Page 281 - The Tribology Handbook
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B24                              Mechanical seals






              SELECTION PROCEDURE                                Example
               In order to make the correct seal selection it is necessary to   Water at  10  bar  3000 rev/min,  45 mm  shaft dia.,  175°C
              know:                                             temperature at seal.
                The product to be sealed                         1.  From the Aqueous Products list:
                Pressure to be sealed                              Vapour pressure curve no. is 21
                Shaft speed                                        Face material  choice is stainless steel  18/8
                Shaft or sleeve dia.                               lead  bronze,  Stellite,  tungsten  carbide,  alumina,  or
                Temperature of product in the seal area            silicon carbide
                                                                   Seal packings  are high  nitrile  rubber up  to  100°C or
                It is assumed that by  knowing the product, its boiling   ethylene  propylene  up  to  150°C  (note temperature  at
               point at the pressure  to be sealed is also known.   seal is  175°C)
                In the procedure  described here, products  to be sealed   Other seal components, 17/2 stainless steel.
               are divided into aqueous products and hydrocarbons.   No special remarks are listed.
                                                                 2.  From the Seal Type Selection curve either a balanced
               1.  Consult Table 24.8 for Aqueous Products or Table 24.9   or unbalanced seal may be used  (at a peripheral speed
                 Hydrocarbons and note:                            of 7 m/sec).
                                                                 3.  For Face Material Selection consider:
                 The vapour pressure curve number,  and possible  seal   (a) a balanced seal when tungsten carbide/carbon and
                 face materials, seal packing and other seal component   silicon carbide/carbon  are possibilities.
                 materials, for compatibility  with the product, together   (b) an unbalanced  seal when silicon carbide/carbon is
                 with any special remarks for satisfactory operation.   the only possible combination.
                                                                 4.  From the temperature and pressure  curves consider
               2.  Refer to the central diagram in the left-hand column of   (a)  balanced  seal and transposing  from curve no. 21,
                 the General Seal Selection Graphs to determine for the   note  that  at  10  bar  and  7  m/sec  the  maximum
                 particular  shaft  speed  and  shaft  diameter  an   operating  temperature  range  is  160°C  to  130"C,
                 appropriate seal configuration with regard to the spring   depending on seal face
                 layout. Note the parameter,  peripheral speed.    (b) unbalanced  seal  diffusivity  maximum  operating
               3.  Depending whether the seal pressure is above or below   temperature is 70°C
                 1 MN/m2 (140 p.s.i.); refer to the Face Material Selec-   5.  Comparison of possible selections
                 tion  curves-balanced   at  the  top  of  the  page  unbal-
                 anced  at the bottom.  For  the particular  seal pressure   Cooling required from  175°C to  160°C or  130°C
                                                                 Balanced
                 and  peripheral  speed,  note  the  face  material  choices   seal   depending on the diffusivity  of the chosen  face
                 available.                                                 seal.
               4.  Move  to  the  right  to  the  temperature  and  pressure
                 curves.  From the centre figure on the right, transpose   Unbalanced   Cooling required from  175°C to 70°C  and high
                 from  the  temperature scale for  the particular  product   seal   diffusivity face seal.
                 corresponding to the vapour pressure curve no.,  to the
                 top  for  a  balanced  seal  and  to  the  bottom  for  an   From these possible selections, the preferred  one is the
                 unbalanced seal, choosing a pressure line relating to the   balanced  seal  using  silicon  carbide  running  against  a
                 diffusivity of the face materials  (see Table  24.5). The   carbon  stationary  seal  ring. The minimum  of  cooling is
                 curves then represent a stability line in terms of sealing   required  and silicon carbide is chemically resistant.
                 pressure  and  operating  temperature  for  particular   Note that nitrile rubber packings are unsuitable.  PTFE
                 speeds (m/sec). The area to the left of the curve is one of   or graphite are suitable however. Other metal seal compo-
                 stable operation: the  area between  the  curve and the   nents are 17/2 or  18/8 stainless steel.
                 saturation curve is one of instability.  If the operating
                 point falls within this latter region, either some cooling
                 is  required  or the seal  pressure  must  be  changed,  to
                 bring  the  operating  point  to  the  left  of  the  stability
                 curve.
               5.  Compare the face material selected from corrosion and
                 compatibility considerations from Tables 24.6,24.8 and
                 24.9 with those from stability considerations as in step 4
                 above and decide on the appropriate combination.
               6.  Decide  what  cooling  (or seal  pressure  change) is  re-
                 quired;  decide  on  the  circulation  system  and  what
                 additional  design  features  are  required  by  way  of
                 quench, safety bush etc.

                 Selection is now complete.



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