Page 281 - The Tribology Handbook
P. 281
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.
B24.5