Fuel prices are prompting plant professionals to consider improving steam system efficiency. The U.S. Department of Energy (DOE), through the Industrial Technologies Program (ITP), instituted the BestPractices program to provide services and information for improving energy use and productivity in areas such as compressed air, motors, process heating and steam. We used one of those tools, the Steam System Assessment Tool (SSAT), to perform a parametric study to quantify energy savings from five common best practices for packaged boiler systems. You can use the link at the end of this article to download the free software and perform plant-specific studies of your own.
We developed a base model to benchmark and analyze the energy savings related to potential system changes. The five best practices investigated were:
- Makeup water temperature
- Steam pressure
- Blowdown rate
- Blowdown heat recovery
- Temperature and quantity of condensate return
Table 1 shows the SSAT input values for the base model used for the study. Although the base model reflects one specific system, projected energy savings can be applied to more complex systems.
Case 1 - Makeup water temperature
As the temperature of your makeup water increases, the amount of boiler fuel you need to convert it to steam decreases. Makeup water can be preheated using blowdown heat recovery systems and stack economizers. The latter approach is more efficient with continuous and automatic blowdown systems. Heat can be recovered from the water and flash steam in the blowdown. A heat exchanger must be used for recovering the heat content in the water because dissolved solids are present. The preheated water from the exchanger can be sent to the deaerator as part of the makeup water.
To collect energy from the steam portion, dump the blowdown to a flash tank, where a portion of the water will flash to steam. The flashed steam can be used in low-pressure applications such as in the deaerator.
When possible, recover heat from the stack. An economizer efficiently recovers wasted stack heat and transfers it to boiler makeup water or any other fluid. An economizer might reduce fuel usage by 5% to 10% by recovering this waste heat. Condensing economizers require more plant modifications and cost more, but provide higher heat recovery potential than conventional economizers. The condensing economizer recovers not only the sensible heat, but also the latent heat in the flue gases.
To examine the effect of makeup water temperature on energy consumption, we made multiple runs of the base model while varying the makeup water temperature from 60F to 200F and the header pressure from 50 psig to 300 psig. The energy input requirement corresponding to changes in these variables is shown in Table 2. It shows that increases in makeup water temperature decrease the energy usage from the base case.
Case 2 Steam pressure
As your operating steam pressure increases, so do your system losses in the form of increased steam leaks from piping and through traps as well as convection and radiation losses from system components.
SSAT assumes gross expected steam trap failure and leakage rates based on your interval between maintenance projects. The software considers each failed trap and steam leak as a 1/8-inch and a 1/16-inch orifice, respectively. Thus, as system pressure increases, more energy is lost through leaks and failed traps. Although SSAT accounts for steam losses, it doesnt calculate convection and radiation losses as a function of pressure. In addition, using a greater operating steam pressure requires more fuel consumption.
Like any mechanical device, steam traps fail over time. Perform periodic testing to locate the malfunctioning traps that waste energy, harm production and damage equipment by allowing condensate or air to contaminate the product or decrease the steam temperature.
Detecting defective traps can involve ultrasonic testing, listening to audible trap sounds, checking condensate vents, opening test valves, visually checking using a sight glass, and measuring temperature differences. Install drip legs and steam trap stations:
- At low points in the piping system
- At changes of direction
- Upstream of normally-closed valves
- Downstream of end-use equipment
Steam systems that go three to five years between inspections will suffer 15% to 30% trap failure, according to DOE estimates. Furthermore, DOE recommends that a system operating at a pressure below 30 psig be tested annually for failed traps. Systems operating at a pressure between 30 psig and 150 psig should be tested monthly or quarterly. Systems with higher steam pressure ought to be tested weekly or monthly.
In some cases, larger leaks get neglected for long periods because many think theyre too obvious to be a problem. Steam should not be apparent in a plant, because its discharged to a condensate return system. High-pressure steam leaks are dangerous because they can be invisible to the naked eye.
To examine the effect of header pressure on energy consumption, we made multiple runs that varied the header pressure from 50 psig to 350 psig. The results are presented in Table 3.
Case 3 Blowdown rate
Its impossible to have boiler blowdown without losing energy, which is why you should minimize blowdown rates. The duration and frequency of blowdown is a function of the quantity and condition of your boiler makeup water. A typical blowdown rate is between 4% and 8% of the boiler feedwater flow rate.