Draft Pressure

[macmet]

I'm on site, in the middle of nowhere, and getting little to no help from our office. I'm in the midst of starting up a solid fuel boiler system which is fired on sawdust (the fuel is fine and less than 10% moisture).

The system is running well, we're getting solid output, good fuel feed, and for the most part everything is working well.

My problem is that our draft transmitter is getting sporadic readings and causing our ID fan to modulate. What we've done is locked our ID fan at a certain frequency which we found kept a negative pressure in the chamber.

Does anyone have any idea what might cause these fluctuations? I've attached a trend file. The trend was taken while our FD air and ID fans were at fixed speeds. Our fuel feed is also constant.

We've also moved our transmitter to a zone and still see this problem.

Our draft pressure always fluctuates somewhat, but not this drastically, nor this suddenly.

This is the second transmitter we've used as well.

Any help would be appreciated.

 

[macmet]

One last point, the draft pressure is shown as inches water column on the plot.

 

[boilerone]

Try to look at the fuel feeding right before going into the furnace and see if the fuel is dropping in batches which causes sudden ignition or dousing of existing fire.

 

[macmet]

boilerone,

I was thinking that also, so I had the pressure transmitter moved but had the same result.

The high and low points also do not correspond to our walking floor movement or anything else I can think of.

 

[boilerone]

With this kind of pressure variation magnitude you should also observe furnace wall pulsation, enough to raise concern... I've seen this before.

My point is that the pressure change is not from ID fan and pressure transmitter feedback and modulating. They are actually trying to reduce the pressure changes. I am suspecting that the feeder is not supplying the fuel evenly into the furnace. When large batches of dry sawdust drop into the furnace, it ignites and burns suddenly and creates a lot more flue gas (if the fuel is too wet it does the other way) so raises the furnace pressure just like small puffs.

 

[hacksaw]

your xmtrs are probably okay, the pressure fluctuations look pretty normal to me. It is not always controllable. There are several things you can consider:

1. check your impulse line for condensate build up, you may need larger dia impulse lines

2. use signal averaging 30-60 sec, so that you only adjust the fan to hold average pressure

3. take the fans of pressure/draft control and slave them off the fuel feed. Just monitor and alarm furnace pressure

your fuel feed is not a flow meas. but variable speed feeder

if you have furnace O2 (may not be available) or combustion air you can trim the fan rates.

 

[rmw]

Wow, sawdust with 10% moisture..... is it really sawdust? Planer shavings are only something like 5-7% moisture off the planer as I remember, and it was trouble when it was fired, by itself or even co mingled with wetter bark fuel.

Fuel this dry is explosive!!!! As in dust ignition type of explosive and I think this is what you are seeing go on in the furnace. Boilerone gives a good description of what is going on. What is the fuel firing system? Are you trying to distribute this dry fuel over a grate with a spreader stoker? You mention grate walking speed. How are you putting the fuel over this grate?

Normally, fuel this dry is burnt with a burner more like a gas spud type burner than a spreader type of stoker. I have seen flame color that gas burners would have killed for coming off the burner tube supplied by dry plywood or particle board trim saws and/or planers. But, this was being fed into a furnace that was predominantly fired by a majority of wetter bark fuel, and the dry feed was just a supplement and didn't have much effect on draft.

The pure dry fuel burners made extensively and only for dry fuel were designed like the old slagging type coal burners and actually had a slag tap on them. Not a furnace type situation.

Obviously you probably aren't going to be able to make any major equipment or configuration changes, so... since you mention that you can put the ID fan on manual and maintain a negative draft, I second Hacksaw's suggestion of signal averaging (or serious detuning of the loop) and spread the reaction of you ID fan controller to a much broader signal average rather than have it chase instantaneous changes due to furnace pulsations caused by 'explosive' fuel firing.

Is this the 'design' fuel? or are you just burning up a bunch of fuel that has been in storage waiting on this start up? Will the fuel for normal operation be this dry consistently?

And one more question; what type of tap do you have in the furnace wall for your draft controller and where is it located?

rmw

 

[macmet]

Well I work for a small company and our technology is similar to the Detroit Stoker "Reciprograte".

Our flame is almost white and it's hard to look at. Our cell temperature is about 1000C but we have high excess air levels. I've been worried about ash fusion so I've always ensured excess air levels are high but that has prevented me from getting a good feel of the actual combustion temperature.

We had a meeting today where I've recommended signal averaging since this is something I was thinking of too. Having other's input has helped my confidence with the idea, although it was not accepted too greatly. I think it will work though and was able to convince everyone that it was worth a shot.

This is the design fuel, and for the most part it burns really well. We have no issues with ash and it is comes from a consistent source. I'm confident that if we can get the fan to run in auto we'll be set.

And to answer your final question, we have the draft port made of a T coupling to allow for cleanout. We've tried two locations and seen the same issue both times. We've also tried two sensors.

The sensor itself is an analog input into our PLC. Do you think using something like a Photohelic by Dwyer might help?

Thanks again to everyone who responded. Hopefully using the average will allow us to minimize the peaks and help the ID fan keep up.