Q.  How can a gasket be designed to provide long-term sealability, as required in an HF Acid Unit?


A.   By focusing on the gasket seating stress.  All sealing depends on gasket seating stress.  The gasket has to be “squeezed” in order to seal.  The amount of gasket stress developed must be high enough to achieve an initial seal upon installation - plus additional stress to offset two additional factors: A) long-term relaxation, and B) Thermally induced load changes.


Q.   How does the ALKY-ONE gasket compare to other gaskets in this regard?


A.   As shown in the graphic above (click on the graphic for a larger image), for the 150# sizes shown, the ALKY-ONE gasket develops an average of 2.0 times the gasket stress as other commonly used gaskets.  While all gaskets may be able to achieve an initial seal at start up, it is this additional stress that assures that the ALKY-ONE will still be sealing tightly years after installation.

“Sealability is
all about
gasket stress.”

Sealability - it’s always important, no matter the flange or application.  But in HF Alky Units, sealability isn’t just important - it’s critical.   

Hydrofluoric acid is an essential chemical catalyst in the formation of useful alkylates from short-chain (gaseous) hydrocarbons.  As such, it plays a crucial role in modern refining processes.  The problem?  Hydrofluoric acid does not play nice!


HF Acid is one of the most dangerous acids known.  It is so lethal that the Centers for Disease Control lists it as a potential Chemical Terrorism Substance.  OSHA limits exposure to just 3 parts-per-million over an 8-hour shift, while NIOSH classifies it “Immediately Dangerous to Life or Health” at concentrations of 30 parts-per-million or higher.  It is unique among acids in its ability to pass directly through the skin to the underlying tissues and bones.  Once there, it not only liquefies the tissues and erodes the bones, but the fluorine anion hijacks the body’s neurochemical processes, causing a cascading failure of essential systems.      

Because of this extreme hazard, the only acceptable standard is ZERO leaks:

ZERO leaks at start up

ZERO leaks during plant upsets

ZERO leaks regardless of the number of startups and shutdowns


This standard requires:        Fugitive Emission Control          

Sealability at Start Up                 

Long Term Sealability

The Flange Assembly Demonstration Unit (FADU) pictured at the right measures and displays
real-time gasket compression in response to the applied bolt loads, giving us a peek at the
actual dynamics that occur when a gasket is tightened in a flange.  The FADU allows us to
easily compare the compressive responses of different gaskets in the same environment to
see which provides the more positive seal under initial bolt-up conditions.


Clicking on the thumbnail to the left will show you one of those
comparisons.  The top line is the compression/load line for the ALKY-ONE
gasket.  The bottom line is the compression/load line for a common
competitive gasket - a spiralwound gasket with an inner cam-profile ring
with PTFE faces.  Take note of the amount of bolt load required to bring
the gasket to full compression - that point in which the gasket reaches a
flat line.  The ALKY-ONE is fully compressed at about 28,000-psi bolt

load, whereas the competitive gasket requires about 53,000-psi.  Since the Yield Strength of a B7 stud is about 106,000-psi, the ALKY-ONE gasket will come to full compression at just 26% of yield, whereas the competitive gasket requires almost 50% of yield.   Many corporate bolt-up standards use 50% of stud yield as their target load - barely enough to compress the gasket to a point of stability!


When fully tightened to Chevron’s recommended 120 ft.lb, how much gasket seating stress do these two gaskets develop?  Due to their differing geometries, the results are astonishing.  Here they are:


At full torque, a 4” 150# spiralwound gasket with a cam-profile inner ring with PTFE facing materials will develop an average full-width gasket seating stress of only 8,890-psi - well under the industry recommended minimum of 12,500-psi for sealing spiralwound gaskets.  


And the ALKY-ONE?  At full torque it develops 18,310-psi of gasket stress - over twice as much!    


Leak-Free Start Up


You now have the option to choose a gasket designed to start up leak-free!  The ALKY-ONE gasket comes to full compression far before competitive gaskets.  At any given torque it is designed to develop several times the gasket seating stress.  


Seals faster…   Seals tighter…       Pretty much a no-brainer, isn’t it?

Fugitive Emissions

Certificate          Test Summary

Fugitive Emission Control

Sealability at Start Up

Long Term Sealability

Click for larger image

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With an OSHA limitation for HF Acid exposure of just 3-parts-per-million over an 8-hour shift, fugitive emission control is more than just an LDAR buzzword.  What styles of gasket construction can achieve “parts-per-million” sealing?  Which can’t?  


In our NPRA paper, we discussed recent test information showing that spiralwound gaskets - made to today’s standards - could not reliably seal below 50 parts-per-million without contacting the outer guide ring.    


In contrast, the KAG style of gasket - the same as used in the ALKY-ONE gasket - has been tested for fugitive emissions, and has passed that test with an average leak rate of just 1 part-per-million!  The Fugitive Emissions Certificate and Test Summary are posted here for your review.

 

It is important to admit that as impressive as these results are, they are only one aspect of sealing.  Comprehensive sealability must also encompass real-world sealability at start up as well as long-term sealability under typical operating conditions.

In Texas, a prominent HF Service Provider reports that upon unit start up - even when the gaskets they install in a turnaround come up leak-free - they see major leaks occurring in flanges they had not serviced.

Click for Larger Image
Leak-free performance is not just a matter of sealing when the gasket is installed. The gasket must continue to seal month after month, year after year.  It must continue to seal in spite of plant upsets.  It must continue to seal regardless of the number of shutdowns and restarts.