OEHLANDT ENERGY OY

Steam Balance Control

What Is Steam Balance Control?
Steam balance control is a control structure that manages the difference between steam production and steam consumption.

Its purpose is to keep pressure stable across the various steam headers and to avoid unnecessary stress on equipment.
The control structure integrates boilers, turbines, reduction stations, steam consumers, and auxiliary components such as
steam accumulators, auxiliary condensers, and vent valves.

Typical Control Components Used in Steam Balance Control

• Boiler fuel controls
• Additional combustion controls for waste heat boilers
• Turbine pressure and load controls
• Reduction station controls
• Steam accumulator controls
• Vent controls
• Auxiliary condenser controls
• Feedwater tank level controls

Why Is Steam Balance Control Needed?
Changes in steam demand occur faster than the production components can respond efficiently.

Why is a Modern Steam Balance Control System needed?

• Factory requirements for steam quality have increased. For example, web speeds are higher and paper
  quality requirements are more demanding.
• Today’s high energy prices do not allow energy to be wasted.
• Reliability requirements in mills have increased, and unplanned shutdowns are no longer acceptable.

In the past, it was difficult to implement a steam balance control system with stand-alone controllers. Today, modern automation systems provide the tools needed to build an optimal and integrated solution.

A Traditional Steam Balance Control System (SBCS)

A traditional SBCS is usually built around only a few closed-loop controls and typically wastes energy. The loops are not integrated to operate as a coordinated system, and each loop often has its own separate pressure measurement. Control is achieved by producing additional energy that, in the worst case, is vented to the atmosphere or sent to the auxiliary condenser. Excess steam may also be
routed to a condensing turbine. This is often inefficient, especially when the steam is produced using high-value fuel such as oil. The result is high energy loss and large pressure variations in the steam headers, both of which disturb overall mill production.

A modern Steam Balance Control System (SBCS)

A modern SBCS consists of several closed-loop controls that are integrated into one coordinated system. Demand pressure settings and operating modes are managed from a central recipe display.
A modern SBCS can switch automatically from one operating mode to another without operator intervention. For example, after a paper machine web break, a turbine can automatically change from outlet pressure control to inlet pressure control. A modern SBCS also aims to optimize energy efficiency and avoid wasting expensive fuel. Because the controls are centralized and optimized, pressure variations are reduced. This can allow, for example, a lower setpoint on the low-pressure steam header, which increases the turbine’s electrical power generation.

Typical Problems

No steam balance control system

The consultant may define the equipment specifications and expect each supplier to provide the control concept for its own scope. In practice, the boiler supplier develops the boiler controls, the turbine supplier develops the turbine controls, and an older part of the mill may remain in operation as well. As a result, no one takes responsibility for integrating all parts into one system that operates safely, optimally, and energy-efficiently under all conditions. Without a complete steam balance control system, startup becomes more difficult, takes longer, and the mill’s profitability is harder to achieve.

The steam balance control system is incomplete

The control system must automatically handle process and equipment limits without exceeding them. When a limit is reached, the system should transition smoothly rather than switch abruptly
to another control component or operating mode. In an incomplete system, operator intervention is required whenever a limit is reached. An operator can never be as precise as a well-tuned
automated system, which leads to conservative limits and less efficient operation. Confidence in the system then declines, more control loops are left in manual mode, and optimal performance is lost.

Sizing and equipment problems:

Requirements are not specified accurately enough. Guaranteed values must be clearly defined. An oversized reduction station may be specified. Two 50% reduction stations usually provide much better pressure and temperature control, improved backup in problem situations, and easier maintenance without shutting down the mill. The actuator type (electric, pneumatic, or hydraulic) or actuator size may not be suitable for the application. After a turbine trip, the reduction station may open too slowly. Reduction stations are often equipped with a quick-opening solenoid that operates with a binary signal, but this is usually incorrect. The station should open to a calculated position rather than fully open. That position should be based on the steam flow through the turbine immediately before the trip. The piping material downstream of the reduction station may not be specified for adequate temperatures. Because reduction stations typically leak some steam, the line may not cool
properly when steam velocity is low and water-to-steam mixing is insufficient. This can raise the temperature enough to trigger a high-temperature interlock and take the station out of service. The spray water line may lack shut-off valves, allowing spray water to leak into the steam pipe. Position signals from important steam valves may not be brought into the automation system. This makes it much harder to identify failures and disturbances caused by malfunctioning equipment. Large pneumatic actuators may have air supply lines that are too small.

Uneconomical operation modes

The auxiliary condenser is used continuously. Steam venting is continuous. The control strategy uses oil or gas instead of lower-cost biofuel. Steam bypasses the turbine. Boiler and turbine trips occur repeatedly.

Uneconomical or incomplete use of the steam accumulator

The accumulator operates as a reduction station, bypassing the turbine and causing a loss of electrical energy generation. When excess steam is available, it is not charged into the accumulator. The accumulator water level is not optimal. Energy remains stored in the water at saturation temperature, and although the accumulator may be correctly sized for optimal use, only part of its capacity can be used if the water level is too low. Charging the accumulator disturbs other parts of the process.

Reduction station control logic problems

Transfers between automatic and manual mode are not bumpless. During an operating mode change, a controller may jump or immediately begin driving in the wrong direction when it becomes active, for example when a limit controller takes over. This causes oscillation. In split-range valve applications, the control may be programmed too simply, which limits optimal valve use. In a correct design, both valves should be freely operable without depending on whether the other valve is in automatic or manual mode. Switching between manual and automatic should also always be possible without disturbing the process. After a turbine trip, the reduction valve may fail to respond as calculated. This can easily cause a boiler trip and, in the worst case, a mill shutdown. The temperature controllers are not configured according to the cascade principle.

Insufficient factory acceptance testing (FAT)

The FAT team may lack sufficient knowledge of steam systems to understand the overall control structure, which can result in an incomplete FAT. In such cases, software faults may remain undetected until delivery to the mill. Because the circuits are complex, development and testing are far more difficult at the plant than in the FAT environment.
The FAT schedule is too short.

Inadequate site supervision

The location of the temperature elements is not optimal. The elements are getting wet and causing so an oscillation of the closed loop temperature control. This is stressing the pipe and disturbing the production.In a dP flow measurement the steam impulse line direction is not downward.In a dP flow measurement the steam condensate pots are isolated.In a dP flow measurement the square root is not systematically applied always in the automation system or always on the flow transmitter.The calibration of the transmitter is not correct. E.g. it is not differentiated between the density difference of the impulse line and the measured
material (caused by temperature difference). In a steam accumulator this causes a too high water level and the demister can't separate all water from steam anymore.

Operator displays
A central operator display for steam balance control, including recipes, is missing. Too many control modes are allowed for the steam balance controllers, such as manual, automatic, local, and remote.
If a recipe display is used, local mode should not be permitted. Alarms, including the suppression of unnecessary alarms, are handled unsystematically. As a result, important alarms may be lost among less important ones

Optimization

By the end of a project, it is often forgotten that system and component design may have taken up to two years. As a result, a system that merely works is accepted even if it is not optimized. Final optimization can only be completed after production has started, when the system can be tuned to operate in the most efficient way.