Cause analysis and preventive measures for EH oil pipeline fracture of steam turbine in power plant
There are 6 coal-fired power units in a power plant, including 600MW 2 units and 300MW 4 units. The steam turbines are all produced by the same domestic steam turbine plant. The digital electro-hydraulic control system (DEH) and high-pressure fire resistant oil (EH oil) are used for hydraulic control. The main steam valve, control valve, bypass system and steam distribution mechanism of feed pump steam turbine are controlled at 14.5 About MPa, it has the advantages of fire prevention, sensitivity and high precision. EH oil system is unstable for many times in operation, which is mainly manifested in cracks or even fractures of EH oil pipeline joints or welds, which seriously affects the safe and economic operation of the unit.
Overview of current situation
From 2015 to 2017, the failure statistics of EH oil pipeline of 6 units are shown in Table 1. From the data in Table 1, EH oil pipeline fault mainly occurs at the joint position of control block and oil pipe of regulating valve hydraulic servo motor, then at the weld position of oil pipe, and then at the position of oil pipe passing through the insulation layer.
Table.1 failure statistics of EH oil pipeline of 6 units from 2015 to 2017
|Site of occurrence||Fault phenomenon||Frequency|
|High pressure regulating valve union weld||Cracks, including 3 casing welds||5|
|Intermediate pressure regulating valve union weld||Fracture once, crack once||2|
|Insulation of oil inlet pipe behind high pressure regulating valve||Crackle||2|
|Medium pressure connecting pipe butterfly valve hydraulic motor Union||Crackle||2|
|Other locations||Welding trachoma||1|
The pipe fitting butt joint mode is unreasonable
In the early stage of EH oil system installation in China, most of the pipe fittings such as tee and right angle elbow are in the form of casing. This structure is convenient for pipe fitting installation. When welding, it is easy to weld from the outside, and the welding dust and slag are not easy to enter the inside of the welding junction, which is conducive to pipe cleaning. However, in recent years, the failure statistics of tubing fracture and crack show that the position failure of this structure accounts for the most. From the section of the broken pipe, the fracture occurs at the step of casing processing at the root of the Union; from the metal detection of the broken part, the section is a typical brittle fracture, the specific reasons are as follows.
- (1) The EH oil pipeline installation and welding process provided by the manufacturer has no clear regulations on the welding of plug-in casing, which results in most stainless steel pipes being inserted into the inner part of the casing directly to the top of the step during the field installation without any clearance. In this way, in normal operation, the pipe sleeve shrinks after being heated, which may cause the part inserted into the pipe sleeve to produce an extrusion stress on the step of the pipe sleeve.
- (2) There is a 90 ° processing step inside the pipe sleeve, which is an obvious stress concentration area, especially the processing difficulty of the 14 mm pipe sleeve. In the process of processing, it is easy to cause obvious scratches due to tool withdrawal, which increases the possibility of stress concentration. Therefore, the step of the pipe sleeve is the weakest area to bear stress concentration. During the operation of the system, the temperature of each oil pipe of the regulating valve is about 60 ℃, the thermal expansion of the pipe increases the extrusion stress at the internal step of the casing, and the impact of external vibration results in the internal crack of the casing at the step.
Poor pipeline welding process
In order to keep EH oil clean, most of the oil pipelines are made of stainless steel pipes, most of which are 1Cr18Ni9Ti. The pipes are made of a variety of specifications, such as〝57mm × 3.0mm, 〝25mm × 2.5mm, 〝32mm × 3.0mm, 〝14mm × 2.0mm and so on. Multiple argon arc welding is used for double-sided argon protection. Due to its high purity and low carbon content, the stainless steel does not have the welding hot cracking tendency of pure austenitic steel, but the Nb bearing steel has a relatively high welding hot cracking tendency, and some problems in the actual welding process lead to poor welding quality. For example: the specification of welding material was changed, but the welding current was not adjusted in time, resulting in excessive welding current; the double-sided argon protection was too cumbersome and the technological process was artificially reduced; the interlayer temperature of the weld was not well controlled, resulting in the intergranular corrosion crack caused by the carbon accumulation under the high temperature; the fillet weld pool used due to the pipe diameter difference during the casing welding was less, which affected the welding strength.
Fatigue failure and fracture of pipeline caused by vibration
Most of the pipelines had long-time, high frequency and large amplitude vibration before crack and fracture, which caused great stress change at the root of the union. The tubing was like doing long-time fatigue failure test. However, due to the influence of welding stress and structure, the heat affected zone is the weakest zone of the whole pipeline, which makes the structure of the base metal of the pipeline and the live joint hard and brittle. Therefore, when the control valve shakes, cracks are most likely to appear near the root of the weld.
Vibration of steam valve body implicates pipeline vibration
Due to the load change of the unit, the steam pressure and the opening of the regulating valve are changing in real time, the shell of the regulating valve is vibrating constantly, and the hydraulic servo motor is fixed on the valve body of the regulating valve, resulting in the implicated vibration of the oil pipe of the hydraulic engine, which makes the oil pipe and the movable joint bear the greater alternating stress. The connection between the hydraulic motor of the intermediate pressure governing valve and the regulating valve adopts a straight pull structure. The vibration is not buffered and directly transmitted to the hydraulic motor and the oil pipe. The hydraulic motor of the intermediate pressure hydraulic motor and the regulating valve of the connecting pipe of the intermediate and low pressure cylinder has a high layout position, long oil pipe, and the relative rigidity of the pipe support is insufficient. It is easy to vibrate together with the steam valve body, resulting in the vibration of the oil pipe and finally the generation of cracks.
Severe vibration of pipeline caused by servo valve failure
Zero zone characteristic difference of servo valve
The output flow of servo valve is not zero when the input current is zero due to the influence of the structure size, electromagnetic performance, hydrodynamics characteristics and assembly of the components. In order to make the output flow zero, an input current must be pre added to make the input current required for the servo valve at the zero position and the percentage of the rated current, which is called the zero bias of servo valve. The zero deviation of the servo valve has a great influence on the zero zone characteristics of the servo valve. When the zero deviation increases to a certain extent, the servo valve will quickly fail, and the servo valve will gradually generate vibration, the system will oscillate, and the system frequency response will greatly reduce. In serious cases, the pipeline will also have severe vibration, which will lead to the fracture of the pipeline welded joint.
Poor resolution and low sensitivity of servo valve
The so-called resolution of servo valve is the percentage of the minimum change value of input current and rated current required to change the output flow of servo valve, also known as insensitivity. The poor resolution easily causes the swing and overshoot of the system, which makes the servo valve unable to respond to the command of the control system quickly. The slow adjustment causes the system to shake and the pipeline to vibrate violently, which causes the system pipeline to crack or even break at the weak point.
Servo valve is blocked and valve core is worn
The blockage of the nozzle, orifice and filter element of the servo valve causes the increase of the system’s zero deviation, the decrease of the system’s frequency response, and the frequent adjustment of the servo valve. Meanwhile, the abrasion of the edge and the valve core of the servo valve will also cause the changes of the servo valve’s zero deviation, gain and other indicators, resulting in the decline of the servo valve’s performance and the vibration of the pipeline.
Pipeline vibration caused by signal
Because the signal cable of the servo valve coil is not well shielded, the signal interference is caused, the servo valve is in the state of frequent adjustment, resulting in severe vibration of the pipeline. The beat phenomenon caused by two linear variable differential transformers (LVDTs) will also cause the servo valve to adjust frequently, and the impact of oil flow will make the pipeline vibrate. The medium pressure valve adopts the direct pull mode of hydraulic servo motor, and the fixed point design of LVDT is unreasonable. In consideration of the neutral problem between hydraulic servo motor and regulating valve rod, the connecting block is used for the connection between hydraulic servo motor and regulating valve rod, the piston rod and connecting block of hydraulic servo motor are loose connection, and the regulating valve rod and connecting block are rigidly connected by two half threads. In this way, there is a problem. LVDT is fixed on the connection block, so the signal reflected by LVDT is mixed with the fluctuation signal generated by the connection block due to looseness, which leads to the control valve shaking and oil pipe impact. At the same time, the performance of the VCC card affects the stability of the output signal. If the performance of the VCC card is poor, the signal of the input servo valve will fluctuate, and the servo valve will frequently participate in the adjustment, resulting in system shaking and pipeline vibration.
External and internal corrosion
Because EH oil pipe is austenitic stainless steel, it is particularly sensitive to chloride ions, and its material is not resistant to intergranular corrosion. Due to the high temperature and other effects during the operation of EH oil, higher ambient temperature will aggravate EH oil cracking, resulting in increased acid value, reduced resistivity, and internal corrosion of the pipeline. Due to the narrow layout space and poor construction technology, some pipelines are contacted by thermal insulation and thermal insulation mud, and the thermal insulation mud contains acid composition, which causes external corrosion of the pipeline and cracks of the pipeline. Some pipe supports are not fixed properly. The contact between stainless steel and common carbon steel supports results in the precipitation of carbon elements, which will also lead to the increase of pipe corrosion.
Measures to prevent pipeline fracture
Strengthen pipeline installation and welding process management
The bending radius r shall be increased as much as possible to reduce the excessive bends. The pipe bending and the whole butt joint procedure shall be free of hot working. The bending die shall be suitable and the process shall be reasonable to minimize the damage to the pipe wall. Increasing the bend radius can effectively reduce scour.
When butt welding is adopted, the gap between two pipe orifices shall be 0.5-1.0mm, and the included angle of groove formation shall be 60 ° – 70 ° (as shown in Figure 1).
Figure.1 Included angle of groove
Welding joint and pipe sleeve processing
For the welded junction with plug-in welding, it is not allowed to insert the small pipe into the root of the outer pipe, and there must be an expansion gap of 1.0-1.5mm (as shown in Fig. 2) to prevent the pipe from squeezing due to expansion after welding. When processing the pipe sleeve, the steps shall be arc transition as far as possible to reduce the large stress concentration area, and the pipe sleeve shall be appropriately thickened.
Fig.2 Expansion gap
Stainless steel tack welding
Due to the large thermal expansion coefficient of stainless steel, it is easy to produce large welding stress during welding, so strict tack welding is required during assembly. For the pipe with D ≤ 56 mm, two-point positioning is adopted, and the length of positioning weld is 3-5 mm.
Welding technical requirements
- (1) In manual arc welding, the welding machine adopts direct current reverse connection, while in argon arc welding, direct current positive connection is adopted. In argon arc welding, the tungsten electrode is cerium tungsten electrode, with specification of 2.5mm.
- (2) Before welding, the oxide skin on the surface of the welding wire shall be removed with abrasive cloth and cleaned with acetone; the welding rod shall be dried in the oven at 300-350 ℃ for 4 hours and used as needed.
- (3) Before welding, clean the oil stain within 25 mm on both sides of the pipe groove, and use acetone to clean the 25 mm range on both sides of the groove.
- (4) When argon arc welding stainless steel pipe fittings, the back must be filled with argon for protection to ensure the back forming. The flow rate of argon filling inside the pipeline is 5-14 L / min, and that of the front is 12-13 L / min.
- (5) During backing welding, the thickness of the weld shall be as thin as possible and well fused with the root. During arc closing, the weld shall be in a gentle slope shape. If there are arc closing shrinkage holes, they shall be ground off with a grinder. The arc must be started and extinguished in the groove. When the arc is extinguished, the crater shall be filled to prevent the crater from cracking.
- (6) Because the stainless steel is austenitic stainless steel, in order to prevent carbide precipitation sensitization and intergranular corrosion, the interlayer temperature and cooling rate after welding shall be strictly controlled, and the interlayer temperature shall be controlled below 60 ℃ during welding, and the cooling shall be conducted immediately after welding (argon can be used for cooling), and segmented welding shall be adopted at the same time.
- (1) Through appearance inspection, there are no defects such as porosity, weld beading, depression and undercut, and the forming is good.
- (2) Tensile and bending tests were carried out on the specimens, and all the mechanical properties met the requirements, and no defects such as lack of fusion and cracks were found.
Post weld heat treatment
After heating to 850-950 ℃, it can be quickly cooled, and then it can be quickly quenched by pure water atomization after welding.
Post weld flaw detection and pressure test
100% RT is required for each weld after welding, and pressure test shall be carried out after putting into operation.
Strengthen equipment inspection and maintenance to reduce pipeline vibration
Strengthen equipment operation inspection
During operation, equipment inspection shall be strengthened, operation condition (shaking) and parameters (zero deviation, s value, etc.) of servo valve shall be checked in time, filter element in the system shall be replaced regularly, and servo valve shall be inspected. If servo valve has fault, replacement shall be conducted in time to reduce pipeline vibration caused by servo valve fault.
Strengthen inspection and maintenance of pipe support
Strengthen the inspection and maintenance of the pipe support, eliminate the vibration source of the pipe, formulate a reasonable plan to strengthen the support, add rubber damping sleeve to some pipes with large vibration, slow down the vibration amplitude and frequency, or disperse the stress to prevent concentration in the weak areas such as welded junction or pipe sleeve.
Strengthen the oil quality management to reduce the corrosion to the pipeline
Strengthen EH oil inspection and management
Strengthen the inspection and management of EH oil, so that the particle level of EH oil is controlled within the scope of NAS level 5, and the untested EH oil is not allowed to be added into DEH regardless of whether it is new or old. EH oil in operation shall be tested and treated regularly for physical and chemical indexes such as particle size, acid value and moisture.
Improve the working environment of the hydraulic servo motor
The needled blanket with good fire resistance and large heat insulation coefficient is used as the heat preservation medium for the rear part of the hydraulic servo motor of the high-pressure regulating valve to insulate the oil pipe and the hydraulic servo motor to prevent local overheating. The installation base of the hydraulic servo motor is changed from the inner package of the original heat preservation material to the outer package in the air to strengthen the heat dissipation. For the newly reformed unit, EH oil pipeline shall be arranged reasonably to prevent local overheating caused by heat radiation and heat conduction.
For the overheating problem of hydraulic cylinder of high pressure regulating valve, the method of installing cooling water jacket on the upper part of hydraulic cylinder can be used to reduce the temperature of hydraulic cylinder of hydraulic motor and the local overheating of EH oil. At the same time, improve the installation base of the hydraulic servo motor, change the original full plate contact to the rib plate contact, reduce the heat conduction area, increase the ventilation, effectively reduce the local overheating effect of the hydraulic servo motor and EH oil, reduce the acid value of EH oil, and reduce the corrosion to the oil pipe and servo valve.
Install anti trampling device
No matter in the process of patrol inspection or maintenance, the staff sometimes accidentally step on EH oil pipe, resulting in cracks or fractures at the root of the pipe. In order to eliminate this situation, it is necessary to install anti trampling device above the pipeline where trampling is easy to occur.
EH oil system, as the actuator of turbine control system, whether the pipeline is safe seriously affects the safety of personnel, equipment and the stable operation of the unit. According to the faults of EH oil system pipeline in recent years, most of them are caused by pipeline installation and welding process and pipeline vibration. Generally, the pipeline starts from small vibration to severe vibration, from small crack to pipeline fracture. After the improvement of pipeline installation process and welding process, the measures to prevent pipeline vibration are taken to prevent pipeline crack and fracture failure, and ensure the long-term and safe operation of the unit.
Source: China Oil Pipeline Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, 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.)
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