OEHLANDT ENERGY OY

Steam Balance Control

What means Steam Balance Control?

Steam balance control is a control structure that is taking care of the difference between steam production and consumption. The purpose is to keep the steam pressures stable on different steam headers and not to cause stress for the equipment. The control structure connects together the boilers, the turbines the reduction stations the consumers and the auxiliary components as e.g. steam accumulator's, auxiliary condensers and venting valves.

Typical control components those are used for the steam balance control.

• Boiler fuel controls
• Waste heat boiler additional combustion controls
• Turbine pressure- and load controls
• Reduction station controls
• Steam accumulator controls
• Venting controls
• Auxiliary condenser controls
• feed water tank level controls

Why is a Steam Balance Control needed?

The consumption changes are faster than the production components are able to handle in an efficient way.

Why is a Modern Steam Balance Control System needed?

• The factory requirements for the steam quality have increased. E.g. web speed has increased and paper quality requirements are higher.
• The high energy prices of today don't allow wasting of energy.
• The operation reliability requirement of the mills has grown. Uncalculated stops are not accepted anymore.

Earlier with stand-alone-controllers, it was difficult to accomplish a Steam Balance Control System. Nowadays, with modern automation systems, experts have tools to create an optimal Steam Balance Control System.

An old Steam Balance Control System (SBCS).

An old SBCS is usually built only with a few closed loop control loops and is wasting energy. The loops are not connected together to work as a system. Normally also every closed control loop has its own pressure measurement. The control is done by producing additional energy that in the worst case is blown out to the sky or it is put in to the auxiliary condenser. The additional steam can also be let to a condensing turbine. This is normally not wise, as this steam is often produced with high value fuel as e.g. oil. The disadvantages of an old SBCS are high energy losses and big variations in the steam header pressures. Those are disturbing the whole production of the mill.

A modern Steam Balance Control System (SBCS).

A modern SBCS consists of several closed loop controls, which are connected as a system together. The operating (adjusting) of the demand pressures and the operating mode is set in a central recipe picture. In a modern SBCS it is possible to move from one operation mode to another without any operations of the operator. E.g. in case of a web break of the paper machine the turbine can automatically change from outlet pressure control mode to inlet pressure control mode. The aim of a modern SBCS is also to work energy efficiently optimally for not to waste expensive fuel. Because of the centralized optimal controls also the pressure variations are smaller. This allows e.g. to put a lower set point on the low pressure steam header. That is increasing the turbine's electric power generation.

Some typical problems.

No Steam Balance Control System.

The consultant has done the equipment specifications and is demanding and expecting that all suppliers also deliver the control concept for their parts. The problem is that e.g. the boiler supplier is making the control concept for the boiler and the turbine supplier the control concept for the turbine. Eventually there is also an old mill that is kept in use. Nobody takes care of connecting all those parts together for them to work in all situations safe and optimal as an energy efficient modern system. If a Steam Balance Control System is not in place, then the start-up is more difficult, takes more time and good profitability of the mill is difficult to reach.

The Steam Balance Control System is incomplete:

The control system must automatically take care of process and equipment limits. It is not allowed to cross any limits. The system must when reaching a limit continue (glide, not switch) with another control component or another operation mode. In an incomplete system the operator is needed when a limit is reached. The operator can never be as accurate as a well tuned system is. That causes to "safe" limits and operations. Also the trust in the system is shaken. After that more and more control loops are kept by the operator in manual mode and are not working in an optimal efficient way anymore.

Sizing and equipment problems:

The needs are not specified accurately enough. (The guarantee values must be clearly defined.)

A too big reduction station is defined. 2 x 50% reduction stations give a much better control (pressure and temperature) behaviour and also more back-up (security) in problem situations. The maintenance without a mill stop is also easier.

The actuator type (electric, pneumatic, hydraulic) or size is not optimal for its purpose.

By a turbine trip the quick opening of the reduction station is too slow. Often a reduction station is equipped with a quick opening solenoid which is working with a binary signal. This is in most situations wrong, because a reduction station must be opened to a calculated position (not 100%). The calculated position is got from the steam flow trough the turbine before the trip.

The pipe material after the reduction station is not specified for adequate temperatures. When the station is leaking steam, as they usually do, it can't be cooled down, because of a low speed of the steam (no mixing with water and steam). This is causing a temperature rise with a following interlock from a high temperature and so the station is out of use.

In the spray water line are no shut-off valves. That causes leaking of the spray water into the steam pipe.

The position signals of the important steam valves are not brought to the automation system. This makes it much more difficult to find failures and disturbances which are coming from not correct working equipments.

Big pneumatic actuators have a too small air supply line.

Uneconomical operation modes:

The use of an auxiliary condenser is continuous.

Venting is continuous.

The control is done using oil or gas and not with a cheaper biofuel.

Steam is bypassing the steam turbine.

Permanent boiler or/and turbine trips.

Uneconomical or incomplete use of the steam accumulator:

The accumulator is used as a reduction. (Bypassing the turbine => electrical energy loss.)

By having additional steam the steam is not dumped to the accumulator.

The water level of the accumulator is not optimal. The energy is bound in the water (at saturation point). An accumulator is sized for the optimal use, but only a part of its capacity can be used if the water level is too low.

The charging of the accumulator is disturbing the other process.

Reduction station control logic problems:

Auto/manual mode changes are not bumbles.

In operation mode change a control is jumping or starts directly after getting active to operate into a different direction, e.g. if a "limit" controller gets active. This causes oscillation.

In a valve split range case the operation is programmed primitively, so that it limits the optimal use of the valves. Correctly both valves could be freely operated without taking care in witch mode (A/M) the other valve is. Also an M/A change should all the time be possible without disturbing the process.

By a turbine trip the reduction valve is not working as calculated. This causes easily a boiler trip and in worst case a mill shut down.

The temperature controllers are not assembled using the cascade principle.

FAT (Factory Acceptance Test) insufficient:

The tester on the FAT test has not enough steam system knowledge to understand the total structure. That leads to an incomplete FAT test. In those cases software with failures is delivered to the mill. Because of the complexity of the circuits, the development and testing work is much more difficult at the plant as in FAT environment.

FAT is too short.

Site supervision is inadequate:

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.

Operation pictures:

A central operation picture for the steam balance control is missing (recipe).

Too many control modes are allowed for the steam balance controllers (manual, auto, local and remote). If a recipe picture is used the local mode should not be possible.

Alarms and the blocking of not needed alarms are done unsystematically. Therefore important alarms are lost among unimportant alarms.

Optimization:

At the end of a project it is forgotten that the system and components planning has taken up to two years. Therefore a "working" system is accepted, even it is not optimized. First when the production has started, it is possible to finalize the optimization of the system and to get it to work in the most efficient way.