Steam Balancing – First Step in Steam System Optimization

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The First Step in

What Is Steam Balancing?

The steam balance of the plant steam system is the most efficient method of gaining knowledge of all aspects of the steam system, such as steam generation, distribution, end users, and condensate recovery systems. The steam balance is always the first step in any steam system optimization and management program.

The valuable knowledge gained from developing or updating a steam balance leads to setting a road map to use the steam system in the most efficient way and provides the understanding to increase steam system thermal cycle efficiency. Every plant needs to achieve the highest steam thermal cycle efficiency possible, and the steam balance provides the information to achieve this goal.

The perfect steam balance system has the end users (steam processes) achieving the correct volume of energy at the correct steam pressure/temperature with the proper steam quality. The perfect steam balance has no energy losses from steam leakage, excessive low-pressure steam venting, flash steam venting, condensate loss, etc.

Establishing the correct steam balance can be very challenging because of all the different dynamics in a steam system, such as modulating steam loads, variable production times, unaccountable losses, insulation inefficiencies, turbine operation, etc.

Shown in Figure No. 1 a close to perfect steam balance which has steam being produced at 200 psig and delivered to the process.  The process consumes the 200 psig (latent energy) of the steam and the condensate (sensible energy) is removed from the process.  The end user process requires the latent energy of steam. When that latent steam is released, there is no temperature or pressure variance, and the condensate that contains the sensible energy is returned to the deaerator operation at a high pressure. The condensate system that is operating at 198 psig returns the condensate (high pressure return system) back to a high pressure deaerator system that delivers the feedwater at a elevated temperature (198 psig = 387^oF) into the boiler operation.  The advantages of the perfect steam balance are as follows:

• High sensible energy content in the condensate
• Reduced flash steam
  1. No need for flash steam recovery. The deaerator will consume the small percentage of flash steam.
• Smaller diameter condensate piping
• Higher feedwater temperatures, thus a higher boiler efficiency
• High steam system thermal cycle efficiency

However, the Figure No. 1 example is hard to achieve due to the process conditions and condensate drainage devices.  But, to achieve a high steam system thermal cycle efficiency is the plant responsibility to strive for achienving a system as close as possible to the example.

However, a perfect steam balance cannot really exist for several reasons, because modulated steam pressures or flows to the process, the economics of condensate drain devices.  The condensate system dynamics can limit the plant on high the condensate pressure can be maintained in the condensate return system.

The consumers, or end users, of the steam demand the correct quantity and 100% steam quality at a given specific pressure/temperature, and it is up to the plant to manage the steam balance to meet the end users’ requirements. The end users’ steam demands and steam quantity constantly vary, so achieving a high steam system thermal cycle efficiency requires plants to have a well-documented steam balance to meet the production requirements.

2.      WHY STEAM BALANCES PRINT(S) ARE NOT ACHIEVED

A high percentage of the time, knowledge of the steam system gets segmented. The boiler plant personnel know the boiler plant operation well, and process personnel understand the process steam operation. Meanwhile, most of the time, the steam distribution and condensate system are left in limbo and not attended to by anyone. Often, the condensate system is omitted from the total steam system balance, even though it accounts for 16% of the overall energy in the steam vapor. The flash steam is a critical part of the steam balance, so the flash steam should be recovered by some means to ensure steam thermal cycle efficiency.

The steam prints are in 35 different PID prints in the CAD library which were accomplished over the years by many different engineers with different formats, thus making it difficult to establish a single or dual set of prints.  CAD prints are not kept updated which makes the task even more difficult to accomplish but the efforts to develop a steam balance print has significant pay back.

3.      Accomplishing a Steam Balance

It is impossible to optimize a steam and condensate system without a steam balance, which requires a complete understanding of the system. The easiest and best method to deliver that steam system understanding is to create a steam balance flow diagram.

The steam balance flow diagram can be accomplished in many different ways, such as a fully devolved document completed in Aspen software to a simple flow diagram completed with Microsoft Visio. The key point is to have a steam balance document for plant personnel.

4.      End Results of a Steam Balance

Implementing a perfect steam balance between steam generation, distribution, end user requirements, and condensate recovery can be an extremely challenging goal in any industrial steam plant operation. An industrial plant can have several different steam-generating sources and a multitude of end users with varying steam pressure and steam flow demands. The steam turbine operation for electrical generation or drive units play an important part of the balance, and the steam pressure letdown valves (pressure-reducing valves) operation needs to be minimized to ensure maximum steam flow to the steam turbine operation.

Four types of condensate recovery systems are added into the balance, making the system dynamics even more complex. Today, to meet production requirements, plants are continuing to update and make process changes that will affect the steam balance. Therefore, steam balancing is a continuous program, not a one-time venture.

The steam balance will eliminate the waste of unusable low-pressure steam being vented to the atmosphere by balancing the steam flow to the end users (steam turbines, heat exchangers, reboilers, etc.), then discharging the condensate/flash steam to the steam cascade systems to successfully end the steam process with the correct amount of low-pressure steam. If this is not accomplished, then the low-pressure steam is thermocompressed to medium steam pressure grids until balance is achieved in the system.

4.1.      Steam Balance Example 1

The following steam balance (Figure 1) was accomplished in a simple format, but it resulted in a substantial reduction in energy and emissions. The steam balance indicated that a pressurized condensate system could easily be instituted into segments of the process system using the current condensate system with minor capital costs, saving millions of dollars per year.

 

4.2.      Steam Balance Example 2

The second example steam balance in Figure 2 determined that a few minor steam piping changes allowed low steam pressures to be consumed by end users, thus reducing the flash steam losses. Changes improved condensate drainage and further reduced the flash steam venting. The result was higher steam system thermal cycle efficiency with a reduction in energy and emissions and increased reliability.

4.3.      Steam Balance Example 3

Figure 3 demonstrates different methods to recover the condensate under pressure, thus saving energy and reducing emissions for the plant operation. Switching from a low-pressure deaerator to high-pressure deaerator with condensate line changes allowed the return of condensate from nonnodulating processes to the high-pressure deaerator, resulting in higher steam system cycle efficiency.

5.      What Are the Gains With a Steam Balance?

1. Knowledge of the steam and condensate system
2. Ability to set a road map for change to improve the system
3. Energy efficiency
4. Reduction in emissions
5. Increase in reliability

A steam balance is like the dashboard of your car: it will provide the drivers of the steam and condensate system with all information required to operate safely and efficiently.

6.      Steps for the Steam Balance

A steam balance flow document provides all the steam/condensate information, such as steam flows, pressures, end users, pressure reduction stations, turbines, boilers, condensate tanks, etc.

The more detail added into a steam balance flow diagram, the more important the document becomes to the plant operation.

Document the following items:

1. All boilers or steam generators
   1. Steam output
   2. Operating steam pressures
   3. Safety valve set pressure
2. Deaerator system
3. Operating pressure
4. Steam flows
5. Make-up water system for the deaerator
6. Flow rates
7. Average temperature
8. Steam turbines
   1. Supply pressures
   2. Extraction pressures
   3. Electrical or drive output
9. Steam headers by operating pressure
10. Steam pressure letdown stations
    1. Inlet and outlet pressures
    2. Maximum and minimum flow rates
11. End users or consumers
    1. Energy requirements
       1. Maximum and minimum
    2. Condensate headers
       1. Operating pressures
       2. Flow rates
    3. Flash tanks
    4. Integration into the system
    5. ASME stamping
    6. Condensate tanks
    7. Venting or not venting
    8. Pressure ratings

The list will grow as the steam balance process continues.

7.      Conclusion

Every plant, regardless of size, from a small food processing plant to a large refinery, needs to prepare a steam balance in a format that works for the plant’s operation. The steam balance does not have to be done in a complex software system to be beneficial to the steam system manager; it can be accomplished in a simple graphic software package. Whatever form is selected, the balance is the first step in increasing steam thermal cycle efficiency, which will reduce energy and emissions and increase