Upgrading wet-running mechanical seals to dry-running systems for greater reliability

Mechanical seals play a crucial role in the chemical industry, protecting the atmosphere while processing mixtures of aggressive and sometimes toxic chemicals, and delivering the highest level of reliability.  

The seal on most top-entry mixing vessels sits in a vapour space below the tank's top cover, above the liquid. While this helps separate the seal from the product, an additional physical barrier offers extra protection.  

Traditionally, that means installing dual, back-to-back wet lubricated seals, such as the John Crane CK725, CK726, CK728 and the 7800 series of high-performance modular mixer and agitator seals.  

There are several similarities and differences between managing a wet-running seal and a dry-running seal. In this blog, we explore how the more recent technology supports a safer and more efficient chemical industry.  

Wet-lubricated mechanical seals 

Beyond needing a process-compatible fluid to lubricate the faces, a wet-lubricated seal requires other elements to function effectively. 

The seal support system should include a pressurised tank and instrumentation to measure level, temperature and pressure. The system’s positioning and condition directly impact seal reliability.  

The shifting regulatory landscape drives the constant evolution of products and solutions, posing challenges for older seals, support systems and those responsible for managing them.  

Older seals are often not reverse-pressure capable, and wet-running support systems do not meet the standards needed for modern certification. This ongoing, incremental strengthening of quality requirements may be one reason the industry began using dry-running seals such as the CK725. 

Dry-running mechanical seals 

Chemical processing demands predictable performance, especially in strictly controlled processes. Modern design standards mean seal faces typically require minimal amounts of readily available plant nitrogen, with little risk of contamination.  

A dry-running seal may be more suitable for applications within the chemical industry. The self-lubricating carbon seal face minimises contamination, and the barrier agent for management containment, plant nitrogen, is widely available.  

Meanwhile, dry-contacting seals minimise impact on the product inside the vessel, thanks to design features such as the debris well. Being less reliant on complicated support systems is another plus. However, improved reliability is the major plus point.  

The simpler seal requires less monitoring, reduced input from plant operators and offers an extended mean time between repair, compared to older wet-running mixer and vessel seals. 

Wet versus dry seals. Is the debate over?  

Wet-lubricated seals had previously dominated mechanical seal technology in the chemical industry. Using a barrier fluid mitigated the increased risk of contamination, and a complex seal support system helped ensure reliability.  

However, dry-running seals were engineered for this environment, and the inert nitrogen gas barrier reduces contamination behind batch losses, thereby improving reliability. 

Discover more about how we support the chemical industry, and how our dry gas seal technology enables cleaner, safer and more efficient processing.