Gas turbine (GT) users are often asking us if we can define the remaining lifetime of filters and if we could offer remaining lifetime assessment services for our filters. As much as we would like to take a few of the filters to our test bench to measure the increased weight and dp as well as do a few microscope pictures to see the material status, we really don’t recommend doing once a year analysis of GT inlet air filters, but we always recommend doing continuous monitoring of the filter condition.
Worst case scenario of once a year inspection would be that you should change your filters immediately
When you go to your dentist once a year you always hope there aren't many holes in your teeth requiring immediate attention. In the best case you get to fear for another year and get a new verdict if you are good to go for yet another year more. It’s the same with intake air filters - you really want to know the condition of your filtration system continuously rather than just once a year.
With the once a year analysis you take a few filters, hoping to get an overview of the status of your filters. Although, for the weight increase, the once a year analysis will give you a fairly good indication of the accumulated dust on your filters but for the more important indicators (dp and material integrity) you have to hope that the random sampling hit the potentially damaged filters – you only need a few damaged filters to see an increased deterioration of your compressor performance – instead of getting a life time expectancy based on the intact filters. The worst case scenario is that the result of the once a year inspection would be that you should change your filters immediately (when you just started your operation season and have 11 months until the next scheduled outage).
So that is why we ALWAYS recommend continuous monitoring of GT inlet air filter condition.
So, what indicators should you be looking for that could reveal the health status of your GT inlet air filters? Actually there are only two variables you need to monitor – the dp of the filters (over each filter stage) and compressor efficiency.
dp of each filter stage
dp is the most important indicator of the remaining expected lifetime of the filtration stage. dp will increase during the lifetime of the filters, as the filter gets blocked by the particles it eliminates from the GT inlet air stream. With each filter material there are different characteristics of how the filter will behave when closing in on its end of lifetime.
Some cheap materials, like cellulose, are quite vulnerable to the effects of humidity and may have very unpredictable lifetimes as even one heavy spike in humidity could weaken the material, causing the pleats to lose their form and dp rising sky-high. ePTFE materials are relatively stable until the end of the membrane lifetime, but typically have a very rapid increase of dp at the end of their lifetime without a proper prewarming. The most reliable materials will indicate the last 25-30% of lifetime by a noticeable change in dp increase rate, enabling an operator to plan for the next outage.
Knowing your filter material behavioral properties helps you prepare for end of lifetime.
dp is the most important indicator of the remaining expected life time of the filtration stage
In addition to material selection affecting the dp increase rate, each filtering stage will have a different rate of blocking depending on how your filtration design has been achieved and how well the filtration efficiencies have been defined. Typically, we recommend having a cheaper and easier to replace pre-filter taking most of the coarse dust load to target 8 000 hrs operational lifetime and have the Fine and EPA filters only taking care of the finer contaminants with 16 000 hrs target operational lifetime.
The final dp setting should also take into account the price of electricity as the increasing dp will reduce the output of your GT – you can look into your GT manual to learn your machine´s specific dp/MW ratio to have the secondary dp setting value optimized.
Unfortunately, the type of contaminants as well as other site-specific factors (such as proximity of industrial plants or coastal environment with high humidity and salt) will affect the lifetime of the filters and that is why it is NOT sufficient to look at the dp alone.
A single sandstrom or high humidity loading from extreme fog may block your semi-loaded prefilters, requiring an immediate replacement. On the otherhand, salt and leaching will perforate your filters with pinholes resulting in the dp value showing a low dp increase value (if any) while letting through a lot of contaminants causing compressor efficiency to drop. This is why you need to monitor also your compressor efficiency.
Compressor efficiency is a health indicator of your filtration system.
Compressor efficiency is your best indicator of the health of your filtration system, especially if you have already upgraded your filtration system to HEPA-class with degradation limited next to none.
You can monitor the compressor efficiency continuously opposite to the other filtration health indicator, which is the off-line washing degradation. With HEPA, the off-line washing degradation isn´t even really applicable, but for F-class filtration you may need to take the off-line washing gain into account as you will be able to detect the change in compressor efficiency drop-rate with F-class filtration as well - it is just harder to notice due to lower filtration levels to start with.
The main rule of thumb is that any sudden change to the compressor efficiency is a clear indicator that something has changed in your filtration system. It is most likely your filtration system has experienced leaching or even a rupture of filter elements, causing contaminants suddenly penetrating your compressor.
You can verify this by looking at your dp value. If you see a sudden spike of dp then a sudden drop of dp, followed by a sudden drop of compressor efficiency (and typically an on-line washing) you know you are in trouble, as your filtration system is seriously compromised.
Rule of thumb: any sudden change to the compressor efficiency is a clear indicator that something has changed in your filtration system
After the double dp spike (up and down), it is the time to take out your production schedule and plan for an unexpected outage to have at least a visual check of your filtration system, hoping for a filter rupture. Remember, with leaching damages you cannot tell which filters are damaged as the damage is just micrometres in size, as opposed to rupture damages, which are easier to identify.
Typically, you have the means to do the continuous filtration condition monitoring on your DCS. But if you only have the local dp gauges, the investment is not that big to add a few remote dp measurements to start continuous monitoring of your filtration conditions. You can also ask if your filter supplier is able to take the monitoring responsibility from your shoulders, at least we do…
And as always, you can consult your friendly GT inlet air filter guy for more information!
Author: Pertti Sundberg