Hello Piet,

The ash/air mixture has 2 different speeds.

Air velocity and ash velocity.

The air velocity is easy to estimate as the air flow is known and the pressure and temperature at the chosen location can be measured.

A correction for the presence of ash is more difficult, as this correction is depending on the ash velocity.

The ash velocity is not that easy to calculate or measure.

The ash velocity is definitely lower than the air velocity and changing with the expanding air along the pipeline and retarded by each bend and accelerated after each bend.

In addition, this velocity also changes with the system pressure and SLR.

As there is a particle size distribution, not all the ash particles are travelling at the same speed. Although they exchange impulse in collisions.

I imagine that a scientific velocity measurement would be a laboratory experiment (electro static principle) and generates information that leaves you with more questions than solutions.

A pneumatic conveying calculation of the installation will explain a lot more about the functioning of the system.

For what reason you like to know the ash velocity at a defined location in the pipeline?

Wear problem?

Have a nice day

Teus ■

Hi Piet, (Must be a Dutch name)

I would be surprised if a contractor can talk his way out with a claim that the ash velocity is higher than designed by the contractor himself.

Another problem would be when the ash is of a different quality than designed for. (Particle size distribution, shape, hardness, etc).

Wear in pneumatic conveying systems for fly ash and cement occur almost always in the bends, whether they are long radius or short radius bends.

This problem is easily tackled with an outer box, welded to the bend.

The difference between the air velocity and the ash velocity is caused by the energy losses due to particle/wall and particle/particle collisions as these are responsible for the loss of velocity.

In straight pipe lines, the difference in velocity generates just enough acceleration to compensate those energy losses.

The air velocity must be higher than the suspension velocity (in vertical conveying) before a particle is accelerated.

Conveying very small particles pneumatically causes remarkably low wear on pipes, due to the fact that the particles are small compared to the boundary layer of the air flow.

A layer of slow moving particles along the wall protects the wall against the impact of faster moving particles.

At a location where the boundary layer is broken (like in bends), the moving particles can wear out the wall by direct contact.

Therefore, the difference in velocity is depending on the material loss factor, the suspension velocity, the Solid Loading Factor and location (f.i. bend).

The generation of wear is a different issue, because that is influenced by the impact of the particle and the sliding force of the particles (Tribo technology)

Under the rules of the EEC, the contractor has to make a technical file of his delivered installation and has to submit that information in case of disputes.

Ask him for the calculation results and in case of later problems, he has to substantiate the deviations from the design that he made.

If the subcontractor states that the velocities are too high, he must also know how high the velocities should be and how high the velocities are under the observed circumstances.

Contacting references with the same kind of installations and having a third party calculate the installation is also an option to prevent future problems.

Have a nice day

Teus ■

Hello Piet,

I do not expect wear problems in a long straight pipe section.

Calculating the expected velocities is a much easier way to go.

If you can supply the installation data (pipe geometry, compressor data), I can calculate the required information for you.

If you prefer, you can also use the e-mail function of this forum.

Take care

Teus ■

Dear Piet,

A way of measuring the velocities is by “Electrical Capacitance Tomography” and using cross correlation techniques (based on measured fluctuations) to derive the velocity between two points in a pipeline.

As an input signal it is also possible to use the density of the flowing medium by measuring the light extinction with a laser.

Search the internet for these subjects.

By far not an easy experiment.

Measuring the airflow with a venturi device in a material flow will be very difficult, because the ash will influence the pressure reading.

Measuring the air velocity without material in the conveying line makes no sense, as a calculation is just as accurate.

Instead of measuring the ash/air mixture velocity in a variety of different pneumatic ash handling plant setups, it is just as easy and much cheaper to calculate the pneumatic conveying installations using a computer program, which calculates the respective velocities at any location in the pipe line.

(in my computer program spaced at every 0.001 second if necessary).

And if you insist on measuring in the field, this calculation exercise indicates the values to be expected, which can help to choose the measuring method to be applied.

Thinking outside the box is already done by many engineers before.

They all returned back into the box.

Have a nice day

Teus ■

Hi Gentlemen

I have a question concerning the pneumatic conveying of ash in a power station application. Is there any way of physically measuring the ash/air mixture flow or speed near (not at) the discharge end of the pipeline?

The system in use is a dense-phase blowtank setup.

Any replies are welcome! Thanks!

Piet

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Dear Piet,

For your dense phase system, the method for measurement of air and solids velocity will be different than that for dilute phase. At the end of a pipeline you may have mixed/dilute phase flow resulting in pipeline wear. This system should be analyzed as a dense phase system.

Regards,

Amrit Agarwal

Consulting Engineer

Pneumatic Conveying Consulting

Email: polypcc@aol.com

Ph and Fax: 304 346 5125 ■