Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
Mechanical Seats 357 Figure 17-2B. During operation without surface instability. ing film and these spots will grow in intensity, generating still more heat. This intense heat may cause the liquid being sealed to flash or vaporize, resulting in unstable operation of the seal. Hie transition from a normally flat condition to a highly deformed surface is called thermoelastic instability (Figure 17-2C). During the mid-seventies, many experiments were conducted to determine if mis could be present in mechanical seals. Burton et al. (1980) demonstrated this phenomenon by running a seal configuration against a glass plate. Hot patches or spots appeared on the surface of the metal primary ring. These spots moved through the entire circumference of the seal. Movement of these spots is believed to be the result of wear. Confirmation of hot patches or spots on conventional materials used in mechanical seals was determined by Kennedy (1984). Localized hot spots from
358 Centrifugal Pumps: Design and Application thermoelastic instability has been observed under both dry and liquid lubricated conditions. Surface distress at a hot spot occurs because of rapid heating in operation, followed by rapid cooling. When a liquid film at the seal faces flashes, the seal opens for an instant, allowing cool liquid into the seal faces. This then heats up only to flash again until the surface cracks. These thermo cracks are attributed directly to the high thermal stresses near the small hot spot. When the seal faces are removed from the equipment, the location of the distressed hot spots may not have any particular relationship to the final surface waviness (Figure 17-2D). Stable operation of a seal is achieved through control on surface waviness and thermoelastic instability. This is accomplished through cooling and seal face design. Figure 17-3 illustrates waviness traces for a set of 3.94-inch mechanical seal faces run in liquid ethane for 3,056 hours without any visible leakage. The pressure and shaft speed were 820 Ibs in.^2 and 3,600 revolutions min" 1 . The temperature was 48°~60°F. The surface waviness of the carbon ring is 75 micro-inches peak to peak, while the tungsten carbide surface has a waviness of approximately 12 micro-inches. These traces are typical of a good running seal. Figure 17-2C. During operation with instability.
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Mechanical Seats 357<br />
Figure 17-2B. During operation without surface instability.<br />
ing film <strong>and</strong> these spots will grow in intensity, generating still more heat.<br />
This intense heat may cause the liquid being seal<strong>ed</strong> to flash or vaporize,<br />
resulting in unstable operation of the seal. Hie transition from a normally<br />
flat condition to a highly deform<strong>ed</strong> surface is call<strong>ed</strong> thermoelastic instability<br />
(Figure 17-2C).<br />
During the mid-seventies, many experiments were conduct<strong>ed</strong> to determine<br />
if mis could be present in mechanical seals. Burton et al. (1980)<br />
demonstrat<strong>ed</strong> this phenomenon by running a seal configuration against a<br />
glass plate. Hot patches or spots appear<strong>ed</strong> on the surface of the metal<br />
primary ring. These spots mov<strong>ed</strong> through the entire circumference of the<br />
seal. Movement of these spots is believ<strong>ed</strong> to be the result of wear. Confirmation<br />
of hot patches or spots on conventional materials us<strong>ed</strong> in mechanical<br />
seals was determin<strong>ed</strong> by Kenn<strong>ed</strong>y (1984). Localiz<strong>ed</strong> hot spots from