Deaerator Level Measurement 1

[surineman]

Hi there,

We have a spray type deaerator in our boiler house.
The existing level measurement on this deaerator is Through a DP cell.
it has been found that slight pressure fluctuations in the top deaerator scrubber drum(i.e. a jump in steam pressure for only few psig), have had a huge impact on level in storage drum(70 to 90%).

Any idea what is happening? for me it does not make sense. the pressure jump should be sensed by both high and low press. connections and should not affect the level measurement unreasonably.

if we have to change the type of level measuremnt, what would be the best choice?
A guided wave radar or a displacer type?

Any kindly advice/exerience would be highly appreciated.

these are the process data:


Vessel Height 12 feet


Units Min Operating Max
Temperature OF 262 298 298
Pressure psig 22 50 50


Liquid heights are measured from the bottom of the vessel.



Regards,

 

[Ashereng]

3 psi is about 80 inches water.

Is your signal bouncing and settling back to the correct reading, or permanently off?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[surineman]

Ashereng,

I am not quite sure, since operations are insisting on replacement I did not ask more questions.
The only thing that I can think of is the tapping points of either high or low (or maybe both) might have blockage or something like that, although there are a lot of bends in tubing too.

Anyway my concern right now is type of instrument for replacement. I was considering displacer type, but the maintenance do not want a 9 ft. displacer type and instead asking for guided wave radar.

What do you guys think?
Thanks in advance

 

[Ashereng]

Give maintenance what they want, if you want the instrument to work.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[surineman]

The only thing is I am worried about is the bubbling surface of Deaerator may cause problem in Radar measurement.

Any experience with same type of service?

Thanks in advance.
Regards,

 

[Ashereng]

Not directly.

I do mostly storage tanks. And, we don't bubble.

In most tanks, the maximum fill level is usually not 100% of the tank capacity. Hence, even with the bubbling, in as long as it is not a large percentage of the total level, it may be okay, depending on accuracy.

I am assuming that your bubbling zone is more or less constant. If so, then it is pretty much just a bias of the signal. If it is variable, then you need to do a bit more work, perhaps vary the bias depending on current level?

Most likely, you need to stay within a range of levels (between the upper and lower operating ranges). If this is the case, the bubbling effect on radar should be negligible.

Also, I have been told that radar is actually fairly good at "looking through" vapor and dust. It is dependent more on density changes to reflect the wave back up. So, I guess if it is just water bubbles, then it should not be too much of an issue? Maybe talk to the radar vendor and see what they say about the application.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[ScottyUK]

Are the two impulse lines adequately lagged and heated? If the steam in the impulse lines is cool enough to condense in one or other line the resulting measurement will be very unstable.


----------------------------------

I don't suffer from insanity. I enjoy it...

 

[JimCasey]

I think that you have a reasonably large leg of saturated water in your sensing lines now, and a decrease in pressure lets the water flash off, changing the zero.

The upper connection should slope upward from the aerator to a datum chamber (condensate pot) . Connection must be above max water level. The datum chamber will continuously fill with condensate and hold a constant level. Excess condensate will follow the slope and dribble back to the DA. Measure the differential from the datum chamber to the connection near the bottom of the DA. Small differential: High level. Large differential: Low level.

 

[jgv]

I will recomend you to have a leveltrol transmitter from fisher governor or other brand, but the same measurement system, you can have water deposits at the upper side of your dp cell, even if you have filled with water this side.

 

[JLSeagull]

Neither the displacer nor open radar sound ideal for this application. Any displacer longer than three-foot is undesirable. The dp issues associated with density would also apply to a displacer, but seen more slowly. I would contact the radar level vendors but suspect that liquid spray would be a problem for open radar. The guided wave radar sounds like a good prospect.

I might have suggested the dp transmitter if you had no problems with the existing installation. DP transmitters are affected by density changes - including the temperature changes and the affects of flashing. Could the existing transmitter impulse line be connected to point where a valve is occasionally opened? The flow would knock the dp for a loop.

Capacitance might work well in this application too.

 

[Bjerke]

This level indication error is well known in the nuclear industy where errors during a pressure transient is important to nuclear safety. I would believe that your error is due to accumulation of non-condensible gases in the condensate pot which leads to a reference leg saturated with gas. At pressure transients gases comes out of solution and canges the density of the reference leg. The remidy is to continously vent the condensate pot or to install an "Advanced condensate pot" which does not accumulate gases. The "Advanced pot" is marketed by Westinghouse Atom.

 

[Bjerke]

An additional comment. There is a very simple way to find out if the condensate pot has a high contet of non-condensible gases. Measure the temperature of the pot surface! If there are no gases the steam temperature in the pot is equal to the steam saturation temperature at the pressure you have in the deaerater. With increaing gas content in the pot the partial pressure of the steam in the pot decreases and the temperature goes down. (The temperature corresponds is the saturation temperature of the steam at its partial pressure.) If the pot is full of non condensible gases, as I suspect, the pot is at ambient temperature. The surface temperature of a gas free pot at your conditions is about 5-15 C lower than the process temperature due to the temperature gradient in the wall of the pot.

 

[hacksaw]

your dearatot pressure cannot be allowed to swing that much. you have a steam supply problem. fix that first

 

[dcasto]

The LP leg is filling up water and flashing. Insulating and electrict heat trace will solve the problem as stated above.
Use two pressure transmitters and subtract the two values to get the level, thats exactly what all new Dp transmitters do. The LP transmitter can be placed above the tap point so the leg will not fill with water.

 

[instrument1717]

if Going for changing the type of level measuremnt, The best choice is guided wave .The displacer instrument also will subject to the pressure surges to some extend .

 

[rajant2001]

Level measurements based on DP are affected by liquid density which in varies with the process Temperature and Pressure. Hence suitable corrections are to be made to the DP transmitter output.

R.Thiyagarajan.
















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[RADT]

Guided wave do work well, however from what I have seen they can be fairly costly. We have also been burnt when there are partial obstructions or, as previously mentioned, turbulence.

Did you try looking into a magneto restrictive float type. Basically the same principle as the displacer however you do not have the torque tube mechanics to deal with. The only problems we have had have been at extreme cold ambient temp (-40 deg) and in start-up conditions (no process heat), but who works when it is that cold.
Check out

http://www.ktekcorp.com

or

http://www.isemagtech.com

we have hundreds of k-teks in our plant and they work well and the cost would be in the ball park of 5000 (CAD)

 

[NakedNed]

Most radar level gauges get confused by boiling / non-collapsed liquid states. Mohr and Associates sells an ultrawideband radar liquid level sensor that measures froth height, froth-liquid interface, void fraction, and determine collapsed liquid level in boiling environments. It was originally designed to assess coolant inventory in nuclear reactor heat exchangers, so it's pretty robust:

http://www.mohr-engineering.com/efp-guided-radar-liquid-level.php

 

[StevedeBritt]

Hi,

I agree with you that the pressure should be equal on both legs, therefore there is a restriction in one of the legs. Have the maintenance people ream out the tubing.

Hacksaw has a good point about the process surges. Try slowing down the rate that the spray operates.

Steve

 

[addy1974]

Hi,
How did you get on have same problem. Due to leg of DP constantly blocking up and was thinking of Guided radar but feel the turbulence may be to great.Is there any reason why a 1:1 pressure repeater cannot be used to eliminate condensate and blocking up of LP side.