Published Mar 2, 2009

Saving energy and improving process performance with flow control solutions for fired heaters

The cost of energy is constantly increasing and its use impacts detrimentally on the environment in the form of increased CO2 emissions to the atmosphere. Identifying energy saving opportunities will increase industry’s profitability and competitiveness. Boilers and fired heaters typically convert between 70% and 90% of the fuel they consume to useful heat. In the hydrocarbon industry’s fired heaters, fuel gas, fuel oil or a combination of both are used to convert fluids, such as crude oil, residues and gas oils, to transportation fuels and other products. Reliable and energy-efficient operation of processes is important for the success of the business in supplying top-class products to competitive markets.

The cost of energy is constantly increasing and its use impacts detrimentally on the environment in the form of increased CO2 emissions to the atmosphere. Identifying energy saving opportunities will increase industry’s profitability and competitiveness. Boilers and fired heaters typically convert between 70% and 90% of the fuel they consume to useful heat. In the hydrocarbon industry’s fired heaters, fuel gas, fuel oil or a combination of both are used to convert fluids, such as crude oil, residues and gas oils, to transportation fuels and other products. Reliable and energy-efficient operation of processes is important for the success of the business in supplying top-class products to competitive markets.

Many fired heater operations are manual or semi-automated and require frequent monitoring to control process variations. Since most refinery heaters are of the natural-draught design, they are more reliable as regards the availability of combustion air. However, heater operation that is manually controlled is typically less efficient. According to K. Vinayagam (Hydrocarbon Processing – October 2007), heater tuning can improve heater thermal efficiency by 1-4%, based on existing operating conditions only. A 1% fuel savings by heater tuning in a 100 MMBtu/hr heat release, saves approximately USD 50,000 yearly in fuel cost.Figure 1. Typical fired heater design.

Proper combustion required to maximize heater efficiency involves control, maintenance and monitoring of the process fluid’s outlet temperature, draught, excess air and fuel-firing rate.1 Flue gas temperature provides the greatest opportunity for improving overall heater efficiency.1 Leakage from the heater pass valve poses a risk to health and the environment and increases maintenance costs. Fuel gas valves are important to the whole process; a failure in the fuel gas system may shut down the entire process unit (Figure 1).

Metso intelligent flow-control technology and services will help the hydrocarbon industries to achieve the most demanding targets. Long experience and application know-how in providing valves and services to the industry is essential when selecting optimal valves that meet the specific requirements of fired heaters. All control valves should be carefully sized and selected to ensure long-lasting performance for the whole life cycle. Follow this article and learn how Metso can help to improve heater performance through intelligent and inherently reliable valve solutions.Figure 2. Metso damper solution.

Metso Damper Solutions

For complete combustion of fuel, oxygen available in the air must be supplied in excess of theoretical requirements. With excess air, fuel energy is wasted in heating inert N2 from ambient to stack temperatures. At higher temperatures it contributes to NOx emissions. When excess air supply is low, the combustion process may not be complete and carbon monoxide is formed. Therefore excess air should be minimized, and at the same time, the combustion should be complete. Optimum air and draught are achieved by adjusting the burner register and stack damper. Closing the damper will reduce draught and excess oxygen demand, and therefore stack temperature and fuel consumption are reduced, which improves the heater’s thermal efficiency. Excess air measured in terms of excess O2 in flue gases should be 3% - 5% dry or as specified. According to K. Vinayagam, as a rule of thumb, every 3% reduction in excess O2 increases heater thermal efficiency by approximately 1%.

Known as Metso damper solutions (Figure 2), our B-series pneumatic actuators and ND9000 intelligent valve controllers have proved a successful answer to providing enhanced damper control in petrochemical, oil refining, pulp/paper and power generation plants. A proven customer success story verifies the results achieved.

 A 3% reduction in excess O2 can reduce the fuel gas consumption by approximately 1%, which means potential annual fuel-gas savings of between USD 50,000 and USD 400,000, depending on heater design and the duty required by the process.

Heater coil flow control

Heater pass control valves have traditionally been globe valve designs. During the heater cycle, some coke and sticky oil may start to accumulate in the rising stem gland packing, especially in heaters processing heavy oils, such as crude oils, residues and heavy gas oils. Leakage and sticking will reduce the accuracy of throughput control and affect heater performance. Leakage from the heater pass control valve poses both environmental and safety issues due to the risk of fire and oil spills, because the sticky residue accumulates on the valve bonnet and the refinery floor. Poorly performing valves in the direct heater system must be maintained be­cause they will have a direct impact on the efficiency of the process. Frequent main­tenance may be required for conventional globe valves, sometimes from every 6 months to every 2 - 3 months. All mainte­nance operations require frequent visits by operators to the plant.

Metso’s rotary gland designs are inher­ently reliable and do not suffer from leak­age problems, thanks to their rotary stem which tends not to move process media into the packing area, as can happen with a rising stem design. For further protec­tion, an appropriate self-adjusting emission gland eliminates leaks and maintenance. Simple rotary designs offer non-clogging flow ports and rugged hard-faced trims. Costly service work, involving the removal of the valve from the line and the need for replacement parts, is eliminated. By using the same face-to-face dimensions as globe valves and upgrading linear valve technology to rotary designs, the need for changes to pipe work configurations is eliminated.

Throughput losses due to sticking and poor control performance will be avoided with high-performance rotary valves. Flow through the heater pass may be changed as the need arises with a maximum rangeability of 150:1. Metso‘s ND9000 intelligent valve controller ensures high positioning accuracy and fast response. Trend data collected by the valve controller and analyzed by FieldCare configuration and condition monitoring software makes it possible to predict and respond to maintenance requirements and avoid unscheduled downtime.Figure 3. Metso valves at heater coils.

Major refineries in Europe and Latin America have tested these concepts by installing Metso valves in their heater coils to replace conventional globe valves (Figure 3). The rotary valves have been in service for several years without requiring maintenance and show no sign of leakage. This also reduces visits to the plant by operators. Cost savings of approximately USD 38,000/valve in a four-year process cycle may be achieved due to reduced maintenance – the precise cost depends on local costs and on the type and size of the valve. This gives potential total cost savings in fired heaters with four to eight control valves of USD 150,000 – 300,000 in four years on maintenance costs alone.

Fuel gas controlFigure 4b. Neles R series valve, a segment ball control valve.

Optimizing fired-heaters’ combustion and heat transfer to the process fluid is not only a matter of saving energy and costs – safety must be considered carefully, because this equipment is operated by people and the risk of disastrous Figure 4a. Neles Finetrol, an eccentric plug control valve.accidents is evident. Various standards have been written that are enforced by authorities around the world. The burner fuel gas train consists of the fuel gas supply, the pilot gas line and the fuel gas line and several instruments. In many cases, pressure relief is required. On large burners, a safety shutoff valve is generally preferred. The fuel-gas safety shutoff valve must be certified for fired heater use. It has a manual reset and a solenoid function that will prevent the burner from re-igniting. This is an extremely important safety feature. Most large industrial furnaces have a gas flow control valve for heat input.

Proper combustion requires maximizing heat transfer to the process fluid, and accurate fuel gas control plays a major role in the operating costs of fired heaters. Wide rangeability is often required, if various operating loads are needed. Metso rotary valves have been widely used in fuel-gas control applications. There are several examples where two globe-valve fuel controls have been replaced by a single Neles Finetrol® eccentric plug valve (Figure 4a). This is possible due to the wide rangeability that eliminates the need to use split range control. With the Neles segmented ball valve (Figure 4b), the rangeability is as high as 1:150. Advanced ND9000 digital valve controllers ensure high positioning accuracy and fast response that reduce process variability, and Figure 5. Neles ESD valves.therefore make it possible to process the fuel gas closer to the set point and reduce fuel gas consumption. On-line trend data collected by the Neles ND9000 and analyzed by Neles FieldCare software makes it possible to predict and respond to maintenance requirements and avoid unexpected trips in the fuel gas supply to burners. Packing construction minimizes any leakages to the environment.

With Neles high-integrity certified ESD valves (Figure 5) and the SIL 3 Neles ValvGuard partial-stroking system, fired heater safety and reliability can be improved. This is assured by the rugged construction of the safety valve assembly, continuous self-testing, metal seat technology and the stem ball design. With this solution no solenoid valve is needed and costs are lower due to reduced engineering work, wiring and commissioning. The knowledge of the exact state of heater safety valves can be identified with the Neles ValveGuard system that supports Foundation fieldbus communication by pre-warning of any upcoming problems, planned maintenance actions and secure plant safety.

 

Conclusion

Metso’s long and proven experience with many fired heaters and boilers around the world shows that there indeed is significant potential to save energy, reduce environmental impacts and improve heater safety by using intelligent valve solutions.

This can be achieved by using fully automatic solutions with monitoring and commissioning services, inherently reliable valves with high rangeability, no leakage and reduced VOC emissions through the gland packing. The ND9000 intelligent valve controller plays a major role in reducing the variability by high positioning accuracy and fast response. On-line trend data collected by the smart controller and analyzed by FieldCare software make it possible to predict maintenance requirements.

Potential cost savings due to reduced excess O2 can save fuel gas consumption annually by between USD 50,000 and USD 400,000, depending on heater design and the duty required by the process. Leakage elimination in fired heaters with four to eight control valves gives potential cost savings of USD 150,000 – 300,000 in four years on maintenance costs alone. With optimum fuel gas control, fuel gas consumption and therefore costs can further be reduced.

 

Text by Sari Aronen

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