The mating pair of counter-rotating seal rings are the heart of the mechanical seal and designers must consider many overlapping physical systems affecting their performance.
There are many factors to consider when designing a resilient mechanical seal ring. This article will look specifically at how mechanical loads, thermal loads, and seal ring deformation all impact a success mechanical seal design.
Mechanical Loads
Designing for resilience starts with determining the loads applied to the mechanical seal rings. Mechanical loads are exerted by other components and by the surrounding fluid pressure.
To maintain the required static equilibrium, fluid pressure and spring force applied on the rear of the mechanical seal ring is supported on the other end by a mix of fluid film pressure and contact between the two seal faces. Fluid pressure applied on the front and rear faces of the mechanical seal ring exerts axial forces that will either open or close the seal faces against each other. Most of the pressure gets “canceled out” (is acting equally on both sides), what matters are the boundaries on either side of the mechanical seal ring where pressure is being sealed.
Thermal Loads
Though viscous shear does take over as the predominant heat-generating mechanism above high viscosities and/or high surface speeds, it is material-to-material friction contact that generates the bulk of the heat load in most mechanical seal applications.
The heat generated by friction between the faces is carried away to a minimal extent by the normal liquid leakage, but mainly it is absorbed by the two mechanical seal ring materials and transferred to the surrounding fluids by convection. After settling into an equilibrium with the surrounding environment and the face heat load, a mechanical seal ring has a temperature distribution that is typically hottest on the ID of the face, cooler on the OD of the face where the fluid flow is, and coolest in the rear of the mechanicalseal ring furthest away from the face.
This temperature gradient causes the mechanical seal ring to expand unevenly, resulting in distortions and internal stresses. Thermal shocks from sudden temperature increases will produce radial stress cracks that cause the seal to fail—some materials like tungsten carbide are particularly susceptible as compared to silicon carbides. Pressure and temperature caused deformations of the face profile can further increase heat generation between the faces. In the worst cases, instability could occur leading to repeated opening and closing of the faces, chipping along the edges, and seal failure.
Seal Ring Deformation
Deflections caused by mechanical and thermal loads can be roughly described as rotations of the cross-section around its centroid. Tangential loads from anti-rotation devices can also significantly flex the seal faces. If you were to greatly exaggerate the actual deformation, the cylindrical ring would look either like an expanding or reducing funnel. Care is taken when designing the seal to make as stiff a shape as possible and to ensure that when deflections happen, they are in the direction that is beneficial to the function of the mechanical seal.
If the seal section rotates so that the faces are converging on the low-pressure side, then more fluid can enter the sealing gap which increases the fluid film pressure, reduces contact, wear, and heat-generation between the faces. This is the characteristic V-gap. However, too dramatic a relative angle between the faces will increase the pressure gradient factor above the balance ratio and result in excessive leakage and seal failure.