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An ever-increasing need for steam at specific temperatures and pressures exists in many modern plants. Fortunately, significant improvements have been made to increase operational thermal efficiency and heat rates by the precise, coordinated control of the temperature, pressure and quality of this steam. But, much of the steam produced in power and process plants today is not at the required conditions for each application, so conditioning is required, often by a desuperheater system.
The sizing, selection, application, installation and maintenance of the proper desuperheating and steam-conditioning equipment, including control valves, is therefore critical to optimum performance. This article will discuss superheaters and associated control valves in detail, but first I will look at common applications and issues in affected industries.
Power Industry
Competing in the modern power market requires a heavy emphasis on the ability to utilize multiple operating strategies. Increased cyclical operation, daily start-stop and faster ramp rates are required to ensure full-load operation, particularly at daily peak hours, and to maximize profit and plant availability. Changes resulting from environmental regulations and economics also are combining to alter the face of power production.
At the same time, these changes are affecting the operation of existing power plants and the design of future plants. Advanced plant designs include requirements for increased operating temperatures and pressures along with stringent noise limitations in urban areas. Steam is used throughout power plants in many ways, from driving to turbines to feedwater heaters.
Hydrocarbon and Petrochemical Industries
Hydrocarbon and petrochemical industries rely on the efficient conversion of low cost feedstock to high profit products. Hydrocrackers, furnaces, distillation columns, reactors and other process units must be designed to meet a range of conditions to accommodate various modes of plant operation. Temperature is a critical factor that must be taken into consideration during the design of each process unit, and it must be controlled precisely to optimize each operation.
Temperature is controlled in many ways in these plants. The most common method is through the use of heat exchangers and process steam. Process steam must be conditioned to a point near saturation before it is transformed into a medium that is more efficient for heat transfer. The proper selection of equipment will ensure optimum plant availability, reliability and profitability.
Other process industries such as mining, pulp and paper, life sciences and food and beverage experience reliability issues caused by steam-conditioning challenges. These industries also use steam for motive force and heat transfer.
Desuperheater Basics
A schematic of a typical desuperheating system is shown in figure 1. A typical system consists of four main components:
Control valve.
Desuperheater.
Temperature transmitters.
Spray-water strainer.
When specifying a desuperheater, it is advisable to consult with the manufacturer because most desuperheater suppliers have multiple models from which to choose. Critical parameters (figure 2) include:
Spray-water temperature.
Spray-water pressure.
Initial steam superheat temperature.
Final steam superheat temperature.
Minimum steam velocity.
Maximum steam velocity.
Pipeline size.
Downstream straight-pipe length.
Steam-pipe liner.
Orientation.
While each components affects operation, a note on orientation is warranted. Orientation can affect the speed of vaporization. Horizontal installations are most common, but vertical flow-up installations perform slightly better because of the positive effect of gravity. Vertical flow-down pipes perform less efficiently because of the negative effect of gravity, which reduces residence time.
Details of the actual control of a desuperheater are beyond the scope of this article; however, suffice it to say that pressure, temperature and flow sensors feed data to a control system that adjusts the spray-water control valve to deal with changing conditions.
Control Valve Considerations
When a desuperheating system is purchased, often each component will be specified and purchased separately. In other words, the desuperheater will be purchased from one vendor, the control valve from another and so on. Unless the process plant has an extensive expertise in the design of superheating systems — not often the case — this approach is problematic due to the complexity of these systems.
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