Problematic tasks can require improved performance in extreme operating conditions.
by Bassem Gabra, FSA Member, Flowserve Corporation; Bob Viloria, FSA Member, Ergoseal, Inc.; Steven Bullen, FSA Member, A.W. Chesterton Co
January 24, 2019

There are applications where the use of a mechanical seal would either not be considered or present major technical challenges. Here are some unusual examples of how mechanical seals can be applied to solve problematic sealing tasks.

Latex

Liquid synthetic latex is an emulsion of polymer particles suspended in an aqueous solution. It is used in making coatings, glues and gloves and more.

image 1 stages latexImage 1. Stages of latex solidification or coagulation when exposed to heat or friction (Images courtesy of FSA)

Sealing latex has historically been a problem for mechanical seals because it solidifies when exposed to either heat or friction (shear). When latex is exposed to heat, water separates from the polymer particles, leading to solidification or coagulation. A more challenging issue with sealing liquid latex is that when it enters the gap between the mechanical seal faces, it gets sheared, which also leads to local coagulation (Image 1).

A double-pressurized mechanical seal seems to be a successful alternative because the fluid film within it is a common barrier fluid. However, latex can still penetrate the seal faces due to thermal and pressure distortions. Both types of distortion cause the inboard seal faces to run convex, producing a gap at the outer diameters. This disrupts the formation of a uniform barrier fluid film across the faces, so latex can penetrate the gap on the outside diameter (OD) where it then solidifies (Image 2).

image 2 convex facesImage 2. Convex faces and latex penetrating the OD, causing face separation

A secondary problem is that latex accumulates around the inboard dynamic O-ring, solidifying and causing seal face hang-up, as seen in Image 3. Special attention should be paid to the design of the dynamic O-ring to make it less prone to seal face hang-up.

image 3 latex accumulatingImage 3. Latex accumulating around the inboard dynamic O-ring

Because of these issues, mechanical seal companies have worked to develop a sealing solution to address these specific problems. In one successful example, starting with a standard dual slurry seal design, a hydrodynamic surface feature was introduced to pump the barrier fluid across the inboard seal faces from the inner diameter (ID) to the OD.

This made the faces run more closely and parallel to one another due to a reduction in thermal distortion. It simultaneously provided a controlled flow of barrier fluid, preventing the latex from penetrating the faces from the outside.

The leakage rate across the inboard seal faces was kept to a minimum by using Finite Element Analysis (FEA) modeling so it would not dilute the product and minimize barrier loss, as seen in Image 4.

image 4 comparison between common slurryImage 4. Comparison between common slurry seal face design and new slurry seal face design

One of the biggest advantages of the new design is that it runs directly in the process without the need for an external flush or an American Petroleum Institute (API) Plan 32 to separate the latex from the inboard seal faces. Some drawbacks of a Plan 32 include:

  • increased operational cost due to the continuous supply of external flush
  • dilution of the final product, as water is usually used as a compatible external flush
  • external flush remains active at all times, even when the seal is static
  • seal performance dependent on the reliability of the Plan 32
  • seal usually fails once the bushing or lip seal separating the Plan 32 flush from the inboard seal wears out

The new seal was tested at a latex production facility in Europe, where it proved to be more reliable compared to the existing seal technology.

Turbochargers & Expanders

Expander machine units have largely been used as waste recovery systems for conversion to power and/or electricity, using different seal solutions to varying degrees of success. Simple seal types used in early development of these machines proved unreliable. One example of these earlier seals is the polytetrafluoroethylene (PTFE) seal ring (scarf-cut design) shown in Image 5. These rings are used to seal and minimize process product loss, and also to prevent cross-contamination of process media and gearbox lubricating oil. The problem with these types of seal rings is short life before cross-contamination of process media with bearing oil.

image 5 ptfe seal ringsImage 5. PTFE seal rings used in early expander units

Process media leakage has proven costly, and cross-contamination of process and gearbox bearing oil after a relatively short seal operation is another critical seal issue. High process gas pressures and temperatures for expander applications limit the use of this type of PTFE seal rings.

Other seal types used for expanders and other waste recovery systems include lip seals, labyrinth and/or noncontacting seals. Applications with more demanding leakage and life requirements restrict use of these seal types.

Mechanical seals can handle more extreme operating conditions.

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