Selecting valves for saturated steam service

By "Apollo" Valves
July 31, 2018

For such a simple substance, steam has a long list of uses. Power generation, sterilization, and of course heating are among the most common. As with any other gas or fluid, valves are essential for regulating how that steam moves through the piping and equipment. It's important to note though that the properties that make steam so useful also create challenges when attempting to manage how it flows.

Saturated, unsaturated, and superheated

Steam generation is surprisingly complicated; there's more to it than just boiling water. Understanding the intricacies means diving into the phase diagram for water, which we won't do here. For now, we'll just note that there are different types of steam. This depends on temperature, pressure, and liquid water content.

Superheated steam is used mostly in power generation. It doesn't offer much in the way of heat transfer but is very effective at delivering kinetic energy.

Saturated steam contains both water vapor and liquid water. However, the temperature and pressure are such that there's a balance between the two. Saturated steam is a very efficient heat-transfer medium. Unsaturated steam is steam that's turning to water, due to a change in either temperature or pressure.

Uses of saturated steam

The main use of saturated steam is heating, particularly in cases where you don't want to use electricity or burn gas. With steam heating, the boiler can be placed some distance from where the heat is needed, with pipes taking it to the process. The food processing and chemical production industries are both intensive producers and users of saturated steam. It's also used for sterilization and cleaning, as well as moisturizing and humidifying. The paper industry, for instance, uses steam to add humidity.

Steam control

Controlling the flow of steam is different than controlling a liquid-like water. That's because steam is compressible. When a valve is almost fully closed, steam is held back under pressure. The steam that does get through moves at high speed, but once through, the pressure drops. That lets the steam expand, which lowers the temperature. This effect means that control valves in steam systems regulate temperature through pressure control.

Steam poses two challenges for valves that aren't experienced with liquids. First, the high velocity that results from a partially open valve causes noise and vibration and accelerates wear in the valve itself. Second, steam is considerably more corrosive than water. This is because it contains a higher level of dissolved CO2. Corrosion of both the stem and the ball are common unless appropriate materials are used.

Valve selection

In on/off applications, a ball valve with no pressure drop is ideal. By not causing any change in pressure, this has minimal effect on temperature. Where flow control is needed, typically to manage temperature, globe valves are usually preferred. Valve component materials are also an important consideration. Always ensure your valve supplier understands your application is for saturated steam and ask that the appropriate materials be incorporated.

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