Inspection and Maintenance of Flame Arresters

Transcription

1 7 Inspection and Maintenance of Flame Arresters 7.1. Need and Importance of Maintenance Inadequate maintenance of flame arresters can result in their failure to perform as required. Failure can result from enlargement of the apertures due to corrosion, erosion, shock waves, or mishandling during cleaning. This results in the inability to stop the breakthrough of flames from deflagrations and detonations. Another cause for failure is pluggage due to entrainment of solids from the process, polymer deposits, freezing of water and organic liquids, and nests built by birds and insects. In this case, the plugged flame arrester may prevent or diminish vapor inflow and outflow from vessels, which can lead to vessel failure due to overpressure or vacuum collapse. Fixed element dry type flame arresters, such as end-ofline deflagration arresters, are especially susceptible to these failures. However, hydraulic (liquid seal) flame arresters, and other types can also fail to operate as required if regularly scheduled maintenance is not provided Mechanical Integrity Issues This section discusses inspection and maintenance practices and procedures that affect the mechanical integrity of flame arrester installations. 147

2 Inspection and Maintenance of Flame Arresters Inspection It is critical to the integrity of flame arresters that they be inspected and serviced on a regularly scheduled basis (preventive maintenance program) so that they continue to be effective. The regularity of inspections and the time interval between inspections depends on the application and company practice. One flame arrester manufacturer recommends that inspection should be made a month after initial operation. Many companies perform inspection and maintenance at least once annually unless operating experience indicates that the inspection period should be changed. Some companies that continuously monitor the pressure drop do not inspect flame arresters until abnormal pressure drop is detected. However, pressure drop may not identify corrosion problems or absence of the element or gaskets. Even if pressure drop indicates no potential problem, it is recommended that all flame arresters be visually inspected on a scheduled basis. Plugging may not be readily detected by pressure drop where the normal vent flow is only a small fraction of the design basis flow. Some flame arrester designs allow in-place inspection (via a removable housing cover plate); however, others have to be removed from the vent nozzle or in-line piping to inspect the flame arrester element. There are some mechanical factors that sometimes hamper inspection. A flame arrester may be in a location that is not readily accessible or does not provide sufficient working space or mechanical aids. This is why, as was mentioned in Chapter 6, proper location and installation is very important. If removal of the flame arrester is required for inspection, then purging may be required before removal. This may be difficult to do easily because of the flame arrester construction or location. Also, some flame arresters may be constructed so that removal of the arrester element for inspection is not possible (i.e., the element is welded to the housing). Another problem is that the flame arrester is in a pipeline or directly attached to a vessel that is in constant use. In some cases, this might even be a problem during a period of a turnaround. In this situation it may be necessary to install two flame arresters in parallel with adequate valving to switch from one to the other and to provide for purging. Before performing any inspection or maintenance on a flame arrester, the associated process equipment and piping should be taken out of service or isolated. The work area must be proven by test to be free of any harmful gases or vapors. It should also be verified that all piping is clean and free of obstructions and debris. All plant, company, local, state, and federal safety and fire codes and standards should be followed.

3 7.2. Mechanical Integrity Issues Current Maintenance Practices Depending on the size of a flame arrester, the cleaning of the flame arrester element may be done in place or in the maintenance shop. Removal of the flame arrester from its location and transport to the maintenance shop may require heavy lifting equipment. Care is needed to ensure that the cleaning method is compatible with the flame arrester in terms of the materials of construction and the arresters overall robustness. Suitable cleaning methods include the use of solvents, water, steam, compressed air, or ultrasonics. For crimped metal ribbon elements, cleaning must be done carefully, because of the small and delicate flow channels involved. It is important not to clean the element with sharp objects that might open the flow channels or distort them, and thus disable the arrester. At the same time, the small flow channels may tenaciously hold solids filtered from the line. In this case it may be desirable to send the element to the flame arrester manufacturer for cleaning. It is advisable to stock spare elements for replacement of the element returned to the manufacturer for cleaning so that the flame arrester may be returned to service quickly. If the flame arrester element is of the parallel plate type some manufacturers caution against disassembling the grid assembly to clean it. This is because such handling may increase the gaps between the plates to larger than the original design rendering the flame arrester ineffective. It is critical upon reassembly to check that all gaskets are properly installed and that the element is correctly aligned in the housing to ensure that there is no bypassing of the element. Failure to do this may result in complete loss of protection. On-line cleaning may be desirable for very large flame arresters as the elements may not be readily removed. One such on-line cleaning installation is provided for a 72-inch-diameter flame arrester assembly, which weighs more than 15,000 pounds and is installed in a 36-inch-diameter piping system (Anon 1999, Dafft 1999). The flame arrester is located in a vent gas collection system, upstream of an incinerator. The vent gas contains sticky solids that could eventually plug the flame arrester if not removed. To remove the sticky solids a cleaning assembly with multiple spray nozzles is provided for on-line cleaning of the flame arrester element with a mixture of steam and hot deionized water while the process gas continues to flow to an incinerator. If a flame arrester element is damaged, its effectiveness will be impaired, and a replacement should be considered. It is important to ensure that the replacement meets all specifications and that it is installed correctly, per the manufacturer s recommendations. It is recommended

4 Inspection and Maintenance of Flame Arresters that a flame arrester be inspected after it stops a flame (see Sections and ). All flame arrester manufacturers have written instructions for installation, inspection, and maintenance, and these should be followed. Some flame arrester manufacturers require that maintenance be done at their facility to avoid invalidating the flame arrester warranty Documentation and Verification of Flame Arrester Maintenance It is important that the inspection and maintenance activities be documented as required per company practice and any process safety management regulatory requirements. Maintenance records should indicate the inspection results and the scope of maintenance work performed, if any. Before the flame arrester is reinstalled in the process, it should be visually inspected to make sure that reassembly conforms to the manufacturer s recommendations. Some companies affix a tag to the flame arrester indicating the inspection and maintenance date Training and Competence Issues for Operating and Maintenance Personnel Proper training on the operation and maintenance of flame arresters should be given to the appropriate personnel so that they will become competent with respect to the operating characteristics of the various types of flame arresters used in a plant, as well as their specific inspection and maintenance requirements. Many flame arrester manufacturers provide training courses on inspection and maintenance of flame arresters, either at their facilities or a client s plant. It is recommended that such courses be scheduled for operating and maintenance personnel. Also, the OSHA PSM standard, 29 CFR (j)(3), should be consulted for requirements for training for process maintenance activities On-Stream Isolation and Switching of Parallel Spares For vapor lines that cannot be shut down conveniently to remove and clean arrester elements, parallel flame arresters are sometimes installed. To be able to switch the flame arresters, suitable isolation valving must be installed upstream of the flame arresters, which will allow the switching to be done without interrupting operations. Care must be taken to assure that

5 7.6. References 151 monitoring instrumentation and/or interlocks remain functional at all times. Procedures or interlocks must ensure that the process system is protected at all times Check List for Inspection Flame Arrester Type Dry fixed element Hydraulic Packed bed What to Look For Plugging, corrosion, physical damage, correct bolt torque, contaminants, correct arrester orientation, functioning instrumentation Proper liquid level, seal fluid contaminants, foam, vessel corrosion, distributor pluggage, functioning instrumentation Packing attrition, corrosion, fouling or plugging, distributor condition (for wet type), functioning instrumentation 7.6. References Regulations, Codes of Practice, and Industry Standards 29 CFR Process Safety Management of Highly Hazardous Chemicals. U.S. Department of Labor, Washington, DC Specific References Anon New Detonation Arrester Stops Flamefronts. InTech, p. 26 (July 1999). Dafft, C. A Personal communication from C. A. Dafft, Rohm and Haas Company, Deer Park, TX to S. S. Grossel (August 1999). Howard, W. B Use Precaution in Selection, Installation, and Operation of Flame Arresters. Chemical Engineering Progress, 88(4),