As the core equipment of modern industry, the reliability of bolt connection of steam turbine is directly related to the safe operation of the unit. Under high temperature environment, the preload force of bolts will be attenuated due to thermal expansion, which may cause leakage or even equipment failure. The traditional torque method has large errors in controlling preload force and is difficult to adapt to complex working conditions. Below we will introduce the preload force control scheme of bolt electric heating rod ZJ-20-8B, which can achieve precise control through the principle of thermal expansion and provide new ideas for steam turbine maintenance.

I. Technical background and scheme design

1.1 Working principle of electric heating rod

The bolt heater ZJ-20-8B is made of nickel-chromium alloy material with high thermal conductivity and high temperature resistance. Its internal design is a spiral heating structure, which generates eddy current effect through electric current to quickly heat up the inner diameter of the bolt. According to the thermal expansion formula ΔL=αL₀ΔT, the change in bolt length is linearly related to temperature, and the preload increment can be accurately calculated by controlling the heating temperature.

1.2 Implementation steps of the scheme

Temperature gradient control: adopt a staged heating strategy, initially heating at a rate of 50℃/min to 300℃, and then at a rate of 20℃/min to the target temperature.

Real-time monitoring system: built-in thermocouples and displacement sensors, which feed back temperature and elongation data to the control unit through wireless transmission, and dynamically adjust the heating power.

Cooling and tightening: After heating, cool naturally for 4 hours. When the bolt stress relaxation rate is ≤5%, the final preload is applied using the torque-angle method.

II. Technical advantages and innovations

2.1 Improved accuracy

Compared with traditional flame heating, ZJ-20-8B achieves an axial temperature gradient of ≤5℃ through electromagnetic induction, and optimizes the heating path by combining finite element simulation to control the preload error within ±10%.

2.2 Intelligent control

Through the intelligent control of heating parameters by the heating rod control cabinet, the heating time of ZJ-20-8B is shortened by 40% compared with traditional methods, and the thread damage rate is reduced by 65%.

2.3 Enhanced safety

Double-layer insulation sheath and overheat protection device are used to prevent local overheating, metallographic changes and hydrogen embrittlement risks.

III. Application cases and effect verification

In a thermal power generating unit, ZJ-20-8B is used to heat-tighten the main steam valve bolts under 300℃ conditions. Data shows that the bolt elongation after heating reaches 2.3mm, and the preload force is restored to 98.7% of the cold state, which is 3 times more efficient than the flame heating scheme. The bolt disassembly time is shortened from 12 hours to 4 hours; the thread repair cost is reduced by 70%.

When looking for high-quality, reliable bolt heaters, YOYIK is undoubtedly a choice worth considering. The company specializes in providing a variety of power equipment including steam turbine accessories, and has won wide acclaim for its high-quality products and services. For more information or inquiries, please contact the customer service below:

E-mail: sales@yoyik.com
Tel: +86-838-2226655
WhatsApp: +86-13618105229

Yoyik offers various types of spare parts for steam turbines, generators, boilers in power plants:
variohm linear potentiometer DET100A
Magnetic speed sensor XS12JK-3P/Y
Flame TV tube SXJZ-70C
Air dryer – water cooled refrigerated SDLW-25
Displacement sensor (LVDT) ZDET-100B
Valve Control E1613
THEMOCOUPLE, RANGE: 0-600 C WZP2-630
Electric Motor 1F0H50G
LIMIT SWITCH XCK-J 20541 H7
Steam turbine speed sensor ZS04-80mm
MAIN IGNITOR FLEXIBLE XDL-6000
LVDT Sensor 4000TDGN-100-01-01
lvit sensor K156.33.31.04G02
LVDT SENSOR TDZ-1-H
Zicronia Oxygen Analyzer ZO-12(Q)+ ZO-12B
types of pressure transmitter PS531SPP10/BB32N3/S3M
Low-voltage differential pressure switch 1F19102
Oil temperature sensor YT315D
Image fire detection probe JSTVHJ-3
Thermocouple WRNK2-332
magnetostrictive level transmitter DQS6-32-19Y
MOTOR MANAGEMENT RELAY WDZ-5232A-521130-D220178
BEARING COVER GT2131307
BENTLY Preamplifier 330180-90-00
SWITCH PRESSURE PSB100AF1A
Ammeter ESS960I
Bo mạch đấu nối in/out van ME8.530.024
BRAUN MONITOR MODULE E1610
PRESSURE TRANSMITTER FLANGED 7MF0340-1GL01-5DF2-Z
bolts heater 1.2311(4)-φ36X1400