No. 65 Steam System Balancing
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WHAT IS STEAM BALANCING?
Creating and maintaining a steam balance is the most efficient way to gain knowledge of all aspects of a plant’s 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 necessary to increase steam system thermal cycle efficiency. Every plant needs to achieve the highest steam thermal cycle efficiency possible.
A steam system in perfect balance 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.
Figure 1 shows a close-to-perfect steam balance that 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, which is operating at 198 psig, returns the condensate (high-pressure return system) back to a high-pressure deaerator system that delivers the feedwater at an elevated temperature (198 psig = 387°F) into the boiler operation.
The advantages of a steam system in perfect balance are as follows:
• higher sensible energy content in the condensate;
• reduced flash steam, with no need for flash steam recovery, because the deaerator will consume the small percentage of flash steam;
• smaller diameter condensate piping;
• higher feedwater temperatures, thus a higher boiler efficiency; and
• higher steam system thermal cycle efficiency.
However, a perfect steam balance cannot exist for several reasons, because of modulated steam pressures or flows to system processes and the dynamics of condensate drain devices. The condensate system’s dynamics can also limit on how high a pressure the plant can maintain in the condensate return system.
Even so, the plant’s responsibility is to strive to create a system as close to the example as possible so it can achieve a high steam system thermal cycle efficiency. 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 BALANCE PRINT(S) ARE NOT ACHIEVED
A high percentage of the time, knowledge of the steam system is fragmented. 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.
In addition, it may be difficult to establish a single or dual set of prints because the CAD library might contain multiple different PID prints that were created over the years by many different engineers using different formats. Some plants do not keep their CAD prints up-to-date, which makes the task even more difficult to accomplish. Even so, plants must remember that developing a steam balance print offers a significant return on the investment.
3. ACCOMPLISHING A STEAM BALANCE
It is impossible to optimize a steam and condensate system without a steam balance, because it 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 ways, from a fully developed 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 plays an important role in the balance, and the steam pressure letdown valves (pressure-reducing valves) need 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 steam balance in Figure 4 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 sample steam balance in Figure 5 showed that a few minor steam piping changes would allow end users to consume low steam pressures, thus reducing the flash steam losses. Changes improved condensate drainage and further reduced the flash steam venting. The result was a higher steam system thermal cycle efficiency with reduced energy and emissions and increased reliability.
4.3. Steam Balance Example 3
Figure 6 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 nonmodulating processes to the high-pressure deaerator, resulting in higher steam system cycle efficiency.
5. WHAT ARE THE BENEFITS OF DEVELOPING A STEAM BALANCE?
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. The knowledge to be gained includes the following:
• a better understanding of the steam and condensate system,
• the ability to set a road map for changes that will improve the system,
• opportunities to improve energy efficiency,
• opportunities to reduce emissions, and
• opportunities to increase reliability.
6. WHAT TO INCLUDE IN A STEAM BALANCE
A steam balance flow document provides all relevant steam and 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 useful the document becomes to the plant operation.
Document the following items:
• All boilers or steam generators
o Steam output
o Operating steam pressures
o Safety valve set pressure
• Deaerator system
o Operating pressure
o Steam flows
• Make-up water system for the deaerator
o Flow rates
o Average temperature
• Steam turbines
o Supply pressures
o Extraction pressures
o Electrical or drive output
• Steam headers by operating pressure
• Steam pressure-letdown stations
o Inlet and outlet pressures
o Maximum and minimum flow rates
• End users or consumers
o Energy requirements
Maximum and minimum
• Condensate headers
o Operating pressures
o Flow rates
• Flash tanks
o Integration into the system
o ASME stamping
• Condensate tanks
o Venting or not venting
o Pressure ratings
The list will grow as the steam balance process continues.
Every plant, regardless of size, from the smallest food processing plant to the largest 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 format is selected, the balance is the first step in increasing steam thermal cycle efficiency, which will reduce energy and emissions and increase profits.
Today, our plant may look like the one in Figure 7, with steam venting to the atmosphere, which can amount to tremendous steam losses.
Our target is for our plant to look like the one in Figure 8 in five or fewer years.
Today is a good day to get started: tomorrow may be too