Huff and Puff

[EmmanuelTop]

We use reverse-pulse air system for online cleaning of turbine air intake (Donaldson GDX) filters. The source of air is the axial compressor itself, and a slipstream of compressed (8barg) cooled air is passed through series of Norgren filters, 40/25/5um particle size, before introduced to the pulse air grid inside the filter housing.

Since the environment is very humid (Equatorial Guinea, Bioko Island), reaching 100% saturation during nights, compressed and cooled air contains approx. 3% liquid water going into the filters. Last night we did a small exercise and tested the system in two ways:

1. Installing a temporary KO pot upstream of the filters, on one of the six turbines. Water collected: 1.5 liters/hr
2. Measuring quantity of water drained from the filter system itself, on another turbine. Water collected: 1.0 liter/hr

It appears that some of the liquid water passes through the filter system and is being blown back on to the main air intake filters during cleaning (huff and puff) cycles.

We have decided to test this system due to the fact that some of the main air intake filters started experiencing high pressure drop, leading to lower power output from gas turbines. We are wondering how much of this DP buildup is due to environmental conditions, and how much due to blowing liquid water onto the filter elements. And also to see what could be the best solution for eliminating this water from the system, e.g:

1. Installing KO pots upstream of the filters, or
2. Installing coalescers instead. Any recommended vendors?

We need completely automated system so that no field operator attention is required.
I would like to hear your suggestions, especially from people with a hands-on experience in this subject.

Best regards,

 

[eadwine]

What your intake filters may be experiencing is a phenomenon known as "Mudding & Caking". This is due to the heavy dirt and dust particulate loading on the filters and then due to the high humidity and wet back purge air creating a "mud" which then "cakes" and dries out creating a dried mud/ceramic compound which is plugging up the filters and no amount of back purging the filters is going to help.

The only thing I've ever found that works, is to swap out the plugged filters with cleaned ones, clean the plugged ones and store for the next swap out.

There is no such thing as a "completely automated system", especially, when it comes to "Operations & Maintenance" in hostile environments.

 

[Compositepro]

All compressed air systems require a dryer if you wish to keep liquid water out of your air pipes. Filter/separators will not be 100% effective. They cannot reduce the relative humidity in the airline below 100% RH. Use a refrigerated drier, desiccant, or pressure swing absorption.

 

[EmmanuelTop]

What is the quality of air required for cleaning air filters by backblowing? I couldn't find any standard with respect to that. I don't believe it has to be 100% dry, but perhaps 100% saturated is not so good idea as well.

 

[Compositepro]

Moisture sensitivity depends entirely on what you are filtering. For your application it may not be a problem. You maybe pulsing too frequently. Pulsing removes heavier accumulations more easily than very light coatings. If you pulse too frequently the coating that accumulates will be more tenacious. You can trigger the pulse on delta P rather than time.

 

[EmmanuelTop]

I'm not sure if I understand well. What makes you think that the pulse system will be effective if it's DP-based while being ineffective when it is timer-based?

Interestingly, I have found an OEM schematic diagram showing that the air quality should meet ISO8573.1, class 3/3/3:

Particle size in micron: max 5
Pressure dew point: max -20C
Oil content: max 1 mg/m3
Recommended dryer to meet ISO standard: desiccant or membrane

This dew point indicates Instrument Air quality requirements (-40C dew point at ambient pressure).

 

[Compositepro]

DP based pulsing cleans only when a filter cake has formed which impedes airflow. A filter cake has mass which increases the force that removes dirt from the filter surfaces due to the impulse that inflates/flexes the filter bag. Adhesion forces usually stay constant.

Timer based cleaning work well for high solids loading in air. Of course, a long period between pulses could be equivalent to DP based, but it would not adjust to changing dirt load conditions.