Steam System Thermal Cycle Efficiency
Steam System Thermal Cycle Efficiency.
First Step in Understanding
Optimizing A Steam System
1. What Is steam system thermal cycle efficiency?
What is steam thermal cycle efficiency, and what affects the efficiency? These are questions that all steam system managers must be able to answer.
The average steam system thermal cycle efficiency is 56.3%, which means that 43.7% of the energy consumed in boilers is wasted or lost. It is impossible to use all the energy input into the boilers. Therefore, the operation will have a few acceptable losses, but a high percentage of losses can be prevented or eliminated. Some plants may be more efficient, and some plants may be less efficient. But not until the steam system is benchmarked will plant management know how much energy is being lost in the steam system.
The total system is shown in Figure 1.
Calculate the thermal cycle efficiency by subtracting the Btu’s recovered and returned to the boiler plant from the total fuel energy or Btu’s input into the boiler to generate the steam.
- When benchmarking the thermal cycle, the quantity of the sensible energy in the condensate that is returned to the boiler is considered the recovered energy.
- The deaerator uses steam from the main steam line maintain pressure and temperature on the deaerator. Therefore, it cannot be the benchmark for recovered condensate.
2. steam system items that can AFFECT the steam system thermal cycle efficiency
The steam system components that can affect the steam system thermal cycle efficiency need to be reviewed. No item can be overlooked. Each item can contribute significant losses if not properly managed.
2.1. Steam Generation
- Steam turbines
2.2. Steam Distribution
- Steam leaks
- Expansion devices
- Steam trap stations
2.3. End Users
- Shell and tube heat exchangers
- Plate and frame
2.4. Condensate Systems
- Condensate lines
- Flash steam losses
- Condensate leaks
- Condensate losses
The first areas that need to be reviewed are involved in generating steam, which can create significant energy losses even before the steam is distributed into the steam system.
3.1. Boiler Flue Gas
The boiler has, on average, a 16.4% energy loss due to the flue gas volume and the elevated temperature of the gases from the combustion process. The boiler can have several devices to lower the flue gas losses.
3.2. Boiler Outer Shell or Casing Losses
The outer shell or casing of the boiler will contribute a low loss percentage (0.5%), which is an acceptable loss as long as the boiler casing does not have hot spots or above-normal temperatures. The plant should have an infrared camera scan of the casing at least once a year if not more. The data is benchmarked to detect fatigue of the boiler internal insulation.
3.3. Boiler Continuous Blowdown
The continuous blowdown is continually skimming the boiler water impurities slightly below the water level and discharging boiler water to a blowdown tank. The estimated energy loss from the continuous blowdown is 1.5%, which will greatly depend on a number of factors that are discussed in other Inveno best practices.
3.4. Boiler Bottom Blowdown
The bottom blowdown occurs periodically from the lowest water containment area in the boiler. The periodic bottom blowdown removes sludge and discharges the water and materials to a blowdown tank. The estimated energy loss from the bottom blowdown is 0.2%, which will greatly depend on a number of factors that are discussed in other Inveno Engineering best practices.
3.5. Steam Turbine
The loss of mechanical work from reducing steam pressures with a control valve and not using a steam turbine for the pressure reduction is not added into the thermal cycle losses. A rule of thumb to use is that 75 lbs. of steam per hour equals 1 horsepower of work from a steam turbine. Steam turbines can be used in many different applications besides electrical generation.
3.6. Total Steam Generation Losses
The steam has left the steam generation area, and the summary of the estimated losses is as follows:
Boiler flue gases: 16.4%
Boiler outer surface losses: 0.5%
Continuous blowdown: 1.5%
Bottom blowdown: 0.2%
Total: 18.6 %
The energy loss is 18.6%, and the steam vapor has not performed any usable work.
To enable the steam to release the contained energy to the process, the next step is to safely distribute the steam to the end users.
Insulation is the most overlooked item that provides energy savings. According to the U.S. Department of Energy Best Practices Steam program, mechanical insulation should be used on any surface over 120°F (49°C). Therefore, all steam and condensate components need to be insulated, and the insulation must be protected to ensure long operational life. The estimated energy loss is 6.4%.
4.2. Steam Leaks
Steam and condensate leaks cost industrial plants millions of dollars in lost energy while increasing emissions, creating safety hazards, and lowering the reliability of plant operations. Steam leaks result in the loss of both latent and sensible energy. While plant personnel would be well advised to pay attention to all utility losses, greater attention should be paid to the costs and problems associated with those losses related to steam. An estimated energy loss from steam and condensate leakage is 7.5%.
4.3. Summary of the Energy Losses to This Point
Steam leaks: 7.5%
Steam generation: 18.6%
Steam distribution: 13.9%
Total energy loss: 32.5%
The energy loss is now at 32.5%, and the steam vapor has not performed any usable work.
Finally, the steam has reached the intended objective, which is to provide the latent energy to the process. The losses from not insulating the heat exchangers, tracer lines, jacket tank heaters, etc. have already been included in the insulation estimate. The steam leaks around the end users from leaking flanges, threaded connections, valves, etc. have already been added into the previous estimate.
5.4. Steam Trap Stations
The failure rates of 18% or more with the steam trap station population have been deemed a normal steam trap station operational performance level, which is totally unacceptable. The failure rate of any steam trap station population needs to be below 3%. The energy loss from failed steam trap stations from steam blowing through and the energy being lost to the atmosphere is estimated at 3.6%.
5.5. Summary of the End User Loss Points
Insulation: 4.4%, already added in the totals
Steam leaks: 6.5%, already added in the totals
Steam trap station failures: 3.6%
Total: 3.6% (insulation and steam leaks have already been added in the previous estimates)
6.1. Condensate Losses
Condensate contains 16% of the energy in the steam vapor (sensible energy). Therefore, failing to recover the condensate is a significant loss. The estimated energy loss from unrecovered condensate is 3.8%.
6.2. Steam Lost to the Atmosphere
The flash and live steam lost to the atmosphere represents the last of the energy losses in the thermal cycle. Venting steam to the atmosphere reduces the thermal cycle efficiency and contributes to an increase in emissions. The benchmark in today’s industrial plants is to have no steam venting to the atmosphere. The steam components that allow steam to be lost to the atmosphere are as follows:
- vented condensate tanks,
- vented flash tanks,
- automatic steam vents, and
- deaerator vents.
Average estimated loss = 7.4%
6.3. Summary of the Condensate System Losses
Steam lost to the atmosphere: 7.4%
Insulation: 4.4%, already added in our totals
Total: 11.2% (insulation not added in the total)
7. STEAM THERMAL CYCLE SUMMARY
The remaining the condensate has been delivered to the deaerator operation, completing the thermal cycle. The deaerator system will use live steam to add the sensible energy to the condensate and make-up water to begin the process of elevating the temperature of the feedwater to the saturated temperature of the operating steam pressure.
- Boiler flue gases: 16.4%
- Boiler outer surface: 0.5%
- Continuous blowdown (boiler): 1.5%
- Bottom blowdown (boiler): 0.2%
- Insulation (team and condensate): 6.4%
- Steam leaks: 7.5%
- Steam trap station failures: 3.6%
- Condensate losses: 3.8%
- Steam lost to atmosphere: 7.4%
Total losses: 43.7%
The steam system cycle is complete. The average energy losses are 43.7%, and only 56.3% of the steam energy was used successfully. Today, an energy cost of 43.7% is unacceptable, and optimization will definitely improve the company’s bottom line. Another tremendous plus to a steam optimizatio