Shell boilers — Part 5: Safety appurtenances and instruments
1 Scope
This document specifies the requirements for setting and selection of safety appurtenances and instruments of shell boilers, including safety valve, pressure measuring device, water level measuring device, temperature measuring device, blow-down and blow-off device, safety protection device and other devices.
This document is applicable to the safety appurtenances and instruments of shell boilers defined within the scope of GB/T 16508.1.
2 Normative references
The following documents contain provisions which, through reference in this text, constitute provisions of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 2900.48 Electrotechnical terminology of boilers
GB/T 12241 Safety valves — General requirements
GB/T 16508.1 Shell boilers — Part 1: General requirements
GB/T 16508.3 Shell boilers — Part 3: Design and strength calculation
GB/T 36699 Specification for liquid fuels and gaseous fuels burners of boilers
NB/T 47063 Safety valves for power station
3 Terms and definitions
For the purposes of this document, the terms and definitions given in GB/T 2900.48 and GB/T 16508.1 and the following apply.
3.1
set pressure of safety valve
set pressure at which the safety valve opens under operating conditions
3.2
ignition safety time
safety time of the ignition flame to be ignited of a burner, that is, the time interval from getting the opening signal to getting the closing signal by the ignition fuel control valve when no ignition flame is formed
3.3
extinction safety time
time interval from the flame extinction signal that the flame detector sends out to the start of closing the safety shut-off valve when the burner is operating
4 Basic requirements
4.1 The safety appurtenances and instruments for boilers shall meet the requirements of safe and reliable operation of the boiler.
4.2 The safety appurtenances and instruments for boilers shall meet the requirements of relevant product standards.
4.3 All necessary parts as required in drawings and technical documents of boilers shall be equipped with measuring devices for pressure, water level and temperature. All those devices shall be safe, reliable and accurate and be provided with proper measuring range and local and remote transmission measuring devices for pressure, water level and temperature as required in this document.
4.3 The calibration and maintenance of measuring instruments shall comply with the regulations of the national metrological authority.
5 Safety valves
5.1 Quantity and type
5.1.1 Each boiler shall be installed with at least two safety valves (including drum and superheater safety valves). In case of any of the following conditions, only one safety valve may be installed:
a) The steam boiler with a rated evaporation not greater than 0.5 t/h;
b) The steam boiler with a reliable overpressure interlocking protection device and rated evaporation less than 2 t/h;
c) Hot water boiler with a rated thermal power not greater than 2.8 MW.
5.1.2 For safety valve of steam boilers, fall-lift spring safety valve, lever safety valve or controlled safety valves (pulsed safety valve, pneumatic safety valve, hydraulic safety valve and electromagnetic safety valve etc.) shall be adopted, and for hot water boilers may adopt, micro-open safety valve may be adopted. All these valves selected shall meet the requirements of corresponding technical standards.
5.1.3 For steam boilers with rated operating pressure not greater than 0.1 MPa, dead weight safety valve or water-sealed safety equipment may be adopted. In case that hot water boiler is equipped with water-sealed safety equipment, safety valve may not be installed. The inner diameter of sealing water tube for water-sealed safety equipment shall be determined in accordance with the rated evaporation (rated thermal power) and rated operating pressure of boilers and no less than 25 mm. The sealing water tubes shall not be provided with valves, but anti-freezing measures.
5.2 Discharge amount
5.2.1 The total discharge of safety valves on the steam boiler shell (drum) and superheater shall be greater than the rated capacity.
5.2.2 The relief capacity of safety valves for hot water boilers shall meet the requirement that the pressure in the boiler after all safety valves are opened shall not be greater than 1.1 times of the calculated pressure in design.
5.2.3 The proportion of discharge from the relief valve at the outlet of the superheater in the total discharge shall ensure that the superheater can be adequately cooled when the relief valve is opened.
5.2.4 The flow passage diameter of steam boiler safety valve shall not be less than 20 mm. The discharge amount of the safety valve shall be determined in accordance with one of the following methods:
a) It is calculated according to the rated discharge amount provided by the safety valve fabrication organization;
b) It is calculated according to Equation (1):
E=0.235A(10.2p+1)K (1)
where,
E——the theoretical discharge amount of safety valves, in kg/h;
p——the steam pressure at the safety valve inlet (gauge pressure), in MPa;
A——the flow passage area of safety valve, which may be calculated by (πd^2)/4, in mm2;
d——the flow passage diameter of safety valves, in mm;
K——the steam specific volume correction coefficient of safety valve inlet, calculated according to Equation (2):
K=K_p?K_g (2)
where,
Kp——the pressure correction coefficient;
Kg——the superheated correction coefficient;
K, Kp, Kg shall be selected and calculated according to those specified in Table 1.
Table 1 Correction coefficients at each inlet of safety valve
p/MPa State of working substance Kp Kg K=K_p?K_g
p ≤ 12 Saturated 1 1 1
Superheated 1 〖√(V_b/V_g )〗^a 〖√(V_b/V_g )〗^a
p > 12 Saturated √(2.1/((10.2p+1)V_b )) 1 √(2.1/((10.2p+1)V_b ))
Superheated 〖√(V_b/V_g )〗^a √(2.1/((10.2p+1)V_b ))
Note: Vb——the saturated steam specific volume, in m3/kg;
Vg——the superheated steam specific volume, in m3/kg;
Tg——the superheating temperature, in °C.
a √(V_b/V_g ) may also be superseded by √((1?000)/((1?000+2.7T_g)))
c) It shall be calculated according to the Equations in GB/T 12241 or NB/T 47063.
5.2.5 The flow passage diameter of safety valve of hot water boiler shall be selected or calculated according to the following regulations.
a) For the boiler with the rated outlet water temperature less than 100°C, it may be selected according to those specified in Table 2;
Table 2 Flow passage diameter of safety valve
Rated thermal power of boiler (Q) /MW Q ≤ 1.4 1.4 < Q ≤ 7.0 Q > 7.0
Flow passage diameter of safety valve/mm ≥ 20 ≥ 32 ≥ 50
b) For the boiler with the rated outlet water temperature larger than or equal to 100°C, the number and flow passage diameter of its safety valve shall be calculated according to Equation (3).
ndh=35.3Q/(C(p+0.1)(i-i_j))×10^6 (3)
where,
n——the number of the safety valve;
d——the flow passage diameter of safety valve, in mm;
h——the opening height of safety valve spool, in mm;
Q——the rated thermal power of boiler, in MW;
C——the discharge coefficient, selected according to the data provided by the safety valve manufacturer or the following values:
where h ≤ d/20, C =135 and when h ≥ d/4, C = 70;
p——the opening pressure of safety valve, in MPa;
i——the saturated steam enthalpy of rated outlet pressure for boiler, in kJ/kg;
ij——the enthalpy of boiler inflow, in kJ/kg.
5.3 Performance requirements
5.3.1 The set pressure of the steam boiler safety valve shall be adjusted and calibrated according to the requirements in Table 3. The boiler equipped with a safety valve shall be adjusted according to the minimum value of set pressure in Table 3; for the boiler with a superheater, the sett pressure of the superheater outlet safety valve shall be adjusted according to the minimum value to ensure that the safety valve on superheater opens first.
Table 3 Set pressure of steam boiler safety valve
Rated operating pressure (p)/MPa Set pressure of safety valve
Minimum value Maximum value
p ≤ 0.8 Operating pressure + 0.03 MPa Operating pressure + 0.05 MPa
0.8 < p ≤5.3 1.04 times of operating pressure 1.06 times of operating pressure
p > 5.3 1.05 times of operating pressure 1.08 times of operating pressure
Note: The operating pressure refers to the operating pressure at the safety valve installation location and for controlled safety valves, it refers to the operating pressure at the control source extended location.
5.3.2 The set pressure of safety valve of hot water boiler shall be adjusted and calibrated according to the requirements in Table 4.
Table 4 Set pressure of safety valve of hot water boiler
Minimum value Maximum value
1.10 times of operating pressure, but not less than operating pressure plus 0.07 MPa 1.12 times of operating pressure, but not less than operating pressure plus 0.10 Mpa
5.3.3 Generally, the opening/closing pressure difference of safety valve shall be 4% ~ 7% of the set pressure and the maximum shall not exceed 10%. Where the set pressure is less than 0.3 Mpa, the maximum opening/closing pressure difference is 0.03 Mpa.
5.4 Installation
5.4.1 The safety valve shall be subjected to strength test, operating performance test and discharge test according to GB/T 12241.
5.4.2 The safety valve shall be vertically installed at the highest position of the shell (drum) and header. Valves and medium access lines shall not be installed between the safety valve and the shell (drum) or between the safety valve and the header.
5.4.3 When several safety valves are installed together on a short pipe directly connected with the shell (drum), the flow cross-sectional area of the short pipe shall not be less than the sum of the flow cross-sectional areas of all safety valves.
5.4.4 The use of threaded spring safety valves shall meet the requirements of GB/T 12241. The safety valve shall be connected with a short pipe with threads, and the connection between the short pipe and the shell (drum) or header cylinder shall be of welded structure.
5.4.5 The devices related to safety valves shall meet the following requirements:
a) The dead weight safety valve shall have a device to prevent heavy sheets from flying off;
b) The spring safety valve shall be provided with a lifting handle and a device to prevent the screw from adjustment by random twisting;
c) The lever safety valve shall be provided with a device to prevent the heavy punch from moving automatically and a guide bracket to prevent the lever moving out.
5.4.6 The controlled safety valve shall have reliable power source and power supply and meet the following requirements:
a) Impulse of the impulse safety valve is connected to the valve on the duct and it shall be kept fully open and sealed with lead;
b) The safety valve controlled by compressed air is provided with reliable air supply and power supply;
c) The hydraulic safety valve is provided with reliable hydraulic transfer system and power supply;
5.4.7 Exhaust steam pipe of the steam boiler safety valve shall meet the following requirements.
a) The steam boiler safety valve shall be equipped with an exhaust steam pipe, which shall go straight to a safe place, and enough circulation cross-sectional area is available to ensure smooth exhaust steam; besides, the exhaust pipe shall be fixed. The situation that proper operation of a safety valve is affected due to thermal expansion or exhaust reaction shall be avoided, and no external force from the exhaust steam pipe shall be applied to the safety valve.
b) The exhaust steam pipe between two independent safety valves shall not be connected.
c) The exhaust steam pipe arranged in the open air shall be equipped with a protective cover, the installation of which shall not hinder the normal operation and maintenance of the safety valve.
d) If a silencer is installed on the exhaust steam pipe of the safety valve, its structure shall have sufficient flow cross-sectional area, suitable support and reliable drain device.
e) Dredge pipe connected to a safe place shall be installed at the bottom of the exhaust steam pipe of the safety valve, and a valve shall not be installed on the drain pipe and dredge pipe.
5.4.8 The safety valve of hot water boiler shall be equipped with a drain pipe, which shall lead directly to a safe place, and enough discharge circulation area shall be available to ensure smooth discharge. Valves shall not be installed on drain pipes, and anti-freezing measures shall be taken.
5.5 Use and calibration
5.5.1 After the installation (including maintenance and replacement) of the safety valve, its set pressure and sealing performance shall be calibrated. For controlled safety valves, control loop reliability test and opening performance test shall be carried out respectively.
5.5.2 After calibration, the safety valve shall be locked or sealed with lead, and its test results such as setting pressure and sealing performance shall be recorded in the boiler safety technical archives.
5.5.3 During boiler operation, the safety valve shall not be disconnected at will, and its set pressure shall not be increased at will, otherwise it will fail.
5.5.4 During boiler operation, discharge test shall be carried out on the safety valve regularly. For controlled safety valves, their control systems shall be tested regularly.
5.5.5 In-use boiler safety valves are calibrated at least once a year, and the calibration is generally carried out under the running state of the boiler. If it is difficult to conduct calibration on site or after the safety valve is repaired, the calibration may be carried out on the safety valve calibration table. The calibrated safety valve shall be free from falling, smashing and collision during its transportation or installation.
6 Pressure measuring device
6.1 Each boiler shall be equipped with a pressure gauge at:
a) Steam space in the shell (drum) of a steam boiler;
b) In front of the feed water regulating valve;
c) At the economizer outlet;
Foreword i
Introduction iv
1 Scope
2 Normative references
3 Terms and definitions
4 Basic requirements
5 Safety valves
6 Pressure measuring device
7 Water level measuring device
8 Temperature measuring device
9 Blow-down and blow-off devices
10 Safety protection and other devices
Bibliography
