[Weekly Accident Case Sharing] Accident Case 1: Pressure tapping issues.

Thread Content

This post was last edited by A student adrift in the sea of knowledge on 2017-8-7 at 19:32. I am sharing here the accident and failure cases that I have collected or experienced, in the hope that they can serve as a reference for everyone! Case 1: The anti-clogging device for furnace pressure sampling became clogged, resulting in erroneous activation of the furnace pressure protection system. I. Fault symptoms: On December 05, 2013, switches for high furnace pressure 1 and high furnace pressure 2 were activated in Unit #5 of a certain company’s boiler plant; this was a false alarm resulting in the activation of the boiler’s MFT. II. Cause analysis: 1. Inspections after the accident revealed that there were high furnace pressures. The pressure switches for high furnace pressure 1 and high furnace pressure 2 are located near the steam soot blower, and they are severely affected by water vapor; in addition, the sampling tubes at these two measurement points were heavily clogged. Generally, the pressure sampling device is installed on the outside of the flue gas flow path, away from the steam soot blower and near the top of the furnace. 2. The furnace pressure anti-clogging device lacks any anti-clogging structure; it is essentially an empty container, resulting in very poor anti-clogging performance. The incident may have been caused by hot coke or ash blocking the sampling port and heating the air within the sampling system, resulting in a rapid increase in pressure and triggering a false alarm. 3. None of the regulations clearly stipulate the relevant requirements in full. Article 3.2.8 of the \"Code for Explosion Prevention in Furnaces of Coal Powder Boilers in Power Plants\" (DL/T 435-2004): \"Furnace pressure monitoring.\" ……There should be no strong air flow disturbances such as soot blowing holes around the sampling point. ’Requirements are only specified for the relative soot blowing holes. Article 4.3.1.4 of the \"Technical Specifications for Construction and Acceptance of Electric Power Generation Facilities – Part 5: Thermal Automation\" (DL/T 5190.5-2004) states that the location of the components used to measure furnace pressure shall comply with the regulations set by the boiler manufacturer; it is advisable to place them above the center of the flame in the combustion chamber’ ； Article 4.3.3: ‘When measuring the pressure of media such as dust or gas-powder mixtures, pressure-taking devices equipped with anti-clogging and/or purging mechanisms shall be used.’ Emphasize clogging prevention and purging. Article 4.1.11 of the \"Technical Specifications for the Installation of On-site Equipment, Pipeline and Cable Design in Thermal Power Plants\" (DL/T 5182-2004) states that \"when measuring the pressure of dusty and turbid media, a pressure transducer equipped with anti-clogging or purging mechanisms should be used.\" Article 4.1.12 specifies that \"the pressure transducer for measuring furnace pressure should be located above the center of the flame in the combustion chamber (the exact location to be determined by the boiler manufacturer), and such transducer must have anti-clogging or purging facilities.\" The requirement is almost the same as the previous one. III. Identified issues: (1) The impact of soot blower steam on the furnace pressure sampling tube was not taken into account. (2) During the infrastructure construction period, the anti-clogging devices were not subject to strict inspection, or their working principles were not understood. (3) Unfamiliar with relevant regulations. IV. Preventive measures: (1) Stay away from the steam soot blower; select an appropriate furnace pressure sampling point in the upper-middle part of the furnace flame area, where air flow disturbances are minimal (in the upper outer part of the flue gas flow, near the furnace top). (2) Replace the defective anti-clogging device or install a purging device. (3) The inclination angle of the sampling cylinder must be greater than 45°, and the opening of the sampling cylinder must be smooth. (4) In accordance with the requirements of Article 5.4.2.3.3 of the \"Regulations for Maintenance and Operation of Thermal Automation Systems in Thermal Power Plants\" (DL/T 774-2015), the regular maintenance tasks for on-site equipment should be specified in more detail; regular soot blowing of the sampling pipelines should be carried out, and the sampling ports inside the furnace should be inspected during shutdown periods to remove accumulated soot and slag.