Quality Staff and Equipment Target Optimized Steam Conditioning

The steam produced in the recovery, oil and bark boilers at the Stora Enso Skoghall in Sweden , is used primarily in mill processes such as drying the 550,000 tons per year board produced on the mill’s two board machines. It also heats chips and pulp in the pulping process.

Skoghall uses steam to drive the backpressure turbines for production of electrical energy. The exhaust steam is used for the process.

The amount of steam to the process is more or less constant, but the requirement for electrical energy via the steam turbine is variable depending on several factors such as unfavorable pricing of the electricity to be sold to the grid.

Excess steam not used by the steam turbine has to be bypassed to meet process demands and this is where a control valve manufacturer with expertise in turbine bypass applications plays an important part in the scheme. See Figure 1.

Figure 1

Recently, control valve manufacturer CCI replaced eight conventional steam conditioning and spraywater valves in the bypass system at Skoghall with eight VST-SE steam conditioning and VD spraywater valves. According to Skoghall staff, training and education was of the utmost importance in the total scope of supply.

Training and education for high-quality operation

Well-trained operators, instrument and mechanical maintenance staff, together with high-quality steam conditioning valves, are paramount for a well-functioning pulp and paper mill from both an economical and operational perspective.

Kermit Karlsson, right, and Stellan Danielsson, left, from CCI Sweden conducted the training and education sessions at the Stora Enso Skoghall mill.

Steam conditioning therefore is essential for the process and one of the factors for determining the successful running of a pulp and paper mill.

From an economical perspective, uniform performance, accurate and reliable processes contribute to profitability at pulp and paper mills and for that matter in any other industry. Viewed from the flipside, non-availability means non-production. No production means no income and therefore no profit.

Operationally, well-trained staff, high-quality and well-installed equipment are the ingredients for optimized steam conditioning. If process steam is supplied at too high a temperature, the equipment and/or the product can be damaged.

Supplying steam at too low a temperature can cause loss of control and damage to downstream equipment and piping due to excess water.

Incorrect operation can damage and cause malfunctioning steam conditioning equipment.

Hands-on sessions

Participants in the training and education sessions included Steam Engineering Operators, Instrument Maintenance, and Mechanical Maintenance staff from each work shift.

Hands-on training and education sessions included a description of the VST-SE steam conditioning valve for Fredrik Jonsson, Stefan Hägglund and Veijo Perätalo, all Steam Engineering Operators.

Steam Engineering Operators are responsible for the daily operation of the power plant. Instrument Maintenance staff works with the actuators, regulators and positioners. The work of the Mechanical Maintenance staff involves welding, disassembling and assembling the valves.

During two, two-hour sessions, two groups of Steam Engineering Operators were presented with a general overview of the function of steam conditioning valves in steam power plants as well as a technical description of the VST-SE, VD and VDZ valves supplied to the mill.

In two other, two-hour sessions, two groups of Instrument Maintenance and Mechanical Maintenance staff were presented with the same information plus a hands-on review of the instruction manuals, service instructions, spare parts ordering and troubleshooting procedures spiced with a lot of practical advice.

Spraywater after multi-stage pressure reduction

Expectations for the new VST-SE steam conditioning valves are running high as benefits resulting from the important design features have been promised which distinguish these valves from those their replacements. See Figure 2.

The forged, fully machined valve body of the VST-SE, both inside and out is specifically designed to handle thermal shock, ensuring reliable service and a long lifetime as required by steam turbine bypass applications.

Spraywater for desuperheating is introduced after the final stage of the multi-stage pressure reduction as another remedy to thermal shock. This method of desuperheating minimizes fatigue, erosion and cracking of the metal elements in the pressure reducing elements of the valve.

The VST-SE incorporates a mechanical system of water proportioning, which reduces temperature and provides a more stable operation with regard to maintaining outlet temperature close to the set point.

Multi-stage pressure reduction also permits operation at noise levels that satisfy stringent requirements for quiet working environments.

Steam atomized spraywater injection

Built-in protection against the effects of thermal shock, high rangeability and quiet operation are just some of the benefits of the VST-SE steam conditioning valves installed recently at the Stora Enso Skoghall pulp and paper mill in Sweden.

The most distinguishing feature of the VST-SE is unique and patented. Unlike all other steam conditioning valves on the market, the VST-SE channels steam through the valve’s central stem where at the end of the stem it is forced through a nozzle to atomise the spraywater at the point of injection.

Steam-atomized water injection creates fine atomization of spraywater and evaporates the water even at low steam flows. This unique feature allows low precision control of the atomizing steam flow to ensure a rangeability of desuperheated steam flow greater than 50 to 1.

Steam-atomization also reduces the risk of water fallout at low steam velocities. Rapid evaporation of injected spraywater simplifies the arrangement of downstream piping.

The high rangeability of the VST-SE provides precise control of downstream pressure and temperature.

Scope of Supply

In addition to the above hardware, the control valve manufacturer supplied training and education, start-up supervision and commissioning.

Published in SOLUTIONS Spring 2003