With today’s competitive international market, all plants need to reduce operating costs, and lowering energy consumption can have a positive impact on the bottom line. A plant’s steam and condensate systems cannot afford to vent any utility steam, blowthrough steam or flash steam to the atmosphere. An additional benefit of not venting steam is a significant reduction in emissions in the boiler operation.
Any steam venting from the steam and condensate system is the top reason for lost energy in today’s steam systems. Can this objective be accomplished? Yes. Many plants have accomplished the goal of not venting steam, and they were rewarded with a high steam system thermal-cycle efficiency.
Of course, lowering energy costs also makes the plant more profitable and better able to compete in today’s international market.
Why Do Plants Vent Steam to the Atmosphere?
There are several reasons that plants vent steam to the atmosphere. With modifications using today’s technology, however, steam and condensate systems do not need to vent steam.
Unbalanced header pressure can be caused by instantaneous process changes, steam turbine operation, and uncontrolled pressure-reduction stations.
Improper or No Steam Balancing of the Steam System. The steam balance is always the first necessary part in any steam system optimization and management program. The valuable knowledge gained from a steam balance can help plant engineers use the steam system in the most efficient way, and this knowledge also provides essential insight that can support efforts to increase the steam system’s thermal-cycle efficiency. The perfect steam balance has no energy losses from steam venting, excessive low pressure steam venting, flash steam venting, condensate loss, and so on.
However, a high percentage of plants do not have a steam balance program, which typically leads to the following results:
- Flash steam being vented.
- Utility steam being vented to meet the process steam demands.
- Blowthrough steam vented from the process blowthrough; bypass valves opened; and steam trap station failures.
- Low pressure nonutilized steam.
A small amount of flash steam being vented to the atmosphere has a significant energy loss.
Ideally, every plant should strive to achieve the highest steam thermal-cycle efficiency possible. The steam balance provides the information needed to achieve this goal (table 1).
The optimal steam balance system ensures that the end users (steam processes) can achieve the correct volume of energy at the correct steam pressure/temperature with the required steam quality.
Flash Steam Vented to the Atmosphere. A typical steam system will incorporate an atmospheric condensate receiver that allows the flash steam to vent to the atmosphere. There are systems such as modulating process steam systems where the condensate system needs to operate at zero pressure; therefore, the flash steam is required to be consumed or vented. Unfortunately, a large number of plants vent the flash, which is a substantial energy loss (table 2).
The lack of a proactive steam trap station management program allows failed steam traps to leak or blow steam into the condensate header.
As table 3 shows, a small amount of flash steam being vented to the atmosphere has a significant energy loss ($26,220 per year).
Blowthrough Steam. Blowthrough steam is generated in two primary ways. Process blowthrough steam is required for a limited number of processes to ensure proper condensate drainage. However, bypass valves around components that allow steam to freely flow into the condensate header — largely as the result of steam trap station failures — are totally unacceptable for steam system operations.
The lack of a proactive steam trap station management program allows failed steam traps to leak or blow steam into the condensate header. Eventually, this steam has to be vented to the atmosphere at the condensate collection tank system (table 4).
The only acceptable steam venting from a steam system operation is the deaerator venting noncondensable gases along with a small percentage of steam.
All of these items are easily correctable.
Unbalanced Steam Header Pressure. Steam header balancing can be a struggle, given that process steam demands frequently change to meet production requirements.
Unbalanced header pressure can be caused by instantaneous process changes, steam turbine operation and uncontrolled pressure-reduction stations.
Unfortunately, an easy way to stabilize the steam header pressures is to vent steam to the atmosphere to reduce or eliminate overpressurized operations (table 5).
Deaerator Noncondensable Vent. In a steam deaerator, steam serves as the scrubbing agent to reduce the partial pressures of the gases being removed.
With the scrubbing action occurring, the deaerator must vent the noncondensable gases to the atmosphere. The only acceptable steam venting from a steam system operation is the deaerator venting noncondensable gases along with a very small percentage of steam.
Many plants have accomplished the goal of not venting steam, and they have been rewarded with a high steam system thermal-cycle efficiency.
With the high cost of steam today, the deaerator vent must be investigated to ensure that excessive steam venting does not occur. All deaerators need to have dissolved oxygen testing conducted at least every three months, and noncondensable venting must be adjusted accordingly to achieve maximum performance.
In conclusion, tomorrow would be a good day to start following the road map to prevent any steam from venting to the atmosphere.