According to The Combustion Engineering Association (CEA), information received from engineering inspection bodies reveals that “approximately 25% of all steam boilers currently fail their annual inspection first time”. The obvious question, is why so many?
CEA director David Kilpatrick explains: “Some 95% of incidents and accidents relating to steam boilers result from the poor management of water treatment. Modern boilers are often forgotten about, particularly with improvements in reliability and safety. And yet for many, without a steam boiler, their process or business would stop.”
He continues: “Water treatment is a particular area that demands attention, even more so in hard water areas. Inside a steam boiler, calcium and other salts precipitate out of hard water under heat and plate the tubes and furnace with scale. Just 1mm of scale on the tubes will reduce boiler efficiency by 10%. And that’s not the worst of it. Scale in the gap between the furnace and outer shell of the boiler will prevent heat transfer. But it doesn’t stop the furnace from heating up: eventually the steel at the bottom of the furnace will melt, causing a furnace collapse. This results in the burner blowing clean off the front of the boiler, akin to a jet, forcing the boiler in the other direction, passing through anything in its path. Anyone in the vicinity lucky enough to avoid impact will likely be scalded to death by the escaping high-temperature steam.”
“Even though the managing director may not even know there is boiler house on site, he or she will ultimately be held accountable in the event of a catastrophic failure,” says Kilpatrick. “Sure enough, it is possible to appoint a competent person, but they must receive suitable training and assessment. Too often, plants no longer employ dedicated boiler operators and instead pass this role to a fitter or electrician, for example, for whom it becomes only a small part of their job.”
Although it is not a legal requirement to have an accreditation for operating steam boilers, there is a lot to learn. Many are even unaware of the annual inspection, which is necessary to comply with the PSSR (Pressure Systems Safety Regulations) and for insurance purposes; see HSE information via www.is.gd/akemej. Aside from health and safety concerns, the cost of repairing steam boilers can run into many thousands of pounds.
For all of these reasons, the CEA - a not-for-profit educational charity - has created a series of 10 guidance documents, including BG01, compiled in conjunction with the Safety Assessment Federation (SAFed) and the Health and Safety Executive (HSE). BG01 replaces the previous PM5 and PSG2 guidance notes from the HSE and SAFed respectively. Its second edition, published in 2019, is available via www.is.gd/yozepi. The CEA delivers several conferences annually across the UK that help teach companies about technical boiler house risk assessments and compliance with BG01. Further events set out to help senior managers understand the importance of correct steam-boiler operation.
“A further issue is the need to ramp-up from cold because the production manager is urgently calling for steam,” explains Kilpatrick. “The operator is then forced into bringing the steam boiler online too quickly, affecting its coefficient of thermal expansion. Steam boilers are essentially large welded cylinders, and while the furnace can quickly reach temperatures of around 1,200°C, the boiler’s exterior remains cold. The common upshot is stress cracking where the end plates of the boiler join the outer shell. If these cracks give way, a large stored-energy explosion will result, which can cause extensive property damage, severe injury and even death. For this reason, as well as annual inspections, NDT [non-destructive testing] of the boiler’s longitudinal weld seams is a statutory requirement every five years.”
Another potential failure issue relates to leaking tubes in the boiler’s passes, again largely due to poor operation, which can lead to stress cracking and other defects.
“All of these factors are linked, and it’s about correct operation as soon as a new boiler arrives,” states Kilpatrick.
Thermal modelling by Fulton, a steam boiler OEM, shows where the main thermal stresses occur in common horizontal tube-fired boilers.
“This type of boiler is welded at both ends, which can be problematic when it comes to material expansion,” states Leigh Bryan, national consultant specialist at Fulton. “Any issues identified with the structure of the boiler will typically lead to a D-patch repair, which can often cost in the region of £20,000. And that’s not taking downtime into account, or hiring a temporary replacement. Scale is another area of concern as there are a lot of horizontal surface where it can accumulate – on the tubes, the bottom of the boiler and the top of the furnace.”
To help negate many of the common issues, Bryan advises avoiding ramping up from cold too quickly and keeping on top of water treatment to sidestep the development of scale and oxygen pitting. However, the company suggests it offers an altogether better solution in the form of vertical steam boilers. Although Fulton manufactures both vertical and horizontal boilers, the latter’s tubes are an easy target for scale build-up.
“A vertical boiler such as our VSRT [Vertical Spiral Rib Tubeless] is still a pressure vessel, so is also subject to annual insurance inspections, but because it doesn’t feature any tubes, there is no need for the five-year NDT test, saving on the cost of a specialist visit,” says Bryan. “Instead, the insurance company’s engineering surveyor carries out a five-year hydro test at 1.5 times the working pressure. Furthermore, due to the fully wetted design and use of thick-gauge materials, the VSRT can heat up from cold and expand freely without developing high thermal stresses.”
At present, only horizontal boilers tend to serve the larger end of the market, typically above 4 T/h capacity, so this design remains commonplace in industry. For plants concerned about the potential of failed inspections, a maintenance contract is always a good option, such as those provided by CFB Boilers, which also specialises in both horizontal and vertical steam boilers.
“These contracts will include two or three service visits a year, depending on the boiler’s running hours,” explains Jeff Ross, technical director at CFB Boilers. “Importantly, one of those service visits will be accompanied by a boiler inspector. At that time we will remove parts at his or her request. We also fire the boiler up and check the combustion to make sure the burner is functioning correctly, and verify all the safety features.”
Ross says that the main issue is usually poor water treatment. CFB Boilers, while not a chemical engineering company, carries a comprehensive section on water treatment in its steam boiler manuals that highlights its importance. However, it is naturally the user’s responsibility to act upon this advice.
“Normally, provided the customer has the correct water-treatment regime in place, it will be rare for a steam boiler to break down,” he states.
BOX: TESTING THE WATER
Advances in industry education, as well as the efficacy and usability of water-treatment products, make clean, contaminant-free water achievable for steam-boiler users. For example, according to water treatment chemicals specialist SolidTek, the recent development of eco-friendly solid-paste chemicals makes water treatment application and control easier and safer than ever before.
“The most important time to consider water-treatment requirements is at the design stage, so consulting with a specialist at this point is highly advisable,” says Steve Crick, managing director at SolidTek. “For example, it is critical that the feed-tank design is correct and follows the guidance of BS2486, as well as BG04 from the CEA. Other factors to consider include make-up water quality, condensate return volume and quality, boiler pressure, and OEM specifications for water-treatment parameters.”