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by:DMS Seals     2020-06-02
No one guessed that a small cone would make the mechanical seal run cooler and last longer.Mechanical seals are widely used to prevent fluid leakageHandling equipment such as centrifugal pump and mixer.In applications where zero leakage is important, such as when dealing with toxic or volatile liquids in the manufacture of chemicals, plastics or pulp and paper, double sealing adds additional safety.
The double seal of Chesterton consists of two pairs of sealing rings.Outside pair (blue-Green) rotate with the shaft;The inner pair (orange) is fixed and contains the channel (yellow) of the barrier liquid (blue ).The seal limits the process fluid (light blue) to the left area.
The cone is visible along the lower edge of the inner sealing ring and the corresponding surface covered by the shaft.A double seal usually consists of two stationary and two rotating rings, and includes a barrier fluid, which is circulating within the seal and remains under pressure above the process fluid.Barrier fluid also acts as a coolant to eliminate the heat generated when the sealed fixing ring and the rotating ring rub against each other.
Usually, water is a barrier fluid, but other substances such as ethylene glycol or oil can also be used.One problem with the double seals is that the cooling action is not always as effective as it is possible.In order to supplement the laboratory test, the engineer ofW.
Chesterton Co.
At school, get up.
, Applying computational fluid dynamics to their heavy-Double sealed working cartridge.They want to study the circulation of the inner barrier fluid in more detail than the laboratory test allows.Chesterton is a manufacturer of mechanical seals, mechanical packaging and liners, and process pumps.
Its products also include hydraulic/pneumatic sealing devices, repair chemicals and materials for arc components.Engineers at Chesterton expect that improving the circulation of barrier fluids will improve cooling efficiency and extend sealing performance.Using the computational fluid dynamics software to simulate the flow of fluid within the seal, they determined that the barrier/coolant fluid did not cycle well to the sealing area that generated heat.
After evaluating some design changes using a computer model, the engineer was able to improve the axial movement of the barrier fluid, thus increasing the heat removal by nearly 50%.Chesterton has now made design changes in many of its products.Generally, mechanical seals are made of two sealing rings, one made of soft materials such as carbon graphite, and the other made of hard materials such as sic.
When pumping the process fluid, a ring rotates with the pump shaft.The other ring is not moving.The interface between the two rings establishes a seal to prevent leakage of the process fluid.CFD images describe the axial circulation of a typical non-cone seal design (left) seal barrier fluid and the improved circulation caused by the cone surface design (right.
Changes in the flow mode resulted in an improvement in heat removal from 0.7 kW to 1.1 kW.When dual seals are used, barrier fluid can be pumped into the seal from a separate tank through the inlet.The fluid then cycles through the seal, acting as a barrier and a shock absorber.
The fluid leaves the seal through the outlet and flows back into the tank.The tank is cooled by natural or forced convection.Heat is usually harmful to the life of the mechanical seal, but friction creates a lot of heat because the rotating and stationary sealing rings are in contact with each other.
If the seal is not fully cooled, the heat will distort the friction surface, causing the unit load to be concentrated in several areas rather than evenly distributed across the entire interface.The concentration of pressure can lead to excessive wear and shorten the life of the seal.In addition, elastic O-The ring used to seal the ring to other parts must allow the ring to move slightly if the seal is to be carried out normally.
The O-The ring performs best at a colder temperature and will harden if it is too hot.If they become hard, they can prevent the necessary movement of the sealing ring and eventually cause a leak.The first step is to know exactly what happened to the coolant inside the existing seal.
This insight is not possible from conventional laboratory tests that can only provide the temperature of the fluid at the inlet and outlet.To understand the temperature inside the seal and the flow pattern inside, the engineers turned to CFD simulation.The software for CFD analysis is Fluent from Fluent Inc.
in Lebanon, N.
Chesterton\'s double sealing effect: the artist\'s rendering shows a section view of the seal installed on the shaft attached to the fluid mixer in the work.The engineer simulated 48-mm-The centrifugal pump shaft with a diameter of 3,600 rpm rotates.It is assumed that the initial operating conditions of the pump process fluid are 687 kPa and 66 °C, and the inlet temperature of the barrier fluid is 1,031 kPa and 38 °C.
These numbers are all within the intermediate range of the operating conditions of this design;Then, the operating conditions of the simulation have changed to a certain extent.The variables studied include the radial clearance between the stationary boundary and the rotating boundary, the cone angle of the flow control surface, the axial speed, the blocking fluid through the flow, and the key thermal physical properties of the fluid.Fluent simulation includes threeSize model of about 160,000 units and on two unitsProcessor machineIn order to achieve the expected level of accuracy, the convergence of the analysis usually requires about 2,000 iterations.
The simulation results are processed using IBM\'s visual data browser software.All CFD and data visualization analysis run on a Silicon Graphics workstation.The simulation results clearly show how seal cooling can be significantly improved.
Barrier fluid enters the seal of the inlet port through the flow channel, which is centered axial between heat sources.Therefore, the cooling efficiency depends on the axial circulation of the fluid to heat-The production area where the sealing ring meets.Graphical representation of CFD results, including color-Coded vectors indicating fluid temperature, speed size, and flow direction show that the circulation to seal these critical areas is limited.
The trajectory of fluid particles released near the inlet of the sealing barrier fluid is described as a ribbon in CFD rendering.The next step is to find ways to improve the axial circulation of barrier fluid.CFD simulations are used to evaluate the effectiveness of each modification considered.
It is found that the biggest change in the axial circulation is to gradually refine the boundary surface of the sealing ring and the shaft sleeve.The axial cone surface pushes the cooling fluid from the flow channel to heat-The origin of the seal.Compared to the original design, the tapered surface is more effective in promoting axial flow.
As expected, increasing the axial flow can better remove heat.The improved design shows that the heat removal is about 50% more than the original configuration1.The heat dissipation ratio of the new design is 1 KW.
The old 7 KW heat is removed.
The engineer also uses CFD to determine the physical mechanism responsible for improving the design performance of the tapered surface.The simulation results show that the fluid near the shaft sleeve experiences a strong centrifugal force outward from the radial direction of the center of rotation.In the case of coneSurface design, this radial load has an axial-oriented assembly that flows the fluid from the end of the fluid seal and the heat-generating area.
A graphical representation of the kinetic energy of the flow turbulence shows a relatively high turbulence area near the sealing interface and near the internal rotating wall, closer to the flow channel.This situation is advantageous because higher turbulence and increased mixing help to facilitate heat transfer where it is most needed.In order to better understand the properties of fluid axial exchange and associated thermal energy, the trajectory of fluid particles released near the inlet was calculated and displayed as a flow band colored by fluid temperature.
The flow trajectory is then animated and the distorted band represents the local level of turbulence or vortex.The animation provides three detailedDimensional perspective of spiral flow pattern features of cone-surface design.One of the techniques used allows observers to walk the course of fluid particles.
The presentation shows the local speed and temperature of the particle, as well as the elapsed time after the particle is released.In 55 milliseconds at the beginning of the animation, the temperature-The low-pressure induced Mapping trajectory can be seen around the exit on the route to the sealing interface area.About one-After two seconds, a path of the accelerating particle follows the rotating radial end-wall trajectory of the domain.
At this point, the particles absorb enough heat, causing the temperature to rise by 12 °c.After absorbing heat from the sealing interface, the temperature reaches 62 °c and reaches the highest temperature along its path.In the process of returning to the flow channel, when the particles move near the inner boundary of the stationary sealing ring, the speed drops by about 25%.
To verify the accuracy of CFD results, physical experiments were conducted using various flow rates, rotational speeds and fluids in the Chesterton sealing test laboratory.When the fluid enters and leaves the seal, measure its temperature and then compare this data with computer analysis.A representative case shows that, from the inlet to the outlet, the predicted fluid temperature in the flow channel area of the cone-shaped surface design has risen by about 11 °c.
The corresponding laboratory data showed that the temperature increased by 10 °c in the range of 1 °c or 10% for CFD simulation.Images from CFD simulations show the path of barrier fluid particles passing through the seal, depicting the flow-belt trajectory when the fluid returns from the sealing interface, reaching the highest temperature.This work led to a design change in some of Chesterton\'s products.
It is not possible to achieve a tapered surface design on all the mechanical seals of the company, because it takes a certain amount of radial space to provide the taper.Some pumps do not have enough space to hold seals large enough.It is true that the seals containing the new design perform well on site and operate at cooler temperatures, which will result in a minimum 30% extension of sealing life.
They also do a good job in the market, and in the usually highly conservative industry, they quickly became one of the company\'s three or four top product lines.The company\'s patent application was finally passed.The new seal design was patented in last August.
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