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Analysis of the Causes of Sticking in ZZYP Gas Control Valves and Corresponding Improvements
Release Date:
2020-12-11
1. Introduction Rizhao Iron & Steel Co., Ltd.’s Power Plant Gas Comprehensive Utilization Power Generation Workshop currently operates eight 50 MW blast furnace gas-fired generating units, with a total installed capacity of 400 MW. It is currently the largest blast furnace gas–based cogeneration plant among domestic metallurgical enterprises, supported by eight 220 t/h boilers whose fuel is the surplus blast furnace gas generated during ironmaking. 2. Causes Analysis Based on years of experience in operating these regulating valves and thorough research, we have identified the following primary causes of valve sticking: 2.1 Improper Installation Method Most valves are currently installed vertically, with the valve disc axis perpendicular to the horizontal plane. While this arrangement ensures that the actuator is located at the top of the gas pipeline, the rear bearing of the valve is positioned at the bottom, where moisture and impurities in the gas tend to settle or accumulate over time. This buildup gradually fills the clearance between the bearing and the shaft, leading to corrosion and eventual jamming of the valve shaft. 2.2 Unreasonable Design Structure Repeated disassembly and maintenance have revealed that the clearance between the valve stem bushing and the valve body is often filled with foreign matter such as sludge, water, and rust deposits. Moreover, the original design features an excessively long bushing, causing the front and rear bushings to bear the frictional forces generated by the reciprocating motion of the valve stem. Over time, dust in the gas and rust particles from the pipeline easily enter these clearances, increasing rotational resistance. The accumulated debris eventually fills the gaps, effectively welding the valve stem, bushing, and valve body together and severely restricting the valve’s ability to move freely. 2.3 Gas Corrosion Currently, all blast furnace gas pipelines in domestic steel plants suffer from severe corrosion. The most noticeable manifestation is frequent pitting and cracking of expansion joints, resulting in gas leaks along the pipeline. Corroded valves and pressure-reducing devices also fail to operate properly. The corrosive nature of blast furnace gas poses serious safety risks to both equipment operation and the integrity of the gas pipeline network. Our own gas pipelines likewise exhibit localized corrosion, including gas leakage through perforations and dripping condensate from expansion joint bellows. These conditions clearly have a significant impact on the valves. To analyze the extent of gas corrosion, we collected gas samples from two separate gas pipelines in the power generation workshop for laboratory analysis. Table 1: Composition Analysis of Blast Furnace Gas (%VOL) High-temperature components such as H2, CO, and H2O also exert considerable corrosive effects on steel. Given the high water content in the gas—particularly since the power generation system is typically located at the downstream end of the gas network—the pipeline often contains substantial amounts of condensate. Testing has shown that the pH of condensate in Rizhao Steel’s gas pipelines ranges from approximately 5 to 6, with an electrical conductivity of about 2762 μS/cm. In addition, the condensate contains high concentrations of iron ions, chloride ions, and sulfate ions. Some studies indicate that Cl⁻ and SO₄²⁻ can induce stress corrosion. In terms of corrosion morphology in gas pipelines and associated equipment, non-uniform corrosion predominates, specifically manifesting as: (1) under-deposit corrosion, also known as crevice corrosion; (2) pitting corrosion; (3) localized corrosion; (4) erosion-corrosion; and (5) stress corrosion. The presence of condensate inherently increases the corrosivity of the pipeline, with higher conductivity accelerating the corrosion rate. The combined effect of contamination and corrosive properties in the gas medium is a major contributing factor to valve sticking. 2.4 Improper Operation Since the current blast furnace gas regulating valves do not operate in a closed-loop automatic control mode, their opening degrees are manually set by operators via the DCS. When the load remains relatively stable for extended periods, the valve disc may stay fixed at a particular angle without regular movement, which can also lead to inflexible valve operation. 3. Improvement Measures 3.1 Installation Method The installation method should be changed from vertical to horizontal to prevent impurities, moisture, and contaminants in the gas from settling in the bushing clearance due to gravity. 3.2 Reducing Gas Acidity Given the significant harm caused by acidic gas to the entire gas pipeline system, large valves, expansion joints, and other components, this issue has drawn the attention of senior management. The company has adopted a neutralization strategy by injecting alkaline solution into the gas pipeline at the TRT inlet. Analysis of condensate samples from the power generation gas pipeline indicates a pH value of around 6. 3.3 Improving the Diseased Bushing Design The bushing dimensions should be reduced: the front bushing length should be shortened from 400 mm to 200 mm, and the rear bushing length from 800 mm to 200 mm. In terms of material, stainless steel, as specified in JB/T1759-2010, should be replaced with cast copper alloy, grade ZCuSn10Pb1. Even if minor scaling occurs inside the clearance, the significantly reduced bushing length will cut friction by one-half to three-quarters, making valve operation much smoother. We are also offering this improvement proposal free of charge to the supplier, requesting that future valve spare parts be custom-made according to our specifications. During maintenance inspections, it was also found that the packing gland on the upper part of the valve stem was overly tight, and in some cases, the packing gland screws broke due to excessive torque. When re-machining the packing gland, the clearance should be appropriately increased, and the gap should be thoroughly greased to prevent gas leakage. 3.4 Process Improvements The operating procedures should be revised to require boiler operators to conduct operational tests on all blast furnace gas regulating valves every shift and meticulously record the setpoint and feedback values in designated forms. This will prevent the valve disc from remaining fixed at a single opening degree for prolonged periods, thereby reducing the risk of scaling and seizing of the valve stem. 3.5 Maintenance Strategy Four spare regulating valves should be procured, and a rotating replacement maintenance approach should be implemented. Whenever there is an opportunity to shut down the boiler, the valve exhibiting the most severe sticking, as determined by the operators’ routine testing, should be replaced. The removed valve should then undergo disassembly, inspection, and refurbishment, followed by lubrication and maintenance before being returned to service, thus minimizing downtime. 3.6 Lubrication and Maintenance Monthly lubrication and maintenance should be performed on the packing glands and front and rear bearings of online-operating blast furnace gas regulating valves, with thorough equipment inspections conducted to promptly monitor operating conditions. Instrumentation and control personnel should ensure that the control components (valve positioners) are protected against rain and corrosion, regularly check air supply pressure, inspect all air connection points for blockages or leaks, and verify the quality of compressed air. In addition, the compressed air supply pressure should be appropriately increased to enhance the output force of the actuators. Instrumentation personnel should adjust the pressure regulator upstream of the positioner to maintain a stable pressure of around 0.5 MPa. Each time a major boiler overhaul is scheduled, the pipeline should be cleaned to remove sludge, debris, and other impurities, thereby reducing wear and corrosion. 4. Implementation Results By establishing revised operating procedures and ensuring that operators continuously monitor the status of the regulating valves, we have achieved comprehensive equipment management—a practical attempt at condition-based maintenance. Following these improvements, the frequency of maintenance has decreased, with the average maintenance interval extending from the original 2–3 months to 6–8 months. The workload for maintenance personnel has been significantly reduced, and crucially, the flexibility and reliability of the regulating valves have been ensured, providing robust equipment support for the safe and stable operation of the boilers. This solution can also be extended to other large valves on the gas pipeline network.
The Cogeneration Workshop of the Power Plant at Rizhao Iron & Steel Co., Ltd. currently operates eight 50 MW blast-furnace-gas-fired generating units, with a total installed capacity of 400 MW. It is currently the largest blast-furnace-gas cogeneration plant among metallurgical enterprises in China, supported by eight 220 t/h boilers whose fuel is the surplus blast-furnace gas generated during the ironmaking process.
1 Introduction
The Cogeneration Workshop of the Power Plant at Rizhao Iron & Steel Co., Ltd. currently operates eight 50-MW blast-furnace-gas-fired generating units, with a total installed capacity of 400 MW. This makes it the largest blast-furnace-gas cogeneration plant among metallurgical enterprises in China today. The facility is equipped with eight 220 t/h boilers, which are fueled by surplus blast-furnace gas generated during the ironmaking process. To regulate the pressure in the gas pipeline network and the power-generation load, each boiler is fitted with eight gas-control valves, each with a nominal diameter of 800 mm. The operating principle of these valves is straightforward: the actuator receives a 4–20 mA current signal from the DCS; the valve positioner converts this electrical signal into a pneumatic signal, which actuates the piston cylinder and, in turn, moves the butterfly-disc valve, thereby changing the effective flow area of the disc and regulating the medium flow rate. However, in actual operation, these gas-control valves frequently experience various malfunctions. According to the workshop’s maintenance records for the year 2010, a total of 98 valve failures were addressed, including eight cases related to instrument signals or positioners and 90 cases of mechanical jamming. Of the mechanical-jamming incidents, only three were caused by excessive packing gland tightness; the remaining 87 were due to jamming at the front and rear bearing sleeves.
In the past, whenever a control valve malfunction occurred, process engineers typically first noticed on the DCS monitoring screen that the valve’s command signal did not match its feedback value. They would then contact the instrumentation technicians, who would verify that the signals and wiring were functioning properly. If no issues were found there, they would call in the mechanical maintenance team to disconnect the pneumatic actuator from the valve body. With the butterfly disc removed, they would apply the control signal and observe that the actuator’s mechanical position matched the signal—indicating that the mechanism was operating correctly. The mechanical technicians’ usual approach involved spraying rust inhibitor and lubricant onto the packing gland, manually applying force with pipe wrenches while turning the stem to facilitate movement, and repeating the process several times. For severe sticking, they would use oxyacetylene torch heating to soften the bearing bushings; in even more serious cases, the unit would have to be shut down, a manway opened in the pipeline, and the valve disc hammered with a sledgehammer, while instrumentation personnel increased the air supply pressure to the pneumatic actuator and manually operated the valve positioner on-site to assist in forcing the valve open. However, these measures only provided temporary relief—they addressed the symptoms but not the root cause—and after 2–3 months the valve would again become inflexible or seize up.
2. Cause Analysis
Based on many years of experience in the operation of control valves and thorough research, we have identified the following primary causes of control valve sticking and seizing:
2.1 Improper installation method
Currently, most valves are installed vertically and upright on horizontal pipelines (with the valve disc axis perpendicular to the horizontal plane). This arrangement ensures that the actuator is located at the upper portion of the gas pipeline; however, the rear bearing of the valve is positioned at the bottom of the pipeline. As a result, moisture and impurities in the gas stream can settle or accumulate at the rear bearing, gradually filling the clearance and leading to corrosion and jamming of the valve stem.
2.2 Unreasonable Design Structure
Through multiple disassembly and repair operations, it was discovered that the clearance between the bushing on the valve-disc drive shaft and the valve body was completely filled with foreign matter, including sludge, water, rust, and other deposits. Moreover, the originally designed bushing length was excessively long, causing the bushings at the front and rear of the control valve to bear the frictional forces generated by the reciprocating rotation of the valve shaft. Over time, coal-gas dust and corrosion-related impurities from the pipeline readily entered these bushing clearances, increasing rotational resistance. The gradually accumulating mixture eventually clogged the gaps, effectively welding the valve shaft, bushing, and valve body together and severely impeding the flexible rotation of the valve shaft.
2.3 Coal Gas Corrosion
Currently, all blast-furnace gas pipelines at steel mills across the country suffer from severe corrosion. The primary manifestations include frequent pitting and cracking of expansion joints, gas leaks along the pipeline, and failure of valves, pressure-reducing valves, and other pipeline components due to corrosion, rendering them inoperable. The corrosive nature of blast-furnace gas poses serious safety risks to equipment operation and pipeline integrity.
Our gas pipelines also exhibit localized corrosion leading to gas leaks and pitting corrosion with dripping at the bellows of expansion joints, indicating that the valves are severely affected as well. To analyze the extent of gas corrosion, we collected gas samples from the two gas pipelines in the power generation workshop for laboratory analysis.
Table 1: Composition Analysis of Blast Furnace Gas, % Vol
High-temperature H2, CO, H2O, and other components also exert considerable corrosive effects on steel. Given that coal gas contains moisture—particularly in power-generation systems, which are typically located downstream or at the end of the entire coal-gas pipeline network—significant amounts of condensate accumulate in the pipelines. Measurements have shown that the condensate in the coal-gas pipelines at Rizhao Iron & Steel Power Plant has a pH of approximately 5–6 and an electrical conductivity of about 2,762 μS/cm, with substantial concentrations of ferrous ions, chloride ions, and sulfate ions. Some literature indicates that Cl− and SO42− can induce stress corrosion. In terms of the corrosion morphology observed in coal-gas pipelines and equipment, non-uniform corrosion is predominant, manifesting as follows: (1) under-deposit corrosion, also known as crevice corrosion; (2) pitting corrosion; (3) localized pitting; (4) erosion-corrosion; and (5) stress corrosion. The presence of condensate renders the pipeline moderately corrosive, with higher electrical conductivity correlating with faster corrosion rates. Moreover, the inherent dirtiness and corrosive nature of the coal-gas medium are major contributing factors to valve sticking and seizing.
2.4 Improper Use
Currently, the gas regulating valve is not operating in closed-loop automatic control; instead, operators manually set the valve opening via the DCS. When the load is relatively stable, the valve disc may remain at a fixed angle for extended periods without frequent movement, which can lead to sluggish or inflexible valve operation.
3 Improvement Measures
3.1 Installation Method
The installation method has been changed from vertical to horizontal to prevent impurities, moisture, and contaminants in the process medium from settling in the shaft sleeve clearance under the influence of gravity.
3.2 Reducing the Acidity of Coal Gas
Given the significant corrosive effects of acidic coal gas on the entire coal-gas pipeline system, including large valves and expansion joints, this issue has drawn the attention of the company’s senior management. As a result, the company has implemented a neutralization strategy by injecting an alkaline solution into the coal-gas pipeline at the TRT inlet. Sampling and analysis of the condensate in the power-generation coal-gas pipeline reveal a pH of approximately 6.
3.3 Improvement of the Poorly Designed Sleeve Section
The bushing dimensions have been shortened: the front bushing length has been reduced from 400 to 200, and the rear bushing length from 800 to 200. In terms of material, stainless steel has been replaced, in accordance with JB/T 1759-2010, with a castable copper alloy designated as ZCuSn10Pb1. Even if minor scaling occurs inside the clearance gaps, the substantial reduction in bushing length will correspondingly decrease friction by one-half to three-quarters, making valve operation considerably easier. In addition, this improvement proposal has been provided to the supplier free of charge; henceforth, when submitting requests for spare parts for such valves, the supplier is required to manufacture them according to our specifications.
During the maintenance inspection, it was also found that the packing gland at the upper part of the valve stem was tightened excessively, and in some cases the packing gland bolts had fractured due to excessive torque. When the gland was re-machined, the clearance was appropriately increased, and the clearance was completely filled with grease to prevent gas leakage.
3.4 Process Aspects
Improve the process procedures by requiring boiler operators to conduct operational tests on all gas regulating valves at each shift and to meticulously record the setpoints and feedback values in designated forms, thereby preventing valve discs from remaining fixed at a single opening for extended periods, which could lead to scaling, solidification, and shaft seizure.
3.5 Maintenance Strategy
Four standby control valves are procured, and a rotational replacement-and-maintenance approach is adopted: whenever a unit shutdown opportunity arises, the valve exhibiting the most severe sticking or seizing—determined through operational testing conducted by process personnel—is promptly replaced. The removed control valves are then disassembled, overhauled, and thoroughly lubricated and maintained before being put back into service as spares, thereby minimizing unit downtime.
3.6 Lubrication and Maintenance
Monthly inspection and maintenance shall be performed on the packing gland of gas regulating valves in online operation, with lubrication applied to the front and rear shafts. Routine equipment inspections must be carried out to ensure timely awareness of equipment operating conditions. Instrumentation and control personnel shall implement rainproof and corrosion-prevention measures for control components (valve positioners), conduct regular checks on air supply pressure, inspect all air supply connections for blockages or leaks, and verify the quality of compressed air.
In addition, the pressure of the compressed air supply should be appropriately increased to enhance the output force of the actuator. Instrumentation personnel shall adjust the pressure-reducing valve upstream of the positioner to maintain the pressure at approximately 0.5 MPa. Whenever a furnace shutdown for major overhaul occurs, arrangements should be made to clean sludge, debris, and other impurities from the pipelines to minimize wear and corrosion.
4 Implementation Results
By formulating and revising process procedures, operators are kept constantly vigilant about the operating condition of control valves, thereby achieving enterprise-wide equipment management—a practical pilot of a condition-based maintenance approach. Following these improvements, the volume of maintenance has decreased, with the average maintenance interval extending from the original 2–3 months to 6–8 months, significantly reducing the physical workload of maintenance personnel. Crucially, the flexible and reliable operation of the control valves is ensured, providing robust equipment support for the safe and stable operation of the boiler. This solution can also be extended to other large valves on gas pipelines.
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