A recent article in the Detroit Free Press highlighted a leak of sulphuric acid from a holding tank at the Cook Nuclear Plant in Bridgman, southwest of Grand Rapids, Michigan. The leak was apparently caused by a faulty gasket on the holding tank’s discharge valve.
The article highlights a key application challenge: achieving reliable sealing with gaskets for strong acid service, including acids like sulphuric acid.
Strong acids, such as sulphuric acid, are extremely corrosive and, unless the gasket material is chosen wisely, the risk of gasket failure with resulting leakage can be high. An important factor to consider in any strong acid service application is the concentration of the acid being serviced, as well as the temperature of the media. For example, applications involving sulphuric acid at concentrations of 96 percent and above can be extremely challenging, and the challenge is exacerbated when hot acid is involved.
Gasket Materials for Strong Acid Service
Materials that should be considered when selecting gaskets for strong acid service include the following:
- Viton® (FKM). Viton® is a brand of fluoroelastomer (FKM) developed by DuPont. Commonly used in the fabrication of automotive seals and parts, Viton® possesses good heat resistance characteristics and is impervious to attack by many aggressive chemicals, including strong acids. Viton® possess very good resistance to many acids, including including sulphuric acid (except fuming), hydrochloric acid, nitric acid (except >50% concentration and fuming), and other acidic media.
- Glass-filled skived PTFE sheet. Glass-filled PTFE (Teflon®) sheet is a good material choice where the application operates at lower temperature ranges (i.e., lower than 65.5°C [150°F]). While glass-filled PTFE is about half the cost, on average, of restructured sheet, the material is not without some disadvantages. The structure of glass-filled PTFE sheet will cause the material to creep and cold flow at elevated temperatures. Therefore, a careful evaluation of the application maximum operating temperature should be made when considering this gasket material for strong acid service. While glass-filled PTFE sheet offers good resistance to many acids, it is not recommended for applications involving hydrofluoric acid or strong alkalies. For more information on the properties of glass-filled PTFE sheet material, please see our blog PTFE Gasket Material: Texolon 9406 PTFE Sheet.
- Restructured manufactured PTFE sheet. Restructured manufactured sheet is now the preferred gasket material for strong acid service where the operating temperature is less than 204.4°C (400°F). Restructured PTFE sheet will do an excellent job in these applications. CRG supplies Tealon™ TF 1570, TF 1580, and TF 1590 restructured PTFE gasket material for moderate and strong acid service. The preferred choice of filler for restructured PTFE sheet for strong acid applications is mineral silica (TF 1590). Tealon™ sheet, manufactured by Teadit North America, is a premium quality restructured sheet that is the exact material equivalent of Gylon®. The unique manufacturing process for Tealon™ results in a sheet that has less creep and cold flow than that normally associated with PTFE gasket material.
- Multi-directionally expanded PTFE sheet. CRG supplies gaskets manufactured from Teadit 24SH sheet. 24SH gasket sheet is manufactured by expanding 100% virgin PTFE using a proprietary process that produces a uniform and highly fibrillated microstructure with equal tensile strength in all directions. The resulting product exhibits characteristics significantly different than conventional PTFE sheet. This style is much softer and more flexible than regular PTFE sheet and thus conforms easily to irregular and rough surfaces. In addition, the material is easier to compress and minimizes creep and cold flow. Gaskets made from 24SH sheet will seal all aggressive chemicals over the entire 0-14 pH range, except for molten alkali metals and elemental fluorine.
No matter the material choice, a good rule to follow for strong acid service is to limit all gaskets used to single use. Reusing gaskets to save a few pennies here and there is inadvisable due to the risks of failure and serious injury to personnel. In strong acid applications, all used gaskets should be destroyed immediately after use to prevent any inadvertent reuse.
CRG can supply gaskets produced from Viton®, PTFE, or other materials especially designed to handle strong acid service. If you have a strong acid application, our sales team will be pleased to review your needs and discuss the material options available. Contact our sales team directly at firstname.lastname@example.org.