Instrumentation process heater control

Increasingly, newer fired process heater installations are adding more fuel-air combustion controls and safety instrumentation systems. However, the decision on the extent of fired heater combustion controls, instrumentation, and safety systems to employ is fundamentally a loss prevention and risk tolerance issue, rather than a fire protection one. The following recommended practices, codes and standards apply to fired heater and dryer controls and instrumentation ... 

API RP 556, Fired Heaters and Steam Generators, provides current guidance on the recommended practice for controls and instrumentation for fired process heaters for liquid and gas stream heating in the petroleum and petrochemical industries. 

TEMPERATURE HIGH Ambient Conditions Fouled or Failed Exchanger Tubes Fire Situation Cooling Water Failure Defective Control Valve Heater Control Failure Internal Fires Reaction Control Failures Heating Medium Leak into Process Faulty Instrumentation and Control... 

Most of the system interlocks are handled by the embedded NI controller. This is similar to that used in other laboratory-scale reactor systems (Mills and Nicole, 2005) except the Siemens 545 PLCs have been replaced with the National Instruments system. The National Instruments controller monitors the status of the AIMS process variables and takes appropriate action if any interlock conditions are found. In addition to these interlocks, the external heater controllers that are used for the system heating tapes and sampling valve box contain hardwired overtemperature interlocks built-in to these controllers. However, these interlocks only interrupt power to the affected heating device, so the NI controller must still take action to shutdown the rest of the process. 

A furnace, or fired heater, is a device used to heat up chemicals or chemical mixtures. Furnaces consist essentially of a battery of fluid-filled tubes that pass through a heated oven. These devices provide a critical function in the daily operation of the chemical processing industry. Process heaters are more technically defined as combustion devices designed to transfer convective and radiant heat energy to chemicals or chemical mixtures. These heaters are typically associated with reactors or distillation systems. Process heaters come in a wide variety of shapes and designs, but the basic styles include cabin, box, and cylindrical. The various parts of a process heater include a radiant section and burners, a bridgewall section, a convection section and shock bank, and a stack with damper control. Modern control instrumentation is used to maintain these rather large and elaborate systems. 

This instrumentation is for heaters located in gas processing sections, Wellhead units have a minimum of controls. 

The auxiliary feed tank (T-IA) holds 2840 L (18 bbl). This tank is equipped with a mixer and a heating coil that is rated at 48 kW (163,800 Btu/ h) and is capable of raising the feed temperature to 150 C (300 F) in 2 h normal operating temperature is expected to be approximately 80 C (176 °F). A sensor in the tank is used as a low-level alarm to shut off the heater. The auxiliary feed pump (P-1 A) is capable of generating a discharge pressure of 517 kPa (75 psi) and a maximum flow rate of 1590 L/h (7.0 USGPM). It may also be used to deliver feed to the other process units or to recirculate the tank s contents. Instrumentation is in place downstream of both feed tanks to control the feed flow rate and to measure the temperature and pH of the feed. Connections are also available for the addition of demulsifier or diluent. 

Manufacturers use two methods of measurements. In the first method called heat flux DSC, the instrument measures this temperature difference (DTA). Through calibration, this temperature difference is transformed into a heat flow, dq/dt. Therefore, there is a thermal factor that may vary with temperature. In the second method, called power compensation DSC, two individual heaters are used in order to monitor the individual heating rates of the two individual ovens. A system controls the temperature difference between sample and reference. If any temperature difference is detected, the individual heatings are corrected in such a way that the temperature is kept the same in both pans. That is, when an endothermic or exothermic process occurs, the instrument delivers the compensation energy in order to maintain equal temperature in both pans. 

Process designs vary among manufacturers, however, there is generally an air flow from the top or sides blowing down or across the bed of material in a zoned area. This zone may have its own batch dryer with a fan, a heater, instrumentation and duct work, or it may be manifolded such that the air flow is regulated to maintain a certain temperature in that section using the evaporative cooling effect to control the outlet temperature from that zone. 

System automation is made easier by the availability of many subsystems that are easily controlled by computer. Process instrumentation of all types and radiometric measurement equipment is available with standard computer interface options. Computer hardware and software is available both for simple and complex control systems. Mechanical equipment for automating the handling of multiple samples includes pumps, valves, heaters, shakers, vibrating plates, and stirring systems for mixing samples. 

A fired heater or furnace is a device used primarily to heat large quantities of hydrocarbons. These systems are very expensive and complex and require a well-trained and dedicated staff. A process technician assigned to these units studies the basic components of the system, traces out each major flow path, and works closely with senior technicians until he or she is qualified to operate the equipment. Modern control instrumentation and high-tech control rooms are designed to monitor and control all vital processes. 

During normal operations, checklists and samples are collected as advanced instrumentation monitors the process. The types of problems a fired heater or furnace system typically encounter include flame impingement on tubes, coke buildup inside the tubes, hot spots inside the furnace, fuel composition changes, burner flameout, control valve failure, and feed-pump failure. Other problems may include incorrect temperature indicator readings, failure of oxygen analyzers, oxygen leaks on the furnace, and the unexpected shutdown of downstream equipment. A fired heater system is designed to run almost continuously, 24 hours a day, 7 days a week. The operational team is in place to ensure that the equipment and systems operate safely, effectively, and produce a quality product that meets or exceeds customer expectations. 

To explore dynamic behaviour, as an example, we will use a simple fired heater as shown in Figure 2.1. It has no automatic controls in place and the minimum of instrumentation - a temperature indicator (Tl) and a fuel control valve. The aim is to ultimately commission a temperature controller which will use the temperature as its process variable (PV) and the fuel valve position as it manipulated variable (MV). 

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