Power facilities are the largest generators of Sulfur Dioxide (SO2), which may cause acid rain. In 2012 Power Plants were required to meet the tougher air quality emission standards. Adding Flue Gas Desulphurization Units (FGD) to clean or scrub the exhaust gases reduces SO2 emissions by 98%. By FGD units reduce the emissions by this amount, they have become the most common type of technology used for reducing sulfur oxide emissions from bituminous and coal fired power plants.
There are two different designs used for FGD systems:
- Dry: This is used to heat up the incoming flue gas to vaporize the liquid in the lime slurry, used to scrub the sulfur dioxides from the gas, resulting in a dry waste stream.
- Wet Scrubber: This uses an excess of slurry and produces a wet waste stream. All internal surfaces as subjected to a saturated environment or are in immersion. This type of system is the most common.
A FGD system and its auxiliary equipment provide a wide range of parameters, which must be evaluated to tailor the proper lining system to the required process conditions. The following is a list of the parameters which should be considered as a minimum: Chemical Exposure, Temperature, Immersion, Impingement and, Sliding Abrasion.
Rubber linings are used where abrasion is a major concern. Typically used in scrubber absorption zones, and slurry piping. There are 3 commonly used rubber linings in the FGD units:
Natural Rubber (60 Shore A Durometer): This has the lowest cost out of the 3 styles. This lining is easy to apply for a qualified applicator and will have the highest degree of abrasion resistance, but is limited in heat / hydrocarbon resistance.
Neoprene (60 Shore A Durometer): This lining is the most costly but will offer a degree of fire and oil resistance, which is the advantage where oil fired start up systems are in place.
Chlorobutyl / Bromobutyl (60 Shore A Durometer): This is the most common lining choice because of the low permeation. The abrasion resistance is not as strong as the NR, but is acceptable and the oil resistance is better than the NR but not as good as the neoprene.
Rubber has advantages so that physical or chemical properties of the scrubbing liquid have any major effects on the service life. Because rubber has good resistance to sulfuric acid, rubber lined steel has been consistently used, especially where abrasion resistance is needed. The key ingredients for a successful FGD lining project are the correct lining specification and the proper rubber lining applicator. With the proper specification and application one can expect to achieve a performance life of 15-20 years with extremely low maintenance costs.
Buck Meadows / Rubber Technologist
Technical Sales Manager
Compressed Air Blotter Test
What is it checking for?
The background behind this specification, which is becoming more and more prevalent in engineering documentation, has to do with the presence of oil and water in the air stream. As you can imagine having oil sprayed onto a surface prior to painting would be terrible. It would immediately create fish eyes, lack of adhesion and all types of terrible things for the paint. Water is just as bad as flash rusting can occur.
Rubber and hydrocarbons for the most part don’t get along. So the presence of oil on a surface will definitely have a negative impact on the adhesion and longevity of rubber on a steel sub straight.
Why would water be in the air line?
Humidity (water molecules suspended in the air) is found in the air all around us. When compressed the moisture forms into water. Most compressors are equipped with a water separator. But water separators are not all created equal. Based on the volume of blasting or painting, your air water separator may not be able to process the volume of water. Winter is a good gauge of if your water air separator is working for your setup. If your lines freeze in winter yours is not adequate for your work load.
Why would oil be in the air line in the first place?
Not all compressors are made the same. Piston compressors are more prone to oil in the air than screw compressors. Although all compressor designs offer various risk to oil in the air lines, there are many inline solution to capturing any residual oil left in the air.
How is the testing done?
You can use a 4″ X 4″ clean cotton cloth. Spray for a duration of 1-5 minutes and look to see if you any oil or water. This only needs to be done once per shift as objective evidence for your client if required. Otherwise it’s good practice to monitor your compressor air as well as you oil separation for long term performance issues.
For the exact specification the standard would be according to ASTM D 4285
The net result of good clean air!
A clean well blasted metal with no surface impurities will enable you to create a perfect adhesive bond that will ensure a rubber coating will last forever.
Good process equals good results…
PH and Rubber Lining
The PH levels in the transport fluids is always a concern when selecting the proper rubber liner for pipe or chutes. If the transport fluid is Acidic, natural rubber can be the right selection. When using natural rubber with acids, swelling occurs dependent on concentration and you may not get the same abrasion resistance you would normally get from a liner. Alternately extremely alkaline solution or caustic materials can also be problematic.
As a general rule of thumb, any high corrosive or caustic application, will change the rubber selection from Natural rubbers to Chlorobutyls or Bromobutyls which can handle high chemical compositions and higher temperatures. The challenge with the Butyl family is that they sometime have difficulties adhering to sub-straights.
Rubber manufacturers know the adhesion problem and will some time use a pure natural gum layer on the bottom of the sheet. This helps adhere the sheet to the sub-straight. Liner applicators must be aware, that the natural rubber layer (tie gum layer) is there. If this is the case closed skives are recommended for all chemical applications.
When a rubber sheet has a tie gum layer it is susceptible to chemical and temperature attack. Many liners have failed due to this layer being dissolved or weakened because of the lower chemical resistance and temperature threshold of the tie gum layer.
When selecting rubber liners an understanding of the PH levels and the effect on the liner is critical to the selection of rubber for the application. Being able to describe the PH, material particle size and velocity can help you in designing the perfect piping system.
Soluble Salt testing is becoming the latest inclusion into EPCM specifications for quality requirements.
What are Soluble Salts and Sulphates?
Soluble Salts and Sulphates are the most dangerous forms of contaminants for paints and coatings. When they are painted over they have the power to draw moisture through Osmosis and cause blistering, detachment and accelerate corrosion of the underlying metal. When steel is repainted, rough or pitted areas are visible after dry abrasive blast cleaning. These may contain soluble salt contamination, especially in the base of the pits. Dry abrasive blasting does not remove these salts. It is wise to check for the presence of soluble salts with specially designed field test kits before painting and if they are present in detrimental amounts, to take additional cleaning steps to remove the salts.
How can you test for Soluble Salts?
The common tools used to test for soluable salts are the Chloride Iron Test Kit for Surfaces. This test looks for remaining chloride levels on a sub-straight prior to painting. The second tool is a Salt contamination Meter. This tests for soluble salts on the substrate surface prior to painting by absorbing distilled water soaked filter paper and then testing it.
How do Soluble Salts Occur?
There are many ways your steel can be exposed to soluble salts and sulphates. The most common way is through transportation. Pipe and steel travelling at sea can accumulate contaminants during travel to the fabricators. The other method is when trucking materials during the winter. Road salts can easily be distributed on to steel during the transportation process.
Is checking for Soluble salts necessary?
Soluble salts became an additional test required and recommended by Nace. As engineers attempt to cover off as many concerns with their specifications as possible, it is becoming a popular addition. The method of testing and the frequency required by this code make production volume of pipe spooling and steel testing costly and difficult. Most pipe from mill come with mill varnish helping protect the surface during travel. New steel will generally not be pitted, where a good sandblast profile will easily eliminate any surface contaminants. There are situations where you are on an oil platform, recoating old steel, in the middle of the ocean where this specification is highly advisable but for most steel processing this is considered over processing. The short answer is where required.
Soluble Salt in relation to rubber lining.
Generally speaking in a rubber lining application, the internal liner will fail far sooner than the steel or coating will deteriorate. Rubber lined product is generally considered a wear product. Warranties are difficult as process flow, materials configurations are always changing. Unless adamantly specified this specification should be avoided.
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