Saturday, February 10, 2007

Effects of Two Phase Steam Flow

Again, it is important to realize that the theoretical and working flow equations for all flow controls and meters are based on homogeneous flows. Depending on the flow controlling devices principal of operation, the effect of non-homogeneous flow can be considerable. Two phase fluids such as wet steam, play havoc with the accuracy of steam flow measuring devices and steam flow controls. For example. With a device that utilized an orifice to measure and or control steam flow, operating at a constant 150 psig going from saturated steam to about 10% moisture could cause data supplied to the control device to be considerably lower than actual flow, depending on how the moisture isdisbursedd within the steam.

Two significant differences with non-homogeneous flows are that first, the density is not easily derived or measured, and second, one phase of one or more of the fluids components may not be moving at the same velocity as the main flow. Therefore, in these cases some of the fluid may actually be flowing along the bottom of the pipe and be separated out, alternating between a separated flow and an entrained flow in different parts of the distribution system. When the liquid droplets separate out of the flow, the phenomena is commonly called slip. Slip is a complex function of viscosity, particle size, density differences, surface temperature and the superficial velocity of each component. The effects of gravity also play a part in slip and a part in altering various flow patterns. Horizontal, vertical and inclined pipes cause differing relative velocities for the varying components, so they each affect slip differently.

Whenever you have a two phase flow which must be controlled and metered, the void fraction, or percentage of each constitute, must be determined in order to predict the quantity or velocity of each component. Therefore, measuring such a two phase flow with a simple orifice designed under homogeneous equations and equations which apply to saturated and single phase flow, will contribute considerable errors in actual field measurements. Additionally, if the steam pressure supplied to a particular device is above the original design pressure of the measuring device and that device does not have pressure compensation, the combination of wet steam and increased steam pressure will generate a substantially reduced reading from any meter or flow control. For example. At 10% moisture content and 160 psig steam in lieu of 125 psig as the original measuring device may have called for, the differential pressure metering device will register only 85% of what the actual flow is.

One way to protect against meter and flow control inaccuracies created by a two phase steam flow is to utilize moisture separators upstream from the measuring device. If such separators have not been utilized and there are indications that excessive moisture exists within the distribution system, one should immediately suspect that meter readings, flow indications and flow control, are all being adversely affected.

If previously installed moisture separators are constantly overloading and damage to existing orifice plates, PRV stations and meters appear to be caused by excessive condensate, immediate action should be taken to determine the cause of the increase in the wetness of the steam.

Moisture in the steam flow that amounts to over 2% wetness has a tendency to also act as a grinding agent on orifice plates and other types of differential pressure measuring devices, causing enlargements in the measuring plate or device. The coefficients utilized therefore in the various equations and algorithms, though correct at the time they were selected since they were related to the design orifice diameters, will be incorrect when applied against the enlarged orifices which have enlarged as a result of the grinding effect caused by the wetness of the steam. The excessive moisture in the steam will also cause orifice plates to become warped. The excessive moisture, even if it is not in the form of large slugs, will have an eroding effect on orifices, control valves and on the PRV valves located as part of pressure reducing stations. Allowing this to happen can cause serious errors and dangers within a steam distribution system.

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
http://www.nrctraining.com/.

Hal