The DA-M Desuperheater

Combined heat and power (CHP) plants in process industries simultaneously generate heat (usually in the form of steam) and power (usually as electricity). Although heat generated by heat recovery equipment such as recovery and bark boilers in the pulp and paper industry is utilized to drive electrical generators, its primary purpose is to provide steam to the process.

Generating electricity can be economically and environmentally beneficial, but steam supplied at the correct pressure and temperature is of the utmost importance to process industries because no steam may mean no production. Therefore, steam is typically available directly from the steam turbine and/or through its bypass valve.

Figure 1: A typical turbine extraction/exhaust desuperheating system.

Two practical examples

CCI visited two pulp and paper mills – SCA Packaging Obbola AB and Mondi Packaging Väja – in Sweden , both of which employ CHP operations to generate electrical power and process steam.

At both mills, medium-pressure superheated steam is exhausted from the steam turbine. The steam temperature varies depending on flow and therefore requires desuperheating before it is supplied to various mill processes such as evaporation, pulp cooking, and drying in the paper machine’s drying section.

If the steam turbine is not available, the bypass valves are used to condition the steam to the exact specifications required for the process. From time to time, steam flow through the turbine may not meet the process demand; in this situation, the bypass valve must respond to make up the difference.

SCA Obbola and Mondi Packaging have one turbine each. During normal operating conditions, steam enters the turbine at around 752°F (400°C). Steam pressure is approximately 50 bar at SCA Obbola and 28 to 36 bar at Mondi Packaging. The temperature of steam exiting the turbine varies from 464°F to 53°F (240°C to 280°C)and pressure is approximately 10 bar.

The pressure of the steam supplied to the various processes ranges from 3 to 10 bar. Generally, the temperature of the desuperheated steam required for the processes is close to saturation, owing to the excellent heat transfer properties of saturated steam.

The standard, however, for desuperheated steam in Swedish pulp and paper mills is about 392°F (200°C), which is the target temperature at both SCA Obbola and Mondi Packaging. The 68°F (20°C)difference between saturated steam (356°F /180°C)and the standard is the margin of safety Swedish mills generally employ.

Uniform temperature and pressure, and the ability to accurately and reliably control these parameters, are the measures of optimized steam temperature and pressure control.

Figure 2: The components in a typical DA-M desuperheating system.

Steam to the PM drying section

Meeting the exact specifications required for the steam to all processes is important. At SCA Obbola and Mondi Packaging, CCI took a close look at the circumstances surrounding the supply of desuperheated steam to the drying section of the paper machines.

At both mills, desuperheating was a problem until CCI installed a multi-nozzle DA-M desuperheater featuring mechanical atomization after the turbine. In both instances, the DA-M replaced pneumatic probe desuperheaters.

SCA Obbola

SCA Obbola Steam Operations Technician Erik Sandberg says that the DA-M was an answer to the problem of excess water in the piping downstream from two probe desuperheaters. (See Figure 3. ) “Water downstream from the probe desuperheaters wasn’t a problem when the steam flow was high. When the flow was low, this was a different story. Water in the piping causes erosion and is therefore a maintenance problem. Variation in steam temperature after desuperheating causes ineffective drying, which affects paper quality. ”

Figure 3: The DA-M desuperheating system at SCA Obbola.

Sandberg points out that the DA-M was not the least expensive option for replacing probe desuperheaters; however, minimal water downstream increases the lifetime of the piping, reducing maintenance costs for overall savings.

Additionally, the Swedish government subsidizes electricity produced from bio-fuelled power stations, so green power was another factor considered when choosing CCI’s DA-M. Says Sandberg, “We can sell electricity at a higher price than we pay for it as it is produced from renewable energy. Since DA-M uses mechanical nozzles activated by process water under pressure instead of steam to produce atomized cooling water, all of the steam out of the turbine or from the bypass valve is used to produce electricity.”

“We’re happy with the steam temperature and pressure control the DA-M and the VST-SE system working together are giving us,” says Erik Sandberg, Steam Operations Technician at SCA Obbola.

Sandberg continued, “We calculate that our earnings from the sale of green power go a long way toward paying for our investment in DA-M. ”

The steam bypass system at SCA Obbola employs VST-SE steam conditioning valves from CCI to convert high-temperature and high-pressure steam to specified pressures and temperatures.

Mondi Packaging

Like SCA Obbola, the major problem with the probe desuperheaters at Mondi Packaging was the amount of water they released into the downstream piping.

CCI interviewed Mondi Packaging’s Instrument Designer Yngve Jonsson, Pulp Process Technician Stig Stattlin, and Technology and Environment Project Leader Per-Ola Bylund, who said that water on the temperature sensor gave a false temperature reading, one that was considerably higher than was desirable. This caused great temperature variation and made temperature control extremely difficult.

Overheated steam in the drying section of the two paper machines caused uneven distribution of steam and heat in the drying cylinders, making paper drying ineffective.

Nozzles positioned around the liner atomize the cooling water well, according to Jonsson, Stattlin, and Bylund. However, they also claim that there will always be water downstream from a desuperheater regardless of how effective it is. To guard against eventual false temperature reading, they shielded the temperature sensor.

The gentlemen are very happy with the results of the DA-M installation. They now experience uniform and controllable temperatures close to saturation to the batch digesters, the evaporator, and the drying section. (See Figure 4.)

Uniform heating, especially to the drying section on the paper machines, affects paper production. All other factors being equal, uniform heating in this application means optimized machine speeds, uniform paper quality, and overall improved productivity.

Figure 4: One day’s results from the DA-M desuperheating system at Mondi Packaging.

Like SCA Obbola, the steam bypass system at Mondi Packaging employs VST-SE steam conditioning valves from CCI to convert high-temperature and high-pressure steam to specified pressures and temperatures.

Both SCA Obbola and Mondi Packaging are required by law to have reserve desuperheating systems. The system at SCA Obbola is a second set of mechanical nozzles and auxiliary equipment on the DA-M. At Mondi Packaging, the former probe desuperheaters serve this purpose.

Published in SOLUTIONS Summer 2005