HP Drum level measurement 4

[npower0073]

Our HRSG is a triple pressure one with an HP Drum that works at pressure of 115 bar at base load of gast turbine. During recent overhaul we saw that the actual level of the drum (red marks on the inside of the level drum) is almost 100 mm higher than the designed one. THe level is measured with differential pressure transmitters , 3 of them, of the same manufactuer, all of them show the same level with a maximum deviation of 15mm , in a total range of 2700mm. Can you please assist on that issue? How can this be corrected?

 

[roydm]

How are you compensating for pressure and temperature?
It seems like there is an error in the correction formula.
Perhaps if you post the maths someone will spopt the error.
A sketch would help also.
Roy

 

[JLSeagull]

dp level transmitters use a fixed density for the liquid. Clearly the assumed density is not the actual density. Consider another technology for at least one of the transmitters to eliminate the same common mode error. I like radar but have not looked for radar at 115 bar.

 

[Mattrock62]

Another possible source of error is static pressure effect on zero and span. It is very difficult to compensate. Manufacturer's rarely talk abaout static pressure errors, especially if they have no compensation.

Try using a dp cell with static pressure compensation.

The error you are reporting is about 3% which is not uncommon for some dp cells.

There are dp calibration standards available that would allow you to determine the error and compensate in your instrumentation.

If you are able to adjust zero at 115 bar with your manifold open between high and low side, you can eliminate the zero error.

 

[hacksaw]

most boilers also require independent level measure such as sight guage and or electrode systems. how do these reading compare with your d/p meaurement?

 

[npower0073]

hacksaw unfortunately the sight gauges are not working correctly and we are in the process to repalce them

 

[roydm]

So you are telling us the site gauges are out as well?
I found this interesting paper on the subject

http://www.tycoflowcontrol-na.com/ld/180.pdf

Roy

 

[Cat3347]

Foxboro now has a density compensating DP transmitter designed for drum level measurement. It's worth a call to your local rep....

Here is a link:

http://www.foxboro.com/NR/rdonlyres...059-574DA2E7E6A3/0/IMV31MultivaribleLevel.pdf

 

[Mattrock62]

You may want consider replacing the sight glass system with a conductivity electrode system such as the Solarton Hydrastep or the Aquarian system.

 

[Standingback]

I do not accept the argument that DP transmitters have a problem with pressure. For example the Fuji Electric FCX series transmitter is now capable to operate with a static pressure of 1035 Barg without adverse effects on the accuracy.

I believe that the issue is a function of definition. The "level" in a steam drum is actually a bubbling mix of water and steam (just try look at a boiling pan of water) and a true level does not actually exist.

The steam load on the boiler has a huge effect. As the pressure drops, the steam bubbles increase in size and the lower the density of the water. The release of bubbles above a dp transmitter tappings is random. So forget trying to pressure compensate for it.

If you change technology to conductivity, ultrasonic or radar, it will not solve your problem, because the physical property that you are trying to measure, is volatile.

The purpose of drum level control is to produce steam and to protect the tubes from overheating. The most effective way to maintain a constant level is to replace the steam produced by the same amount of water (maintaining a mass balance). This is the aim of 3 element level control, which uses steam flow and water flow as fast response inputs before trimming to an "average level".

When this perspective is taken. The requirements of the "level transmitter" are far less and demanding.

To illustrate common solutions to this many level probe manufacturers advise that probes should be installed in "protection tubes" or "a still well". In effect, these are mechanical ways of averaging what is physically happening in the drum.

No amount of compensation will help in this application, as I explained earlier, the formation of steam bubbles is random and pressure and firing rate dependent. If you really want to see what I mean, try getting hold of a copy of a video taken by Spriax Sarco, where they video's the internal of a boiler under load conditions. It will change your idea about boiler "level".


I trust this helps,
Mlv

http://www.coulton.com

 

[jasonf]

Mag gauges can be used as a secondary level indicator for drum level. At which point one could use a magnetostrictive or guided wave radar for the transmitter level.

 

[roydm]

I suspect the level in the drum will be higher than the level in gauge glass or standpipe because the density is less due to deltaT. So whatever device you measure with is going to indicate a lower level than actual unless it's right inside the drum. The density inside will also be lower because of the bubbles rising to surface.
100mm in 2700 is 3.7% not a lot of error
if your normal is 50% error would be ~7%
The easiest solution is just reduce the transmitter span by a similar amount.
This is a good explanation I found

http://www.tycoflowcontrol-na.com/ld/180.pdf

Roy

 

[Standingback]

I agree with Roy that the issue is the the density of the water in the drum. The article that Roy refers to is very useful and confirms my suggestion that balancing the steam flow against the water flow should be used as the primary measure for level control (see figure 2 in the Tyco article. In addition Fig2 also shows how the drum pressure should be used to compensate the DP transmitter signal to compensate the for the change in density due to boiler pressure.

Nice find Roy.



I trust this helps,
Mlv

http://www.coulton.com

 

[npower0073]

roydm, thanky you very much for your post, we use on site the three element level control. And it works OK, during ent big outage we will combine the revision of the transmitters span as well.

 

[JimCasey]

At 115 Barg the water in the drum only has a specific gravity of 0.667. The water in the tubes connecting to your DP cell is going to have specific gravity close to 1.0. you get a zero shift AND a span shift.

Jerguson and Yarway both offer drum level indicator /transmitters that have temperature-compensated datum chambers. Jerguson's is called "Trulevel" to bring attention to the reading being the same on the gage as the inside of the tank.

Here's a link to the Jerguson unit.

http://www.nciweb.net/tru-.htm

Look at installation "C".

 

[Standingback]

Jim,
I am unsure how you come to a SG of 0.667 At 115 Barg in the drum.

The "water" in a steam drum is actually a bubbling mix of water and steam. The density of which is totally unpredictable and changing as steam bubles are released off the tubes in an uncontrolled manner.

To add to the problem the firing rate of the boiler, the feedwater makeup rate, the FD and ID fan speeds (and most other things that you would like to mention) will all change the way in which steam is released of the heating surfaces. The result is unknown density.

Internal or external chambers do not solve this issue, they just provide a calmer environment for level measurement. The level in chambers (compensated or not) are trying to equal the "level" in the drum.

This takes me back to my origional posting where i suggested that the issue is in the difficulty in defining "drum level". In that a true level does not actually exist.

The following link attempts to explain this in a more articulate way than I have managed so far.

http://www.spiraxsarco.com/resource...oiler-house/water-levels-in-steam-boilers.asp

I hasten to add, that I have no affiliation with Spirax, but I worked for them in the past as a boilerhouse specialist.


I trust this helps,
Mlv

http://www.coulton.com

 

[JimCasey]

Specific volume of water at 115Barg (saturated liquid) is 0.024 Ft3/lbm
Specific volume of 20C water is 0.016 ft3/lbm (SG=1)
Hence SG of (Liquid) at 115Barg saturated is 0.667

You are right about the bubbles stirring things up. A Stilling well to trap a column of single-phase liquid would be a way to get a measurement of the "Liquid " level.

Referencing the DP cell with an external leg at ambient temperature will undoubtedly create a false reading, and I believe that accounts for the observed discrepancy in npower0073's original post.

Yarway and Jerguson have been reading boiler levels for a lot longer than your and my combined ages, and the thermally compensated Datum chamber I referenced was developed specifically to address THIS phenomenon, because they realized that even a sight glass gives a false reading on a high-pressure boiler.

And you are undoubtedly correct that there is not a true surface of the liquid in a steam drum, but the boiler manufacturers know what the >READING< needs to be so that they don't get a lot of liquid carryover into the Superheater-where slugs of liquid could cause catastrophic failure due to thermal shock.