The Basics of Desuperheating and Steam Conditioning

Control valves are critical components in desuperheating and steamconditioning systems. They require an understanding of system operation before they are specified.

Two primary uses for steam in the power and process industries are performing mechanical work and as a heat transfer medium. Superheated steam Is preferred for mechanical work because it does not contain entrained water droplets, which can damage rotating equipment. For heat transfer applications, steam at or near saturation - produced by processes referred to as desuperheating and steam conditioning - provides more efficient heat transfer than superheated steam.

Desuperheating systems are used to reduce the temperature of process steam by spraying water Into superheated steam, thereby reducing Its enthalpy. Steam-condltlonlng systems combine the desuperheating function with pressure reduction.

This article will examine desuperheating and steam-condltlonlng Installations, some considerations for proper operation, and a look at how desuperheating and steam conditioning equipment can affect related systems.

Desuperheating in Detail

Figure 1 depicts the main components of a desuperheating and steam-condltlonlng system.

During operation, steam Is fed into a pressure-reducing valve, and this steam flow enters the desuperheater section. There, it receives fluid from a spray-water valve to cool it. Temperature and pressure readings are taken downstream of the desuperheater section. These readings are used as inputs to the control system, which regulates the actions of both the pressure-reducing valve and the spray-water valve as needed to maintain the desired temperature and pressure.

The desuperheating setpoint can be either a temperature or an enthalpy. It Is used in conjunction with the downstream temperature reading to dictate the amount of cooling water injected into the steam. The amount of water added to the superheated steam is regulated by a control valve. This spray-water control valve needs to respond quickly to required travel changes and should be capable of Class V shutoff when closed. Also, the high pressure drop In the water system often requires the valve to incorporate anticavitation trim to protect it from damage during operation.

Desuperheating Challenges

The main challenge In desuperheating Is the creation of temporary two-phase flow in the steam pipe through the introduction of spray water. Proper design of a desuperheater alms to limit any negative effects by reducing the amount of time the steam resides In the system.

Steam...