Hello, I have extensive experience with gas seal installations, but I am currently facing an issue that I can't seem to resolve. The new gas seal we just installed on a re-boiler pump, which serves to reheat the product being boiled off in a distillation column, is causing problems under heavy vacuum. The nitrogen gas flow to the John Crane 2874 Double Cartridge seals on the Durco Mark II 3X4X8 pump increases significantly during heavy vacuum, leading to a vacuum system breakdown. Despite inspecting the seal for damage and finding nothing significant, the production engineer is pushing to replace it. Have others encountered similar issues with gas seals in vacuum service applications?
A seal serves its purpose but may not have enough evidence to determine any issues. Are there pressure gauges to monitor the seal's performance? It's important to consider the seal's capabilities in handling specific pressures. For instance, a double seal with one stationary seat on each end and a rotating element in the middle may operate differently depending on which seal fails. Checking for balancing holes in the impeller and listening for unusual sounds can provide clues to any potential problems. It's also essential to account for the pressure generated by the product being pumped. Analyzing various factors such as process data, gauge readings, and seal pot pressure can help troubleshoot any seal issues. Additionally, make sure the seal is suitable for handling nitrogen gas, a dry type of gas that could impact the seal faces if not appropriately sealed.
Thank you for your email, Dave. I apologize for the delay in responding. I have been analyzing trends in our DCS to identify the cause of the heavy N2 leak. It appears that during deep vacuum operations, there is an increase in N2 flow rate, while during normal operation, a high amount of N2 flows through the seal. After discussing with the production engineer, it seems that we are pumping multiple fluids at or near their vapor pressure, leading to off-gassing and vapor entrainment issues during deep vacuum. Our double seal in Tandem operation consists of two stationary rings and two rotational rings made of Tungsten on Carbon. The size of a dry gas seal is larger than a wet seal to provide enough surface area for hydrodynamic lift. The seal gland covers the connection port to the seal chamber, which is why I have requested it to be machined to accommodate a 3/8" pipe for a pressure gauge attachment. By monitoring the stuffing box pressure, we can better understand the seal pressure correlation. While we have calculated the stuffing box pressure theoretically, there may be discrepancies. Balancing holes could help remove gas from the seal cavity, but I would need assistance from an expert in this area for our current pump. John Crane recommends maintaining N2 pressure 30-50 psig above the seal chamber pressure, although our in-house N2 pressure is limited to a maximum of 40 psig. Booster pumps have been installed to increase pressure, although their reliability is a concern. The nitrogen gas used is filtered with a coalescing filter to remove moisture and is suitable for the seal. Despite not seeing significant damage, it is possible that the seal faces are being pulled open, causing the leakage issue. For further details, please refer to the attached Seal Drawing document.
For more information, please refer to the documents provided below: - Check out the impeller with "balancing holes" on Page 5 of this document: https://www.sulzer.com/en/-/me...ngleStage_E10083.pdf - Find out about the stuffing box pressure difference with balancing holes on Page 8 of this presentation: http://peg-az.com/wp-content/u...TAR-PRESENTATION.pdf Another option to consider is a throttle bushing, which is a machined flat washer that is mounted on the back side of the impeller. This component helps maintain stable stuffing box pressure by providing clearance between the rear cover shaft opening. For more details on how throttle bushings work, check out this article: http://www.fluidsealing.com/sealingsense/aug08.pdf In addition, if you are dealing with high temperatures, be sure to account for vapor pressure in your calculations. Based on my calculations, the Total Dynamic Head (TDH) should be around 17'. To ensure a 30-50 psi increase, you should aim for a minimum of 47-50 psi to be 30 psi higher than the discharge pressure. If you have any further questions or concerns, feel free to reach out. Thank you, Dave.
I've had a similar experience with a Double Cartridge seal during heavy vacuum operations. It's possible your issue lies in the seal support system rather than the seal itself. The seal might be fine, but due to heavy vacuum, seal support systems are typically subject to more challenging conditions that can lead to failures. As you've mentioned a significant increase in nitrogen gas flow, it could be an issue with the flow control mechanism, potentially a faulty valve or regulator misreading system pressure. Have you cross-checked the flow meter calibration and seal plan configuration to confirm they're set up correctly for this application? That could be a good place to start before considering seal replacement.
Based on my understanding, it seems that the seal you're using might not be designed for heavy vacuum applications. The increase in nitrogen flow could be a response to maintain the seal integrity under vacuum conditions. This could lead to system overload and subsequent breakdown. Before considering replacement, it might be worthwhile to check the specifications of the seal in relation to vacuum operation. You need to ensure that the seals you're using are rated for the vacuum level you're running in your setup. If they're within the specification and still having problems, replacement could be necessary. Alternatively, the pump internals, alignment, or even the seal support system could also be contributing to the problem, so don't rule that out.
I've come across a similar issue in the past, and it turned out that the problem wasn't with the seal itself, but rather the pressure differential across the seal. This increased during heavy vacuum, causing the nitrogen flow to rise. Before replacing the seal which might not be necessary, I'd suggest checking your pressure control system. Ensure it's responding adequately to changes in vacuum levels. Additionally, consulting with the seal manufacturer could give you specialized guidance as they might have dealt with similar problems in various applications.
From my experience, the issue could potentially be stemming from the design of the seal support system, rather than the seal itself. It's important to note that gas seals in heavy vacuum applications require a pressure control system that's notably fine-tuned. I'd have a close look at your pressurization system settings, maybe ensure the supply of nitrogen gas is consistent and that the flow isn't being affected by any possible faults or contamination. Also, do cross-check that the system pressure is appropriately higher than the seal chamber's maximum pressure to avoid gas venting into your pump. A system diagnostic might help you pinpoint any performance-related anomalies. Don't rush to replace the seal, it might not be the root of the problem.
I can totally relate to your frustration with gas seals in vacuum applications; they can be tricky beasts! In my experience, excessive nitrogen flow under heavy vacuum often points to an imbalance in the pressure differential or possible wear on the inner seals that might not be immediately visible. Have you considered checking the alignment of the pump or reviewing the system for any airflow obstructions that might be affecting the seal's performance? Sometimes the issue could be upstream, impacting how the seal maintains its integrity. Good luck!
I've encountered similar issues with gas seals in vacuum applications, and it can be quite tricky. It sounds like the vacuum might be overwhelming the gas seal design, leading to excessive nitrogen flow. One thing to consider is whether the seal is properly configured for the specific vacuum conditions you're experiencing; sometimes, seals need adjustments or specific settings to handle extreme conditions. It might also be worth checking if there’s a way to optimize your nitrogen supply to alleviate the pressure on the vacuum system. Have you thought about reaching out to John Crane for their insights on performance tweaks specific to your setup? They often have valuable recommendations for these kinds of scenarios.
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Answer: Answer: Common issues include increased gas flow, system breakdowns, and potential seal damage under heavy vacuum conditions.
Answer: Answer: Troubleshooting steps may include inspecting the seal for damage, checking gas flow rates, and consulting with production engineers for potential solutions.
Answer: Answer: Challenges can include managing gas flow during heavy vacuum, ensuring seal integrity, and avoiding system breakdowns that may impact production processes.
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