February 2019

Maintenance and Reliability

Best practices in boiler maintenance and replacement

Refineries cannot afford a sudden boiler failure (FIG. 1). Regularly assessing the condition of boilers is vital.

Refineries cannot afford a sudden boiler failure (FIG. 1). Regularly assessing the condition of boilers is vital. The following are best practices for maintenance, boiler replacement and upgrades based on several refinery and petrochemical projects conducted by the author’s company.

FIG. 1. View of a typical boiler.
FIG. 1. View of a typical boiler.

Maintaining boiler health

The maintenance staff should keep a close eye on the condition of any refinery boiler. This includes valves, feedwater and steam lines, boiler corrosion, tube scale, fouling, economizers, turbines and control systems. Personnel should also pay attention to boiler performance, fuel consumption and other parameters that might indicate a non-optimum condition.

Effective maintenance and repair can extend the life of an aging boiler. Tuning can counter gradual output and efficiency degradation due to issues like soot or corrosion buildup.

Maintenance best practices include:

  • Continuously measure conductivity or total dissolved solids (TDS) to determine blowdown rates.
  • Institute automatic control of boiler excess air to maximize efficiency.
  • Check daily that water treatment is functioning properly.
  • On at least a monthly basis, look for and eliminate steam system water hammer.
  • Every quarter, monitor and assess fuel costs for steam generation. Measure steam benchmarks, boiler efficiency and steam quality to ensure that dry steam is being generated.
  • Conduct a thorough annual inspection of all boiler plant equipment (boiler, deaerator, feedwater tank, chemical treatment equipment), distribution equipment (piping, valves, pressure reducing stations), end-use equipment (piping, heat exchangers, coils, air vents, vacuum breakers) and recovery equipment (flash tanks, condensate pumps, piping, valves, fittings). Act on what you find.
  • Develop a maintenance schedule to address areas such as record-keeping, finance, personnel and training. A properly maintained boiler can outlive the personnel
    in the plant, so be sure to include the transfer of knowledge and proper documentation.
  • If the air flow is inadequate, carbon monoxide (CO) forms instead, releasing less than one-third the amount of heat. Conversely, when there is excess air, the fuel burns completely but much of the heat gets carried away by the excess stack gas. Therefore, efficient operation depends on maintaining the proper air-to-fuel ratio. Ensure that only the exact amount of air is provided to fully convert the carbon into carbon dioxide (CO2) to release the maximum amount of heat.
  • Keep heat transfer surfaces clean. Over time, deposits of chemicals such as calcium, magnesium and silica coat the water sides of the heat transfer tubes. This scale has a much lower thermal conductivity than bare steel. It retards heat transfer and leads to overheating and tube failures. An 1/64-in. layer of iron plus silica scale formed by high-pressure steam can produce a 3.5% fuel loss. Such a miniscule quantity of deposition may be missed during a cursory inspection. However, one way to detect scale or deposit formation is by a rise in flue gas temperature. If the boiler load and excess air are kept constant and the flue temperature rises, it may indicate the presence of scale. This would then be verified by a visual or ultrasonic inspection during a shutdown and removed by mechanical means or an acid wash.
  • Try to prevent scale. While cleaning and washing are sometimes required, the best approach is to prevent buildup by using water softeners, demineralizers or reverse osmosis to remove scale-forming minerals from the makeup water. Additionally, proper blowdown practices and the addition of scale retardant chemicals to water are further preventive measures.

Refineries with old boilers can extend their lifespan via good maintenance practices. This must be supported by minor repairs as inspections reveal issues with components. However, there comes a point in any boiler’s life when it becomes more expensive to keep it running than it is to upgrade or replace it. The following are important points in boiler selection and specification to consider before any upgrade or replacement.

Choosing a boiler

Generally, boilers fall into two categories: Package and stick-built.

Package boilers are fabricated in a factory and shipped to the site for installation. They are standardized into specific sizes that are produced in volume and are less expensive than alternatives. If the needs of refinery processes happen to fit within the limits of available sizes, they make an excellent fit. However, refineries typically work back from their process needs and tailor related equipment to match those requirements. Thus, package boilers may not fit the bill.

Another limitation for package boilers is transportation. Sometimes, there comes a point when it is no longer feasible to ship a boiler from the factory to a large refinery due to size. Large packaged units can be difficult, if not impossible, to transport.

The usual alternative is the stick-built boiler. They are tailored to the individual facility and built onsite. The various components are shipped. However, assembly and welding are done at the facility. Compared to a package boiler, a stick-built model is more expensive. They also take longer to install, as they are customized rather than off the shelf.

Hybrid boilers combine elements of package and stick-built boilers. A boiler is designed for a distinct refinery application to customer specifications. It is preassembled and tested at the manufacturer’s site. It can then be transported in one or more pieces for installation and commissioning. Some boilers are small enough to ship in one piece, but it is often the case that they have to be shipped in more than two pieces. Due to pre-testing of the entire assembly, the work that needs to be done at the refinery is greatly reduced. By minimizing site work, the cost is lower compared to traditional stick-built units.

Getting boiler sizing right

In this era of maximizing efficiency and keeping budgets tight, some try to precisely size boilers with little margin. Smaller boilers that are constantly run near capacity will certainly be cheaper. However, reliability eventually becomes an issue due to constant wear and tear.

A best practice is conservative sizing. By running the boiler well below capacity, the likelihood of a breakdown is greatly reduced. Warranty problems can also be avoided. All that is required is the specification of a boiler with a larger steam drum than is technically required. The refinery gains a more reliable boiler for a small increase in cost. In the long run, this is far cheaper than trying to keep a small, hot rod boiler going.

Why is that the case? Smaller boilers that run near capacity can suffer from water carry over from the steam drum to the superheater. Over time, the superheater blows out. A larger steam drum offers more separation. Thus, water can more easily be removed from the steam before it arrives at the superheater.

Furthermore, if a boiler loses feedwater flow, the presence of a large steam drum means water issues can be mitigated before steam levels fall. Instead of having a minute or two to avoid a trip, the facility gains extra minutes to react to any feedwater situation that may exist.

Refinery sizing

Take the case of a boiler at a large facility. The specifications called for 290,000 lb of steam/hr at 250 psig and 555°F of superheat. In addition, the boiler needed to be ramped up or down rapidly in response to demand. These requirements created certain problems.

The proper design of a boiler that will cycle up and down is to have a maximum heat release of less than 80,000 Btu/hr/ft3. That limits the size of a boiler that can be shipped on a truck or rail to approximately 200,000 lb/hr. In this case, they needed 290,000 lb. One option would be to buy two smaller package boilers. A second possibility was to buy an undersized package boiler and put a larger burner in it. Erecting a custom, stick-built boiler onsite was a third option. The latter choice would meet the performance requirements, but not budgetary ones. The solution was a modular, custom-built boiler. The convection section and the mud and steam drums were manufactured in the shop and shipped to the field for installation. The tubes were attached in the field. This turned out to be about half the price of a stick-built boiler of the same size.

The boiler can ramp from 20% to 100% load in about 5 min. without breaking down due to thermal stress. Oversized downcomers are positioned out of the heat path. This eliminates deviation from nucleate boiling where steam goes up the downcomers and fights with the water coming down. This is the most common cause of rupture in steam boilers. Finally, the boiler has an oversized steam drum as a safety factor. In the event of an unscheduled power outage or incident, having more steam in the drum leaves more time for an orderly shutdown.

Take emissions regulations into account

Around the world, governments are rolling out stricter environmental regulations. For example, the US state of California has set aggressive targets to reduce greenhouse gas (GHG) emissions to 40% below 1990 levels by 2030. This includes a 20% reduction in emissions from oil refineries. In such a climate, older coal-fired boilers can be a burden. The price tag for the installation of clean coal technology is likely to be much higher than shifting boilers over to natural gas. Some facilities go as far as replacing aging coal-fired boilers with a combined-cycle plant in which both steam and electric power are produced. In some cases, older steam turbines can be repurposed as part of the new design. In other cases, they need to be replaced.

The final design depends on refinery needs. If only process steam is required and not power, a combined-cycle plant is an unnecessary expense. If lower-cost electricity is to be generated onsite, it may be wise to invest in a combined-cycle facility complete with gas turbines, steam turbines and heat recovery steam generators (HRSGs). In any case, a switch from coal to natural gas will significantly reduce the emissions of a refinery.

Replacement to lower operating costs

Just as old household appliances are far more inefficient than the latest models, it is the same with boilers. Some boilers have been operating for several decades. For example, one US facility had several boilers dating back to the 1940s. These inefficient, 50,000-lb/hr, induced-draft boilers were costly to run, leading to escalated fuel costs. They were replaced with a modern, high-efficiency, 200,000-lb/hr unit. This came with several benefits: Natural gas usage was greatly reduced, and the presence of a larger furnace provided the platform for reducing emissions and eliminated problems with flame impingement.

Contemporary boilers come with many features that were unavailable to their aging relatives. Selective catalytic reduction (SCR) technology and low nitrous oxide (NOx) burners are integrated into the boiler so that all components work together to curtail NOx production. New designs lower the amount of refractory, so operators do not need to worry about replacing refractory seals or rebuilding refractory walls. Correct superheater placement improves reliability. Modern superheaters should be buried in the convection section. This design protects them from the hot exhaust gas entering them and extends lifespan. Additionally, duct firing in the HRSG allows production of either more steam or steam at a higher pressure.

Automatic blowdown control

Automatic blowdown controls entails the installation of instrumentation and controls on steam boilers to automatically control blowdown conductivity.

Some firms recommend high conductivity, as this helps lower boiler feedwater expense and chemical use, but it can be a false economy. High conductivity increases boiler system fouling and corrosion. It also inhibits heat transfer. Low conductivity in a boiler can be similarly destructive. It is advisable to establish an accurate way to control conductivity within a desirable range. Doing so via automatic blowdown controls reduces costs, lowers maintenance requirements and boosts efficiency. HP

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