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Courtesy of A.W. Chesterton
Mechanical seals are commonly used in a wide variety of industrial applications
for sealing process fluids in rotating equipment. Often, the presence
of suspended solids or gas bubbles in fluid sealing environments can be
detrimental to seal life and operating efficiency. Conventional ways to
manage or limit the adverse effects of particulates tend to rely on seal
chamber design and clean liquid flush systems. While suchmethods can be
effective, operating costs may be prohibitive if large volumes of flush
fluid are required.
Using CFD, novel new concepts and proven devices are being explored that
offer solids control in sealing environments. Recent results show dramatic
improvements as designers provide cleaner fluid environments for mechanical
seals without expensive flush systems.
Figure 1 shows process liquid sealed at the interface between contacting
seal rings. One ring (orange) rotates with the shaft, while the other
ring (green) mounts to the nonrotating parts of the seal. The f lush (inlet)
port above the interface is shown in cross-section. At the impeller end
of the seal chamber is an EnviroSeal SpiralTrac spiral groove bushing
(dark blue), which, together with the rotating sleeve of the Chesterton
seal (dark gray), helps rid the chamber of solid particles.
Figure 1. A 3D Unigraphics model of a typical mechanical seal installation
for a centrifugal pump – the seal mounts to the rotating shaft and
the drive side of the pump seal chamber
The FLUENT model (Figure 2) represents the seal chamber, the spiral groove
bushing, and the seal. A hybrid element CFD model consisting of 1.2 million
cells was set up. First, the flow field was solved using the standard
k-e turbulence model, requiring about 20 hours on an SGI Octane workstation.
The particle motion was then calculated using the discrete phase model
(DPM). Effects of particle size, shape, material, and density were examined
for different shaft speeds, chamber sizes, and flush fluid rates. Using
Data Explorer, the DPM results were then converted into time dependent
animations revealing solid particle and fluid element behavior.
Figure 2. The geometric model of the fluid sealing cavity used to study
the flow of liquid, and the associated behavior of solid particles –
the annular seal chamber is between the spiral groove bushing (left) and
the mechanical seal (right)
When flush liquid is used, it enters the system through nine (yellow)
circumferentially spaced inlets, circulates through the flow domain, and
is discharged through the (magenta) annular outlet. For the case shown
in Figure 2, however, a clean liquid flush was not used. In this mode
of operation, with particle-laden water flow in the seal chamber, the
magenta annular outlet of the bushing admits liquid. Even so, solids are
expelled from the system via this outlet into the flow behind the impeller.
In Figure 2, spherical solid particles with a specific gravity of 1.15,
colored by their velocity magnitude, are shown for a shaft speed of 2700
rpm (Re = 140,000). The vector arrows, also colored by velocity magnitude,
depict the motion of neutrally buoyant fluid elements approximated using
the DPM. As shown in the figure, solid particles migrate along the spiral
groove of the bushing to the outlet plane of the system.
Figure 3 shows particles being driven through the grooved bushing, a
snapshot of the action well into the 6.2 sec simulation. By this time,
only about 11% of the solids originally present in the chamber remain
in the flow. The particles, in this case 15% heavier than the liquid,
centrifuge radially outward from the rotating shaft and into the groove
where they are driven to the outlet by the induced flow.
Figure 3. A profile of the spiral groove geometry showing velocity mapped
particles and fluid elements
Another view of the fluid/particle motion is shown in Figure 4. Here,
the observer is inside the seal chamber looking at the intake of the spiral
groove bushing. The particles and fluid elements are seen moving with
the complex pattern of swirling flow in the chamber. In the distance,
solids can be seen entering the spiral groove as they begin their journey
to the outlet.
Figure 4. A view from inside the seal chamber, looking along the rotating
shaft (below) toward the intake of the spiral groove bushing
Insight from these studies is helping seal analysts and designers provide
longer product life and greater operating efficiency, resulting in lower
cost solutions to slurry sealing applications for end users and customers.
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