How to solve the problem of control valve leakage?

The regulating valve is the main type of actuator, by accepting the control signal output by the regulating control unit, with the help of dynamic operation to change the fluid flow. Control valves are generally composed of actuators and valves. If the power used by the actuator is used, the control valve can be divided into three kinds: pneumatic, electric and hydraulic. Here we talk about several common failures and solutions of control valves.

In the modern factory automation control system, the control valve, as the terminal executive device of the automatic regulation system, accepts the control signal to realize the regulation of the chemical process. Its operating sensitivity is directly related to the quality of the regulating system. According to the actual statistics of the site, about 70% of the faults come from the control valve. Therefore, summarize and analyze the factors affecting the safe operation of the control valve and its countermeasures in daily maintenance.

The following is the analysis of several common leakage fault phenomena, and put forward countermeasures.

Valve leakage
Valve stem length is not appropriate. If the valve stem is too long, the upward (or downward) distance of the valve stem is not enough, resulting in a gap between the valve core and the seat, which cannot be fully contacted, resulting in loose closing and internal leakage. Similarly, the valve stem of the gas-closed valve is too short, resulting in a gap between the spool and the seat, which cannot be fully contacted, resulting in loose closing and internal leakage.
The solution:
The control valve stem should be shortened (or extended) so that the control valve length is appropriate, so that it is no longer internal leakage.

Packing leakage
After packing into the stuffing box, axial pressure is applied to it through the gland. Due to the plasticity of the packing, it produces a radial force and is in close contact with the valve stem, but this contact is not very uniform. Some parts of the contact is loose, some parts of the contact is tight, and even some parts are not in contact. During the use of the control valve, there is a relative movement between the valve stem and the packing, which is called axial movement. In the process of use, with the influence of high temperature, high pressure and strong permeability of the fluid medium, the control valve packing box is also a part of the leakage phenomenon. The main cause of packing leakage is interface leakage, and there will be leakage for textile packing (pressure medium leaks outward along the small gap between the packing fibers). The interface leakage between the valve stem and the packing is caused by the gradual decay of the contact pressure of the packing, the aging of the packing itself, and so on. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem.
Solution:
In order to make the stuffing easy to load, chamfer at the top of the stuffing box, and place a metal protection ring with a small clearance against erosion at the bottom of the stuffing box (the contact surface with the stuffing can not be inclined) to prevent the stuffing from being pushed out by the medium pressure. The metal surface of each part of the packing box and the contact part of the packing should be finished to improve the surface finish and reduce the wear of the packing. The filler is flexible graphite, because of its good air tightness, small friction, small change after long-term use, small wear and burn loss, easy maintenance, the friction force does not change after the cap bolt is re-tightened, good pressure and heat resistance, not by the erosion of the internal medium, and the metal in contact with the valve stem and stuffing box does not pitting or corrosion. In this way, the sealing of the valve stem stuffing box is effectively protected, and the reliability and long-term nature of the sealing of the stuffing is ensured.

The spool and seat are deformed and leaking
The main cause of spool and seat leakage is the strengthening of corrosion due to casting or forging defects in the control valve production process. The passage of corrosive media and the erosion of fluid media can also cause leakage of the control valve. Corrosion is mainly in the form of erosion or cavitation. When the corrosive medium passes through the control valve, it will produce erosion and impact on the spool and seat materials to make the spool and seat oval or other shapes, and over time, lead to the spool and seat are not matched, there is a gap, and the leakage occurs when the valve is not tightly closed.
The solution:
The key is the selection and quality of the material of the spool and valve seat. Choose corrosion-resistant materials, and resolutely eliminate products with pitting, trachoma and other defects. If the spool and seat deformation is not too serious, it can be grinded with fine sandpaper to eliminate traces and improve the sealing finish to improve the sealing performance. If the damage is serious, it should be replaced with a new valve.