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Liquid-Lubricated Double Seals for Increased Stability in High-Speed Pumps in PTA Facilities

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Purified Terephthalic Acid (PTA) is the predominant raw material for production of high-purity polyester resin widely used in the production of polyester fiber, polyethylene terephthalate (PET) bottle resin, polyester film, and engineering plastics.

Sundyne Pump in operation

Sundyne HMP-series centrifugal pumps have been the preferred choice for HP Reactor Feed pumps at PTA purification plants all over the world for many years. These integrally geared high-speed centrifugal pumps are used during the purification stage of crude terephthalic acid (TA). Sundyne pumps deliver TA slurry, containing TA powder suspended in demineralized water at a high temperature into a hydrogenation reactor, where contaminants are removed from the solution via reaction with hydrogen.

Operational reliability of the HP Reactor Feed Pump is critical for maintaining stable operation of the PTA purification plant, and mechanical seals are among the most critical pump components due to high-speed and high-pressure service requirements.

For this application, a centrifugal pump, configured as a horizontally mounted, integrally geared two-stage pump that has a single double-ended output shaft at rotational speed of 6,200 rpm with impellers attached on each end was selected. The two stages are piped up to operate in series to develop the required head rise, and the first stage discharge feeds the second stage pump suction boosting the stage 2 seal chamber pressure up to 1,160 PSI (80 bar).

The seals for the application were engineered as a cartridge design double seal face-to-face arrangement for stage 1 and as a tandem oriented face-to-back dual seal arrangement for stage 2 which allows a split of the total differential pressure between two seals and maintains suitable pressure velocity (PV) parameter levels.  The seal support system utilizes flush supply to both pump stages protecting the product side seals from plugging with TA slurry.   

Technical Challenges

One of the main technical challenges pertained to the barrier/buffer fluid that was presented by the end user, one of the world’s largest PTA producers who remains proactive on maximizing the efficiency of their PTA process technology. Instead of using the more common ambient temperature demineralized water as a barrier/buffer liquid that is usually supplied from the PTA plant centralized seal support system, it was requested to utilize plant return water at normal temperature of 158 ˚F (70 °C), with a maximum temperature of 176 ˚F (80 °C). The potential problem with using plant return water under these conditions as barrier/buffer liquid is an adverse seal environment such as inadequate heat dissipation and poor lubrication of the seal faces due to a loss of fluid film caused by vaporization.

An additional technical challenge was reverse pressurization of the stage 2 process side seal during the pump startup and shutdown. During the startup sequence, this seal is reverse pressurized by the buffer fluid introduced into the seal support system before the pump main driver is turned on. Under transient conditions, while the pump is ramping up to full speed and reaching full discharge pressure, the pressure applied to the seal is reversed causing the seal to hang-up. The same occurred in opposite order during pump coast down to shutdown. The original seal design was modified to incorporate new features to overcome seal hang-up associated with the secondary seal.

End-to-End Expertise Required

“In view of this demanding specification, we could clearly see at an early stage that in-depth know-how was needed - and that we would have to design a special application-specific solution,” said Eric Vanhie, project leader at EagleBurgmann in Houston, Texas.

Once the performance specification had been drawn up at a kick-off meeting between Sundyne and EagleBurgmann, the development and design stage started at EagleBurgmann’s headquarters in Wolfratshausen, Germany. The operating points were analyzed in detail with the R&D department. This provided precise performance calculations and a computer-aided design for the sliding elements.

The new double seals were based on a proven special high pressure seal from the existing product portfolio. Specifically, the team opted for the highly efficient high pressure seal SH(V), which has been used in high pressure applications for years. In contrast to conventional mechanical seals from the standard range, high pressure seals have one important special feature: in these seals, the seat rotates on the shaft while the seal face - with its spring backing - is stationary in the housing. This seal concept provides additional stability at high speeds. At sliding velocities of 66 ft/s (20 m/s) or more, the springs should be stationary so that they do not absorb vibrations and thus deform.

Optimized Design

The design of the proven SH(V) seal was also optimized for this application. Specific design improvements were made to guarantee stable running across the entire operating range. These included the use of ultra-high performance materials. While the regular SH(V) seal uses the proven silicon carbide ceramic material for both seal face and stationary seat, the stationary seal face for the Sundyne solution was based on the silicon carbide variant BuKa 30. This ultra-high performance material from EagleBurgmann has a high carbon content, making it the perfect solution for media with poor lubricating properties, such as water. BuKa 30’s effective emergency running properties and tolerance to dry running also make it an optimal choice in this case.

EagleBurgmann SH Double seal in tandem arrangement. Yellow parts are rotating, blue are stationary, gray parts show pump shaft and housing.

The seal was further optimized to guarantee functional reliability, even in the marginal ranges. A loosely-fitted seal face provides additional safety against tipping and tilting. One of the special technical features of the high pressure seal developed for the PTA application is the incorporation in the seal faces of high-precision grooves calculated by R&D. The depth and geometry are specified with incredible accuracy.

The grooves that EagleBurgmann has been using for years in the dry gas seal field also support safe operation of liquid-lubricated double seals in critical phases. At low pressure, the grooves promote lift-off of the seal faces by creating a positive pressure cushion, and thus very quickly establish a stable operating state.  At high pressure, the grooves have a stabilizing effect as they prevent the gap from opening further.

Proven in the Test Field and in Practice

Combining all of these measures resulted in sophisticated sealing systems in both tandem and back-to-back versions. These cover the range of applications up to 1,450 PSI (100 bar) and 9,000 rpm and guarantee the functional reliability that Sundyne required. The liquid-lubricated double seals easily cope with all operating parameters, and their constant sealing performance is impressive, even when exposed to considerable pressure, temperature and speed fluctuations.  This was tested and confirmed in dynamic test runs in the test field in Wolfratshausen, where the seals, which are designed as easy-to-fit cartridge systems, were extensively tested and then accepted by the Sundyne engineering team.

Test rig at EagleBurgmann R&D center in Wolfratshausen, Germany.

The new double seals have proven their worth in the many Sundyne HMP model integrally geared pumps that were brought into service in China in 2014 in one of the world’s largest PTA facilities.

For more information on SH(V) seals, contact us at info.us@eagleburgmann.com.

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