Gasket FAQ
From www.gasketwiki.com
The following page answers some gasket related frequently asked questions.
Q. I selected a gasket material that was within the manufacturers' pressure and temperature maximum specifications, but the gasket failed.
A. This is often a common mistake that is made when selecting a gasket material. Although the application temperature and pressure may be below the listed maximum, it still may be too demanding for gasket material to withstand. This is because temperature and pressure are correlated in determining the gasket's true threshold. Often, the higher the temperature of the application, the less pressure the gasket can tolerate, and vice-versa. The best way to determine if the gasket material is suitable is to reference a temperature/pressure graph (PxT graph) for each specific material. It will indicate if the gasket can tolerate both the temperature and pressure of the application.
Q. Should I coat the gasket with anti-seize before installation to prevent the gasket from sticking to the flange during removal?
A. No, this often causes the gasket to prematurely fail. Anti-seize can adversely affect the rubber binder in the gasket which drastically changes the gasket material's capabilities. It is best to look for a gasket material that has a non-stick coating such as Klingersil NST products.
Q. We are using a compressed synthetic fiber gasket (CSF) in our application. After a period of time the gasket developed a leak, so we re-torqued the bolts. The gasket continued to leak and we re-torqued the bolts again and again, but the leak got worse. Why did this happen and what should we do?
A. Unlike the old compressed asbestos gaskets that were tolerant of re-torquing while in service, compressed synthetic fiber gaskets are not. The reason for this is the higher percentage of rubber binder in the gasket material. Once the gasket has been fitted and bolted into the flange and experiences a nominal working temperature, the rubber will begin to stiffen. If the bolts are torqued up again, the stiffened rubber will likely break apart, creating leak paths and cause an eventual gasket failure. Traditional asbestos gaskets contain a much lower percentage of rubber binder, so they remain more flexible during heating and can be re-torqured. It is critical to determine before hand, the gasket's maximum compressive strength and the minimum seating stress for the application in order to calculate the required torque values before installing the gasket. Remember that CSF gasket require more careful thought and planning to create the perfect seal.
Q. Why are there so many different types of gasket materials? Isn't there just one type of gasket material that I can use?
A. Unlike the traditional asbestos gaskets that could almost be used universally in every application, compressed synthetic fiber gaskets, PTFE gaskets and graphite gaskets each have their own strengths and weaknesses. It takes a large cross-section of gasket materials just to cover the broad range of applications that the old asbestos gaskets could handle. Be sure to check closely that each application is matched with the most suitable gasket material for a successful and reliable seal.
Q. At what temperature does a graphite gasket oxidize?
A. graphite gasket will oxidize at different temperatures depending on the environment it is exposed to. In an environment exposed to air the graphite will begin to oxidize at 575ºf according to the WRC (Welding Research Council) Elevated Temperature Characterization Of Flexible Graphite Sheet Materials For Bolted Flanged Joints study Bulletin Number 419, please refer to this document for further details. There are several type of corrosion inhibitors that would help a graphite gaskets withstand higher temperatures than the recommended 575ºF. also thickness and metal inserts play a significant role on pressure and temperature resistance.
It doesn't matter how much area of the gasket is exposed to air, oxidation appear uniform throughout the gasket due to the high porosity of the graphite, although graphite seems to be adopted as a gasket of general characteristics and properties there are a lot of grades and they shouldn't be consider as equals, due that studies have shows that different graphite sheets from different suppliers have tested very poor in comparison to others.
the glue used in the manufacture of graphite sheets and its treatments affect greatly a graphite sheet performance. NOTE: Tests should be consider before changing to graphite gaskets or to change graphite sheet brands or style. Carlos Chapa - RAISAMEX.COM.MX
Q. Our application is proving to be very time consuming and laborious for installing compressed sheet gasket. The application is in a very difficult to access area and requires a lot of time and patience. Is there an easier way to seal this application?
A. Yes, use pure expanded PTFE in a thick tape form with an adhesive back for easy installation. Compresses into a thin, wide ribbon, which conforms to surface irregularities.
Q. At what point do I need to use a compressed synthetic fibre gasket over a pure rubber gasket.
A. Whenever the application demands more than the rubber can offer. You must select a CSF gasket over a rubber gasket when the temperature, pressure and/or chemical medium is not compatible. In particular, rubber gaskets are usually only good up to 150psi service pressure.
Q. What is PMI for gaskets?
A. PMI stands for Positive Material Identification. Sometimes semi-metallic gaskets (Spiral wound, Maxiprofile, DJs, Corrugated Metal Gaskets) and metallic gaskets (Ring joint) are required to have PMI testing to ensure that they are comprised of metal suitable for a specific application. PMI testing is usually performed by a material inspection company by means of a non-destructive method that uses a handheld x-ray fluorescence technology. The device can determine the content of specific elements that the material is made of, which can be used to identify the specific metal type.
Q. Which gaskets can I use in a food application?
A. There are only a few gaskets that are suitable for contact with food and are FDA compliant: White nitrile rubber, virgin mechanically expanded PTFE and modified PTFE materials. These materials do not contain carbon black, a common additive to many rubber and gasket materials, which is not compliant with FDA standards.
Q. Why is the sponge rubber with PSA not sticking to a surface for my application?
A. There are many factors involved with poor bonding of PSA to surfaces. First of all, two main types of PSA's exist - both of which provide different bonding characteristics. The common PSA used with sponge rubber is a rubber based adhesive which provides a high initial tackiness/stick, but its considered to be non-permanent. The other type of PSA is acrylic based which has a low initial tack but creates a more permanent bond. However, it needs to have a positive pressure(clamped in place) acting on the sponge rubber and bonding surface for a period of time to allowing for a curing period. Selecting the type desired bond is an important step in selecting a PSA.
Secondly, the type of surface and surface preparation are very important factors that contribute to the success or failure of an adhesive bond. The type of surface to be bonded to often dictates how effective the PSA will be. Painted surfaces will often prove to be very difficult or impossible to achieve the desired bond. Proper surface cleaning will help prepare the surface for the best possible bonding. There are chemical cleaning and etching agents that are commercially available for various types of surface types, but often all that is needed is an alcohol wipe to clean away oils and dirt.
Q. When deciding upon the gasket dimensions, does a wider gasket create a better seal (wider is better)?
A. Not necessarily. This seems to be a common thought, however excessively wide gaskets prove problematic for proper installation. In order to achieve the proper clamping force on the gasket from the flange, the bolts must be torqued to a certain value. Typically the bolts should be torqued to around 75% of their yield value so that they can deliever their force to the flange while maintaining their 'stretched' state. That being said, the bolts will have a maximum clamping force value that they can provide to squeeze the gasket. The gasket always requires a specific amount of compression to create and maintain a seal which is based on application crieteria such as pressure, temperature and chemical medium, referred to as E min. Generally speaking, the wider the gasket becomes, the larger the surface area of the gasket is. And a larger surface area requires more bolt force to acheive E min. If the gasket is too wide, the maximum bolt force allowable will not be adequate to create and/or maintain a seal.
Q. I followed the standard torque values for installing a non-asbestos gasket, but it leaks. What happened?
Standard torque guidelines will work in most applications, but do not encompass all of them. Many standard torque values may be insufficient to produce the adequate gasket seating stress that is required. This is because factors such as temperature, pressure, chemical medium, liquid or gas, poor flange conditions/alignment may require a higher torque value. Steam applications can often prove more demanding and require custom torque value calculations. It is recommended to inform your gasket supplier of ALL application information and conditions before purchasing the gasket. This will ensure that you will get the right gasket for the job and that an appropriate torque value will be assigned to your application.
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