Troubleshooting of Copes Vulcan bypass valve

Complete guide for on-site debugging and troubleshooting of Copes Vulcan turbine bypass system

In thermal power generation, combined cycle, and cogeneration plants, the turbine bypass system is a key equipment that connects the boiler with the turbine, reheater, and condenser. It not only undertakes the steam bypass function during unit start-up, shutdown, and load shedding, but also is responsible for quickly reducing the main steam pressure and temperature to an acceptable level for downstream equipment, while protecting the reheater tube bundle from being burned out. As a global steam regulation equipment supplier with over 90 years of experience, Copes Vulcan's HP (high pressure) and LP (low pressure) bypass systems are widely used in the industry. However, as the operating years of the unit increase, problems such as blockage of bypass valves, erosion of internal components, delayed response of actuators, and inaccurate temperature control gradually become apparent. This article is based on the official technical information and on-site service practice of Copes Vulcan, systematically sorting out the selection points, typical fault causes, and standardized troubleshooting methods of turbine bypass valves, providing a practical reference manual for thermal control and maintenance engineers in power plants.

Function of turbine bypass system and Copes Vulcan product system

Modern steam turbine bypass systems typically adopt a two-stage configuration of high-pressure bypass (HP bypass) and low-pressure bypass (LP bypass), and some supercritical units also have a medium pressure bypass. Its main functions include:

Decoupling debugging of boiler and turbine: allowing the boiler to operate at full load (100% capacity bypass) before the turbine is connected to the grid, reducing start-up time.

Cold, warm, and hot start-up metal temperature matching: By reducing temperature and pressure, the steam parameters are synchronized with the turbine metal temperature to reduce thermal stress.

Prevent boiler overpressure during load shedding or tripping: Avoid damage to the sealing surface and loss of condensate caused by the spring type safety valve tripping.

Cool the final stage superheater during sliding pressure operation.

Maintain the cooling flow rate of the reheater to prevent dry burning.

Maintain stable boiler outlet pressure at partial load.

Protect the condenser from the impact of high temperature and high pressure steam.

Copes Vulcan offers a variety of valve series for different application scenarios:

Full name of valve model, typical application, pressure rating, structural characteristics

DSCV Direct Steam Conditioning Valve HP/LP bypass, back pressure machine bypass up to Class 4500 forged steel structure, customizable temperature and pressure reduction integration, online replacement of internal components

TE-PRDS Top Entry Pressure Reducing&Desuperheating Medium Pressure Auxiliary Bypass, Temperature and Pressure Reduction Station Class 150~2500 Cast Valve Body, Top mounted Interior, Maintenance without Disassembling Valve Body

PRDS Pressure Reducing&Desuperheating Conventional Temperature and Pressure Reducing Station Class 600~2500 Classic Casting Structure, Cage Guidance, Multi level Noise Reduction Optional

HP Bypass Valve High pressure bypass valve Main steam to cold reheat maximum Class 4500 forged steel, multi-stage HUSH noise reduction orifice plate+outlet diffuser, cold end water spray

LP Bypass Valve: Low pressure bypass valve for reheating steam to condenser, made of forged steel up to Class 900 with a large flow area and optional discharge/diffusion pipes

The vast majority of faults encountered by engineers on site are concentrated in four aspects: the actuator, internal component erosion, cooling water regulation, and control logic of the above-mentioned valves. Expand one by one below.

Typical faults and on-site handling of HP bypass valve

The HP bypass valve is the most stringent valve in the system, which requires the main steam (up to 220 bar/590 ° C) to be instantly depressurized and cooled to cold reheat conditions (usually around 40 bar/300-400 ° C). When the steam turbine trips, the valve must be fully opened within 1-2 seconds to prevent the boiler safety valve from tripping.

2.1 Common Fault Phenomena and Causes

Possible causes of fault phenomena and diagnostic methods

The valve cannot be quickly opened due to insufficient oil/gas pressure in the actuator; Pilot valve jamming; Mechanical spring fatigue inspection of HPU accumulator nitrogen pressure (normally 60%~80% of system pressure); Conduct a full stroke step response test

After opening, the pressure fluctuation is mostly caused by blockage or erosion of the HUSH orifice plate; The sealing ring of the balance plug is damaged. Disassemble and inspect the internal components, and measure the pressure before and after each throttle stage; Check the sealing surface of the valve seat

The outlet temperature exceeds the limit and the cooling water nozzle is blocked; Linear difference of cooling water regulating valve; The low water level in the hot well causes the suction port to be empty. Check the corresponding nozzle status for the cooling water introduction method (suction spray/mechanical atomization/steam atomization); Verify the Cv characteristics of the cooling water valve

High temperature thermal oxidation caused by leakage of valve stem and packing leads to carbonization of packing; Replace the flexible graphite packing due to surface strain on the valve stem and check the hardness of the valve stem (recommended ≥ 40HRC)

Internal leakage valve seat/disc sealing surface erosion after closure; Failure to close properly (mechanical limit drift of actuator), pressure test or ultrasonic leak detection; Re calibrate the actuator stroke

2.2 Key Internal Component Inspection Points

The multi-stage HUSH noise reduction orifice plate is the core of the HP bypass valve. During on-site maintenance, it is important to focus on checking:

Whether there are cracks or corrosion at the edge of each throttle hole (especially when the steam temperature exceeds 540 ° C, the nickel based alloy overlay may experience thermal fatigue).

Is the fastening bolt between the orifice plates loose (due to high temperature creep causing a decrease in pre tightening force).

If obvious erosion is found, it is recommended to replace it with a new orifice plate welded with cobalt based alloy (Stellite) and check the upstream filter (the aperture should not exceed 1mm).

The selection of cooling water introduction method directly affects the stability of outlet temperature. Based on on-site experience:

Aspiration: using steam flow rate to generate negative pressure and suck in cooling water, suitable for high pressure difference and low water flow conditions, with no moving parts and strong anti clogging ability. The common fault is scaling of the suction nozzle, which requires regular chemical cleaning.

Mechanical spray: Water is atomized through multiple nozzles, suitable for medium water volume. The nozzle is prone to clogging and requires the installation of a Y-shaped filter.

Steam atomization: using auxiliary steam to atomize water, with the widest adjustment ratio (up to 50:1), but the system is complex and requires an additional steam source. Malfunctions often occur in the fluctuation of atomization steam pressure or leakage of check valves.

2.3 Failure mode of actuator (HP bypass)

HP bypass valves typically require fail open, meaning that the valve should automatically open to protect the boiler when power is lost. There are three types of executing agencies:

Pneumatic piston+spring reset: The most common fault is spring fatigue causing the fully open time to exceed the standard (normally ≤ 2 seconds). Inspection method: Cut off the gas source and measure the time from closing to fully opening the valve. If it exceeds 3 seconds, replace the spring or increase the spring preload.

Pneumatic+accumulator: Damaged accumulator skin or nitrogen leakage can lead to insufficient storage capacity. Check the nitrogen pressure every six months and replace the bladder if the pressure drops by more than 20%.

Electro hydraulic actuator (HPU centralized or self-contained): Malfunctions often occur due to servo valve blockage, high pressure difference in high-pressure filter element, and hydraulic oil emulsification. On site oil samples should be taken to check the acid value and moisture content (phosphate ester fire-resistant oil should control moisture content<0.1%).

Special issues of LP bypass valve and condenser protection

The inlet of the LP bypass valve is reheated steam (usually at a pressure of several kilograms to tens of kilograms and a temperature of 300~500 ° C), and the outlet is negative pressure of the condenser (about 0.05 bar abs). The steam specific volume expands violently, and the valve size is often very large (such as DN600~DN1000). The faults mainly focus on high noise, vibration, and cooling water suction issues generated under high flow rates.

3.1 Vibration and Fracture of Dump Tube

After the LP bypass valve, a discharge pipe is usually installed into the condenser throat, with a large number of small holes on the pipe wall ("small hole technology" is used for noise reduction). Common on-site issues:

Cracking of discharge pipe weld: Due to steam reaction force and thermal expansion displacement, fatigue fracture occurred at the connection between the discharge pipe and the condenser.

Small hole blockage: During shutdown, rust or foreign objects enter, causing an increase in back pressure and insufficient flow during operation.

Cavitation: Fish scale erosion pits appear on the inner wall of the discharge pipe, usually in the wet steam zone.

Solution:

Use bellows expansion joints or sliding guide brackets at the connection between the discharge pipe and the condenser to release thermal displacement.

The diameter of the small hole should not be less than 8mm, and its orientation should avoid the pipe wall and condenser bundle (to avoid direct flushing).

Use an endoscope to inspect the inside of the discharge pipe and clean up debris during each minor repair.

3.2 Dilution water cannot be effectively mixed

Due to the limited internal space of the condenser, there is often not enough straight pipe section after the LP bypass valve to allow steam to fully mix with the desalinated water. Copes Vulcan recommends two solutions:

A diffuser cartridge is installed at the valve outlet, and the final pressure reduction and cooling are achieved through multi-stage expansion plates.

Replace traditional thermocouple temperature feedback with algorithmic temperature control (ATC) (see Part 6 for details).

If there is a severe fluctuation in the outlet temperature on site, first check the linearity of the action of the cooling water valve, and then confirm whether the correct inlet pressure and temperature signals are used for calculating the steam enthalpy value in the ATC logic.

Common debugging problems of PRDS and TE-PRDS temperature and pressure reduction stations

PRDS and TE-PRDS valves are widely used in auxiliary steam systems or industrial steam supply pipelines. Typical faults include:

4.1 Cage type guide valve stem jamming

When the steam cleanliness is poor or rust is generated during shutdown, the gap between the valve core and the cage sleeve (usually 0.15~0.25mm) may be stuck by hard particles, causing the valve to fail to operate or operate slowly.

On site emergency response:

In the shutdown state, inject specialized cleaning solution (such as a mixture of kerosene and penetrant) from the valve body discharge port, soak for 12 hours, and then repeatedly move the valve stem.

If it still gets stuck, the valve cover needs to be removed, and the throttle cage sleeve needs to be removed for grinding or replacement (pay attention to the thickness of the chrome plating layer).

4.2 Water droplets sprayed from the cooling nozzle are not completely vaporized

Manifested as water accumulation at the bottom of downstream pipelines, even causing water hammer. The main reason is that the steam flow rate is lower than the minimum atomization speed (generally required to be 25-30 m/s); Or the nozzle installation position is too close to the temperature measurement point.

Countermeasure: Recalculate the number and aperture of nozzles based on the design flow rate; Ensure that the length of the straight pipe section after the temperature reducer is not less than 10 times the diameter of the pipe.

4.3 Selection of throttling components and excessive noise

When the pressure difference before and after the PRDS valve exceeds 100 bar, if multi-stage pressure reducing components are not used, the noise can easily exceed 110 dBA. On site inspection should be conducted to ensure that the valve internals are HUSH multi-stage orifice plates. If it is a regular balance plug, it can be considered to be modified into a 3-5 stage throttling sleeve, with each stage's pressure reduction ratio controlled between 2-3.

Selection and troubleshooting of pneumatic and electro-hydraulic actuators

The actuator is the core of the dynamic response of the bypass system. Copes Vulcan offers two solutions, pneumatic and electro-hydraulic, each with its own advantages and disadvantages in practical operation.

5.1 Pneumatic actuator - rapid response transformation

Standard pneumatic diaphragm or piston actuators often require 5-10 seconds of full stroke time due to air capacity limitations, which cannot meet the requirement of fully opening in 2 seconds during machine trip. On site, the time can be shortened to 1.5-3 seconds by installing a pneumatic accelerator (Booster) and a quick exhaust valve.

Fault phenomenon: The actuator operates slowly, with a step response time greater than 2 seconds.

Inspection steps:

Measure the minimum pressure of the gas supply pipeline (should not be lower than 5 barg).

Check if the filter in front of the accelerator is clogged.

Manually operate the quick exhaust valve and confirm that the valve can instantly release the pressure on the other side of the cylinder.

If all of the above are normal, consider the wear of the piston seal ring (gas leakage) and replace the seal.

5.2 Electro hydraulic actuator HPU system

Common faults of centralized hydraulic power unit (HPU):

Frequent pump start-up: The accumulator bladder is damaged or the nitrogen charging pressure is too low, resulting in a short holding time. Under normal operating conditions, the pump should be started every 1-2 hours to replenish leaks; If it is started every 10 minutes, it is considered that the accumulator has failed.

Excessive oil temperature (>60 ° C): scaling of the cooler or excessive viscosity of the oil. Phosphate ester fire-resistant oil is temperature sensitive and will accelerate decomposition beyond 55 ° C. The oil/water heat exchanger should be cleaned regularly.

Large pressure fluctuation: The pilot stage of the servo valve is blocked. It is necessary to disassemble and wash the servo valve (replace the filter element), and install a 10 μ m absolute filtration precision filter at the outlet of the oil pump.

5.3 Self contained electro-hydraulic actuator

Common problems with integrated electro-hydraulic actuators (including motor, oil pump, oil tank, and accumulator) directly installed on valves:

Frequent motor start stop: severe internal leakage, mostly due to piston seal or one-way valve leakage. After manually pumping, observe the pressure drop rate. If the drop exceeds 5 bar per minute, it needs to be disassembled for inspection.

Unable to operate manually: The check ball valve in the manual pump is stuck or the pump is short of oil. Add fuel and repeatedly pump the handle to release air.

Principles and Troubleshooting of Algorithmic Temperature Control (ATC)

Traditional temperature control relies on plug-in thermocouples, but there are unevaporated water droplets adhering to the thermocouple sleeve at the outlet of the bypass system (especially the LP bypass), resulting in significantly low temperature measurement. After the controller is fully turned on to reduce the temperature, the temperature actually drops and oscillations occur. The Algorithmic Temperature Control of Copes Vulcan directly determines the required cooling water volume through feedforward calculation, fundamentally solving this problem.

6.1 Working principle of ATC

The system collects the following input signals:

Steam temperature T1 and pressure P1 in front of the valve → enthalpy value h1 obtained by checking the table

Steam flow Q1 (calculated from throttle orifice or valve position flow curve)

Valve pressure P2 (condenser pressure) → Determine outlet saturation enthalpy value h2

Cooling water temperature (set as a constant, with minimal impact from changes)

Calculation formula:

Q w=Q one×h one−h two h two−hw 

Q w=Q one× h two−h w h one−h two

After the PLC calculates the required cooling water flow rate Qw, it directly controls the cooling water regulating valve to the corresponding opening (based on the valve characteristic curve). This process does not rely on outlet temperature feedback, so it is not affected by water droplet adhesion. In actual operation, a slow PI loop is used to fine tune residual errors.

6.2 Common on-site issues

Problem cause handling

The actual outlet temperature is much higher than the calculated cooling water temperature. Assuming a large deviation (such as a 10 ° C increase in summer water temperature), hw is used as a variable input and connected to a water temperature transmitter

The opening of the cooling water valve is always 100%, but the temperature is still high. The pressure of the cooling water is insufficient or the nozzle is blocked. Check the outlet pressure of the water supply pump and clean the nozzle

The flow Q1 signal is inaccurate, and the range setting of the orifice plate differential pressure transmitter is incorrect; Re calibrate the differential pressure transmitter if the valve position feedback does not match the Cv curve; Download the correct Cv table to PLC

After stabilizing the control, there is a sudden jump in the steam inlet pressure P1 and a significant fluctuation. Check if there is a condensate column or blockage in the main steam pressure transmitter in front of the bypass valve

6.3 Special Applications: No Calculation Cycle Delay (Case Inspiration)

In the bypass system of the back pressure machine, Copes Vulcan applies the "2-second look back" logic: the PLC samples the steam parameters every 2 seconds and continuously stores the latest values. When the trip signal arrives, the system calculates the valve position using the last valid parameter before the trip to ensure undisturbed switching. If it is found on site that the pressure overshoot is too large after switching, the "look back" window duration should be checked to ensure it matches the system inertia (generally recommended 1-3 seconds).

Case Analysis of Back Pressure Turbine Bypass System (96TW20706)

This case is an HP back pressure machine driven by a synthesis gas/CO ₂ compressor, downstream of which is a catalyst unit, with extremely high requirements for steam pressure and temperature stability. The bypass system needs to achieve "disturbance free" switching when the turbine trips, directly supplying steam to downstream processes.

System configuration:

A 6 × 12 "Class 2500 HP DSCV bypass valve (forged steel F22)

A 2 "Class 2500 cooling water regulating valve (WC9)

Centralized HPU electro-hydraulic actuator, fault open

Key points of control logic:

During normal operation, the PLC continuously monitors the flow rate, pressure, and temperature at the inlet and outlet of the steam turbine, and pre calculates the target opening of the bypass valve and desuperheating water valve when the turbine trips.

The calculated value is locked in the PLC output register and not output.

When the DCS sends a digital signal for tripping, the switch switches and the valve moves directly to the locked position within milliseconds.

Maintain this position for about 10 seconds (adjustable), then switch to the regular PID slow adjustment mode.

Typical on-site fault: If the valve fails to accurately stay in the predetermined position when the machine trips, it is usually due to the locking register being erroneously updated in the previous sampling cycle. Check logic: Ensure that the 'maintain output' function is only updated when there is no trip signal and the operating conditions are stable.

This case ultimately achieved a pressure fluctuation of less than 1 bar, proving the effectiveness of the feedforward+preset position strategy.

Integrated steam regulation system for combined heat and power (CHP)

Modern CHP factories often require steam supply at multiple pressure levels, and Copes Vulcan can provide a complete set of equipment from the main bypass valve to the end temperature and pressure reduction station. Common integration issues on site include:

Multiple cooling water valves share the same water supply main, causing mutual interference: when two cooling water valves operate simultaneously, water pressure fluctuations can cause unexpected changes in the flow rate of the other valve. Solution: Install a one-way valve and a stabilizing accumulator in front of each cooling water valve.

The VOII type suction spray cooler has insufficient suction capacity under high vacuum: when the pressure on the LP side approaches absolute vacuum, the suction spray effect decreases. It can be changed to steam atomization and the injection steam flow rate can be increased.

Individual nozzles of MNSD multi nozzle desuperheater are blocked: During daily operation, perform regular full open close operations and use steam to blow the nozzles.

Preventive maintenance cycle and spare parts recommendations

Based on Copes Vulcan and industry experience, it is recommended to conduct maintenance according to the following cycles:

Key inspection contents of component cycle

Check the erosion amount and weld layer thickness of the HUSH orifice plate every 3 years or 500 cumulative actions of the bypass valve internals; Measure the gap between the balance plug and the cage sleeve

The sealing surface of the valve seat shall undergo dye penetrant testing annually (if frequently operated), with a grinding depth not exceeding 0.5mm

Disassemble and clean the cooling water nozzle every 6 months, check the wear of the nozzle hole diameter (replace if it increases by more than 10%)

Pneumatic actuator checks diaphragm/piston seals every six months, tests full stroke time, and verifies limit switches

HPU hydraulic oil is sampled and tested annually, with a moisture content of ≤ 500ppm and an acid value of ≤ 0.2 mgKOH/g (phosphate ester)

The nitrogen pressure of the accumulator should be checked quarterly using a specialized nitrogen filling tool and replenished to the predetermined value (calculated as 60%~80% of the system working pressure)

During each major overhaul of the discharge pipe, the endoscope checks for small hole blockage and weld seam cracks

Emergency spare parts list:

HUSH orifice plate sealing gasket (high-temperature graphite reinforced type)

Valve seat/disc grinding tool and spare welding parts

Servo valve (for electro-hydraulic systems)

Quick exhaust valve and sealing ring repair kit

Flexible graphite packing ring (high temperature grade)