Steam Quality 2

[NeilatNEL]

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

 

[kyong]

Many heat exchanger designs for plants assume steam quality of 100%. This is quite rational because in most cases, saturated steam is the best for heat transfer purpose and therefore plants will intend to operate steam system so that saturation steam can go into each equipment. Actually steam traps are usually used to drain condensate from piping. By doing that, they can maintain steam quality of 100%(I think it's a little less than 100%, but practically can be regarded 100%)

kyong

 

[rmw]

The people who can tell you about moisture in saturated steam are people who are running steam jets on saturated steam without the benefit if a moisture separator in front of the jet. The moisture in the "thought to be dry, saturated" steam is accelerated to supersonic speed through the throat of the jet, and severe damage occurs. All this from "dry" steam. Check some steam jet manufacturers websites for their recommendations and experience. You should see a lot of comments about wet steam.

 

[TBP]

I've found the largest error in steam metering is due to lack of proper pressure compensation, with gross oversizing coming in second place. Without pressure compensation, the meter will read low if the pressure increases, and high if the pressure drops. Most industrial/institutional steam metering is terrible, and generate readings that are little more than a guess.

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".

 

[NeilatNEL]

Thank you all for your words of advice.

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

 

[TBP]

The only plants that generate superheated steam are operations that are driving turbines, or in the old days, steam engines. Generating superheated steam is very much NOT desired for heat transfer applications like heat exchangers, unit heaters, etc. Superheat is sensible heat, and you have to get past that to the latent heat, which is really where the "action" is. The little bit of superheat that occurs across most PRVs is not normally an issue. Most saturated steam systems have a little bit of moisture in the steam, and what occurs in practice is that instead of superheat, you get nice dry steam a few feet downstream of the PRV station. If nothing else, generating superheated steam costs money upfront for a boiler with a superheater section, and depending upon how much superheat, can affect a lot of other system components, like flanges, valves, expansion joints, etc and drive their cost up considerably.

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.

 

[rmw]

TBD,

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

 

[NeilatNEL]

Most of the people I have been discussing this with to date have been at oil refineries using steam from CHP plant. They all seem to use (or at least meter) superheated steam. In fact they have serious problems if they get any wetness in their steam. I guess from what you're saying that they are not typical steam users.

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

 

[TBP]

I don't have any refinery experience, but I assume that those operations likely generate steam to drive turbines, or run gas turbines to generate steam in a HRSG. The gas turbines I have experience with had steam injection. Dry steam is critical for that kind of operation. It must make sense for refineries to desuperheat the steam (if required) when it arrives at it's destination. For general industrial or institutional heat transfer operations a little moisture in the steam, while hardly desirable, is not a big problem.

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.

 

[rmw]

Neilatnel,

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.