You will need to know the volume of air coming in from the blower, the volume of air that would leak from the rotary valve, and the internal cross-sectional area of the conveying line. With this information you can write an equation that would calculate the conveying velocity and display it on your control panel. In this calculation you would have to consider compressible gas flow.

I can help you in this calculation if you would like.

Regards,

Amrit Agarwal (Tim)

Pneumatic Conveying Consultants

polypcc@aol.com ■

Dear Bob,

If you have a positive displacement blower, its incoming volumetric flow rate is fixed. Its discharge flow rate will depend upon the pressure at the blower outlet. You already have on-line pressure measurement. You can use this pressure measurement to convert the intake flow to discharge flow. This could be done by a simple algorithm stored in the DCS.

Using a flow meter will do the same thing if that would cost less.

One thing to remember is that solids velocity in a conveying line is always less than the gas velocity. By the flow meter or through the DCS calculation you can get the gas velocity but not the solids velocity.

Solids velocity in a conveying line depends on a number of variables such as particle size, shape, density, pipe line bends, etc. Let me know if you have any questions.

Regards,

Amrit Agarwal (Tim)

Pneumatic Conveying Consultants ■

For lobe type positive displacement blowers the incoming gas flow depends upon the slip or "leakage" between the rotor tips and the casing. This slip increases as the discharge pressure increases resulting in reduced incoming gas flow. Blower curves given in the vendor catalogs are based on this slip flow. Most vendors can also give equations for calculation of the slip flow based on the standard clearances.

The discharge flow from the blower depends upon the discharge pressure and temperature.

Regards,

Amrit Agarwal (Tim)

Pneumatic Conveying Consultants ■

I would suggest to install particle moving detectors in your line.

This detectors are available in different sizes and types.

The simplest are adjusted by your Pls and indicate if the particle flow out of the desired range. This sensors are available for about 700 $ or less.

If you want to determined the velocity exactly you have to install a more comfortable system (the sensors are the same).

Please visit the following websites for more informations:

http://www.dynatechnik.com

or

http://www.swr-engineering.com

Best regards

Klaus Schneider (schneider@enviro-engineering.com) ■

Its not possible to measure the solids velocity in pneumatic conveying systems. We can measure the gas velocity but not the solids velocity.

I think that what you have is for measuring emission of solid dust particles from dust collectors, etc. to comply with emission regulations.

Regards,

Amrit Agarwal lTim)

Pneumatic Conveying Consultants ■

Dear Mr. Agarwal,

It seems that you have not visited the websites I recommend in my last post. Please have a look for example to the Dynatechnik-website. They sell equipment for measuring particel velocities since several years.

The technique is not new and well known since about 20 years.

Best regards

Klaus Schneider ■

Dear Mr. Schneider,

I am fairly conversant with devices such as made by your company. I have not seen applications of these devices in pneumatic conveying lines. It may be a good idea if you give some examples of commercial installations where these devices are being used for ACCURATE measurement of solids conveying velocity in pneumatic conveying.

Best regards,

Amrit Agarwal (Tim)

Pneumatic Conveying Consultants ■

Dear Mr. Agarwal,

I am not supplier of such devices. During this afternoon I spoke with Mr. Dybeck (Managing Director of Dynatechnik) and asked him for an additional post and answers to the questions you have requested for.

I´m shure that he will convinced you concerning the advantages of the Dynatechnik technologies.

best regards

Klaus Schneider ■

Hi Bob,

As most of your problems appear to be filter related, have you considered simply fitting a DP cell across your intake filter, and a pressure gauge in the receiving vessel? This is a simple solution, that will at least give some indication of the condition of the filters. These can then be replaced when they start blocking up. Alternatively, simply replace the filters on a fixed schedule. Filters are cheep when compared to the cost of blockages.

Obviously, there are more sophisticated solutions possible, but this one is quick and inexpensive, and easily maintainable.

Regards,

Dave ■

Bob I'm not sure if measuring line velocity would give you enough warning. In the past I've used the Magnehelic pressure and vacuum switches to monitor the system differential and when it gets to a preset high point shut the feed into the system off until it drops to a low setting. If it doesn't drop with in a short period of time shut the system down and sound the alarm.

The problem with monitoring the differential across filters is that the connections to the switches will typically fill with what ever is being conveyed effecting the switch performance.

Presently I'm involved in setting up our test lab pneumatic system and one of the tasks I have before me is accurately measuring line velocities. Two things have come to mind - one the the best way to measure the velocity is probably to use either a manometer across a couple of pressure taps or rely on a pitot tube inserted into the end of a test section of line that hopefully will reduce the turbulence before the pitot tube and give me some reasonably accurate or at least consistant readings.

I wouldn't think accuracy is all that important in your case you're really looking for sudden changes in flow.

You could contact Dwyer- they have a fairly good line of measuring gear.

If you want you could call me at work and I'd be happy to discuss this with you further - perhaps something else will come to mind.

Number is (610) 814 - 2400 Ext 662 - promise not to try and sell you anything.

Good Luck

Al Powell ■

"The problem with monitoring the differential across filters is that the connections to the switches will typically fill with what ever is being conveyed effecting the switch performance."

The solution to this problem is to purge the instrument taps. At this time, I am helping a client establish a temperature and pressure profile of their process. Connecting the instruments to the DCS allow you to develop an understanding of how material is being conveyed through your system. With a knowledge of the pressures and how they relate to system performance, you can then develop interlocks and permissives to control the addition of solids to your pneumatic conveying system. The addition of a VFD to the rotary feeders can provide additional control.

A little operator training in maintaining purged taps and monitoring system performance will go a long way to avoiding plugged lines. When the operators understand that proper attention to the details can limit the use of hammers and shovels, they will try to maintain the process within operating limits. ■