Fitting the correct filter to a gas turbine can save money in terms of availability, reliability, power output efficiency and heat rate loss. But how do operators evaluate and select a filtration system that will deliver what they need in terms of performance and budget? By Daniel Burch
On paper many filters may seem to offer the same or similar performance.
Standard filter ratings are, however, based on laboratory tests and may not allow for the specific challenges a particular gas turbine installation faces.
With intense time pressure, however, it is often difficult for onsite personnel to evaluate all the criteria they need to outside of what the paper specification tells them. This can be especially true in a competitive power marketplace. As experienced personnel reach retirement and are not replaced, the amount of resources available on site reduces; adding time pressure to every operation.
First of all, let’s take a look at why filters are so critical to plant performance. If a filter does not perform as it should, a site can face a number of challenges in its efficiency and profitability.
|A comparison of two gas turbines shows the impact filtration can have|
Turbines consume vast amounts of air and the filter helps protect their performance and reduce the need for maintenance shutdowns. Fouling and corrosion caused by airborne contaminants reaching the turbine can significantly reduce turbine efficiency and, in some cases, lead to complete turbine shutdown.
Fine particles entering a turbine can stick to turbine blades. As this fouling builds up, it starts to affect the aerodynamic performance of the turbine. This is seen in a reduction in output power and rise in heat rate. To restore its performance, a turbine will need to be taken offline and washed to remove the particles. Other contaminants may cause corrosion or erosion of turbine parts, resulting in costly repairs. Overall, the reduced turbine efficiency and the lost production time when a turbine is taken offline have large cost impacts in terms of lost MW output.
Indeed, poor air filtration can account for approximately 60-80 per cent of overall gas turbine losses. Filtration systems should not be treated as commodities. There is a bigger picture to consider in terms of overall turbine performance and health.
Power plants are situated in many varied environments, each presenting its own challenges. Particularly demanding installations may include those located near to coastlines as these can experience a bombardment of contaminants from both land and sea.
Salt is a particular concern as it can cause extreme, accelerated corrosive damage to turbine internals as sodium in the salt combines with sulfur in the fuel in a process known as sulfidation.
Dusty environments, such as those found in or near deserts, quarries, industrial or agricultural areas, can also place high demands on a filter. Where very high dust levels are present a filter may need to be self-cleaning to keep maintenance levels sustainable. Of course salt and dust are by no means exclusive and dusty coastal environments can be the most challenging of all for filtration systems.
Other climatic conditions that need careful consideration in filter house design include extreme heat or cold. Low temperature climates where ice and snow are an issue can introduce a level of unpredictability in the conditions a filter house may be exposed to.
|Data comparison: It’s important to analyze turbine performance data to understand how filtration choices are performing. There can be variations depending on media, environment and operating conditions.|
Even in a relatively benign environment, other factors may need to be considered. For example, if a cooling tower is nearby, the direction of the filtration house may be important so that contaminants are not blown in from the tower as the wind changes direction.
Local conditions may also mean that additional equipment needs to be installed to support the filter itself. This could include weather hoods to protect against rain and snow, mist eliminators, moisture separators or coalescers. Indeed, how an overall filter system handles moisture and other elements is vital to the performance of the final filter stage and to the protection of the gas turbine.
Finally, the best filter for an application will also depend on a plant’s operational goals. These too need to be fully understood and factored into the selection process to ensure a solution matches areas such as budget, criticality of the process (level of availability) and practical maintenance schedules.
The EN779/ASHRAE 52.2 and EN1822 standards give an indication of the level of performance of a filter but the ratings they produce are based on only dry particulates in the air flow.
However, in the real world, the inlet air is rarely dry and seasonal variations are common. One of the first additional areas to query, therefore, is whether the filter has been tested for wet and dry performance.
How a filter handles moisture can be one of the most critical factors for an installation. Moisture can affect a filter as it appears with high humidity levels, mist or fog.
|Microglass vrs membrane: A thickness comparison of microglass media and PTFE membrane media. Media performance can vary widely by environment type|
Therefore, in the often harsh environments in which gas turbines are installed, the results of additional testing for dust and salt removal efficiency; impact of humidity, mist and fog; strength burst testing in wet and dry conditions; simulated life testing, and durability through rough handling testing are also worth reviewing. Ultimately, data on how a filter performs in similar real-world installations give the best view of which to choose.
It makes sense to think that higher efficiency particulate air (HEPA) filters offer greater protection to gas turbines as they capture smaller particulates. To do this, however, they use finer filter media which can become easily blocked in the presence of moisture and hydrocarbon mists.
When using some HEPA filter types, blockages can occur very quickly to result in sudden pressure spikes that may severely damage equipment, not to mention the financial bottom line in terms of unplanned outages and loss of potential revenue. So care needs to be taken that the filter can efficiently handle local installation conditions as well as dry, fine particulates.
Generally HEPA filters are available with two different media types: glass fiber or ePTFE membrane. Both can achieve (H)EPA ratings of E10-E12, the common HEPA ratings found in gas turbine inlet filtration. One of the main differences between the two, however, is the thickness of the media. Membrane media are extremely thin (typically around 0.05mm) and their excellent performance in removing sub-micron particles gives them a very high efficiency rating. If there is moisture or rain present for any length of time, however, the pores in this thin material can become easily blocked.
Glass fiber media is around ten times thicker than an ePTFE membrane and its greater pore volume makes it naturally less prone to such sudden blockages. This greater resistance to blockages means any pressure increases happen much more slowly, giving the operator plenty of time to take action and making this type of media much more predictable in its performance, especially in heavy industrial applications.
Although it is well accepted that an efficient filtration system is needed to protect the performance of the turbine, in-depth investigations into the optimum air inlet filter may not be the highest priority or concern especially with ever-growing pressure on increasingly limited personnel.
Indeed, the time to review the many options available may simply not be possible and, instead, operators simply select what has been used before or accept the latest claims in sales literature – all of which appears to promise the same things with huge cost savings and great results.
The wrong inlet solution for the local environmental conditions of the installation can, however, cause severe detrimental impact on turbine efficiency, availability and reliability.
The answer to this conundrum in a way is very simple: a trusted partner that fully understands filter performance and the challenges of individual installation conditions.
This partner should not just offer the latest trend in filter technology or be limited in the filter types they can offer. Instead, they should help to fully evaluate site conditions and find a high quality, reliable filtration solution that fits the unique needs of an individual turbine installation.
This involves listening and understanding, analyzing plant data, and taking every engineering, operational and business factor into consideration. Ultimately, the only data that should matter to the operator about a filter is how a gas turbine performs when it is fitted. A filtration solution should always be designed with this in mind.
Once a filtration system is installed (and great care should be taken to ensure correct installation), the partnership should not end there. Working together with a filter supplier over the lifetime of an installation can help ensure maintenance procedures are optimized, keep solutions updated with latest technological developments if they are of benefit, and continue to make sure turbines are protected and perform reliably.
Ultimately, finding the right filtration partner to understand your plant and turbine needs could save a great deal of time and money.
CLARCOR offers personalized filtration plans that consider all factors for optimum turbine performance. It evaluates environmental and locational conditions, determines necessary plant economic conditions, compares industry tests to give a baseline if this is needed, and proposes a filtration solution that is specifically tailored for a plant’s success.
The result will be a turbine inlet solution that will meet operational goals such as raising power output, lowering operating costs, reducing heat rate and increasing turbine availability. Solutions will not only protect against turbine damage from contaminants but also optimize filter life and result in easy, realistic and practical maintenance schedules.
Daniel Burch is Product Marketing Leader at CLARCOR Industrial Air.