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There are five elements to each mechanical seal that must be evaluated for each application. The first element is referred to as the Primary Sealing Element. This is simply the primary ring and the mating ring, both commonly referred to as the seal faces. The seal faces must be properly chosen for their durability as well as their compatibility with the service.
This article contains excerpts from the paper, "Caustic processes: Application and implementation of successful sealing strategies", by Kyle Stoner of John Crane and Akso Grooms of Akso Nobel Functional Chemicals at the 2017 Turbomachinery & Pump Symposia in Houston.
Seal faces must be capable of withstanding the viscosity of the fluid, any particulates in the fluid being sealed which may be abrasive to a seal face material, as well as the pressures and velocities exerted on them which can all lead to excessive wear. Seal vendors have a variety of seal face materials to choose from, but the most common materials are Carbon, Silicon Carbide and Tungsten Carbide. As these materials can be combined in a variety of ways, it is important to consider the best material pairing possible for each service.
For caustic services, namely NaOH, API 682 recommends a particular classification of Silicon Carbide known as Sintered Silicon Carbide for both seal faces. This classification of Silicon Carbide is “chemically inert in virtually all corrosive environments” due to its manufacturing process (API 682 4thedition, B.2.3.3). In addition to its chemical compatibilities, it is also a hard material that improves its abilities to handle processes with abrasive particles as well as fluids that crystallize.
This material combination is strongly recommended but after evaluation of the process fluid, the seal arrangement and the support system chosen, other material combinations can be selected. In a dual unpressurized seal arrangement, this may be the proper combination of seal faces for both the inner and the outer seal faces as the outer seal will also see traces of caustic process in the buffer fluid that has crossed the inner seal faces.
In a dual pressurized seal arrangement, this particular combination may only be necessary for the inner seal as the outer seal will only see the barrier fluid. The second mechanical seal element to consider is referred to as the Secondary Sealing Element. This element comprises of multiple individual components that seal the process from the atmosphere where ever two components join together. In a mechanical seal, these components are elastomeric O-rings, PTFE components such as wedges, chevrons, etc., metal and elastomeric bellows, press formed graphite foil rings, and gaskets found between the mechanical seal gland and the face of the seal chamber. API 682 suggests using a perfluroelastomer in caustic applications.
This classification of elastomer has a wide chemical compatibility range with a broad temperature range as well, but of particular concern to some customers, it is often more expensive than other elastomer compounds. As stated before, with careful consideration of the process and potential characteristics, other less expensive elastomers can be evaluated for use in a caustic service. If an application requires an API 682 qualified seal, then it should be noted that according to API 682 4th edition; “(w)edge, u-cup, or v-ring shaped elastomer or PTFE material also has to be energized by external means, such as a spring, but reliability concerns associated with fretting and increased friction eliminate these as suitable alternatives within this standard.”
The third mechanical seal element to consider is referred to as the Drive Element. The purpose of this element is to provide a positive connection between the shaft and the rotating face to overcome seal face friction. This element can be a simple pin that keeps a seal face from rotating, or a retainer that houses a primary ring, or a set screw or key that transmits torque from the rotation of the shaft in to the rotation of a seal face. If these components are not compatible with the process, then they will not be able to properly perform their role in the mechanical seal. These components are often made of a metal that is easy to replace, but some services require the use of exotic materials such as titanium, Hastelloy, or Monel to resist any chemical incompatibilities with the process. For thermal growth concerns and full seal compatibilities, the materials chosen for this element are often used for the remainder of metal components in the seal design.
The fourth mechanical seal element to consider is referred to as the Load Element. The Load Element is used to maintain the minimum prescribed seal face pressure during static and dynamic pump conditions. This element is simply the springs found in apusher style seal and the bellows of a metal bellows seal. Corrosion of these elements will result in loss of static sealing capabilities in a pusher seal and in the case of a metal bellows seal, this will result in both static sealing and secondary sealing concerns. The fifth mechanical seal element to consider is referred to as the Adaptive Hardware. The Adaptive Hardware is everything that is used to adapt a mechanical seal to the pump. This includes the sleeve, gland, and drive collar. The sleeve and gland are always in direct contact with the process and may require the use of exotic materials if the process is found to be aggressive with othermaterials. Both the sleeve and the gland house a Secondary Sealing Element and if these two components are not compatible with the processleading to severe corrosion, then a leak path can be formed by the secondary seals. All five elements must be equally suited for the process conditions. Corrosion of a metal component or chemical attack of a seal face or secondary seal will result in a seal leak.