No. 54 – Venting Steam
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The Largest Energy Loss in Steam Systems
|Figure 1. Example of Steam Plant Operation: No Steam Venting|
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.
2. WHY DO PLANTS VENT STEAM TO THE ATMOSPHERE?
There are several reasons that plants vent steam to the atmosphere. However, with modifications using today’s technology, steam and condensate systems do not need to vent steam.
2.1. 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 following:
- process blowthrough,
- bypass valves opened, and
- steam trap station failures; and
- low-pressure nonutilized steam.
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.
|Figure 2. Example of an Unbalanced Steam System|
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.
Table 1. Example of Steam Venting Costs
|Steam cost per 1,000 lbs.||$3.75|
|Steam loss (pph)||12,784|
2.2. Flash Steam Vented to the Atmosphere
|Figure 3. Flash Steam Venting|
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. Example of Flash Steam Venting Costs
|Steam cost per 1,000 lbs.||$6.21|
|Steam loss (pph)||3,196|
|Figure 4. Flash Steam Venting to the Atmosphere|
A small amount of flash steam being vented to the atmosphere has a significant energy loss ($26,220.00 per year).
Table 3. Example of Flash Steam Venting Costs
|Steam cost per 1,000 lbs.||$6.11|
|Steam loss (pph)||510|
2.3. 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.
|Figure 5. Blowthrough Steam|
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.
All of these items are easily correctable.
Table 4. Example of Energy Losses From a Small Steam Trap Station Population
|Steam cost per 1,000 lbs.||$8.45|
|Steam loss (pph)||2,125|
2.4. Unbalanced Steam Header Pressure
|Figure 6. Steam Venting From an Unbalanced Steam Header|
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. Example of the Cost of Steam Venting From an Unbalanced Steam Header
|Steam cost per 1,000 lbs.||$4.90|
|Steam loss (pph)||2,111|
2.5. Deaerator Noncondensable Vent
|Figure 7. Deaerator Venting|
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.
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.
Table 6. Example of Energy Costs for an Aggressively Overventing Deaerator Vent
|Steam cost per 1,000 lbs.||$5.45|
|Steam loss (pph)||292|
3. How to eliminate steam venting
There are two types of steam systems in plant operations today.
3.1. Flash Steam Vent Condensate
|Figure 8. Flash Steam Vent Condensate|
Modulating Steam System
A modulating steam/condensate process means the process application has a steam control valve that modulates the steam to the process. The steam control valve can operate from 0% (closed) to 100% (fully open) and anywhere in between.
The modulating steam system’s operational design requires the condensate from the processes to be recovered by a gravity (0 psig) condensate system.
The condensate system will incorporate a condensate receiver that allows the flash steam to vent to the atmosphere. The venting of the flash steam ensures the condensate receiver is never pressurized.
To prevent the flash steam loss to the atmosphere, plants install devices such as flash steam vent condensers in the flash steam vent line.
|Figure 9. Steam Vent: Air Condenser Design|
A flash steam vent condenser is incorporated in the system to recover the flash steam by using an external heat exchanger (condenser).
The vent condenser (heat exchanger) will consume the flash steam by heating air, water, or some other process fluids. The vent condenser is designed for the application to ensure proper operation.
The process fluid consumes the flash steam and allows the condensate to drain back into the condensate tank. Therefore, the flash steam is consumed, and the condensate is recovered.
All vented condensers are engineered for the application.
3.2. Nonmodulating Steam Systems
A nonmodulating steam condition means there is no control valve modulating steam flow to the process. If there is a steam control valve, it always maintains a steam pressure to the process that is higher than the pressure in the condensate recovery system. This system is classified as a nonmodulating steam process.
A nonmodulating process steam system provides a steam pressure to the process that will provide a differential pressure on the drain device (steam control valve or steam trap station).
Examples of nonmodulating steam processes include the following:
- steam tracing,
- processes with temperatures above 240°F,
- drip leg steam traps,
- process heaters,
- corrugators, and
The typical plant operation is not designed to accommodate the higher condensate temperatures and additional flash steam generated by the nonmodulating steam system. In these operations, the condensate and flash steam are delivered to a vented condensate tank system. The flash steam is then vented to the atmosphere, and a substantial energy loss occurs.
However, flash steam can be recovered by using a cascade system that incorporates a flash tank system or by using a thermocompressor system.
|Figure 10. Flash Tank System|
The two-phase flow (flash/condensate) from the process discharge can be directed to the pressurized flash tank for separation. The flash steam then can be delivered to a lower-pressure steam header system. This method is referred to as a cascading flash steam system.
|Figure 11. Flash Steam Recovery With a Thermocompressor System|
A large number of plants do not have a need for low-pressure steam; therefore, the cascade steam system is not a benefit. Another method to recover the flash steam is to use a thermocompressing system. Thermocompressing takes the low-pressure steam and produces a higher, usable steam pressure.
The thermocompressor is a simple device that has existed for many years. It has a nozzle where high-pressure steam is accelerated into a high-velocity fluid. The high velocity entrains the low-pressure steam from the flash tank by momentum transfer and then recompresses it in a divergent venturi. The result is an intermediate steam pressure that is useful to the plant operation.
4.1. Process Outlet Temperatures Above 240°F
There are processes where the outlet temperatures are at or above 240°F, which means the steam control valve will never reduce the steam pressure below the steam pressure equal to the process temperature.
- Process outlet temperature: 310°F
- Steam pressure @ 310°F: 90 psig
The steam pressure will not be lower than 90 psig; therefore, the condensate can be discharged into a flash tank, and the flash can be used in a cascade system or thermocompressing system.
Tomorrow would be a good day to start following the road map to prevent any steam from venting to the atmosphere.
|Figure 12. Plant Operation: No Steam Venting|