### Tech Briefs

Initial seal design

Optimized seal design

Optimizing a Mechanical Carbon Face Seal

Seals are abundantly used in industry and their proper functioning at high load and temperature conditions is extremely important — indeed, their failure can result in loss of life, e.g., consider the failure of turbine engines of airplanes. Hence very reliable and accurate analyses of seal designs is of paramount importance.

In this Brief, we are looking at an interesting application using ADINA where very precise results are needed for optimizing a mechanical carbon face seal for a small gas turbine engine*. The seal face deflections are in the millionths of an inch. The seal is used for oil sealing of a bearing sump (lubricating oil reservoir) as shown in Figure 1. The seal operates at 60,000 rpm which results in a rubbing velocity of 390 ft/sec and a maximum calculated PV (pressure-velocity) of 258,000 lb/in2-ft/min. The seal is required to accommodate the axial translation of the shaft which occurs with variations in power settings of the engine.

Figure 1  The carbon face seal and its operating conditions

Figure 2  The finite element model consists of the carbon seal ring, the seal rotor, the bearing inner race, and two clamping sleeves which load the rotating parts with 10,000 lbs.

Figure 3  The entire thermal and deformed model results for the original design

The thermo-mechanical analysis of the initial seal design shows excessive seal face coning, see Figure 4. The degree of coning is predicted by ADINA and is 158 micro-inches across the seal face which is excessive. The worst case carbon temperature is predicted to be 601 °F which is high enough that it can cause the seal to burn up or to coke the oil excessively leading to a short term failure. The excessive coning causes two problems which are:

1. The seal face pressure profile (opening force) is reduced which increases the seal face load 32%.
2. The seal rubbing contact occurs on the internal diameter corner of the carbon seal ring which causes the high local heat flux and raises the temperature to over 600 °F.

Figure 4  Initial seal design showing excessive seal face coning

Figure 5 shows the optimized seal reduces the coning down to 30 micro-inches which is within blueprint tolerances. This brings the temperature down to 411 °F which is acceptable.

Figure 5  Optimized seal design showing significantly reduced seal face coning

Figure 6  The seal face pressure for the initial design and optimized seal designs. Note the optimized design reduces the total face load from 5.8 lbs. to 4.4 lbs.

The two movies above show the seal face temperature for the full range of operating heights which varies the spring load from 1.5 lbs. to 4 lbs.

In the design of seals, accurate simulations using fluid-structure interaction and thermo-mechanical coupling capabilities in finite element programs are very important. ADINA offers unique strengths for reliable and accurate simulations of these critical components of structures.

Keywords:
Seals, mechanical seal, hydrodynamic seal, carbon face seal, gas turbine engine, coning, oil sealing, bearing sump

*Courtesy of Chuck Alten (c.alten@cox.net) of ABSt, LLC.