packing and valve trim. Pneumatic actuators are divided into diaphragm type and piston type. The diaphragm type mainly consists of diaphragm chamber, diaphragm, spring, push rod, spring adjusting element, limiting device, handwheel mechanism and other parts. It is simple in structure and reliable in operation. The diaphragm type pneumatic control valve is usually installed on the small caliber pipeline requiring fast reaction rate. In case of failure, the diaphragm, sealing ring, fastening push plate, valve rod fastening bolt, diaphragm chamber bolt, spring and preload shall be checked Valve stem finish and bending, actuator working air pressure; The piston type is mainly composed of cylinder block, piston, sealing ring, valve stem, handwheel mechanism and other parts. It can control the opening and closing of the valve to provide a large stroke, provide sufficient closing force, and have a certain opening and closing speed. Compared with the membrane type, the actuator allows higher operating pressure, so the output thrust is large. It is generally installed on the pipeline with large caliber and sensitive response, but the price is also high, In case of failure, check whether the piston sealing ring, inner wall of cylinder block, plunger push rod are worn, the working air pressure of actuator, tightness of air chamber and valve stroke.

3.3 Valve body failure

Main inspection contents in case of failure: pressure resistance and corrosion resistance of the inner wall of the valve body; The valve seat is loose or damaged due to corrosion on the inner surface of the thread; Loose valve core caused by corrosion and wear; Aging and damage of diaphragm, O-ring and sealing gasket; Damage caused by aging and drying of sealing packing.