by Henri Azibert, FSA
May 24, 2012

Circulating Devices

End users should also be careful in situations in which a circulating device built into the mechanical seal is used to drive fluid through a heat dissipating external device. This external device can be a heat exchanger or a reservoir depending on whether the user installs API Plan 23 (single seal circulating fluid through a heat exchanger) or Plan 52/53 (dual seal circulating a buffer or barrier fluid through a reservoir, which can also include a heat exchanger). 

When these plans are used simply to handle the seal generated heat, there is likely to be little impact. Because the flow rate of the pumping device and the heat generated by the seal are proportional to the pump speed, the decrease in flow rate is compensated by the decrease in heat generation. However, when these plans are also used to cool the seal environment because of high process temperatures, the decrease in speed results in elevated seal temperatures that can adversely affect seal life and operation. In these circumstances, the lowest speed is the most critical case that needs attention. 

 

Severe-Duty Applications

Finally, severe-duty applications may also require special considerations. When the pressure, temperature and speed are elevated, a custom design with rigorous analysis might be required. For example, a boiler feed water application has the original conditions of 5,500 rpm, 170-millimeter shaft,  seal face velocity over 11,000 feet per minute, (56 meters per second), and 190 psig (13 bar), with a process temperature of 350 F, (175 C). Figure 1 is a cross-sectional view of the sealing system used in the application. 

A standard design was used and verified to be able to handle these conditions. FEA analysis predicted a convergent profile to promote face lubrication. The operation then changed from a constant speed of 5,500 rpm to vary from 5,500 rpm to 3,300 rpm on a daily basis. Analysis of the same geometry at 3,300 rpm showed that the seal did not have an optimal face profile at the reduced speed with a slightly divergent profile. Both profiles of this original design are shown in Figure 2.

Although the seal could operate under this regime, an optimized face geometry was developed to handle both conditions (see Figure 3). Note also the reduced temperature under both regimes.

sealing2
Figure 2. (top) FEA analysis — standard design; Figure 3. (bottom) FEA analysis — optimized design


Pumping Ring Efficiency

Finally, consideration must be given to the efficiency of the pumping ring. The role of the pumping ring is to circulate fluid to a heat exchanger to dissipate heat generated from the seal faces and to cool the process fluid in the seal cavity that is constantly heated from the heat soak. At the lower speed, enough circulated must be present to bring the temperature down to the desired level. However, at high speeds, there can be cavitation problems if the design of the pumping ring is too aggressive. Achieving a balance can be challenging and may require flow analysis (see Figure 4).

 

Figure 4. Flow velocity vectors at the exit of pumping ring

 

Consider the Seals

VSDs significantly improve pump efficiency, but their potential impact on mechanical seals must be recognized and considered. While in most cases varying the speed of the pump will have no adverse effect on the mechanical seal, certain situations require careful analysis. These are primarily cases in which environmental controls are used with the mechanical seal. The purpose of these controls must be well understood so that the system design can properly function under the varying operating conditions produced by variable speed drives.

 

Next Month: What are the current industry best practices for the assembly of bolted flange connections?

We invite your questions on sealing issues and will provide best effort answers based on FSA publications. Please direct your questions to: sealingsensequestions@fluidsealing.com.

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