The right chemistry: Why choosing the right mechanical seal and support system is vital to risk management in ATEX environments

How can the mechanical seal and support system protect the atmosphere while mixing aggressive and flammable liquids and reducing risk in atmospheres explosible (ATEX) environments?

The top-entry mixer and similar agitating equipment are standard for the efficient blending, stirring and homogenisation of liquids in tanks and vessels in multiple industries.

Similarly, John Crane customers in chemical processing, pharmaceutical production and other industries, using top-entry mixing and agitating equipment favour John Crane vessel and agitator cartridge seals such as the Type CK725, CK726, CK728, CK736, CK738, the Type 32 and the Type 5280 agitator seal ranges.

But what about the supporting technologies, such as the seal support system? How do we ensure what we use meets the ATEX directives' specific regulatory requirements?

What is the context?

Unlike pump mechanical seals, which sit inside the pumped liquid for top-entry mixers, an optimised mechanical seal will sit on top of the vessel and work in a vapour space. That creates a risk, as sources such as static electricity and surface temperature can ignite the vapour.

So, how does that work?

Managing volatile elements means considering two principal factors: choosing safe equipment, namely seals and supporting systems, and creating ATEX-compliant environments.

The international standard IEC 61140 and its British equivalent, BS EN 61140, use Class I and II to classify electrical equipment safety. They rely on an alternate path for electricity and double insulation, respectively, to ensure safe operation.

Meanwhile, ATEX Zones 0, 1 and 2 classify environments. The increments signify the increasing likelihood of an explosive atmosphere, from Zone 0 (continuous presence of flammable gas or vapour) through to Zone 2 (a short-term or unlikely presence). The T Class determines the autoignition temperature of the product in the mixing vessel.

Most of the risk guidance is prescribed, but other tangible variables, such as sealant gas and construction materials, process temperature, speed and pressure, also contribute to the risk equation.

Seal faces: Their role in a safe environment

Proprietary systems moderate sealant temperature, which, in turn, controls the temperature on dual seal faces. While a wet system is typically suitable for higher pressures and speeds, dry-running vessel seals are uncooled and can therefore only run in lower energy applications.

In this case, temperature rises at the seal faces are calculated through extensive testing and applied across a wide range of applications, often improving reliability. Contamination-averse environments often use non-contacting gas seals, which create no heat themselves.

Support systems: A key role

A liquid reservoir system, such as the GR1, GR2 or GR3, utilises a thermosyphon effect or optional circulating pump to circulate buffer or barrier fluid to support wet-lubricated dual mechanical seals in mixing vessels. For the highest specification of support system, or when API 682 specification is required, the John Crane Plan 53A pump seal support system is compatible for both API Plan 52 and 53A.

Meanwhile, a seal gas supply system, such as the John Crane GP-S/GP-D, can offer additional reassurance against leakage and containment issues. This control panel conditions and regulates the gas supply for dry gas seals in pumps and compressors. Meanwhile, a seal gas supply system such as the GP/S would be used with both dry-running and non-contacting seals to provide the supply of clean gas required to keep the seal running.

The API Plan 74

For the highest specification of support system, or when API 682 specification is required, the API Plan 74 system provides the highest levels of monitoring and premium materials and components, enabling a non-contacting, gas-lubricated seal to contain volatile or hazardous fluids with a high degree of reliability, and, crucially, with zero-emissions.

Category 1 applications require at least two separate forms of instrumentation, such as a filter regulator, pressure gauge and flow indicator, while Category 2 applications need only one.

What does all this mean?

Seal faces, whether wet, dry-running or non-contacting, are a key component and an essential element of the product mix.

There is no one-size-fits-all solution to managing the risk posed by the gaseous environment on a top-entry mixer. Instead, it is a situation-specific equation that operators solve through product selection.

That means selecting the seal, support system and additional instrumentation that suit the situation in terms of environment-specific factors, such as temperature, pressure and speed.

Learn more

Our portfolio of high-performance cartridge, vessel and agitator seals includes models mentioned here. We also offer products and solutions for specialist sectors such as the chemical industry.