Steam Quality
Steam Quality
(OP)
I am currently doing a review of tseam 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
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





RE: Steam Quality
kyong
RE: Steam Quality
RE: Steam Quality
Many plants have installed low-water content boilers. If these little boilers are not piped correctly (and a great many are not), steam quality can be terrible. In addition, they have very little tolerance for even minor water treatment upsets, often causing large amounts of carry-over. Older style fire-tube or water-tube boilers typically deliver better quality steam, and have much greater tolerance for mechanical or chemical upsets.
There are also a lot of steam distribution system issues that contribute to what is often considered "wet steam", but actually are just poor sloping/traping of the lines. Slugs of condensate build up ahead of closed control valves. This hot water is then shot through the valve when it starts to open, shortening it's lifespan by a factor of many, and causing all kinds of other problems. Steam quality that is actually good or excellent often gets the blame for being "wet".
RE: Steam Quality
I'd agree with TBP that pressure (and temperaure) correction is a big issue for flowmeters.
Can anyone give an indication of when superheated steam is used and when saturated steam is used? Is this related to the size of the steam distribution system or how well it is maintained or cost limitations?
Has anyone got a feel for typical wetness values? How wet is very wet?
Thanks,
Neil
RE: Steam Quality
I worked in the district heating business for a number of years, and we didn't bother with temperature correction on our steam meters, just pressure. The steam flow meters were on the high pressure side of the system (125 PSIG operating pressure), which meant that they could be considerably smaller and cheaper than if they were after the PRV (10 - 12 PSIG) stations where some superheat might have been present. The cost of a 2" meter body is substantially less than a 6" meter, especially when you're trying to find 10 or 15 diameters of straight pipe upstream, and 5 more downstream in existing buildings. The increased meter size/cost combined with with installing and insulating significantly larger diameter pipe adds up pretty fast.
RE: Steam Quality
I like your stuff. Now you need to go to another current thread in this forum called "superheated steam in heat exchangers" and post the same information for the benefit of that inquirer.
rmw
RE: Steam Quality
One of my contacts suggested that 90% of steam metering systems operate below 350F and 250 psig (177 deg C and 17 bar). Is this reasonable?
Thanks for your input again. This discuss is quite helpful to me.
Cheers,
Neil
RE: Steam Quality
Every industry has it's own little spins, quirks & requirements, and it's important to be familiar with those before anybody buys and installs equipment.
RE: Steam Quality
Let's go back to your original post.
As TBD has pointed out, the situation starts with why the plant is generating steam. Those who are going to use it in prime movers such as turbines want to generate as much superheat as possible, because turbines like superheat, and have more potential to do work for a given pressure rating.
Other users of this steam, then, have to accomodate the superheated steam, or desuperheat it for their use.
Nuclear plants, as you observe, due to the nature of the beast, only generate saturated steam, and it only gets wetter as it expands down through the turbine. Yes, they extract the flow and run it through a device called a MSR, a moisture separator reheater, which uses main steam to dry, and reheat the wet steam so it can continue down the turbine path, but in general, the steam is wet or wetter.
Plants that have no turbines generally generate saturated steam, and if it is lucky it leaves the boiler dry, or mostly dry, but keeping it dry is the hard part.
In theory, if piping is sized right, and insulated well, steam passing down a header, as it loses pressure it will superheat slightly, offsetting the heat loss to the atmosphere. Unfortunately, most plant steam systems don't work like the theory says. Insulation quality is less than optimum, piping has low points causing pockets, size is either too small or too large.
The user devices pay the price in damage done by the moisture in the steam, such as the jets I mentioned in an earler post. Erosion cuts piping, and heat exchanger tubing, etc.
So, there is no perfect system. Usually you are fighting too much superheat, or too much moisture. And the jet vendor is screaming for d&s steam, like it is supposed to exist.