This article describes how the selection, installation and maintenance of desuperheating and steam-conditioning equipment, including control valves, is critical for optimum performance.
An ever-increasing need for steam at specific temperatures and pressures exists in many modern power and other process plants. But much of the steam produced for these applications is not at the required conditions, so conditioning is needed, often by a desuperheater system.
Mark Nord, director of the power/OEM industry group at Emerson’s Fisher Business Unit in Marshalltown, Iowa, writes that the sizing, selection, application, installation and maintenance of desuperheating and steam-conditioning equipment, including control valves, is critical to optimum performance. His article in the Apr 2019 issue of Process Heating, Desuperheater Application Best Practices, begins with common applications and issues in affected industries.
Increased cyclical operation, daily start-stop and faster ramp rates are required to ensure full-load operation, which is especially needed at daily peak load hours.
Mark says other changes are also 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 turbines to feedwater heaters.
Hydrocarbon and Other Industries
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.
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.
Per the diagram below, a typical desuperheating system consist of multiple components:
A typical desuperheating system consists of multiple components. When specifying a desuperheater, it is advisable to consult with the manufacturer because most desuperheater suppliers have multiple models from which to choose.
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. Mark says unless the process plant has expertise in the design of superheating systems, this approach is problematic due to the complexity of these systems.
Given these reasons, the control valve and desuperheater should be sized and selected together, preferably by the same vendor.
Once the equipment is received, it is critical for it to be installed according to the manufacturer’s specifications, for example the distance from the desuperheater to the temperature sensor.
The second critical item is to verify the distance that the temperature sensor is installed downstream of the desuperheater. This distance is determined by the manufacturer, and installation must be at the dimension specified or greater.
Third, the desuperheater itself must be installed in the proper orientation to the steam line. It is not uncommon to see desuperheaters installed backward, or off one bolt circle segment on the mounting flange.
In most cases, desuperheater problems are caused by the spray nozzles. Problems caused by improperly maintained spray nozzles include:
A damaged spray nozzle in operation (left) and a spray nozzle operating as designed (right) demonstrate the differences in spray patterns, which have a significant effect upon operation.
Proper maintenance calls for nozzles to be inspected with spray-pattern tested annually using a desuperheater nozzle test skid. During an inspection, check for cracks, loose nozzles, broken nozzles and even missing nozzles.
Nozzles should be inspected and spray-pattern tested annually using a desuperheater nozzle test skid. A desuperheater nozzle test rig with integral camera is one option.
Although all desuperheaters share many of the operating characteristics described, there are three basic types: insertion-style, ring-style and steam-conditioning valves.
An insertion-type desuperheater has either an integral control valve (far left) or a separate control valve (left). A ring-style desuperheater (center) is used when very large amounts of spray water flow are required. A steam-conditioning valve (right) is used when pressure and temperature downstream of the valve must be controlled.
An insertion model is so named because it is inserted into a pipe instead of being mounted inline like the other two types. Insertion-style desuperheaters are only used for temperature control. A ring style desuperheater is used when very large amounts of spray water flow are required.
A steam-conditioning valve is used when pressure and temperature downstream of the valve must be controlled. These valves are commonly used in process steam letdown stations, turbine bypass and auxiliary steam applications.
No matter the type, care must be taken in design and operation.
Proper specification, installation and maintenance of a desuperheater will ensure optimum temperature control performance, long desuperheater life, correct operation, long life of downstream equipment and minimal unexpected shutdowns. Desuperheaters are complex devices, however, and should be selected with the aid of the manufacturer.
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