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What causes gas turbine compressor fouling?

The typical causes of gas turbine compressor fouling are:

  • Passage of oily vapours through the filtration system
  • Ingestion of oily vapours through breaches in the air inlet plenum
  • Leakage of lubrication oil directly into the compressor
  • Leakage of oil from oil-bath type filtration systems
  • Passage of very fine particulate matter through the filter system
  • Passage of larger particulate matter through breaches in the plenum
  • Re-ingestion of exhaust gasses through the filtration system and breaches in air inlet plenum casing
  • Ingestion of saturated salt droplets or dry salt crystals through filters and/or breaches in plenum casings
  • Ingestion of seasonal tree and plant gums
  • Ingestion of a wide variety of chemicals and other pollutants generated at the site of gas turbine operation

What are the signs of a fouling process gas compressor?

There are a few signs that a process gas compressor could be suffering from fouling:

  • Flow
    Fouling will result in increased frictional losses and/or internal recirculation. This increase in resistance effectively reduces the capacity of the process gas compressor. Over time, a fouling process compressor will show a gradual reduction in flow.
  • Power
    The efficiency of a fouling process compressor will drop. In order to maintain the required discharge pressure and flow the power demand will increase. Hence, a gradual increase in power requirement could signal a fouling process compressor.
  • Pressure ratio
    The process compressor head is directly proportional to the pressure ratio. Trends obtained by monitoring the pressure ratio will be similar to monitoring the head. A fouling process compressor will show a gradual drop in pressure ratio.
  • Speed
    A process compressor driven by a gas turbine, steam turbine or variable speed drive system may see a gradual increase in speed because of fouling. The process control system will try to maintain the required discharge pressure and flow and can only achieve that by increasing the speed of the process compressor.
  • Temperature rise
    Monitoring temperature rise will be an indication of total work input. If the temperature rise goes up for a given flow and speed, then this is an indication that the efficiency of the process compressor has gone down.
  • Thrust bearing load
    Build-up inside the process compressor could affect axial rotor balance. The axial thrust (movement) of the rotor will be absorbed by the thrust bearing. A gradual increase in thrust-bearing load (displacement, temperature) could be an indication of a fouling process compressor.
  • Vibration
    A gradual increase in radial vibration could signal a fouling process compressor. Foulant build-up inside the impeller(s) may cause rotor turbalance thereby increasing radial vibration levels. Flow instability could be another factor affecting the smooth operation of the process compressor which may result in increased radial vibration.

What are the risks of process gas compressor fouling?

Apart from a clear decay in process compressor performance, there are also some risks associated with process compressor fouling.

Because of fouling, the process compressor operating point will gradually move in the direction of reduced capacity. This could increase the risk of the surge which, when not properly safeguarded, could result in a catastrophic equipment failure.

Fouling will also increase the risk of flow instability which could potentially cause increased vibration levels. Very often high axial thrust of the rotor assembly will cause a unit trip which, in the case of a single unit, will result in a complete shutdown of the facility.

Fouling build-up on the rotor assembly itself could potentially also cause increased vibration levels with similar consequences.

The proper functioning of the shaft end seals may also be compromised because of fouling.

What are the methods of cleaning fouled compressors?

There are basically two methods to clean a fouled process compressor: on-line and off-line cleaning. You can read more about them here. 

What is online gas compressor cleaning?

On-line gas compressor cleaning is carried out during normal gas turbine operation and includes the following methods as options:

  • Injection of abrasives (e.g. crushed nutshells) into the compressor air stream to displace blade deposits by high-velocity impingement
  • Injection of plain water to remove water soluble deposits
  • Injection of special chemical solutions (solvent & aqueous based) to chemically dissolve and remove surface deposits from the blades

The final on-line cleaning option is the preferred cleaning method for process compressors. The advantage of on-line cleaning is that the process compressor can be cleaned during normal operation. There is no need to shut down the unit and the cleaning operation can be performed in a matter of minutes.

A suitable process compressor cleaning chemical is injected by means of a specially designed nozzle system installed in the process compressor suction line. The atomized cleaning chemical will be entrained by the process gas thereby wetting all the process compressor internals. On-line compressor cleaning will not only clean the gas passages but also between the impellers and diaphragms.

Depending on the application and fouling level, on-line compressor cleaning can be performed periodically or on a continuous basis. For process compressors where build-up on the diaphragm surface is a problem, continuous flushing with a cleaning chemical may be required.

Given the fact that the atomized cleaning chemical is evenly distributed inside the process compressor casing, there is no risk of flow instability. Any material build-up will be gradually removed thereby minimizing the risk of increased vibration levels and/or accumulation in downstream process equipment.

The positive effect of on-line process compressor cleaning will be visible instantly. The process parameters can be monitored and the cleaning operation can be adjusted as required.

At what frequency should on-line cleaning be performed?

The on-line cleaning frequency very much depends on the degree and speed at which fouling of the process compressor occurs. Some processes, such as gas crackers, are known for their particularly troublesome fouling problems. Costs associated with cracked gas compressor fouling are high and they can increase exponentially as conditions deteriorate.

The key is to control fouling as it initiates rather than to implement a treatment program for an already fouled process compressor. In other words, regular on-line process compressor cleaning will be much more effective than postponing on-line cleaning until such a moment that the process compressor performance has already significantly dropped.

The optimum on-line cleaning frequency will be determined in cooperation with the customer by collecting field operational data serving as input for regular performance verifications.

What are the effects of on-line cleaning on the downstream process?

Fouling inside process gas compressors can be the result of two possible causes:

  • Foulants being entrained by the upstream process gas and deposited inside the process compressor (and other equipment)
  • Fouling build-up inside the process compressor resulting from (a) polymerization due to high stage temperatures or (b) seal oil reacting with process gas components (in case oil seals are being used).

A cleaning chemical mixed with good quality (demineralized) water will be injected in the process gas upstream of the process compressor (suction side). The foulants inside the process compressor will be washed away (diluted) and leave the process compressor with the process gas downstream of the process compressor (discharge side).

During on-line cleaning, the cleaning solution and foulants will end up in the process system downstream of the process compressor.

When we consider a process whereby the fouling mechanism falls into category 1, on-line cleaning will most likely not affect the downstream process. In this case, the fouling is not limited to the process compressor alone, but already present throughout the complete process system both up and downstream of the process compressor. It is assumed that the customer has taken appropriate measures to control the degree of fouling and/or implemented a preventive maintenance schedule to periodically clean the complete process system.

When we consider a process whereby the fouling mechanism falls into category 2, on-line cleaning could affect the downstream process. In this case, the fouling is only limited to the process compressor and not present throughout the complete process system. The possible implications of foulants originating from the process compressor being entrained by the process gas into the downstream process system shall be studied.

What are the advantages and disadvantages of online compressor cleaning?

Let's start with the disadvantages:

  • Can have limited effect if the compressor cleaning chemical is not properly formulated
  • Can have limited effect if the chemical is not injected properly
  • Some on-line gas compressor cleaning chemicals are quite expensive and results may not justify the costs in some cases
  • Some on-line cleaning systems are very expensive and are priced against what the customer may save in operation costs rather than what it actually costs to develop and manufacture the equipment

Now the advantages:

  • No engine shutdown required
  • Can arrest or slow down the rate of gas compressor fouling while the gas turbine remains in operation, thus maintaining heat rate and saving large amounts of money by avoiding additional fuel burn
  • Helps maintain power output to avoid loss of production revenue and unnecessary use of standby machines
  • There is no interruption to production or operation routines as washing can be carried out at full speed and load
  • The cleaning process usually only takes a matter of minutes
  • There is no waste chemical or water to deal with after the wash
  • Assuming the chemical is properly formulated and injected it should not contribute to any additional atmospheric pollution
  • By avoiding shutdowns for off-line washing, thermal cycles are also considerably reduced thus extending the life of the engine
  • No erosion, thermal shock, bearing damage, cooling system blockage (if a properly designed system is used)

What is offline gas compressor cleaning?

To enable off-line gas compressor cleaning, the process compressor must be shut down and isolated then one of the following options followed:

  • Crank-soak chemical washing using in-built chemical injection/water rinsing systems
  • Crank-soak chemical cleaning using hand-held hose or lance
  • Partial hand cleaning (e.g. struts, IGVs, 1st stage rotor & stator blading) using chemicals, rags, brushes and water rinse
  • Full hand cleaning with compressor covers removed using chemicals, various types of abrasives or even light shot blasting techniques
  • Steam cleaning

Off-line gas compressor cleaning is an alternative option to on-line gas compressor cleaning. However, a major disadvantage is that the process compressor must be taken out of service which, in case of no backup, results in a complete plant shutdown. The costs associated with such an unscheduled shutdown could be enormous and adversely affect plant profitability.

Another negative aspect of offline processor cleaning is its effectiveness. The process compressor casing can only be filled below the shaft end seal level. The top half of the casing, containing the top half of the diaphragms, cannot be soak washed. Given the fact that any material build-up in this area most likely cannot be removed, off-line cleaning could result in the additional risk of flow instability which could potentially cause increased vibration levels. Furthermore, chunks of dirt may eventually dislodge from the top half and move within the process compressor thereby aggravating the risk of increased vibration. Some will accumulate in the aftercooler inlets or knockout drums resulting in high-pressure drops.

What are the advantages and disadvantages of offline compressor cleaning?

Let's start with the disadvantages:

  • The gas turbine must be shut down completely
  • Time consuming process
  • Labour intensive process
  • Costly problems in disposing of waste chemicals and rinse water
  • Lost power output cannot be recovered
  • Shutdown/start-up thermal cycles for off-line washes are damaging
  • Extra wear and tear on starting system during off-line washes
  • Can unintentionally wash salt and corrosives into inaccessible parts of the engine
  • Only a short-term cure and not prevention for compressor fouling

Now the advantages:

  • When carried out correctly, this kind of cleaning can effectively clean the compressor and restore the majority of lost performance

Is the Rochem cleaning procedure safe?

The Rochem FYREWASH® system has been designed with safety in mind from conception to build, installation and implementation. The patented nozzle design allows for precise atomization of the spray pattern to eliminate erosion of the engine. FYREWASH® chemicals are specifically designed for use with gas turbine compressors to target foulants without harming equipment.

What about cost effectiveness?

The cost of installing and operating a Rochem FYREWASH® system is minimal compared to the potential savings in fuel, engine efficiency and output, and the avoidance of downtime by keeping the gas compressor clean while the engine remains in normal operation.

How much time is required to clean a gas turbine compressor?

The time required to clean a gas turbine compressor is dependent on its size. A typical wash takes around 10-20 minutes and consists of chemical injection and soaking period which allows the chemicals to break up stubborn deposits. This will be followed by one or two water only rinses. The whole process can happen during normal operation.

What are the financial benefits of on-line gas compressor cleaning?

The costs of fouling can be significant and adversely affect plant profitability. Costs of USD4.8M per event are common which can be attributed to:

  • Reduced equipment availability and reliability
  • Loss of revenue resulting from reduced plant throughput
  • Plant outage costs resulting from unscheduled maintenance
  • Direct and indirect equipment overhaul costs
  • Man hour costs

Regular on-line compressor cleaning helps avoid these issues and costs.

What process compressor types are suitable for online cleaning?

In principle, on-line cleaning is limited to continuous flow compressors of axial and centrifugal design or a combination thereof.

What are the questions to ask when selecting a gas compressor cleaning system?

  • How long has the vendor been in business?
  • Can they supply an installation reference list?
  • Is the system known to engine manufacturers? Do they approve it or have no objection to its installation and use?
  • Is the system a recommended option or installed as standard in new gas turbines by any manufacturers/packagers?
  • Does the vendor design, manufacture, install, service and guarantee the system themselves? If not, why not?
  • Are the materials of the system good quality?
  • How long does the vendor say it should take to install the system? Some can take a few hours to install, others can take weeks to install if it involves drilling thick castings etc.
  • Is the design safe? Could it possibly damage the engine or injure those using it?
  • Does the vendor have sufficient liability insurance?

What does the Rochem FYREWASH® system consist of?

The system consists of a uniquely formulated chemical and a specially developed method of application.

The cleaning fluid is formulated to remove oil, grease, carbon, salt and general atmospheric fouling from the gas compressor whether axial or centrifugal type. Complete compatibility with engine materials, including special coatings, is assured by the correct chemical formulation and proven over many thousands of operating hours in a wide variety of engine types. The gas turbine cleaning fluid has been tested and approved by many of the world’s leading engine builders and also meets stringent U.S. military specifications for compressor washing fluids.

The injection system is designed to deliver the cleaning chemical into the air stream in a highly atomized mist-like form. In every case it is specifically designed to suit the particular engine type and air inlet trunking system.

For industrial engines a set of specially designed atomizing nozzles connected by a common manifold ring are normally permanently fitted to the air intake plenum chamber and fed from a pressure injection tank or pump system. In all cases the entire system is designed for simple and low cost retrofit.

How does the Rochem FYREWASH® system compare in safety and efficiency with other compressor cleaning methods?

Water washing is generally ineffective because it has no power to remove the oily/greasy contamination which is the major problem affecting the loss of compressor efficiency. Even for salt removal purposes – an absolute necessity in marine operations – plain water washing is often of no benefit whatsoever since the salt is usually trapped in and under the layer of oily deposit that the water cannot remove.

Crank-soak solvent cleaning can be effective but it requires a complete shutdown, is labour intensive, contributes significantly to wear and tear on starting systems, and does nothing to solve the problem of power loss and increased fuel consumption while the engine is becoming fouled in normal operation.

Abrasive cleaning methods can result in serious blade erosion problems, bearing damage and the removal of very costly blade coatings. Also, since abrasive cleaning can only dislodge the contamination back into the air stream rather than chemically dissolve it, the contaminant can simply be redeposited in later stages of the compressor and bearing system.

In comparison, the Rochem FYREWASH® system has none of these disadvantages:

  • It is used while the engine remains in operation.
  • It can be used as frequently as necessary so contamination is not allowed to build up meaning that output and specific fuel consumption are not affected.
  • The FYREWASH® chemical range will keep the compressor clean of any salt, oily and greasy deposits and contamination whilst being fully compatible with blade coatings.
  • It also leaves no residue in the engine, nor redesposition of the removed deposits.
  • The FYREWASH® patented nozzles apply the wash solution in a highly atomised mist-like form at a controlled flow rate so that there is no danger of blade erosion.

How many installations have Rochem carried out?

In excess of 10,000 nozzle manifold installations worldwide on practically every type of gas turbine.

Has there ever been a failure of a Rochem nozzle?

Never in more than 40 years and millions of engine operating hours in every environment has a Rochem nozzle failed.

How many injection nozzle systems are required when operating two or more process compressors in series?

Each section of the process compressor should be equipped with a dedicated injection nozzle system.

A section comprises of all the process compressor stages between the suction and discharge nozzle. A section is sometimes also referred to as a 'stage group' or 'process stage'.

In case of a single casing process compressor comprising of two sections, two injection nozzle systems will be required.

In case of two or more process compressors in series (each comprising of one or more sections), a corresponding number of injection nozzle systems will be required.

In most cases, a single wash delivery skid will be sufficient to supply the cleaning solution to the injection nozzle systems. However, there may be applications where the installation of additional wash delivery skids could be required, e.g. large compression trains with multiple injection nozzle systems.

Is a shutdown required to install the injection nozzle system?

Yes. The specially designed injection nozzle system will be installed in the process compressor suction line. Normally, a suitable piping or instrument connection close to the process compressor suction flange can be used for this purpose.

To enable the installation of the injection nozzle system, the process compressor should be taken out of operation.

How much time is required to install or retrofit a Rochem system to a stationary engine?

This depends onto which engine the system is being installed. Some systems can be fitted in a matter of hours while others can take a couple of days. Generally, the systems are fitted during an engine overhaul or outage.

Are there any restrictions regarding the type of shaft end seals being utilised?

Process compressor shaft end seals can be divided into three main categories:

  • Clearance seals (e.g. labyrinth seals, restrictive-ring seals)
  • Oil seals (e.g. liquid-film seals, mechanical (contact) seals)
  • Self-acting dry gas seals (e.g. dry running seal)
On-line process compressor washing

In principle, process compressors equipped with oil seals or self-acting dry gas seals can be cleaned on-line. However, this shall be studied and confirmed on a case-by-case basis, since each process compressor application is different and designed according to site specific conditions. Minor modifications of the shaft end seals and/or seal support system may be required (in case of self-acting dry gas seals).

In case of process compressors equipped with self-acting dry gas seals, the primary seal gas shall be supplied at the right temperature, i.e. at least 20 °C above the dew point. The dew point margin must be maintained throughout the seal system and not just at the supply connections.

On-line cleaning of process compressors equipped with clearance seals should be avoided. There is a high risk that foulants being washed away by the cleaning solution will accumulate in these seals thereby affecting their proper functioning.

Off-line process compressor washing

In principle, process compressors equipped with clearance seals, oil seals or self-acting dry gas seals can be cleaned off-line. However, this shall be studied and confirmed on a case-by-case basis, since each process compressor application is different and designed according to site specific conditions.

At what process operating conditions should on-line cleaning be performed?

The operating zone of a process compressor is a region constrained by the following boundaries:

  • Surge limit line
  • Choke or stonewall line
  • Line of maximum power
  • Lines of maximum and minimum pressure
  • Lines of maximum and minimum speed

By adding control margins to this region, a stable zone of operation will result, i.e. the actual operating zone of the process compressor.

During on-line cleaning, one must ensure that the operating point of the process compressor stays well within the limits of this stable zone of operation.

What is the requirement for the water used in the compressor washing process?

The water must be of high quality, i.e. demineralised or deionised water.

Is on-line process compressor cleaning in recycle mode an option?

On-line washing of the process compressor should only be performed during normal operation, i.e. with the recycle valve in fully closed position.

Foulants being washed away and entering the recycle line could adversely affect the proper functioning of the recycle valve thereby jeopardizing the safe operation of the process compressor.

Furthermore, on-line cleaning in recycle mode would circulate these foulants back to the suction scrubber and could potentially end up inside the process compressor again.

What are the advantages and disadvantages of using plain water for compressor washing?

The advantages of using only plain water for process compressor washing are:

  • Can be used when the gas turbine is operating (if injected correctly)
  • Simple to use
  • Relatively inexpensive
  • Can have some cleaning effects if fouling is purely water soluble 

The disadvantages of using only plain water for compressor washing are:

  • Not effective against oily/greasy contamination
  • Not effective against lacquered or hard carbonaceous deposits
  • May move deposits to the hot section of the engine and cause corrosion
  • May directly cause corrosion if not inhibited
  • May cause thermal shock/blade stress if not injected correctly
  • Not effective for off-line cleaning (unless the deposit is water soluble)
  • Must be high quality water with very low TDS and suspended solids

Do I need to use cleaning chemicals and detergents for on-line and off-line compressor cleaning?

Rochem suggests the use of specialist cleaning chemicals and detergents for on-line and off-line compressor cleaning. The chemicals break up the carbonaceous deposits and particulate matter that water alone cannot remove. Think of it as trying to clean a greasy pan with no washing up liquid!

What type of cleaning chemical is used for on-line cleaning?

In most cases, we recommend using the Rochem FYREWASH® F1 chemical, a high purity hydrocarbon solvent and surfactant formulation for heavy duty on-line process compressor cleaning.

In case of environmental limitations, Rochem FYREWASH® F4, a highly bio-degradable water based detergent for on-line process compressor cleaning, could be an alternative option.

The type of cleaning chemical to be used will depend on the nature of the foulant and environmental circumstances of the installation concerned and will be determined in cooperation with the customer.

What are the questions to ask when selecting a compressor cleaning chemical?

  • Does the system manufacturer also make special cleaning chemicals for use with the system? Are they tried, tested and approved?
  • Was the chemical on offer solely developed for on-line cleaning or was it originally developed for some other application not directly related to gas turbines?
  • Does the vendor offer a choice of chemicals (i.e. solvent-based and water based) to suit particular fouling and/or environmental requirements?
  • Is the chemical supplied as a concentrate to save storage and transportation costs? Paying for water in ready-to-use chemical solutions can be very expensive and unnecessary.
  • Can the chemical offered also be used safely and effectively for off-line compressor - washing?
  • Does the vendor offer ex-warehouse availability of the chemical?
  • If the vendor only supplies chemicals, are you sure it is safe to use them in your injection system?

Recommendation: be very wary of using any chemical, especially for on-line, fired washing unless it has been properly tested and approved and has a good long-term safety record behind it.

Can the cleaning chemical damage the process compressor internals?

Based on the information provided with the Process Compressor Cleaning Questionnaire, one of our specialists will verify the material compatibility of the process compressor internals.

No material compatibility issues have ever been reported by our customers.

What quantity of cleaning chemicals is required for online compressor cleaning?

On-line cleaning is normally performed by mixing one-part of Rochem FYREWASH® concentrate (25 litres) with four parts of demineralized water into a stable solution (total quantity 125 litres per process compressor section).

The flow rate at which the chemical solution will be injected is determined on a case-by-case basis and is dependent on process compressor size and operating conditions. Normally, it is recommended that a post rinse with demineralized water is performed to ensure that both the injection nozzle system and process compressor are thoroughly cleaned. The above mentioned on-line cleaning procedure may need to be repeated depending on the degree of fouling and process compressor configuration.

The optimum cleaning chemical quantity will be determined in cooperation with the customer by collecting field operational data serving as input for regular performance verifications.

Can the cleaning chemical adversely affect Viton O-rings in shaft end seals?

Viton O-rings will have good chemical resistance to Rochem FYREWASH® F1 with little or no swelling/degradation even when used in a 100% concentration.

What kind of information is required to prepare a proposal?

The information requirements are listed in the Process Compressor Cleaning Questionnaire which should be completed by the customer. The collected information will be analysed by one of our specialists, preferably followed by a site visit and technical review meeting (optional).

Once all the technical details have been clarified with the customer, Rochem will be able to submit a commercial proposal.