Steam Quality

[NeilatNEL]

I am currently doing a review of steam flow metering systems. I know a reasonable amount about flowmeters but I'm a relative novice on the subject of steam systems. I believe that one of the primary sources of flowmetering error is that steam quality is rarely known. This is obviously not an issue in superheated steam but it is in saturated steam. What I would like to know is what are typical operating conditions for steam systems. Based on various conversations I've had I am coming to the following tentative conclusions -

1. Relatively large scale systems (e.g. in refineries or chemical plants) usually transport superheated steam (and then desuperheat at the point of use)
2. Nuclear and geothermal systems can be very wet (less than 90% quality).
3. In smaller scale systems (e.g. building heating, hospitals etc) the quality depends very much on how old and how well maintained the system is. A well maintained new system will probably operate in the 90 to 97% quality range). An old system may be much worse than this.

Does anyone have any comments on the above or further information.

Thanks for your help.

Neil

 

[TrevorP]

Measuring steam flow is notoriously difficult if the quality is unknown and varies over time. Often at the end of a long line, heat has been lost to atmosphere, and condensate is formed and collected in the main, hence the reason for let down stations at the end of long lines. Usually this consists of a condensate separation system - incorporating a steam trap - and a downstream pressure reducer. Usually only a slight drop in pressure is necessary to re-saturate or slightly superheat the steam. The quality of the steam can then usually be reasonably accurately measured. I have used a vortex flowmeter to very good effect on steam in the past using this approach. For further information I suggest you contact a supplier of steam trapping and regulation equipment, such as Spirax Sarco.

 

[cub3bead]

A venturi designed for ASME PTC-6 would be suitable for this.

 

[MJCronin]

neil,

Here is an article praising ultrasonic devices use for "district heating systems" (saturated service)

http://www.flowresearch.com/articles/InTech 7_21_00.pdf

Accuracy of steam flow measurement devices in these types of systems is of great importance, as money typically changes hands......

"Vortex flowmeters" also compete with ultrasonics for this saturated steam service.....see

http://www.flowresearch.com/articles/PDF_Files/Market watch-Vortex-LO.pdf

My personal experience with superheated steam has always been with a venturi device....

keep us informed about what you discover...

MJC