Internal Circulating Fluidized Bed Boiler Bed Level Measurement

It seems odd that the line clogs so fast since there shouldn't be any flow through if it's only measuring static pressure; therefore it's time for the usual set of questions. Is this a new problem, since the last shutdown, last overhaul, or long-standing? What has/was changed since the problem started, different fuel type/blend, new sand/limestone source, etc.? Does the DP sensor give readings that are expected or are they not to be relied upon? Were there any screens on the high pressure line that may have not been replaced? If there is more than one bed-level sensor loop is it having the same problem?

Without being there or waiting for your answers, I suspect that your DP sensor integrity has been breached allowing flow from the high pressure zone and the fines are being carried along. There is also the possibility that the bed level is oscillating such that the fines are being shaken-out to a level below the sensor port, or the bed is thicker than the instruments indicate and is, again, getting too close to the port. Have you considered installing a camera to view what's happening below the bed?

We await your answers.

All sensing lines I have ever seen in 31 years at power plants have a 4-way instrument valve to blow down the sensing lines. Over time the the internals of these valves wear and leak through putting a small flow of blow down air on one sensing line and that sets up a flow that creates a suction on the other sensing line usually causing it to plug shortly after blowing the sensing lines down.

I recommend these valves at www.conantcontrols.com. I first came across these valves while serving in Submarines, they are good precision valves that are better than anything on the market. I installed them on the sensing lines at a 1300 MW plant I worked at.

Ridge-Runner

One option you may look at is having the pressure transmitter with a connection that is at a 45 degree angle allowing for dust / bed particles to settle down and slide away from the transmitter, also you could set up a blow out system activated by a solenoid valve such that periodically it blasts air in and maintains the line unclogged as possible.

Ok, thanks to RAM, Ridge, and vargaalex for your replies. I am new to CR4 (as well as my career) and appreciate the help and look forward to being an active, contributing member to this community.

Now, let me reply to the questions.

1)"Is this a new problem?"

I spoke with the supervisor who has been here since the plant was built (2005) and he said that the DP sensor gave accurate readings for about 2-4 hours after clearing. About a year and a half ago, the situation has gotten to the point that clogging occurs very shortly after clearing the line.

2)" What has/was changed since the problem started?"

We burn waste coal so the quality of coal is always changing. The limestone and sand has remained relatively constant. One thing that was altered was that the screens in the tuyures were removed to achieve higher air flow.

3)" Does the DP sensor give readings that are expected or are they not top be relied upon?"

The DP sensor can only be relied upon for a very short time after clearing the line.

4)"Were there any screens on the high pressure line that may have not been replaced?"

No screens now. No screens in the original design.

5)"Is there more than one bed-level sensor loop?"

No, only one.

RAMConsult- I am confused by your statement:

"There is also the possibility that the bed level is oscillating such that the fines are being shaken-out to a level below the sensor port, or the bed is thicker than the instruments indicate and is, again, getting too close to the port."

Do you mean that the fines have piled up to the sensor? I have sketched the setup to give a better idea of the appx. geometry of the boiler.

6)" Have you considered installing a camera to view whats happening below the bed?"

We have not looked into this, but if you believe that will shed some light on the problem we will consider it.

Ridge-Runner- "All sensing lines I have ever seen in 31 years at power plants have a 4-way instrument valve to blow down the sensing lines. "

Yes this is our setup as well. Thanks for your reply, we will look into the internals to attempt to find a leak. I have not looked into conantcontrols yet, but will today.

vargaalex- "One option you may look at is having the pressure transmitter with a connection that is at a 45 degree angle"

Are you suggesting setting the sensor line at a 45 degree angle instead of parallel to the floor?

"also you could set up a blow out system activated by a solenoid valve such that periodically it blasts air in and maintains the line unclogged as possible."

We have actually considered this. The head engineer/owner has expressed concern of using air as this would create a hot spot near the sensing line. He said that if we were to condider this approach, to use a inert gas (argon). Would you guys agree with this?

Ridge Runner's reply seems very plausible.

Purging the impulse lines is done for slurries. The diagram shows purging for an orifice plate in a pipe, but the concept is the same, it keeps the impulse line open.

If a purge needs inert gas wouldn't nitrogen be suitable? I assume it costs much less than argon.

Typical purge layout:

Hi

Similar type of boilers I have worked in , we ensured that tapping pipe is 45 degrees (up) to the boiler wall and instrument impulse tubing is taken in upwards direction ( i.e. 90 degrees) to this tapping pipe. This ensures that sand /bed material never enters the tapping point , or if it enters the tapping point , with downward slope of 45 degrees it falls back in the bed. Even with this kind of arrangement , once in a while tapping point will be clogged . So we gave purging connection to tapping point, though I don't remember the exact air pressure used for purging.

Hi MaxT,

We have a solution to your problem. The Clearguard Autorodder. For more information please navigate to www.clearguard.com. The Autorodder keeps the tapping point from plugging by periodically stroking a scraper through the port, and at no time blocks the port, so your instrument measurement is online the whole time. The Autorodder is ideal for fluidized beds (including chlorinator beds, etc..). Each Autorodder is custom made to suit the required application, and as such, the material of each part is carefully selected to match the application (duplex, inconel, hasellloy, 316 S/S, etc..)

HIsmelt had an Autorodder operating on their furnace that was running at 1450c and we have worked with processes with operating pressures up to 100 Bar.

Regards

Ben Cammack

Senior Engineer

Clearguard

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