Analysis and solution of valve failure mode

In view of the safety accidents and adverse effects caused by valve failure, different failure modes and failure causes of valves are briefly described on the basis of valve failure types and failure analysis. Finally, the research and development trend of valve reliability are prospected.

General

The valve is the control component in the fluid conveying system, which has the functions of cutting off and dredging fluid, adjusting flow, dredging, check, stabilizing pressure, dispersing fluid, overflow and pressure relief, etc. There are many kinds and specifications of valves, including globe valves and ball valves with simple structure, as well as complex valves suitable for automatic control system.
With the expansion of production demand and the improvement of industrial automation technology, in order to ensure the smooth progress of production and avoid accidents, the failure analysis and solutions of valves have attracted more and more attention, especially in aerospace, nuclear power, thermal power, nuclear power, high temperature and high pressure and other fields. Valve plays an important role in industrial production, so it is very important to study the failure forms and causes of valve for practical applications such as stress, strength and fatigue.
At present, many scholars have done a lot of research on valve reliability analysis. One part is from individual to whole, that is, first conduct a complete analysis for the valve with specific environment and specific needs, study its failure form and principle, and then expand the overall field. The other part starts from the basic theory, first comprehensively designs and analyzes the valve reliability from different directions such as strength and sealing, and then carries out experimental research on specific valves according to different situations. Due to many types of valves and complex working conditions, there is no complete specification for design guidance. Therefore, based on the analysis of valve failure types, this paper briefly describes the causes of different failure modes of valves, and puts forward solutions from the perspective of material, design, test and manufacturing.

Failure mode and cause analysis

Leakage

Leakage is mainly divided into external leakage and internal leakage. External leakage mainly refers to the flow of fluid medium to the outside of the system due to sealing failure, otherwise it is called internal leakage. The main cause of valve leakage is seal failure. Wu Weizhi et al. found that the valve of 300MW steam turbine in thermal power plant had leakage. Through systematic inspection, it was found that the valve seat was worn due to multiple opening and closing, resulting in the decline of valve sealing performance and lax closing, resulting in leakage. In low temperature environment, with the decrease of temperature, the tensile and compressive properties of rubber sealing materials will decline sharply and harden, resulting in the reduction of preload on the contact surface and leakage. Under the condition of high temperature and high pressure, the valve will deform and the valve stem may fall and jam. Regardless of the working conditions, once the valve seal fails and causes leakage, especially involving high temperature and high pressure, corrosive, radioactive, flammable and explosive media, poisoning, fire, explosion, personal casualties and other accidents will occur. The common leakage parts and causes of valves are shown in Table 1.
Table 1 common valve leakage parts and causes

Leakage position

Sealing surface

Filler

Seal ring

Body and bonnet

Cause of leakage

① The surface of the sealing surface is uneven due to the manufacturing material or strong extrusion;
② The bending of the valve stem causes the assembly position to deviate;
③ Not selected as required.

① The packing is not selected as required or aged, resulting in the decline of sealing performance;
② The valve stem problem caused the packing to be damaged.

① Not selected according to the requirements, and the material has poor fatigue resistance;
② Corrosion and heat treatment problems.

① Bolts and fasteners are loose or have different tightness;
② Insufficient material strength or defects in processing technology.

The reasons for valve leakage due to seal failure are as follows.
(1) Unreasonable design. Huang Hanhui found through the joint simulation of ANSYS and MSC. Fatigue that the fatigue life of the steam drum continuous exhaust valve is prolonged with the reduction of load, the improvement of surface finish and good heat treatment. The analysis results are consistent with the basic engineering knowledge, which verifies the feasibility of using the analysis software to analyze the fatigue life of the continuous exhaust valve. Wu Zongping conducted finite element analysis on the bellows, established the valve leakage model, and calculated that when the surface roughness Ra value is 0.1 ~ 0.4 μ M, which not only meets the requirements of service life, but also solves the problem of valve leakage.
(2) Unreasonable structure. Xiao Dinghao designed the sealing structure of slag lock valve on the basis of working environment, sealing specific pressure and structural size, as shown in Figure 1. The finite element model is established for simulation, and the flow model and leakage model are used for analysis. The effective sealing interval of design parameters and the leakage grade of valve seat under different parameters are obtained and verified. After analyzing the U-shaped seal (Fig. 2), Mao Jianfeng found that the maximum creep strain occurred in the upper and lower arm span of the U-shaped seal and the root of the U-shaped groove, and there was no leakage.
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Fig.1 sealing structure of slag lock valve

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Fig.2 U-shaped seal structure

(3) Unreasonable material selection. Zhang Tao trial produced a439d-2c austenitic nodular cast iron. Its mechanical properties and composition are shown in Table 2 and table 3. The spheroidizing rate of austenitic nodular cast iron is higher than 95%, its mechanical properties and composition are better than those of the same domestic cast iron, and its plasticity index is increased by 3 times. After the material is applied to the double-sided sealing structure, the service life of the valve is significantly improved, and the manufacturing cost and manufacturing difficulty are reduced. Wei HONGPU et al. studied and developed a cobalt free iron-based alloy powder, which was covered on the stainless steel substrate by laser cladding technology. The results show that the hardness of the stainless steel matrix covered with the powder is significantly improved, which is about twice that of the uncovered matrix.
Table.2 mechanical properties of austenitic nodular cast iron

Number

Tensile stress at tensile strength / MPa

Elongation at break at non proportional elongation (standard) /%

Modulus of elasticity (chord cursor) / GPA

Tensile stress at yield (offset 0.2%) / MPa

Load at tensile strength / kN

1

507.236

31.602

93.145

259.286

9.255

2

464.776

21.091

109.142

235.615

9.126

3

478.537

33.006

95.900

235.504

9.396

Table.3 composition of austenitic nodular cast iron wt%

Technical standard

C

Si

Mn

P

Cr

Ni

ASTM

≤2.90

1.00~3.00

1.80~2.40

≤0.08

≤0.05

21.00~24.00

A large number of scholars have analyzed and improved the sealing structure of the valve from the aspects of design, structure and materials, but there are still problems. Firstly, the sealing is analyzed qualitatively under specific conditions, and the calculation method of quantitative analysis under most conditions can not be given. Secondly, sealing involves many micro phenomena in the working environment, and the principle and performance of micro phenomena caused by different influencing factors are different. Therefore, it is very important to study the seal failure caused by many factors and the superposition of different factors, which can provide the direction and way to further improve the seal structure and seal performance.

Valve body rupture

The main causes of valve body rupture are as follows.
(1) Improper selection of valve body material or material defects, such as sand holes, bubbles, shrinkage cavities, etc.
When installing wafer butterfly valve, if rubber gasket or other gaskets are added to increase the sealing surface at the end of the valve body, cracks and cracks will occur during the use of the valve body. Zou Shengwu analyzed the stress, material, torque and other aspects of the valve body in this state, and believed that the valve body material should be nodular cast iron without defects, so as to ensure the normal use and safe operation of the valve body. Su MALIANG analyzed the influence of casting defects on the strength of hydraulic valve body, established the casting defect model by using ProCAST software, compared the stress values of valve body in ideal state and defective state under different pressures, and found that the valve body with shrinkage defects will produce greater local stress, as shown in figure 3. That is, shrinkage will reduce the local effective wall thickness of the valve body, produce stress concentration, reduce the bearing capacity, strength, fatigue and service life of the valve body.
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Fig.3 variation curve of maximum stress with pressure

(2) External environmental factors cause cracks in the valve body.
The external environment such as low temperature, high temperature and high pressure will cause creep, fatigue and impact of the valve body, which will lead to the rupture of the valve body. It is mentioned in the literature that the main reason for the rupture of the valve body of the coke oven riser is due to the combined corrosion of high temperature and other working conditions for a long time, resulting in deformation, hole, jam and so on. Ma Wenju found that the valve body of the check valve at the end outlet of the mud lift pump is cracked due to repeated forward and reverse water hammer impact of the fluid with a large amount of sediment and a 30m drop, which greatly destroys the strength and reduces the service life of the valve body.
(3) Strong vibration, resonance or violent installation make the valve bear uneven force and produce cracks.
In the process of use, the valve body with cracks due to vibration and other factors will be impacted by the medium, which will prolong the cracks and eventually lead to the damage of the valve body. Niu Chuangui investigated the impact of vibration and noise on the main steam isolation valve of the nuclear power plant, and found that the steam flow rate exceeded the recommended 40 ~ 60m / s to 87m / s due to diameter reduction, resulting in sound noise, forming an acoustic resonance cavity, causing fretting fatigue damage to the valve (Fig. 4). At the same time, cavitation also accelerated the damage speed of the valve. Zhao Yanan mentioned that high cycle fatigue vibration and low cycle fatigue vibration will reduce the service life of the valve and cause certain damage to the power generation system, pipelines and instruments.
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Fig.4 Relationship between fretting fatigue caused by noise and ordinary fatigue strength curve

Jamming

(1) Deformation and bending due to material selection and stress superposition, or valve action is stuck due to physical reasons such as excessive and tight packing. Jiang Xiaohong et al. found that the valve failed many times during the fatigue life test of the valve. Through the disassembly of the valve, it was found that the main reasons were: the material strength of the valve rod was not enough, the thread was seriously damaged, and the “bite” between the valve rod and the valve rod nut; The surface finish of parts does not meet the requirements; Due to the thermal stress, the bolt pair is welded, which leads to the failure of the valve fatigue life test. Zhang Wenbin et al. found that the opening and closing pressure of the valve in the stuck state is higher than that in the normal state, and the opening and closing characteristic curves of the valve are shown in Fig. 5 and Fig. 6. After disassembly and assembly of the valve, it is known that the large distance error between the rams will also lead to valve jamming. The disassembly and inspection data are shown in Table 3.
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Fig.5 valve opening and closing characteristic curve under stuck state

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Fig.6 valve opening and closing characteristic curve under normal state

Table.3 Comparison of disassembly and inspection data mm

Feature size name

a

b

L

Nominal value

Measured value

Deviation

Nominal value

Measured value

Deviation

Nominal value

Measured value

Deviation

Ram

44.50

44.60

0.10

0

0.35

0.35

92.50

93.25

0.75

Ram

44.50

44.70

0.20

3.50

3.60

0.10

Deviation

0.30

0.45

0.75

(2) The valve is stuck due to pollution or chemical corrosion caused by the working environment. Wei Guojian et al. mentioned that the liquid oxygen control valve in the main engine of the space shuttle was stuck due to stem corrosion. Xu Guoqiang et al. found that due to the lack of new oil film coverage and replacement on the surface of the distributor slide valve, the remaining oil film deteriorated and cemented under the action of the hot air of the transmission box for a long time, resulting in the corrosion of the valve rod and the stagnation of the valve.

Valve vibration and noise

In the actual production process, the vibration and noise of the valve usually accompany each other. Medium vibration, unstable flow and vortex caused by diameter reduction will lead to vibration of the valve, reduce the control performance, affect the service life of the valve, and even cause accidents such as valve leakage.
The main causes of valve vibration and noise are as follows.
(1) Mechanical vibration.
Unreasonable number, spacing and layout of process pipes, elbows and valves will lead to vibration and pressure pulsation of medium fluid in the pipeline (Fig. 7). Resonance between external environment and medium fluid will also lead to valve vibration. 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 increases greatly, resulting in the vibration of the valve disc, resulting in the vibration of the valve disc. When studying pipeline vibration, Liu Li et al. found that fluid pulsation is the main cause of pipeline vibration. Qiao et al. studied the dynamics of cantilever pipeline under different flow rates and concluded that when the medium velocity exceeds the critical velocity, the pipeline will vibrate periodically and the valve will vibrate accordingly.
(2) Cavitation vibration.
Cavitation vibration mainly refers to the phenomenon that occurs when the medium is liquid (Fig. 8). When the fluid flow rate increases, the pressure decreases and is lower than the saturated vapor pressure, the liquid will produce bubbles, and when the saturated vapor pressure recovers, the bubbles will burst, resulting in valve vibration with the repeated generation and rupture of bubbles. Ma qiaochun et al. confirmed through theoretical analysis and diagnostic experiments that the vibration of boiler reheater control valve is mainly caused by serious cavitation caused by valve deviation from design.
(3) Hydrodynamic vibration.
Large medium flow, unstable flow and diameter reduction will lead to vortex. Due to random occurrence probability, complex vortex calculation method and uncertain energy, resonance will occur when the fluid frequency is consistent with the valve frequency. Xu Dengwei mentioned that although it is very complicated to calculate the vortex frequency, subjectively, the vortex will fall off at a specific frequency. When the falling off frequency is consistent with the natural frequency of the valve, it will lead to resonance and noise. Xu Zheng et al. used fluent combined with three-dimensional modeling and finite element method to simulate and analyze the flow channel of main steam isolation valve, studied the flow characteristics of three-dimensional turbulent flow field based on time homogenization flow field calculation and large eddy simulation, and found out the flow field inducement causing vibration and noise.
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Fig.7 vibration caused by pressure pulsation

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Fig.8 vibration caused by cavitation

From the main failure forms of valve body fracture, jamming, vibration and noise, most of them are caused by vibration except for design and material factors. Although a large number of studies on valve vibration have been carried out at home and abroad, most studies only analyze and calculate the failure of specific valves, and do not form a unified method or criterion. Moreover, due to the complex fluid flow, complex calculation of vortex energy and frequency, random probability of resonance phenomenon, environmental corrosion and damage and other factors, It is difficult to fundamentally solve the vibration problem.

Resolvent

Several important forms of valve failure can be solved from the aspects of material, design, test and production.

Materials

Material and its stress, strength and corrosion resistance in practical application are the key factors to solve the problem of valve failure and improve the reliability of valve.

  • ① Advanced technology is used to develop composites.
  • ② Heat treatment of valve fittings and components to improve their performance and internal metal structure of materials, so as to improve the corrosion resistance and strength of valves.
  • ③ Advanced surface technologies such as spray welding and cladding are adopted to improve the wear resistance and corrosion resistance of parts.

Design

The strength calculation and structural design of the product are carried out by using advanced dynamic simulation, parameter method technology and finite element analysis. In addition to special process requirements, the influence of environmental factors, such as chemical corrosion, thermal stress corrosion, resonance, frequency, pressure fluctuation, etc., shall also be considered. Stress limit calculation and fatigue performance analysis related to system operation and working condition conversion shall be carried out.

Test and production

According to the requirements of valve characteristics and actual functions, a test platform suitable for valve working environment is simulated and established. Conduct valve action and performance tests on some key valves under simulated actual working conditions to master their reliability under actual operating environment conditions. Improve the valve manufacturing process and manufacturing equipment to avoid material defects, strength reduction, dimensional deviation and other problems caused by insufficient manufacturing level. Zhang Wenbin solved the problem of valve sticking by correcting the distance between valve rams.

Conclusion

  • (1) Through the summary of common valve failure forms, the causes and mechanism are analyzed.
  • (2) The R & D investment of new materials or composite high-strength materials shall be increased, the strength calculation and structural design of products shall be carried out by using advanced design means and methods, and the process level and equipment manufacturing level shall be improved to improve the quality of valves.
  • (3) Establish valve reliability database and reliability analysis system to find and eliminate the factors affecting reliability as soon as possible in the early design stage.

Author: HU Yi-jun

Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Stainless Steel Valves, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@sfutube.com

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