Better Understand Boilers

A number of factors affect selection, performance, reliability and safety

By Amin Almasi, mechanical consultant

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Some boilers instead use a spiral-wound furnace, with inclined tubes as opposed to vertical tubes. Such an arrangement reduces the number of tubes in the furnace and, hence, raises the fluid mass in each tube. At the same time, it increases the individual tube length; each tube passes through every part of the furnace heat transfer surface. This smooths out variations in heat input between, for example, mid-wall and corner locations because each tube passes through both regions.

Many different options exist for dealing with part-load operation, with the proper choice depending upon the specific boiler application.

Safety, Reliability And Operation

Each year, numerous boiler accidents and failures occur. Most stem from malfunction of different parts, error in operation, poor maintenance, corrosion, etc. Properly functioning control and safety devices are absolutely essential. In addition, you must establish and enforce regular testing and verification regimes to provide confidence the safety and control features will work when needed.

Safety or relief valves usually serve as the primary safety feature on a boiler; these valves prevent dangerous over-pressurization. Safety valves are required in case there’s failure of pressure controls or other devices designed to control the firing rate. If something goes wrong, the safety valve is designed to relieve all the pressure generated within the boiler. So, you should think of the safety valve as the last line of defence. It should have sufficient relieving capacity to meet or exceed the maximum burner output. Several factors, such as internal corrosion, restricted flow, etc., can impede the ability of a safety valve system to function as desired. Internal corrosion is probably the most common cause of freezing or binding of safety valves. Keep all safety valves free of debris or foreign materials and test their operation regularly. It’s not good practice to operate a boiler too close to the safety valve setting. This may cause the valve to leak slightly, resulting in an internal corrosion build-up that eventually will prevent the valve from operating. As a very rough indication, a boiler’s steam pressure often is maintained at approximately 75–80% of the safety-valve set pressure.

Water flow or level control and low-water fuel cut-off usually serve as the other important control and safety features of a boiler. These devices perform two separate functions. However, on very simple and small boilers, they often are combined into a single unit that provides both a water control function and the safety feature of a low-water fuel cut-off device. However, for many boilers, two separate sets of devices should handle these two functions. Usually, a boiler, particularly a medium or large one, should have two independent low-water fuel cut-off devices (a primary and a secondary). Many codes and jurisdictions require two such independent devices on steam boilers.

Modern fuel systems for boilers are complex assemblies, consisting of both electronic and mechanical components. Many things can go wrong with a boiler’s fuel system. For instance, ignition transformers may deteriorate or fail; ignition electrodes may burn and become coated; fuel strainers and burner equipment may clog up; fuel valves may get dirty and leak; air/fuel ratios may drift out of adjustment; and flame scanners may become dirty. A fuel system should incorporate many safety features. The burner system, in particular, requires periodic cleaning and routine maintenance. Failure to maintain the equipment in good working order could result in many problems such as excessive fuel consumption, loss of heat transfer or even an explosion.

The flow of water in different parts of the boiler as well as the temperature profile in the furnace and hot gas sections demand care; these are key operational parameters that require measurement. For example, stack temperature reflects the temperature of the flue gas leaving the boiler. A higher-than-usual stack temperature indicates the tubes may be getting a build-up of soot or scale and inefficiencies exist in the heat transfer regime.

The majority of boiler troubles, failures and accidents are avoidable. One of the most effective tools to prevent such problems is condition monitoring. Most boiler problems and issues don’t occur suddenly but develop slowly over a long period of time. The best way to detect important changes that may otherwise go unnoticed is to comprehensively record condition data and carefully evaluate those data periodically.

For the best performance, safety and reliability, maintain the fire in the furnace section as uniformly as possible to avoid an excessive rate of combustion, undesirable variations in temperature, and possible explosions. The destructive force in a boiler explosion comes from the instant release of energy whether in combustion system or steam sections.


 

AMIN ALMASI is a mechanical consultant based in Sydney, Australia. Email him at amin.almasi@ymail.com.

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