The Failures of the Valve & Its Cause Analysis (Part One)

Valve failures and cause analysis
1. Leakages
Leakages are mainly divided into external leakages and internal leakages. External leakages mainly refer to the fluid flowing to the outside of the system due to sealing failures and other reasons, and while the fluid flowing to the inside of the system is called internal leakage. The main reason for the leakage of the valve is the sealing failure. The valve of a 300MW steam turbine in a thermal power plant leaked. After conducting a full inspection, it was found that the valve seat was worn due to multiple opening and closing, which led to a decrease in the sealing performance of the valve, and a leak was caused by the valve not being closed well. In a low-temperature environment, as the temperature decreases, the tensile and compressive properties of the rubber will drop sharply and it will harden, which leads to a reduction in the pre-tightening force of the contact surface, resulting in leakages. Under high-temperature and high-pressure conditions, the valve body will be deformed, and the valve stem may fall and jam. Regardless of the working conditions, once the sealing of the valve fails and causes leakages, especially when high-temperature, high-pressure, corrosive, radioactive, flammable and explosive media are involved, accidents such as poisoning, fire, explosion, and personal injury will occur. Table 1 shows the common leakage locations and causes of the valve.
 
Table 1 Common leakage locations of valves and their reasons

 
The reasons for leakages of valves caused by sealing failure are as follows:

(1) Unreasonable design

It is found that the fatigue life of the steam drum continuous exhaust valve is prolonged with the decrease of load, the improvement of surface finish and good heat treatment after using ANSYS and MSC.Fatigue to simulate. The results obtained by the analysis were consistent with the basic common sense of engineering, which verified the feasibility of using analysis software to analyze the fatigue life of the continuous exhaust valve. Finite element analysis was performed for the bellows and a valve model with leakage was established. It was calculated that when the surface roughness Ra value was between 0.1 and 0.4μm, it not only met the requirement for the service life, but also solved the problem of the leakage of the valve.
 
(2) Unreasonable structure
The sealing structure of the slag lock valve is designed based on the working environment, sealing specific pressure and structural sizes, as shown in Figure 1. The finite element model was established for simulation, and the flow model and leakage model were used for analysis. The effective sealing interval of the design parameters and the valve seat leakage level under different parameters were obtained and verified. After analyzing the U-shaped sealing (Figure 2), it is found that the maximum creep strain occurred at the upper and lower arm spans of the U-shaped sealing and the root of the U-shaped groove, and there was no leakage.

 
Figure 1 The sealing structure of the slag lock valve
 
 
Figure 2 The U shape sealing structure
 
(3) Unreasonable selection of materials
A439D-2C austenitic ductile iron was trial-produced, and its mechanical properties and composition are shown in Table 2 and Table 3. The spheroidization rate of austenitic ductile iron is higher than 95%, and its mechanical properties and composition are better than those of the same type of cast iron in China; its plasticity index is increased by 3 times. After the material was applied to the double-sided sealing structure, the service life of the valve was significantly improved, and the manufacturing cost and difficulty were reduced. A cobalt-free iron-based alloy powder was researched and developed and was covered on the stainless steel substrate by the laser cladding technology. The hardness of the stainless steel substrate covered with the powder was significantly increased after testing, which was about twice the hardness of the uncovered substrate.
 
Table 2 Mechanical properties of austenitic ductile iron

                  
Table 3 The composition of austenitic ductile iron wt%

 
A large number of scholars have analyzed and improved the sealing structure of the valve from the perspectives of design, structure and materials, but there are still problems. First, the sealing is qualitatively analyzed under specific conditions and the calculation method for quantitative analysis can't be performed for most conditions. Second, sealing involves many microscopic in the working environment, and the principles and manifestation of microscopic phenomena caused by different influencing factors are not the same. Therefore, it is very important to study the sealing failure caused by multiple factors, which can provide directions and ways to further improve the sealing structure and improve the sealing performance.
 
2. Broken valve bodies
The main reasons for the breaking of the valve body are as follows:
(1) Improper selection of materials for valve bodies or material defects such as sand holes, bubbles, shrinkage holes, etc.
When a wafer butterfly valve is installed, if rubber gaskets or other gaskets are provided to increase the sealing surface of the valve body end, it will cause cracking and breaking in the valve body during use. The force, material, torque and other aspects of the valve body in this state were analyzed. It is believed that the valve body's material should be ductile iron, and no defects are allowed to ensure the normal use and safe operation of the valve body. The impact of casting defects on the strength of hydraulic valve bodies was analyzed. The ProCAST software was used to establish a casting defect model, and the stress values of the valve body in the ideal state and defective state under different pressures were compared. It is found that the valve body with shrinkage holes would produce more partial stress, as shown in Figure 3, that is, shrinkage would cause the partial effective wall thickness of the valve body to be reduced. Stress concentration would occur, which would lead to a decrease in the load-bearing capacity of the valve body and strength, and a significant decrease in fatigue and service life.
 
 
Figure 3 The curve of the greatest stress changing with the pressure