Gas Turbine Air Inlet Filtration dP - back-to-back filters. 1

[Natz1235]

Hi,

I think I've got a measurement issue but I'd like to make sure that what I am seeing is not a "real" phenomenon...

In the air intake of one of our gas turbines we have installed 2 sets of filters back-to-back so that one of them sits the opposite way to normal, a bit like the picture below;
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/|| / || 3| ||2 | 1 <-Air flow
\ || /
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With differential pressure (dP) measurement points as indicated by the numbers 1, 2, and 3, in order to give an indication of filter loading.

With this setup, we are seeing an increase in dP across the second filter (3-2) but the dP across the first filter (1-2)is remaining roughly constant (there was a very minor increase in dP when first installed but this has since levelled off). When the filters are examined there is some fouling on both filters, though neither appear saturated.

Could the apparently static dP be giving a "false" reading for loading due to the proximity of the probe to the surfaces of the filters? Or, is this a true phenomenon caused by the closeness of the filters to one another - i.e. that the second filter is actually loading before the 1st somehow?

Also, does anyone know if positioning filters this close to one another reduces the effeciveness of the filters?

The filters are a box-type W - shape.

Any ideas are welcome.

Thanks in advance.

 

[dicer]

Since turbines are such an air driven engine, and such huge amount of CFM is involved, and of course depending on dirt conditions,(I suppose is bad else you wouldn't consider filters)
the filters will need constant attention and cleaning. It will be interesting how much they derate the engine. Recips shine where air conditions require filtering.

As far as the pressure issue, why not post the pressures measured? By increase of Pd, and since its a differential, do you mean the pressure is going up or going down. Since a difference can mean either one.
I'm not a turbine person, but I would wonder how they react with no ram air. And with an inlet restriction can combustion pressure somehow bleed through the compressor?

 

[rmw]

I've thought about this post for a while now. First, I don't know the answer, but I have some conjecture.

Filters actually work by building up a layer of filtrate on the filter medium and the layer of filtrate does most of the 'heavy lifting'. Filters are least efficient when they are brand new and don't have a layer of filtrate built up yet. They get more efficient as they go along until the filtrate layer gets too thick and begins to cause excessive pressure drop.

Filters are also not 100% efficient and what gets through is material too fine to build up on the medium to form the filtrate. I am a turbine person and I know that turbines with first class filter systems have to have water wash systems for the compressors to wash off what gets through the filters.

So I am wondering if the second bank of filters has loaded up with fine material that is too fine to build up a layer of filtrate on the surface of the medium but rather has become entrapped in and 'saturated' the pores of the medium causing it to plug?

Pure conjecture.

rmw

 

[Cocorocox]

Probably , you have leaks near your measure point of pressure, when the air pass trough your filter and there are no hermeticity , you get a wrong measure.

Check your instalation, if you have problems you should buy double packing for your filter.

Good Luck

 

[abeltio]

What i do not understand is: which one of the two filters is installed "backwards"?

Typically filters do not like to be installed "backwards", they have been designed and tested to have the flow in the "correct" direction. Sometimes they have like a membrane first and then a composite material behind it, installing them backwards makes the membrane useless and the efficiency is greatly diminished.

if you want to have the two filters "in series" then they have to be installed with the correct orientation.

The measuring point does not even need to be too far into the airstream because the intent is to measure "static pressure differential".

Also, to eliminate errors it is best to measure with just one instrument both differentials (with adequate valving). If the signal is wired to a control system, then you'd need two differential pressure transducers.

Please correct me if i am wrong, but i would guess that the first filter (measurement 1-2) is the one installed backwards. Isn't it?

saludos.
a.

 

[ElRingo]

Hi,

these "piggy-packs" are often installed when problems occur lateron e.g. if you have a fertilizer plant nearby and all the salts dissolved in the environmental humidity pass your F-class-fine filters and make your vanes white and clog the holes. Then a piggy-pack e.g. consisting of an F9 and an H11 filter is installed, because an H-grade-filter catches the salt.

The direction of air flow through the filter is negligible, this is just a box.

I'm used to install the DP-measuring you describe that way, too. Problem is, that you measure only one filter as reference for the complete stage and this filter has to have a proper sealing to his opposite collegue for proper measuring.

Regards

El