Darren Smith, Zenith SAS, UK
Industrial chimneys are important items of plant whose continued operation is often critical to production. Owners, operators and users of industrial chimneys have duties to protect the health and safety of their employees and others affected by the operation of the chimney. Part of those duties is the responsibility to ensure that the chimney is properly maintained and remains in a safe condition.
Industrial chimneys are subject to deterioration because of high temperature and chemical attack as well as the effects of the weather. For industrial chimneys remain safe and fit for continued service they need to be inspected and maintained at regular intervals.
Chimney contractors have the specialist skills, training, knowledge and experience to work at height.
Damage or deterioration to industrial chimneys may not be readily apparent, partly because access to the damaged areas may be difficult, and partly because the damage may not be visible until it becomes serious. Neglect of maintenance can lead to the need for expensive emergency repairs and may even result in structural failure.
The consequence of a neglected chimney becoming unsafe or even collapsing may greatly exceed the cost of the chimney repair or replacement. Consequential damage to surrounding buildings or plant, loss of production, or even injury or loss of human life, may result from material falling from the chimney or structural failure of the chimney.
Industrial chimneys need to be inspected and maintained at regular intervals.
A responsible pro-active approach to chimney inspection and maintenance, namely to the windshield and flue plus normal chimney ancillary items, plays an important role in maintaining the chimney in a state that is structurally sound and does not present a danger to personnel or plant on site.
Inspection of industrial chimneys may be carried out either as part of a planned maintenance strategy or in response to an obvious problem. Planned maintenance allows for inspections to be carried out during plant shut-downs in association with other maintenance work; minor maintenance can be carried out at the same time and more extensive repairs scheduled for a future outage. An inspection precipitated by an unforeseen problem may require an unscheduled plant closure with associated costs due to loss of production.
Specialist skills and equipment, as well as knowledge and experience, are necessary to gain access and carry out industrial chimney inspections and repairs. This work must always be entrusted to chimney contractors with the appropriate training and qualifications for the work. Chimney contractors are qualified to work at height and are competent to use the most modern safety measures.
Modern investigation techniques
A range of investigation techniques is available. Some of the more common techniques include visual examination, such as photographs and video filming, and physical examinations of the concrete chimney windshield by hammer testing. The taking of concrete cores allows a visual inspection of the quality and condition of the concrete and reinforcement and subsequent testing. Tests on concrete cores may include chemical tests and strength tests.
Ultrasonic thickness measuring devices are used to measure the thickness of the steel plate in a chimney windshield or flue. Other investigation techniques are available for particular cases. These include à¢€Ëœhot camerasà¢€â„¢ which can be lowered into an operating chimney flue à‚— starting at the termination point of the stack and suspended from wires, the à¢€Ëœhot cameraà¢€â„¢ is able to take detailed digital photographs of the capping arrangement, internal brickwork, corbels, baffle walls and flue inlets as it descends under control to floor level.
Following the inspection, the camera is brought back up to the top of the chimney and is removed externally. This type of inspection allows engineers to identify defects prior to a shutdown and to plan accordingly, should any repairs be required. In addition, thermographic photographs show the variation of surface temperatures on the outside of a chimney and ultrasound measurements are used to determine the integrity of concrete or brick structures.
Typical maintenance and repair methods include steel repairs, concrete repairs, brickwork repairs, renewal of paint systems, repair and replacement of linings and maintenance of ancillary items such as expansion joints and liner support system guides.
Recently Zenith successfully completed the relining of a 200 metre- high reinforced multiflue concrete chimney at Ratcliffe Power Station in Nottinghamshire, England, operated by E.ON. The existing acid resistant brick lining was removed due to the installation of flue gas desulphurization (FGD) where the low temperature of the flue gas lining is below the acid dew point.
A low-temperature borosilicate glass block lining was used as an alternative lining for the full height of all four flues. Zenith is in the process of completing an identical project at AES Kilroot Power Station in Northern Ireland for the main contractors Alstom and AMEC. Flue gas ducts and chimneys in coal and oil fired power stations are subjected to highly corrosive conditions, especially when FGD is installed. The installation of cellular borosilicate glass blocks offers reliable protection.
Challenges posed by hyperbolic cooling towers
Hyperbolic cooling towers present a particular challenge due to shape of the tower. Zenith has developed unique safe working access methods enabling quick and cost-effective solutions to complex inspection and repair requirements when working on the shell, whether internally or externally.
Zenith utilizes a bespoke design cathead system based on the cornice detail at the top of the tower. The cathead is engineered to suit individual projects, supported by documentation to satisfy regulations and project requirements. There is a centrally located counter anchorage. The system can be adapted to complete work on load to satisfy market demands supported by specific risk assessments.
The cathead system is constructed piecemeal at the top of the tower. The manageable components are hoisted into position via a winch and lifting stick. The system is assembled in sections to form one unit. The system is countered with an internally located counterweight or anchorage. The system is tested on completion to a pre-determined safe working load, normally 375 kg, but it can be adjusted up to 800 kg.
The cradle platform follows the tower profile by the use of profile wires fixed into the shell and tensioned at ground level. When moving around the perimeter of the tower the load is marginally released and the system can be literally pushed to the next working position. Zenith successfully used this method of access while carrying out major concrete repairs on four 113 metre-high hyperbolic cooling towers at British Energyà¢€â„¢s Eggborough Power Station.
Natural draught cooling towers
Eight natural draught cooling towers exist at the power station. These were constructed using an early method of à¢€Ëœjump formingà¢€â„¢, which produced an irregular shape. Two different techniques were used to form the cooling towers. Six towers have a composite skin of 125 mm reinforced concrete, with an additional top surface of 50 mm sprayed gunite material added several years later. Two towers have an overall thickness of 178 mm of reinforced concrete.
Movement of the cooling towers had occurred in the past together with natural cyclic movement from the various weather seasons. Ongoing maintenance included crack repairs, removal of defective sections, which have separated from the underlying reinforced concrete shell and spalling repairs. The immediate concern was the durability of the towers including the condition of the gunite layer on four of the towers, that is to say the integrity of the connection of the gunite to the tower shells.
The consequences of a neglected chimney can be dangerous. Inspections need to be carried out during planned shutdowns.
Typical findings on this project included identifying areas of extensive delaminating requiring between 1000 and 2000 bolts per tower. Zenith favoured continuous bolting in one direction. The bolts were located at 1200 mm intervals on the horizontal plane and between 600à‚—900 mm on the vertical plane.
Bolting started to the right lower side of the delaminating area until each vertical row was complete before moving along to fit the next row in sequence. An area of at least 1000 mm was bolted beyond the delaminating area on both the vertical and horizontal planes. This had marginal results, with the worst case being that minor chasing occurred.
Exemplary safety record
The skills developed whilst working on chimneys have been utilized to carry out complex high-level tasks on other tall structures such as flare stacks on oil refineries. Zenith has developed methods to remove and replace flare tips without the use of cranes.
Zenithà¢€â„¢s safety record is exemplary and operates to the highest industry standards, adhering to the required health and safety legislation. Site safety audits are routinely undertaken and all operatives have CSCS (Construction Skills Certification Scheme) cards and safety passports.
Investment in its progressive training programme remains a priority. All the companyà¢€â„¢s steeplejack technicians are Construction Industry Training Board trained, while rope access personnel hold Industrial Rope Access Trade Association certificates. Zenithà¢€â„¢s Quality and Safety Management System has been judged against the most rigorous best practice standards to ISO 9001 accreditation from DNV.