On November 24, 2001, at about 18:28, an explosion accident of the expansion joint of the heat transfer oil pipeline occurred in someone's plate making company. There was a sea of fire at the scene, causing one death and eight injuries, and a direct loss of 1.3 million yuan. The company has a 360 kcal (4.2MW) heat transfer oil furnace, with a design pressure of 1.0MPa and a rated temperature of 320 ℃. The heat transfer oil is used to dry wood fibers and hot pressed MDF. The normal working pressure of the heat transfer oil furnace is 0.2 -- 0.4 MPa, and the working temperature is 250 -- 260 ℃. During the installation and construction of the heat transfer oil pipeline, without formal design, the design drawings could not be provided. An expansion joint was installed on the oil outlet pipe and oil return pipe from the heat transfer oil furnace to the dryer. At that time, the expansion joint was purchased from a boiler accessories store. The manufacturer of the expansion joint and its performance parameters were unknown. The expansion joint of the oil outlet pipe from the heat transfer oil furnace to the dryer had burst, and the expansion joint of the oil return pipe had not burst.
According to the field measurement, the linear distance between the heat transfer oil furnace and the dryer is 42m, φ two hundred and nineteen × 6. The pipeline is 3.25m above the ground, using ordinary axial expansion joints, which are connected with the pipeline by flanges. There are elbows at both ends of the pipeline to the ground, and there is no fixed support at the elbows. The bellows of the expansion joint burst into three roughly equal pieces, and the bellows were pulled along the flange axially, and the ripples have been straightened.
1. Analysis and calculation
2.1 Preliminary analysis
The purpose of selecting expansion joints is to compensate the thermal expansion of the pipeline. For ordinary axial expansion joints, the pipeline is extended and the expansion joints are compressed. Usually, fixed supports are set at the valves and elbows. The corrugated expansion joints do not bear the pressure thrust (i.e. blind plate force). The design of expansion joint shall not only consider the working pressure, compensation amount and fatigue life, but also consider the support, blind plate force and installation position of the pipe system. It can be seen from the site that the installer did not understand the working principle of the expansion joint, and thought that it could be installed just like an ordinary valve. The pipeline had no fixed support, which caused the bellows to bear the blind plate force, which was the direct cause of the explosion accident. For overhead pipelines (3.25m above the ground) Π Natural compensation for T-shaped pipes, or multiple connecting rod type and hinge type expansion joints.
2.2 Material analysis and geometric dimension calculation of corrugated pipe
The corrugated pipe is made of stainless steel. Zhejiang Provincial Metallurgical New Product Quality Supervision and Inspection Station was entrusted to conduct chemical composition analysis. The contents of Cr, Ni and Ti are 18.30%, 8.73% and<0.05%, respectively. Therefore, the material is 18-8 austenitic stainless steel (such as 1Cr18Ni9, SUS304, etc.), which can be used as expansion joint corrugated pipe. Measure the wall thickness with vernier caliper as δ= 0.8mm, the expansion joint is made of single-layer (m=1) corrugated pipe, and the distance of the straightened corrugated (making mold trace) measured with a ruler is l=66mm. Since the expansion joint of the oil outlet pipe has burst, the geometric dimension of the ripple cannot be measured. To make a quantitative analysis of the expansion joint burst, the same specification was purchased with reference to the geometric dimension of the expansion joint of the oil return pipe. Because the oil return pressure and temperature are lower than the oil outlet pipe, the expansion joint has not burst yet, but the ripple has slightly extended, and there is bulge in the middle. The measured wave number n=6, wave root diameter D0=223mm, wave distance w=42.5mm, and wave height h=21.4mm, Then the ripple extension distance l '=2h+(π/2-1) w=67mm, l' ≈ l, it can be seen that the measured ripple geometric dimensions are basically correct. Take the working pressure P=0.4MPa, the working temperature T=250 ℃, and the allowable stress at 20 ℃ and 250 ℃ respectively according to 18-8 austenitic stainless steel[ σ]= 137 MPa 、[ σ] T=122 MPa, the elastic modulus at 20 ℃ and 250 ℃ is E0=196000 N/mm2, E1=180000 N/mm2 respectively, and the average linear expansion coefficient at 20 ℃ to 250 ℃ α= 0.01166mm/m℃。
2.3 Stress analysis of expansion joint
According to literature [1], [2] and [3], circumferential membrane stress, longitudinal membrane stress and longitudinal bending stress of internal pressure bellows are:
The axial stiffness of the expansion joint is
The longitudinal membrane stress and the longitudinal bending stress caused by the axial displacement are respectively Where According to the literature {[1], [2] and [3], the waveform coefficient Cp=0.417, Cd=2.54, Cf=0.754 is checked, and the working pressure P=0.4MPa, then the stress generated by internal pressure is S1=40.54MPa, S2=5.6MPa, S3=65.4MPa, which has not yet damaged the expansion joint.
Blind plate force of pipeline
Where Di=Do-2 δ， Then Fp=15400N, according to the stiffness calculation formula, the axial stiffness of the expansion joint KN=752.3N/mm
The expansion joint is stretched under the blind plate force, en=Fp/kn=21.23mm, and the elongation per wave is e=3.54mm. If the above formula is substituted, S4=27.41MPa, S5=760.15MPa. It is obvious that the stress caused by the expansion joint elongation due to the pressure blind plate force accounts for a large proportion and is greater than 3[ σ] T=366 MPa, if frequent startup and shutdown, fatigue damage will be caused.
2.4 Calculation of thermal expansion and fatigue life
Assuming that fixed supports are set at the elbows at both ends of the expansion joint pipeline, the thermal expansion amount of the oil outlet pipeline from the heat transfer oil furnace to the dryer
Δ L= α L Δ T，L=24mm， Δ T=250℃， Δ L=122.43mm, the compression amount of expansion joint per wave e=20.4mm, according to the fatigue life formula in [1], [2] and [3]:
In the formula, SR=0.7 (S2+S3)+(S4+S5), temperature correction coefficient Tf=E1/E0, nf=15, then N=32 times, allowable fatigue life [N]=2 times. Obviously, this design is incorrect, and generally the allowable fatigue life is more than 1000 times.
3. Conclusion
Based on the above analysis, the following conclusions can be drawn:
3.1 The expansion joint is made of 18-8 austenitic stainless steel, and the material selection is correct.
3.2 When installing ordinary axial expansion joints, fixed supports are usually set at the valve or elbow. If there is no fixed support for the pipeline, the bellows will bear the blind plate force, which is the direct cause of this explosion accident.
3.3 Assuming that the fixed support is set at the pipeline elbow, the allowable fatigue life is only 2 times through calculation, it is obvious that the expansion joint selected in the design is incorrect
