Fly ash conveying limitations

Wayne W
(not verified)
Posted in: , on 18. Mar. 2008 - 06:13

Our flyash system has recently decided to continuously plug lines on the way to the ash silo. The ash is conveyed pneumatically via a steam vacuum eductor on what should be the clean side of a baghouse. I say should be because many years ago we gutted the inside of the baghouse and it just became a wide space in the pipe. For many years this has worked just fine, but about a month ago the main ash conveying lines repeatedly started plugging up. The ash system vacuum is 8'-12'' but that is about where it has always operated. We actually have two systems that are tied together immediately before our ash silo. One system is a very fine pulverized coal ash which is not, and has never plugged. The other is an ash from a bark and wood waste boiler which is where all of the pluggage is occuring. At what volumes and particle weights will a system like I have described start to fail? Any rules of thumb, calculations, test procedures, or any other wisdom, would be beyond welcome at this point. We have made no modifications or changes that we can point to that would be causing these problems.

Re: Fly Ash Conveying Limitations

Posted on 18. Mar. 2008 - 08:40

Dear Wayne,

If a system is performing well for a long period of time and suddenly (or gradually) the system performance is changing, then one or more parameters are changed or changing.

Plugged the lines in a pneumatic conveying system is caused by a too high Solids Loading Ratio (SLR)

SLR= material mass flow / gas mass flow

A too high material mass flow causes a high SLR and eventually plugging

A too low gas mass flow causes also a too high SLR and also eventually plugging.

A too low gas flow can also cause a too low velocity in relation to the suspension velocity, leading to sedimentation and plugging.

At a too high vacuum (too low absolute pressure), the gas mass flow decreases so much, that the SLR exceeds the limit of the system.

Normally a system gets plugged at a certain vacuum. If this vacuum is reached, the vacuum

increases suddenly and the SLR increases drastically followed by plugging.

Try to measure the vacuum where plugging occurs and the related SLR.

Then try to calculate the system under the condition when it worked satisfactorally in the past.

The difference between the actual measurements and the recalculated history could indicate the problem area(s)

Also check whether the pipeline ids still open and free of obstructions and/or leaks.

Also check the functioning of the eductor, whether enough mass flow is generated.

The interaction between the two systems (especially when they are running in parallel on the same eductor needs a thorough evaluation)

More data might be helpful to help you further.