Sizing and Selecting Control Valves for Desuperheaters

According to Wikipedia, superheated steam is steam:

…at a temperature higher than its vaporization (boiling) point at the absolute pressure where the temperature is measured.

It has:

…tremendous internal energy that can be used for kinetic reaction through mechanical expansion against turbine blades and reciprocating pistons, that produces rotary motion of a shaft. The value of superheated steam in these applications is its ability to release tremendous quantities of internal energy yet remain above the condensation temperature of water vapor; at the pressures at which reaction turbines and reciprocating piston engines operate.

With this property, superheated steam is used in many electrical power producing applications using steam turbines. Desuperheaters perform the function of reducing superheated steam temperature and recovering useful heat in the process.

Process Heating: Desuperheater Application Best PracticesIn a Process Heating magazine article, Desuperheater Application Best Practices, Emerson’s Mark Nord describes the role and importance of proper specification, installation and maintenance of control valves in this application.

Mark opens highlighting the growing operating challenges for power producers.

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.

Steam is an important operational component and:

…is used throughout power plants in many ways, from driving to turbines to feedwater heaters.

He highlights the critical parameters to design, measure or control:

  • 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.

Consideration in control valve selection in desuperheaters include understanding the turndown ratios and flow coefficients (Cv) of both the control valve and desuperheater, and the differential pressure across the nozzle as well as the control valve with anti-cavitation trim if required.

Given these considerations Mark recommends that both the superheater and control valve should be sized and selected preferably by the same supplier.

Read the article for more on his guidance on proper installation tips, maintenance requirements, and types of desuperheaters including insertion style, ring style, and steam-conditioning valve. Supplier such as Emerson, can provide an integrated and properly sized desuperheater and control valve for reliable and efficient superheat temperature control.

Visit the Steam Conditioning and Desuperheating section on for more on the technologies and products to improve overall operational performance. You can also connect and interact with other valve experts in the Valves, Actuators & Regulators group in the Emerson Exchange 365 community.

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