Monday, July 10, 2006

Two Phase Steam Flow

Wet steam takes on the characteristic of a two phase fluid. Since two phase flow creates metering and control problems, changes in the moisture content of steam for any reason in a distribution system becomes very important and must constantly be evaluated. Two phase non-homogeneous fluid flow causes critical deviations from steam metering and control formulas which are based on single phase homogeneous flow.

The thermodynamic state of the steam in distribution systems is commonly defined by utilizing temperature and pressure as the two most important parameters. Pressure and temperature uniquely define the properties of superheated steam. For saturated steam, the additional parameter of steam quality must also be determined.

The problem with steam metering and steam flow control occurs when the steam is left as unsaturated wet steam which is a situation commonly found in many steam distribution systems. Under these conditions the pressure and temperature fail to define the steam properties. This situation occurs because the system actually contains a two phase fluid and not just a finely dispersed vapor fluid combination. Under this two phase flow situation, with wet steam flow, the mixtures specific volume could change radically depending on the ratio between the liquid and vapor within the steam.

From a practical standpoint, one never knows the exact liquid to vapor ratio in flowing steam. Therefore, there will always be a question concerning the make up of the mixture and the homogeneous nature of the mixture along the distribution system. Depending on the velocity of the steam within the pipeline, different amounts of condensate are picked up at different times and entrained along with the steam in different quantities.

The density of this two phase mixture varies along the length of the piping system. This density variation occurs due to steam velocity changes or changes in condensate development rate which occurs in different segments of the steam distribution system. Therefore, along the distribution system, the steam flow is made up of a two phase fluid which has within it water vapor existing as slugs, a non-uniform mixture of droplets or a total separated flow where the liquid itself has separated out of the vapor portion, accumulating on the bottom of the pipe. The effect therefore of this two phase flow on differential pressure meters and controls is very different depending on the device location in the steam distribution system.

Be aware that any kind of flow that is not homogeneous, whether it is slug flow or flow with randomly occurring droplets of water, will affect the accuracy of any type of pressure differential flow measuring device; especially those utilizing an orifice device or annubar, which appears to be the bulk of the measuring devices utilized today. The affects of this type of flow can be the loss of large sums of money in a commercial facility or very dangerous over or under shooting of sensitive controls in industrial and military facilities.
For more information on this subject please see the papers I have written and the book I wrote, Steam Distribution and Flow: A Guide for High, Low and Medium Pressure Systems, available at www.nrctraining.com.

Hal