One of our inspectors sent this photo of scorched feed pump controls from a failed LWCO.
By Anzar Hasan, BPC’s chief inspector
Since the advent of boilers, there have been severe accidents and explosions related to them and there continue to be hundreds of such incidents every year around the world.
Some of these incidents are caused by manufacturing defects. The development and implementation of boiler codes globally, such as the ASME and NB codes implemented in the United States, have greatly reduced incidents related to such manufacturing defects.
Most incidents relating to boilers, however, are attributed to malfunctioning safety devices, operator error, poor maintenance or corrosion. Various governments, states and municipalities around the world have laws pertaining to construction, installation, operation and inspection of boilers.
I’ve outlined some important tips related to inspection procedures to ensure the safety of the inspector and assess the condition of the boiler for continued reliable and safe operation. Because inspections may be either internal or external, for clarity, the below tips are limited to inspection procedures for fire and water tube boilers utilized for heating and process and excludes special purpose boilers such as chemical recovery boilers, biomass boilers, steam generators utilized in the steel mills, etc. Most boilers are subjected to these inspections periodically, although the exact interval of inspections varies in different jurisdictions.
- To prevent asphyxiation and contact with hazardous residues, the atmosphere inside the boiler (confined space) must be tested prior to entry.
- All sources of energy must be isolated, tagged and locked out.
- All valves must be closed and there should be no leakage past the valves.
- Must have an escort at all times to prevent inadvertent activity around the boiler that could compromise the safety of the inspector.
- Personal Protective Equipment and protective clothing must be worn as appropriate for the type of boiler. These include coveralls, gloves, eye protection and safety shoes. Special PPE is required for chemical recovery boilers, large utilities coal burning boilers, etc.
- Special attention should be paid to boilers stored with nitrogen blanketing. It should be verified that the boiler has been properly purged and there is a minimum of 19 percent of oxygen inside the boiler before entry.
Internal inspection preparation:
The boiler must be properly prepared to get access to all areas for proper inspection as applicable such as:
- Inspection plugs in water columns removed.
- All manhole and cover plates removed.
- All washout plugs removed.
- Float chambers of LWCO devices removed.
- All fireside access panels and doors opened or removed as applicable.
Fire tube boilers:
- Closely inspect for structural integrity of the boiler such as presence of corrosion, scale, oil, cracks, grooving, metal wastage due to corrosion and erosion.
- Examine stay bolts for even tension, metal wastage, fastened ends for cracks.
- Check stays with telltale holes for signs of leakage through the hole that could be indication of a broken stay.
- Examine for oil contamination. This is usually evident along the water line.
- In water tube boilers, examine the baffles as they often become dislodged.
- Examine the fire-side surfaces for deformity, bulging or blistering as the result of flame impingement and improper combustion.
- Examine the furnace in fire tube boilers for overheating stains, distortion and minor cracks.
- Boilers often have sweating and drip marks from around the rolled ends that could be mistaken for leakage. Recommend NDT when in doubt.
- Examine for ligament cracks between the tube holes. Sometimes the fine cracks are not visible with the naked eye. Discoloration and stain may be signs of cracks. Recommend NDT or hydrostatic testing the boiler to verify the integrity of pressure parts.
- Examine the opening to the safety valves, water columns, valves, etc. to ensure there are no deposits or obstructions.
- Examine the operating mechanism of the float type LWCO and feed water regulator.
- Examine for deteriorated refractory around the burner and on the doors.
- Look through the tube to check for obstructions, bulging, etc.
- It is important to have a proper lighting source available and an inspection mirror to enable examination of areas not visible or accessible for inspection.
Water tube boilers:
- Follow a sequence for the inspection, usually starting from the steam drum at the top and follow through at each level. Enter spaces that are properly secured for entry.
- Inspect supports and hangers in economizer, generation and superheater sections, and signs of distortion, bowing, sagging, external corrosion and erosion, deposits, leakage, etc.
- Examine for broken and/or cracked welded joints – hangers, tube welds, skin thermocouple weld joint.
- Tube sampling for trending the rate of metal wastage, corrosion analysis and effectiveness of the water treatment program.
- Inspection of the fin tubes or studded tubes in the chemical recovery boilers for cracks, broken fins and studs, and cracks and grooving around the welds.
- Inquire if thickness testing of the generating and superheater tubes is part of the maintenance program and if the user keeps the trending results for timely replacement of the tubes.
- Examine the refractories around the burners, air register and dampers as applicable.
- Examine the soot blowing tube for deformation. Also examine the supports, welding joints and general condition.
- Examine thermowells for corrosion, erosion or cracking.
- Inspect baffles and screens inside the steam drum for proper securement and any missing bolts and studs.
- Inspect nozzles and manway weld joints for corrosion and cracking.
- Inspect internal surface for scale, fouling, discolorations and cracking, especially at the longitudinal and circumferential welded joints and the manway landing and weld.
- Review testing procedures for safety valves to ensure they are in compliance with jurisdictional, ASME and NB codes.
External inspection procedures:
- Walk around the boiler to check for water leakage, casing distortion, hot spots of the external casing, leakage past the safety valves, gauge glass connections, etc.
- Review the operating certificate of last inspection and/or the boiler nameplate to verify the MAWP, capacity and jurisdictional/NB/ASME stamping.
- Inspect the safety valve nameplate to verify the size, setting and capacity of the safety/relief valve is adequate.
- Verify when the safety valves were tested. The testing should be in accordance to the requirements stated in the NB Code.
- Review records to ensure the LWCO devices are flushed and tested periodically. The devices should be rapid drain or simulated tested daily and slow drain tested preferably annually by actually lowering the water level in the boiler. This is best done when the boiler is taken out of service for internal inspection.
- Check the water level gauge glass both at the boiler or remote water level indicators to ensure the levels are adequate. For remote water level indicators, verify the indicators are tested periodically to verify proper operation.
- Check all the pressure and temperature gauges to verify the boiler is operated within its designed parameters.
- Review feed water treatment program to verify the results are within the desired parameters to prevent scale, oxidation, etc. on the water side surfaces of the boiler.
- For boilers that produce superheated steam for STG’s, establish the program for steam sampling and testing is established.