Role of fired heater safety systems
A fully automated burner management system operating as a SIS for burner control can meet minimum safety targets, improve system availability and lower costs
NIKKI BISHOP and DAVID SHEPPARD
Emerson Process Management
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Safety and risk mitigation have always been, and always will be, an important topic for any operating company. Safety risks can be lurking anywhere throughout a facility and the consequences of an event range from minor incidents to catastrophic failures that lead to a loss of life. Protective systems are put in place to reduce the likelihood of occurrence of safety incidents and to take action in the event that unsafe conditions arise. While the safety and well being of personnel is of utmost importance, the financial impact of safety systems cannot be ignored. With so many options available, selection of a safety system can prove challenging but can also be rewarding.
Minimum safety goals must be met, but a safety system can go beyond meeting safety requirements to improve overall operations and profitability. This article will discuss common risks associated with fired heaters, the role of the safety system, applicable standards and practices, safety instrumented systems (SIS) and the benefits of an automated burner management system (BMS), including an example of cost savings.
Fired heater operation and risk
Process fired heaters present significant safety risks. Common in refineries and chemical facilities, they are used for heating, vaporisation and thermal cracking of various process fluids. Heat energy, provided by the combustion of fuel, is transferred to a charge or feed in a controlled manner. The primary functions of a process fired heater are to maintain the desired outlet temperature at the desired charge rate. Besides maintaining temperature and charge rate, control and safety systems are designed to maintain efficient combustion of fuel and safe operation throughout the full range of conditions the heater experiences. Figure 1 shows a typical process fired heater.
Burners in the heater transfer energy into the process through the combustion of fuel. As with any combustion process, care must be taken to ensure safe operation. Fuel can accumulate when burners are off but should be on and also from substoichiometric conditions. Fuel must not be allowed to accumulate in the firebox as subsequent introduction of an ignition source could be catastrophic. In addition to combustion risks, fired heaters present risks associated with the process. Unlike boilers, where the process stream is water, the process stream for most fired heaters is highly flammable hydrocarbons. Overheating or overfiring can cause process tubes to exceed metallurgical limits and rupture. In cases where tube leaks occur, resulting explosions can destroy process equipment and pose a threat to human life. The release of the process stream into the surroundings can pose an environmental threat. Even minor events can result in extended downtime for repair, impacting production.
The purpose of the fired heater safety system is to prevent disastrous combustion of accumulated fuel and to prevent overheating and the subsequent catastrophic release of the process stream. It sounds simple enough to inhibit the admission of fuel when unsafe conditions exist, but the determination of a safe state requires careful monitoring of many conditions. In particular, conditions such as fuel gas pressure and flow, furnace draft pressure, flame detection, process stream flow, combustion air flow, tube skin temperature, stack temperature, per cent oxygen and combustibles all pose an operational threat if limits are exceeded. The safety system must continuously monitor for unsafe conditions and take action when necessary, making it critical to understand all of the possible equipment failure modes and the potential impact to both the operating unit and personnel.
Safety system design and selection
Every fired heater must have some type of safety system in place. It may be as simple as a written procedure for manual intervention or it may be a fully automated emergency shutdown system. The design and selection of a safety system starts with the evaluation of risk factors and risk tolerance. It can be quite a daunting task to assess risk factors, assemble the interlock and permissive conditions and then determine which safety system to implement that meets both safety and financial targets. Fortunately, organisations such as NFPA, ISA, IEC and API publish documents that offer guidance for protective systems, which inform a user how to avoid a situation where the fuel supply should be off but is not, where the flame should be on but is off, where the process equipment is overheated, and where the protective system itself is prevented from working as it should. These standards also describe possible actions that the protective system can perform when it detects any of these situations. Each document has been developed based on experience and offers valuable information. The fact that accidents and disasters are as infrequent as they are is due to the long experience that has been incorporated into the various standards and recommended practices.
The National Fire Protection Association (NFPA) publishes NFPA 86 Standard for Ovens and Furnaces, which covers protective systems for process fired heaters. It applies to heated enclosures regardless of heat source. NFPA 86 is a prescriptive, conservative standard written like instructions with few options. While NFPA develops the standards, it does not enforce compliance to the standard. Insurers or local authorities may, in certain cases, enforce compliance.
IEC 61508 Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems, developed by the International Electrotechnical Commission (IEC), provides the framework and core requirements for safety-related system design of hardware and software, independent of industry sector. IEC also released the document IEC 61511 Functional Safety – Safety Instrumented Systems for the Process Industry Sector, which defines the functional safety requirements established by IEC 61508 for the process industry sector specifically. The International Society of Automation (ISA) released a document very similar to IEC 61511, ANSI/ISA 84.00.01-2004, and the two documents were merged into one standard, IEC 61511 - Mod. This standard is a performance-based, rather than prescriptive, standard that applies to SIS regardless of application, with no specific functions defined. S84-2004, as the merged standard is more commonly known, focuses on the safety lifecycle. Steps include identifying risks, assessing the risks and then reducing the risk by means of a SIS. Figure 2 shows the safety lifecycle as defined by S84-2004. The standard clearly defines the steps for designing the SIS and requires that users have a good understanding of their process hazards and risks. ISA also published a technical report, TR84.00.05 Guidance on the Identification of Safety Instrumented Functions (SIF) in Burner Management Systems (BMS), which offers specific guidance on SIS used as BMS. This technical report offers recommendations for assessing SIF within a BMS and provides some example safety assessments.
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