Accident causes and protective measures of plate type heat exchanger

Accident causes and protective measures of plate type heat exchanger

25 Feb 2019

In petroleum, chemical, light industry, pharmaceutical, energy and other industrial production, plate type heat exchanger is often used to heat low temperature fluid or high temperature fluid cooling, liquid vaporization into steam or steam condensation into liquid. A heat exchanger is a unit of equipment, such as a heater, a cooler, a condenser, etc. It is also an integral part of a process equipment, such as a heat exchanger in an ammonia synthesis tower. Heat exchanger is an important unit equipment in chemical production, according to statistics, the tonnage of heat exchanger accounts for about 20% of the whole process equipment, some even as high as 30%, it can be imagined how important it is.

Due to the working conditions of the heat exchanger is under high temperature and high pressure line, such as the working medium pressure up to 250 MPa, the operating temperature up to 1000 ~ 1500 ℃; Some of its working fluid has flammable, explosive, toxic, corrosive characteristics, in addition to chemical, petrochemical production requirements for processing large, strong continuity, therefore, this will bring certain difficulties to the normal operation of the heat exchanger, a little careless will occur accidents, and endanger the lives of workers.

According to the statistical data of foreign chemical equipment damage, the heat exchanger damage rate accounts for the largest proportion of all the chemical equipment damage rate, which is 27.2%, far higher than the tank, tower, kettle damage rate (17.2%). According to the incomplete statistics of small petrochemical production from 1949 to 1982, there were 21 explosion accidents of heat exchanger and 102 casualties. From 1973 to 1983, a total of 16 heat exchanger explosion accidents occurred in small nitrogen fertilizer, with 12 casualties. According to the incomplete statistics of the special equipment safety supervision bureau of the general administration of quality supervision, by the end of 2012, 218 serious leakage and explosion accidents of heat exchangers had occurred in 29 provinces, autonomous regions and municipalities directly under the central government, and 412 people were killed.

The main types of heat exchanger accidents are combustion explosion, serious leakage and tube bundle out of control. Among them, unreasonable design, manufacturing defects, improper material selection, severe corrosion, operational errors and poor maintenance and management are the main causes of heat exchanger accidents.

I. Combustion and explosion
1. Cause of the accident:
1) the homemade heat exchanger blindly changes the structure and material of the equipment to a large extent, resulting in poor manufacturing quality, non-compliance with the pressure vessel specifications, and greatly reduced equipment strength.
2) poor welding quality, especially the welding joint is not fully welded, and the welding seam inspection and blasting pressure test are not carried out, resulting in leakage or fatigue fracture of the welding joint, and then a large number of flammable and explosive fluid medium overflow, and explosion will occur with the fire.
3) the compression strength decreases due to corrosion (including stress and intergranular corrosion), resulting in tube bundle failure or serious leakage, and explosion in case of open fire.
4) when the heat exchanger is used for air tightness test, combustible refined gas is used for oxygen make-up pressure test, causing physical and chemical explosion.
5) operation violation, operation error and valve closure lead to overpressure explosion.
6) if sewage is not discharged for a long time, excessive accumulation of inflammable and explosive substances (such as nitrogen trichloride) and high temperature will lead to violent explosion of heat exchanger (such as liquid chlorine heat exchanger).
7) peroxide explosion.

Ⅱ. Preventive measures
1) the design and manufacture of heat exchangers shall meet the requirements of national pressure vessels. The modification and change of drawings shall be approved by the competent department, and the quality shall be qualified.
2) during the manufacture of heat exchangers, the welding quality shall be guaranteed and the welding seams shall be strictly inspected.
3) when the fluid is corrosive medium, attention should be paid to improving the material quality and welding quality, increasing the thickness of the pipe wall or adding pre-corrosion preparation into the fluid, regularly checking the surface corrosion of the pipe and testing the equipment that is prone to corrosion damage, and taking effective measures.
4) dry air, nitrogen and other inert gases must be used in the air tightness test of the heat exchanger. Oxygen or combustible gases are strictly prohibited to be used in the test or supplementary pressure test.
5) it is strictly prohibited to operate in violation of rules and regulations, and strictly implement the operation rules.
6) the easily scaling fluid can be cleaned regularly to remove scaling.
7) strictly control the oxygen content.

Two, serious leakage
The most vulnerable parts of the heat exchanger are the welding joint, the connection between the sealing head and the tube plate, the connection between the tube bundle and the tube plate and the flange. Welding joint leakage is the direct cause of poor welding quality, such as: welding not penetration, not fusion, the existence of stomatal slag inclusion, welding without inspection, or even did not do blasting experiments, only parts of the water pressure test and the use of many cutting welding, resulting in metallographic changes, increased internal stress, greatly reduced strength. Pipeline leakage will cause gas proximity, such as the pipe half water gas into the inter-pipe change gas, so that the carbon monoxide increased, affecting the normal production. The main causes of tube leakage are corrosion, frequent opening and stopping, large temperature change, rapid expansion or contraction of heat exchanger, leakage at tube expansion of flower-plate and manufacturing defects of equipment. In short, fires, explosions, asphyxiation, poisoning and burns are caused by leaks. Flammable and explosive liquid or gas spill due to leakage, in case of open fire will cause combustion explosion accident; Toxic gas will cause asphyxiation poisoning; And strong corrosive fluid leakage will lead to burns accident.

1. Cause of the accident
1) serious corrosion (such as vapor droplets, hydrogen sulfide, and carbon dioxide) causes tube leakage.
2) due to frequent start and stop, temperature change is too large, the equipment expands or contracts rapidly, so that the tube plate expands and the tube leaks.
3) manufacturing defects of heat exchanger itself and leakage of welding joints.
4) due to the increase of operating temperature, bolt elongation, loose fastening parts, resulting in flange leakage.
5) leakage caused by thermal shock caused by loose assembly site of tube bundle, pipe vibration, start and stop, emergency stop, and mechanical shock caused by improper operation of regular maintenance.

2. Preventive measures
1) clean the pipe regularly, choose corrosion-resistant pipe materials, such as corrosion and oxygen inhibitor in the fluid, control the flow rate in the pipe, and decide to stop or replace or take blocking measures according to the leakage situation.
2) careful operation, the temperature of control system do not be changed too often.
3) ensure the quality of welding and carry out careful inspection of welding.
4) reduce flange connection as far as possible, and tighten the bolt in time after heating up, so as to facilitate the fastening operation.
5) welding assembly should be adopted for the heat exchanger with no leakage in the part of the expanding tube.