Figure 1. A film of air or noncondensable gas that
is only 0.001" thick has the resistance of a 3" wall of iron.
Entrained air and noncondensable gases are some of the major problems in a steam system that can cause production problems for a plant’s operation. Air in a steam system dramatically reduces the system’s performance. For example, the thermal conductivity of air is 0.000049, compared with 0.002 for water, 0.20 for iron, and 0.96 for copper. Not removing air and noncondensable gases from the steam system can reduce heat transfer efficiencies by 21 percent or more, depending on the air concentration in the steam system.
Feedwater contains a certain small percentage of noncondensable gases in solution. When the boiler water changes state (liquid to vapor), the noncondensable gas is released and is carried with the steam into the plant. Steam will release the latent energy to the process and condense down to condensate in the heat transfer area, but the noncondensable gases do not condense. These gases stay in the heat transfer component unless some method or action removes them. Fortunately, during the steam system operation, the percentage of steam in the system will be close to 100 percent steam vapor with a small percentage of noncondensable gases.
Figure 2. Stagnant air, condensate, rust or
corrosion, and scale are among the obstacles to effective heat transfer in a
steam system.
On heat transfer units, it is extremely important to have functional vacuum breakers on the heat transfer units to enable the condensate to drain out of the unit by gravity. If vacuum breakers are not on the heat transfer unit, then the vacuum will hold the condensate in the unit and cause another set of issues. On a steam line that does not have vacuum breakers, the vacuum that will occur will draw air in from components on the system (such as air vents, valve packing and flanges) and fill the void in the line with air.
Therefore, when energizing or restarting a steam system or steam heat transfer components, one of the first goals should be to vent the air out of the steam system or components.
Figure 3. The air venting device typically is a
thermostatic balance pressure bellows unit with a very low subcool.
How Does Air Affect the System?
Air Reduces the Heat Transfer Efficiency. The release of latent energy (change of state) to condensate in the steam components takes place on the heat transfer surface, which is where heat is being transferred due to the temperature difference (steam to the process). The steam component transfer is consuming the latent energy, and the steam is condensing to a liquid (condensate); the condensate is drained away by gravity, but the noncondensable gas and air remain.The noncondensable gases form a stagnant film on the walls of the heat transfer surface, which creates resistance. Heat energy transmitting through the heat transfer surface has to pass by conduction through these films of resistance. A film of air or noncondensable gas that is only one-thousandth of an inch (0.001") thick has the resistance of a 3" wall of iron (figure 1).
The latent heat energy of steam must pass from the steam heat transfer area to the area where the process material is being heated. To do so, it must pass through several obstacles (figure 2):
- On the steam side, a stagnant film of air/steam.
- The condensate film.
- Any buildup of rust or corrosion material.
- Heat transfer metal wall.
- On the product side, rust or scale.
- A stagnant film of material on the process side of the wall.
Figure 4. During the startup period for steam lines,
the drain valve off the line drip pocket should be open to vent air.
The pressure reading is in absolute units. For example, suppose the total pressure of a mixture of air and steam is 114.7 lb/in2absolute (337.87°F). In addition, suppose the mixture is made up of 75 percent steam and 25 percent air or noncondensable gases.
- The partial pressure of steam is 114.7 x 0.75, or 86.02
lb/in2 absolute (317°F).
- The partial pressure of the air is 114.7 x 0.25, or 28.56 lb/in2 absolute.
There are other steam system issues with the concentration of air and noncondensable gases. The buildup or volume of air and noncondensable gases in the heat transfer area is not constant. The thickness of a stagnant film of air can vary due to velocities, baffles, flow direction, metal finish and other heat transfer internal designs. This factor provides even more problems with uneven heating of the products.
Plants increase steam pressures to overcome the issues with air and noncondensable gases. In turn, the higher cost of producing higher steam pressures increases the plant’s energy cost.
Figure 5. A shell-and-tube heat exchanger typically
has a port on the top of the shell to accommodate a vacuum breaker and air
vent. The process steam side needs to have the air purged to ensure proper
startup and temperature control.
Eliminating Noncondensable Gases or Air
Several methods can be used to vent air from the system or heat transfer unit, including manual valves, automatic valves, air venting devices and steam traps.Manual Valve. While a manual valve requires an employee to operate the device, which can be a negative at many plants, the plant employee can ensure all of the air has been vented from the system. Using a manual valve, the plant employee also can drain condensate from the system during startup. Manual valves typically are used on steam and pressurized condensate lines.
Automatic Valve. An automatic valve requires electric or compressed air connections, is more complex, and has no method of ensuring all the air has been removed from the system. These negative factors may be offset for some facilities, however, by the fact that a plant employee does not have to physically go to each valve location and actuate the valve.
Air Venting Device. One factor differentiates an air/steam mixture from 100 percent steam: temperature. Therefore, an automatic air vent can be provided with the use of a thermostatic device (figure 3).
The air venting device typically is a thermostatic balance pressure bellows unit with a very low subcool. The device will be able to sense air or noncondensable gases due to the resulting temperature suppression. An air venting device can be used on the heat transfer units that have shutdowns and startups during the operational week.
Though reliability can be a issue if the proper air venting device is not purchased, plant personnel does not have to be at the location during startup, and the air venting device can greatly improve the operation of the heat transfer system.
Steam Traps. Steam traps are never considered to be the primary air venting mechanisms due to the methods steam traps incorporate into their design to accomplish this task. Therefore, the steam trap is always considered a secondary air vent mechanism.
There are two methods to vent air in any steam trap: a leak path and a thermostatic mechanism. The steam trap design has a leak path incorporated into the operational design. The steam trap leak path is a very small path to ensure no significant steam loss occurs during operation. Due to the small leak path, the steam trap is not able to provide sufficient air venting capabilities.
In addition, a thermostatic element inside of the steam trap can offer a high capacity of venting air at startup due to the orifice size. In process applications, the preferred steam trap is a float and thermostatic steam trap; this design incorporates a thermostatic air venting mechanism.
The plant may decide to use one or more of the methods described to remove the air from the steam system during startup or operation.
Figure 6. Steam coils will have a vacuum breaker
located at the entrance of the steam into the coils.
Installing Air Venting and Devices
When the steam lines are activated or started up, one of the main tasks will be to remove the noncondensable gases. During the startup period, the drain valve off the steam line drip pocket is open for venting air from the steam line. Its other major function is to remove the condensate. In some cases, to ensure all air is removed from the system, a second manual air venting valve is installed on top of the steam line (figure 4). The strainer blowdown valve also should be open at startup to allow the strainer to be blown down.Air Vents and Vacuum Breakers Locations. A key factor in the location of air vents on process equipment is to understand the design of the unit. For example, a shell-and-tube heat exchanger has a port typically on the top of the shell for the placement of a vacuum breaker and air vent. The process steam side needs to have the air purged to ensure proper startup and temperature control (figure 5).
Steam coils will have a vacuum breaker located at the entrance of the steam into the coils. The preferred method is to have a port on the top header for the devices. Steam coil manufacturers will incorporate the port on the top header for the devices (figure 6).
For process application steam traps, the preferred steam trap design is the float and thermostatic, which incorporates a thermostatic air vent mechanism. The steam trap thermostatic air vent mechanism becomes the secondary air venting device.
When designing or starting up a steam system, to ensure noncondensable gases and air are removed, survey all process equipment and ensure units have air vents and vacuum breakers. Also, survey all steam lines’ drip legs and ensure proper blowdown valves are installed. Finally, have standard operating procedures (SOP) for the use of manual air vents on steam lines and process applications. Following these tips will help maximum steam system efficiency.
