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

(2) Cracks in the valve body due to external environmental factors
The external environment such as low temperature, high temperature and high pressure will cause the valve body to undergo creep, fatigue and impact, which will cause the valve body to break. Some literature mentions that the reason for the breaking of the valve body of the coke oven ascending tube is mainly the long-term compound corrosion of high temperature and other working conditions, resulting in deformation, holes, jams and others. It is found that the check valve body of the muddy water lift pump end's outlet was cracked due to repeated forward and reverse water hammer impacts from a fluid containing a large amount of sediment and a drop of 30 m. The strength of the check valve is greatly damaged, reducing the service life of the valve body.
 
(3) Uneven force on the valve caused by strong vibration, resonance or violent installation
During use, the valve body that cracked due to vibration and other factors is impacted by the medium, which will extend the cracks and ultimately lead to the damage of the valve body. The impact of vibration and noise on the main steam isolation valve of nuclear power plants was investigated, and it is found that the reducer caused the steam flow rate to exceed the recommended 40 to 60m/s, reaching 87m/s, resulting in acoustic noise, forming an acoustic resonance cavity, and producing fretting fatigue damage for the valve (Figure 4). Meanwhile,  cavitation also accelerated the valve's damage rate. High-cycle fatigue vibration and low-cycle fatigue vibration will shorten the service life of the valve, and cause certain damage to the power generation system, pipelines, and instruments.
 


3. Jamming
(1) Deformation and bending were caused by the selection of the material and stress superposition, or the valve was stuck due to physical reasons such as too much or too tight packing. When the fatigue life test of the valve was performed, failures occurs many times. After disassembling the valve, the main reasons were that the stem material was not strong enough and the thread was severely damaged; the stem and stem nut were seized; the surface finish of the parts did not meet the requirements; melting welding of the bolt pair occurs due to thermal stress, and the fatigue life test of the valve couldn't be carried out normally. It is found that the valve's opening and closing pressure in the stuck state is higher than that in the normal state after comparing the hydraulic gate valve under normal state and stuck state. The characteristic curves of the valve opening and closing are shown in Figure 5 and Figure 6. After disassembling and assembling the valve, we knew that the great distance error between the valve plates will also cause the valve to jam. The disassembly and inspection data are shown in Table 3.
 
 
Figure 5 Characteristic curves of the valve opening and closing in a jamming state


Figure 6 Characteristic curves of the valve opening and closing in a normal state
                                                    
Table 3 The comparison of disassembly and inspection data mm

 
(2) The valve is stuck due to pollution or chemical corrosion caused by the working environment. The liquid oxygen regulating valve in the main engine of the space shuttle was stuck due to the corrosion of the valve stem. It is found that due to replacement and lack of new oil film on the surface of the distributor slide valve, the remaining oil film was deteriorated and glued under the action of hot air in the transmission box for a long time, causing the valve stem to rust and the valve to jam.
 
4. Vibration and noise of valves 
In the actual production process, vibration and noise of valves are usually accompanied by each other. Medium vibration, unstable flow, and vortex caused by reduced diameter will cause vibration of valves, reduce control performance, affect the service life, and even cause leakage and other accidents of valves. The main causes of the vibration and noise of valves are as follows:
 
(1) Mechanical vibration
The unreasonable number and spacing of process pipelines, elbows and valves will cause vibration and pressure pulsation in the pipeline (Figure 7). The resonance of the external environment and the medium fluid will also cause the valve to vibrate. The reason for the mechanical vibration of the valve also includes the vibration of the valve disc. When the medium flows through the valve disc, the pressure difference before and after the inflow was greatly increased, causing the valve disc to vibrate, thereby causing the valve to vibrate. When the research was conducted for the vibration of the pipeline, it is found that the fluid pulsation is the main cause of the pipeline's vibration. Another research was performed for dynamics of cantilever pipes at different flow rates and it is found that when the medium velocity exceeds the critical velocity, the pipe would periodically vibrate and the valve would also vibrate.
 
(2) Cavitation vibration
Cavitation vibration mainly occurs when the medium is liquid (Figure 8). When the fluid flow rate increases. The pressure decreases and is lower than the saturated vapor pressure, the liquid will generate bubbles. When the saturated vapor pressure is restored, the bubbles will burst. As the bubbles repeatedly generate and burst, the valve will vibrate. The vibration of the boiler reheater regulating valve was mainly caused by severe cavitation caused by the deviation of the valve design after theoretical analysis and diagnostic experiments.
 
(3) Fluid dynamics vibration
Large medium flow, unstable flow, reduced diameters and other reasons can cause vortex to occur. The occurrence probability is random. The calculation method for the complicated vortex is complicated. The energy is uncertain. Resonance will occur when the fluid frequency is the same as the valve frequency. The calculation of the vortex frequency is very complicated, but subjectively, the vortex will fall  at specific frequency. When the falling frequency is consistent with the natural frequency of the valve, it will cause resonance and noise. FLUENT combined with three-dimensional modeling and finite element method to simulate and analyze the flow passage of the main steam isolation valve. According to time-averaged flow field calculation and large eddy simulation, the flow characteristics of the three-dimensional turbulence field were studied and the flow field inducement of vibration and noise was found.

 
Figure 7 The vibration caused by the pressure pulsation


Figure 8 The vibration caused by the cavitation
 
From the perspective of the main failure modes of valve bodies such as breaking, jamming, vibration and noise, most of them are caused by vibration, except for design and material factors. Regarding vibration of valves, although a large number of studies have been carried out at home and abroad, most of the studies only analyze and calculate the failure of specific valves, without forming a unified method or criterion. It is difficult to fundamentally solve the vibration of the valve due to the complex fluid flow, complex calculation of vortex energy and frequency, random probability of occurrence of resonance, and factors such as environmental corrosion and destruction.