Lawrence Chan
MANE-6960: FRICTION, WEAR AND LUBRICATION OF MATERIALS
Fall 2012
12/13/12
Friction and Wear of Mechanical Seals
Pumps are an unnoticed, yet vital part of our everyday lives. From large oil pumps
working on offshore oil refineries to simple water pumps that provide irrigation to farming
communities, a pump can affect many. As with any mechanical component, wearing
components will play a large role in the lifetime and effectiveness of a pump. For a pump, the
major wearing parts are the bearings and seals and it is no surprise that tribology has numerous
applications in both. The most simple and common seal is a flat piece of rubber (gasket) or a
cylindrical piece of rubber (O-ring). These are effective in sealing between two static surfaces
but cannot be used for long between moving surfaces. This interface of a dynamic surface
typically includes the shaft of a pump, between the driver and the working fluid. A basic and
effective solution utilizes material to "pack" the space between the rotating shaft and static
surface. Packing requires monitoring and frequent adjustments to be effective. In many
applications, packing is sufficient but for some applications, minimization of maintenance is a
priority. Mechanical seals provide an effective and, if correctly designed, low maintenance
solution to this problem. This paper will investigate advancements in mechanical seal
technology and focus on steps taken to maximize the effective life of a mechanical seal.
The most simple solution to prevent leakage is to press two surfaces together until no
fluid is leaking. While this is effective for static applications, it is important to understand the
challenges and differences with two surfaces that have relative movement while in contact.
Many tribological factors including friction, wear and thermal generation/dissipation need to be
considered. Along with the coefficient of friction, a common unit used to measure expected life
of a seal is the Pressure x Velocity (PV) limit. A high PV limit corresponds to an ability to
operate under conditions of high loads and velocities. See Figure 1 for a cross sectional view of
a typical mechanical seal with the point of contact of sealing rings highlighted.
Point of Contact for sealing rings
Shaft
Figure 1. Cutout of a Typical Mechanical Seal attached to rotating shaft (1)
The selection of materials on these faces are subject to great research and are aimed at
minimizing friction and wear with the end goal of maximizing life of a seal. A common seal
face combination contains a high duty mechanical carbon running against hard silicon carbide.
Jones in [2] ran typical seal face combinations, a carbon grade against a corrosion/wear resistant
silicon carbide, to elaborate on the critical features of these surfaces that affect friction and wear.
The study in [2] confirmed the importance of formulating a stable surface film of composed of
wear debris which are from breakdown of the carbon surface. A failure of this film corresponds
to an increase in the coefficient of friction, which in turn cause failure of the contact film. The
results of Reference 1 demonstrate that the PV capability is critically dependant on the stability
of the contact film formed. Jones proposes that the steady state wear rate of mechanical carbon
is a function of the dynamics of the contact film disintegration regeneration process and the
consequent rate of loose particle expulsion from the contact zone.[1] Ironically, to avoid severe
wear on two dry seal faces, the seal face must begin with and be able to replenish worn particles
in a contact film. Solutions to replace the source of the contact film are typically used.
Regardless of the type of contact film, control of this film has proved to be the major challenge
and focus of mechanical seal research. A reduction in friction, reduction in abrasive wear and
thermal degradation have been proposed to address this problem.
Jones in [2] clearly establishes the importance of a contact film, but highlighted the issues
of relying on the wearing face to provide a contact film. The solution to the abrasive wear of
surface on surface contact is to provide the film from another source. The most common source
of this film is the actual pumped fluid which provides both pros and cons.
The major benefits of using a fluid film is the lower friction and higher thermal transfer
properties of the wet film source when compared to simple contact dry faces. A flush connection
is typically provided directly to the seal faces to aid in thermal transfer and also to remove any
abrasive particles that when left to sit on the faces, can cause wear. The obvious issue with
relying on the pumped fluid, is that it requires a controlled leakage of the very fluid you are
attempting to prevent from leaking. The control of the film thickness and leakage requires
understanding of the forces involved. A standard mechanical seal with a flush plan to the seal
faces, is shown in figure 2 below.
Figure 2. Common mechanical seal with wet seal faces and flush plan to the sealing surfaces. Pumped fluid is
shown in brown.
If the pumped fluid is hazardous or if the loss of fluid is unacceptable, a dual mechanical
seal is can be used. A dual mechanical seal has two pairs of seal faces. One pair of faces seals
the process fluid from an injected clean fluid and the other face seals the clean fluid from
atmosphere (see Figure 3).
Sealing surface #2
Sealing surface #1
Pumped fluid
Clean fluid
Atmosphere
Figure 3. Dual mechanical seal [6]
In this common mechanical seal, the pressure in the lubricating film is composed of
hydrostatic and hydrodynamic contributions. The pressure difference between the pumped fluid
and the atmosphere provides a hydrostatic force while the pressure generated as the surfaces
slide provides the hydrodynamic forces. Hydrostatic forces can be carefully balanced with
knowledge of pressures and geometry. On the other hand, hydrodynamic forces in seals are
harder to control. With two perfectly flat parallel faces (see Figure 3), the fluid between the
faces will be held at a constant height due to no hydrodynamic forces. This known height of the
film is preferred as the designer can rely on the fluid to provide enough lubrication, and control
the amount of leakage. Because of this, mechanical seal surfaces are machined to be at flat as
possible through the process of lapping.
Figure 3. Velocity distribution of a fluid film between two surfaces, one moving with relative velocity V0. Note that the fluid
near the surface tends to adhere to the face and move at the same velocity as the surface itself.
[1]
For surfaces that are not parallel, a pressure distribution will form that is will be a factor
of fluid viscosity, rotational speed and force pressing the faces together. Since the pressure
distribution will change the axial forces, and the axial forces affect the pressure distribution, it is
tough to maintain a stable film which, as mentioned earlier in [1], is critical to seal life (see
Figure 4).
Figure 4. Slightly tilted moving surface creating a pressure profile due to hydrodynamic effects
While the industry has been able to obtain very flat surfaces that are within microns of
perfectly flat, they are never perfect. In reality, there will always be hydrodynamic forces in a
mechanical seal.
While balancing the hydrodynamic and hydrostatic forces in a standard wet seal can be
challenging, research has found ways to use them to as an advantage. Accepting the fact that
hydrodynamic forces exist, researchers have developed surface texturing methods which provide
hydrodynamic and hydrostatic lift and which have been proven to reduce friction torque on
mechanical seal faces and provide more stable inter-surface films
[4]
. Etison in [3] investigated
small pores etched into wearing surfaces and found a significant benefit to friction torque with a
textured face vs a lapped face (see Figure 5).
Figure 5. Regular micro-surface structure in the form of micro-dimples (a) and Comparison of the friction torque vs. face loading for textured
and non-textured SiC/SiC seals in water (b) [3]
In [5], Etison tested textured faces relative to hydrostatic pressures and found up to a 90%
decrease of friction in textured faces vs untextured faces[5] (see Figure 6). While a lower friction
torque at the same pressure is evident, the results presented in [5] also suggests the ability of
textured faces to withstand higher pressure past the untextured limit (see Figure 6).
Figure 6. Friction torque vs. sealed pressure for untextured and textured seals [5]
A common failure of seals involve the formation of surface cracks that are commonly
referred to as blisters (see Figure 7)[4]. In [4], micro dimples were formed in controlled patterns
on seal face surfaces with a laser. Tests were conducted with texture patterns on a carbongraphite face or on a mating silicon carbide face. Fewer blisters were detected with textured
silicon carbide faces in comparison to silicon carbide faces that were not textured
[4]
. In effect,
the solution is no longer simply to have flat wearing surfaces (Figure 3), but perhaps also have
strategically textured surfaces (Figure 5a).
Figure 7. Blistering magnified on a carbon mechanical seal face [1]
For low viscosity fluids, such as gas or hot water, instead of pores another solution is to
control the hydrodynamic forces with hydrodynamic tracks. Tracks are created in the wearing
surfaces that guide the lubricating film and create a controlled pressure distribution during
operation (see Figure 6 and 7).
(a)
(b)
Figure 6. Hydrodynamic tracks in seals rings for hot water (a) and Hydrodynamic wedges in gas seal face (b).
[1]
When designed correctly, the film thickness can be controlled during operation.[1] The
largest gas seals in industry today are being developed using this technology and have been rated
up to 6,500 psi using technology contained in Figure 7 [6]. Etching a surface intentionally to
create a non uniform surface is counterintuitive to a basic seal analysis (see Figure 3) but if great
care is placed on geometry and placement of these "asperities", mechanical seals can and have
been proven to benefit.
(a)
(b)
Figure 7. (a) The sealing dam at the inside of the seal face allows the gas to compress and provides a necessary force to separate
the seal faces and (b) pressure distribution of the seal faces that contain hydrodynamic tracks [9]
Mechanical seals are an extremely complex component with many variables contributing
to an effective design. All aspects of tribology are present and researchers have been
continuously challenged as larger systems with more severe conditions require even more
complex seals. As demand grows for these severe applications, mechanical seal technology will
certainly follow.
References
[1] Mechanical Shaft Seals for Pumps; Grundfos; 2009. Available at
<http://net.grundfos.com/doc/webnet/waterutility/_assets/downloads/shaft_seals.pdf>
[2] Jones, G.A., On the tribological behaviour of mechanical seal face materials in dry line contact
Part I. Mechanical carbon, Wear 256 (2004) 415–432
[3] I. Etsion, Y. Kligerman & G. Halperin (1999): Analytical and Experimental Investigation of
Laser-Textured Mechanical Seal Faces, Tribology Transactions, 42:3, 511-516
[4] Pride, S., Folkert, K., Guichelaar, P., and Etsion, I., 2002, ‘‘Effect of Micro-Surface Texturing
on Breakaway Torque and Blister Formation on Carbon-Graphite Faces in a Mechanical Seal,’’
Lubr. Eng., 58, pp. 16–21.
[5] I. Etsion (2002): A Laser Surface Textured Hydrostatic Mechanical Seal, Tribology
Transactions, 45:3, 430-434
[6] John Crane 28EXP, John Crane Inc. Available at
<http://www.johncrane.co.uk/Prod_ProdPage_Layout.asp?r=uk&l=en&ObjectItemID=144&ele
ment=Prod_ProdPage_Seals_1 >
[7] SDVT Mechanical Seal, TREM Engineering, Available at
<http://www.tremseals.com/products/mechanical-seals/sdvt-bellows-seal>
[8] Khonsari, M, Improving Reliability With Cooling Face Seals, Machinery Lubrication. Available
at <http://www.machinerylubrication.com/Read/1295/cooling-face-sealsorces>
[9] Joe Delrahim, John Crane Type 28 Dry Gas Seal Reliable Favorite for Ethylene Plant, John
Crane Inc., Available at <http://www.johncranetoday.com/johncranetoday/index.cfm/news/johncrane-type-28-dry-gas-seal-reliable-favorite-for-ethylene-plant/>