How to design and monitor quench systems for seals?

Properly design and monitor quench systems for optimum seal reliability and to ensure that moisture does not enter the bearing housing. The proper function of seal quench systems is to remove solid particles from the lower seal face to prevent premature seal wear.

The term ‘quench’ originally came from the use of steam to buffer the outer seal chamber, between the seal and throttle brushing, to dilute the hydrocarbon fluid leaking from the seal to a safe non-flammable level. Today (2010) this practice is not acceptable and dual seals are required to be used for all hydrocarbon services.

When a quench is used today (2010) either steam (where solid hydrocarbon particles can form) or water (where water soluble particles can form) are used. It is solely for the purpose of removing solid particles from the lower seal face. Both of these alternatives expose the bearing housing to entrance of water vapor which will impact bearing reliability and MTBFs.

Reliability of mechanical seals

Proper system design to control the amount and condition of the quench fluid and the mandated use of a bearing bracket oil condition monitoring bottle is essential to the reliability of mechanical seals employing quench systems.

A quench (known as an auxiliary flush plan) is a flush plan that uses a medium (steam, nitrogen or water) on the atmospheric side of the seal to wash away any solids buildup from the faces. The buildup is drained to a collection system.

A quench is most commonly used in single seals with steam as the medium, if the seal fluid is hot and can form coke particles, and/or if the seal fluid is flammable or toxic.

Note: Many countries today require dual (tandem or double) seals to meet environmental requirements if the seal fluid is flammable or toxic. The steam should be regulated to a pressure of approx. 20 to 33 kPa (3 to 5 psi), which is just enough to wash the solids accumulation off the atmospheric side of the faces. It is essential for the steam to be superheated (dry), to prevent flashing of water at the faces, causing premature failures and to ensure that moisture does not enter the bearing chamber.

We have experienced plant fires as a result of water contamination in the bearing housing resulting in a hot bearing which served as an ignition source for a single seal leaking a flammable vapor. We recommend that an ‘oil condition bottle’, to  monitor water in the oil, always be installed in the bearing housings when a steam or water quench is used.

Failure to regulate quench fluid conditions and to monitor the operation of seal quench systems has resulted in low mechanical seal MTBFs (lower than 12 months). In one case, it resulted in a refinery fire when the steam quench became saturated and displaced all of the oil in a pump bearing bracket. An excessive hydrocarbon seal leak was ignited by the ‘red hot’ bearing bracket.

This best practice has been used since the mid-1990s when I was faced with contamination of pump bearing brackets with saturated steam from malfunctioning steam quench systems in refinery service. Since that time, this best practice has optimized mechanical seal quench system safety and reliability.