air pressure transducers

[Ronnie Sullivan]

You know that feeling when you realise that you don't know anything about what you are being asked to do...... That's me at the moment.

I have been asked to renew a very old and antiquated control panel for a air/fume/smoke extractor system.

This is how it basically works: A 37KW fan motor pulls air/fumes/smoke from above a molten zinc galvanising bath, through a filtration system and outside into the fresh air.

I assume, as the pressure gauges don't work that when the pressure gets to a certain point (indicating the filters are getting blocked) - a shake down of the filters occurs, dropping all that is caught in them to fall into bags below.
At present, there are 2, 6mm air pipes that are connected to a specially made pressure unit and 2 air pressure gauges. (none of them work) so the operatives run the shakedown manually when they notice that the fumes are not being extracted properly.
As you can imagine - these fumes are pretty toxic and operatives are lazy and don't clean it until everyone is choking.

The ducting for the fumes is about 1M wide (circular) and the air pressure detecting pipes go - one before the filters and one after.

I want to replace these with air pressure transducers (4-20ma or 0-10V)
But I have no idea what pressures or even vacuum will be in these pipes.

As they are expensive, I don't want to get the wrong ones.

Has anyone any experience with this type of system and can tell me what type of transducer - or what pressures I am likely to get?

 

[AutoMax]

You might want to look at differential transducers, which are a single transducer with two probes and a single analog output. The output is based on the difference in pressure between the two probes. I've used a few brands, they all worked but none of them wowed me to the point I want to endorse them.

 

[tragically1969]

You know that feeling when you realise that you don't know anything about what you are being asked to do...... That's me at the moment.

I have been asked to renew a very old and antiquated control panel for a air/fume/smoke extractor system.

This is how it basically works: A 37KW fan motor pulls air/fumes/smoke from above a molten zinc galvanising bath, through a filtration system and outside into the fresh air.

I assume, as the pressure gauges don't work that when the pressure gets to a certain point (indicating the filters are getting blocked) - a shake down of the filters occurs, dropping all that is caught in them to fall into bags below.
At present, there are 2, 6mm air pipes that are connected to a specially made pressure unit and 2 air pressure gauges. (none of them work) so the operatives run the shakedown manually when they notice that the fumes are not being extracted properly.
As you can imagine - these fumes are pretty toxic and operatives are lazy and don't clean it until everyone is choking.

The ducting for the fumes is about 1M wide (circular) and the air pressure detecting pipes go - one before the filters and one after.

I want to replace these with air pressure transducers (4-20ma or 0-10V)
But I have no idea what pressures or even vacuum will be in these pipes.

As they are expensive, I don't want to get the wrong ones.

Has anyone any experience with this type of system and can tell me what type of transducer - or what pressures I am likely to get?

When i was in the process side of things (a while ago now) my go to was IFM for this sort of thing, give them a call they will be able to advise on the right sensor for the application:

http://www.ifm.com/ifmuk/web/pmain/040.html

 

[Andybr]

Standard pressure transmitters are unlikely to work well with the very low pressures you will want to detect to operate an air filter pulse cleaning system. There are people who make transmitters designed specifically for this sort of thing. I have used Furness Controls in the past and their transmitters have worked well.

 

[T Gibbs]

We have similar setup in our sawdust extraction system. Each filtration house has a small tube run from each side of the filters down to the base where I have placed a differential pressure transducer. We monitor the pressures on the HMI. I also set up alarm points at 3 different levels to warn us when it gets clogged. We have to manually activate the cleanout because we don't want it shutting of in the middle of a shift.

 

[AustralIan]

You can get an idea with a manometer http://uk.rs-online.com/web/p/digital-pressure-meters/9205188/

Maybe program in an open-loop mode, where it just shakes every so often regardless of the pressure.

Also, see someone about ways of keeping the pipework clean. Might need to blow compressed air down them every so often, you could possibly automate this.

 

[danw]

>what type of transducer - or what pressures I am likely to get?

As has been stated, you will need a differential pressure (DP) sensor. DP's have two ports, a high side and a low side.
Internally, the low side pressure is subtracted from the high side pressure and the result is the pressure difference.

Presumably the impulse sensing lines are 'across' the filter, one upstream, one downstream. A new filter creates a low DP across the filter because there is little restriction to air flow. As a filter clogs, the DP increases with the increasing restriction of air flow.

The range is likely to be mm or inches of water column. It is important to get the right sensing range.

The old instruments are likely to have an ID tag with a model number which can be decoded for a range.
Or if they were gauges with a switch the gauge scale can give you an idea of the range.

If the range of the existing units are unidentifiable, the cheapest way to determine the DP is with a water manometer (the link above is for an electronic indicating manometer). Some cheap Tygon tubing and some hose barb fittings to go in where the existing impulse tubing is can do this can give you a visual indication of the pressure difference in mm or inches of water column.

How to read a water manometer:

Actual installation with one end of the tube upstream, other end downstream of the filter.

For replacement instruments, you get what you pay for. All the way for cheapie (rubber sensing diaphragm) to rugged industrial.(stainless sensing diaphragm).

You should re-route the impulse lines if necessary so the transmitter is above the pressure tapping points so that any particulate has to travel 'up' the dead headed impulse line which keeps the instrument free of clogging with accumulated particulate. You might even consider increasing the impulse line diameter to 3/8" (9.5mm) or 1/2" (12mm) to prevent it from clogging internally.

 

[JohnCalderwood]

You could get someone like Endress & Hauser or an agent of theirs to come in, show them your situation.
They would get a rig up to check the pressures and differential pressure across the filters and then specify what DP cell you would need for that duty.
You may pay more, but put the onus on them to supply an instrument for your needs....

 

[Bit_Bucket_07]

You haven't said whether this system will include a PLC.

If not, then you might want to consider a setpoint relay meter, rather than an analog differential pressure transducer.

 

[Doug-P]

You should re-route the impulse lines if necessary so the transmitter is above the pressure tapping points so that any particulate has to travel 'up' the dead headed impulse line which keeps the instrument free of clogging with accumulated particulate.

Just curious, is it ever done that some sort of filter is added in the lines to prevent contamination of the instrument?

 

[Bit_Bucket_07]

Just curious, is it ever done that some sort of filter is added in the lines to prevent contamination of the instrument?

If you did that, then you'd have to perform rigorous maintenance on the filters to assure that you were measuring pressure drop across the process filter, rather than the instrument's filters. I've seen purge air used to keep instrument tubing free of debris.

 

[Dirt]

Just curious, is it ever done that some sort of filter is added in the lines to prevent contamination of the instrument?

If you have a demanding process regarding clogging and money isn't a problem there's always this:

 

[Ronnie Sullivan]

Yes, it will have a plc with analog inputs and an HMI

Thanks for all the possible solutions so far.
I have already learned that I need to read the difference in pressure between the front and back of the filters to determine when to shake down.
I say shake down because it is 4 motors connected to concentric pulleys and arms that literally shake the filters.
The zinc oxide (or whatever it is that is shaken into the bags) is a profitable sideline for them as they sell it on.