The latter part of the 20th Century saw the introduction of more stringent boiler manufacturing standards, particularly in relation to welding technology and inspection, and the evolution of digital control systems that now provide an unsurpassed level of reliability. The modern, micro-processor based, high-integrity, “Boiler Control System” (BCS) has the ability to perform many of the tasks traditionally undertaken by the qualified operator allowing extended periods of automatic operation without any human intervention whatsoever. A key feature of modern packaged boilers is that they are designed to operate unsupervised for extended periods.
The degree of human supervision and operator training requirements are determined by the level of sophistication of the “Boiler Control System” (BCS). They are defined in local boiler codes and standards and differ widely between jurisdictions. For example, in some, a high-pressure boiler is defined as any boiler operating above 1 BAR (approximately 15 psig) pressure and, regardless of all other considerations, it must at all times be under the control of an appropriately qualified operator. Other jurisdictions impose capacity constraints on unsupervised operation but in many, boiler codes and other relevant legislation have been revised and extended to accommodate the demand from industry for this type of boiler.
Industrial boilers of all types generally fall into one of three categories in terms of their operational and qualification requirements. They are:
• Attended Boilers;
• Limited-Attendance Boilers;
• Unattended Boilers.
What is a “packaged boiler”?
The term “Packaged Boiler” is used to describe a boiler that is factory built and delivered either fully assembled or, as pre-built modules that can be easily assembled on site. It is essentially a self contained unit complete with a control system and all equipment required for operation. The boiler manufacturer assumes responsibility for all components and after delivery all that is required is for it to be connected to the steam distribution system, water, fuel, and electricity supplies. Almost all small and medium sized boilers manufactured today are packaged boilers and they may be the conventional firetube or watertube type, including their derivatives or, hybrid boilers such as once-through coil and electrode boilers.
What is an attended boiler?
Until relatively recently, the vast majority of steam boilers operated in the “Attended” mode. That is, they were required to have an appropriately qualified operator on-site and in the immediate vicinity of the boiler at all times that it was operating. Since the introduction of micro-processor based technology to boilers from around 1990 onwards, many traditional attended boiler owners have upgraded their control systems. Given also that micro-processor based controls are now an almost universal feature on new packaged-boilers, the traditional boiler attendant is becoming an endangered species in small to medium sized plants supplying process steam. At the very least, their role has changed from that of direct, hands-on operation to one of supervising the operation of the BCS and performing tasks that ensure its continuing integrity and compliance with the requirements of boiler codes and standards.
Boiler operation has evolved to the point that in many jurisdictions, fully attended operation is unusual outside of large, complex plants. The exceptions are large power utilities, chemical recovery boilers and similar integrated processes where the boiler is a key component.
what is a limited-attendance boiler?
The Limited-Attendance mode of operation allows for periodic checking of the boiler at set minimum intervals rather than continuous attendance. A key feature of limited-attendance boilers is that they may at any time be brought under the control of an appropriately qualified operator and operated in the fully attended mode under continuous supervision.
Many older packaged boilers and those that have been upgraded from attended status are classified as limited-attendance simply because their control systems do not achieve the same standards of integrity and reliability as do modern micro-processor based systems. Many also include provision for manual intervention which in most jurisdictions disqualifies them for consideration for unattended status.
What is an unattended boiler?
An Unattended Boiler is essentially a fully automatic boiler that operates under the continuous supervision and control of a sophisticated, micro-processor based, BCS. The BCS controls the entire boiler function including the energy input management system (on oil and gas-fired boilers commonly known as the “Burner Management System” or, BMS), the water level management system, the alarm system, the pressure controls, trip devices, all instruments and circuitry.
A key feature of this type of boiler is that in most jurisdictions boiler codes and standards dictate that a boiler that is classified as unattended must be under the continuous supervision and control of the BCS at all times that it is in service. The unattended boiler may only be operated in unattended mode. It may not be operated in attended mode under any circumstances even for brief periods and even if a qualified operator is available. In practice, the BCS employed on an unattended boiler has no provision for manual operation. In the event of a control system fault or a potentially unsafe condition, the BCS will shut down the boiler and lock-out the energy input system. It will prevent a boiler restart until such time that the control system fault has been repaired or, the potentially unsafe condition has been addressed and all of the BCS inputs indicate that the boiler is in a safe condition.
The BCS is equipped with some means of recording all major events that cause the alarms to activate. On early unattended boilers this was typically a simple printer that recorded such events on a line-by-line basis. More modern systems however, store the information in the onboard micro-processor memory for an extended period. They typically record a much wider range of information to provide improved oversight of boiler performance and, are sometimes integrated with a plant-wide information system which facilitates the remote storage of data for extended periods.
The unattended boiler is a relatively recent phenomenon. Traditionally most boiler installations operated under the continuous supervision of a qualified operator although in many jurisdictions, some small boilers were permitted to operate in limited-attendance mode. Until very recently, in most jurisdictions unattended boiler operation was subject to strict constraints. Typically, permissible maximum operating pressure and design output were severely limited and in many locations those constraints still apply, at least on paper. In recent years however, boiler codes and standards in many jurisdictions have been extended to accommodate this type of boiler and in others it has become quite common for local regulatory authorities to grant exemptions on a case-by-case basis.
With the advent in recent years of improved manufacturing standards and the rapid evolution of sophisticated, increasingly reliable micro-processor based control and monitoring systems, most small to medium sized packaged boilers from reputable manufacturers now incorporate a high-integrity BCS as standard equipment. Where unattended operation is permitted, they may only require inspection and operational certification by an accredited inspection body to ensure compliance with National and local standards.
Limited-Attendance vs Unattended Operation
To achieve Limited-Attendance or Unattended classification, the boiler is subject to strict statutory oversight covering operation, maintenance, servicing and record keeping. Boiler codes and standards require that boilers in either category be equipped with a minimum, specified inventory of safety equipment that will automatically shut down the boiler in the event of a potentially unsafe condition.
The classification applied to a particular packaged boiler will depend largely on the sophistication of the BCS or, sometimes, the type of fuel that is fired. For example, a 10 MW boiler firing natural gas may achieve unattended classification where a similar 10 MW boiler from the same manufacturer but firing solid-fuel may be classified as limited-attendance. Firing solid-fuels, particularly those with a variable moisture content, is sometimes something of a black-art requiring a degree of skill and experience. In this author’s jurisdiction, unattended solid-fuel boilers are nevertheless not uncommon even for quite large boilers over 20 MW capacity and often in multiple boiler installations.
The key differences between the Limited-Attendance and Unattended modes of operation are:
• An appropriately qualified operator or responsible person must be on-site at all times that the boiler is in service;
• Codes and standards usually require that the boiler be continuously supervised during startup and shutdown periods;
• During normal operation, the limited-attendance mode allows for periodic checking of the boiler at set minimum intervals rather than continuous supervision;
• Limited-Attendance boilers may at any time be brought under the control of an appropriately qualified operator and operated in the fully attended mode under continuous supervision;
• The operator or responsible person must be on-site at all times that the boiler is in service and must be immediately available if required.
• Where unattended operation is permitted, a qualified operator is not a mandatory requirement. The boiler may be supervised by an appropriately trained “Responsible Person”;
• The operator or responsible person may leave the site provided that he or she is available to respond to boiler outages.
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That is usually achieved by a pager or cellphone signal transmitted by the BCS;
• In many jurisdictions, unattended boilers may start up and shut down automatically under the control of the BCS provided that they are suitably equipped to the satisfaction of an approved inspection body;
• Unattended boilers are allowed to operate unsupervised for extended periods and in most jurisdictions this is currently 24-hours. Recent control and monitoring systems innovations by some manufacturers however, have seen that period extended in some jurisdictions. The 72H notation in Europe for example, signifies that the boiler is approved for unsupervised operation for periods of up to 72-hours;
• Boiler codes in most jurisdictions specifically forbid the provision for manual intervention in the operation of unattended boilers. They must at all times be under the control of the BCS. That is, they may only be operated in unattended mode. They may not be operated in attended mode under any circumstances even for brief periods and even if a qualified operator is available.
Boilers that are certified for unattended operation do not require a qualified operator. This relaxation of the manning requirements imposes substantial obligations on boiler owners, controllers, manufacturers and inspection bodies to ensure compliance with boiler codes and other relevant legislation and, that boiler supervisory and maintenance staff receive adequate training. Non compliance will almost always be interpreted as a breach of Occupational Safety and Health legislation which can result in severe penalties particularly if the non compliance is discovered as a result of an incident or accident.
Accident investigation reports demonstrate that the most common contributing causes to boiler accidents are operator intervention with the safety controls, inadequate maintenance, control device failure following maintenance by unqualified personnel, isolation or deliberate disablement of the safety devices, inappropriate operating practices and inadequate water treatment. Automatic controls are not a complete substitute for supervision. A measure of supervision by a qualified operator or a trained Responsible Person familiar with the BCS and the operation of the boiler plant is an essential feature of safe operation.