April 2020

CHANGE-HF for HF Alky Sealing Prevent flange face crevice corrosion in hydrofluoric acid alkylation service with the

Change-HF gasket.

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CHANGE-HF for HF Alky Sealing Prevent flange face crevice corrosion in hydrofluoric acid alkylation service with the Change-HF gasket.

Alkylation is a process reacting light olefins with isobutane to produce alkylate, an intermediate product used for high-octane gasoline blending. Hydrofluoric acid is one type of liquid catalyst used to accelerate the reaction. Carbon steel is the standard specification for flange metallurgy in HF alkylation facilities and preferred over a cost-prohibitive exotic alloy. However, despite carbon steel’s compatibility, a phenomenon known as crevice corrosion is known to occur in an HF alky unit.

Crevice Corrosion Flange face crevice corrosion is corrosion that initiates in the gaps and small spaces, or crevices, left by

standard gaskets. Historically specified HF alkylation gaskets are spiral wound gaskets conforming to ASME B16.20 dimensions and restrictions of having a 0.175” thick spiral wound gasket with 1/8” thick inner ring. Common materials are Monel winding, graphite filler and either a carbon steel or virgin PTFE inner ring. The standard 1/8” thick inner ring leaves a gap between the ring’s ID and the bore as illustrated in Figure 1. Natural flange bending upon compression also ensures a gap above and below the inner ring as depicted in Figure 2. This ever common setup is normally not problematic, but when paired with HF acid and carbon steel flanges, crevice corrosion such as that shown in Figure 3 may occur.

Figure 1. Standard spiral wound

with inner ring – crevice at ID Figure 2. Crevices above and

below spiral wound inner ring Figure 3. Crevice corrosion at the

ID of a flange

Seal integrity is eventually compromised when corrosion reaches the spiral wound sealing element and beyond. When a leak occurs, plants always consider risk to personnel and the surrounding environment. However, demands on production typically prohibit shutting down for repairs. To maintain operability while managing risk, refineries apply an industrial clamp injected with a sealant that solidifies to contain the leak until maintenance can be performed. It’s common to find dozens of such clamps in an HF alky unit that utilizes traditional spiral wound gaskets described above.

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After years of managing extensive corrosion repair, it may seem routine, but HF Alky maintenance and repair turnarounds (TAR) are lengthened and more costly due to crevice corrosion. Clamp removal & cleaning, bolted joint disassembly & inspection, and flange repair & replacement are all required activities that further extend the life and cost of a TAR. Also consider the wild card of discovering unknown flange damage after disassembly requiring unforeseen repair, cost and delay. Further downtime and productivity loss only adds to already escalating costs. All the above can be eliminated with the right gasket.

Gasketing Challenge To prevent the onset and propagation of crevice corrosion, one must eliminate all gaps and crevices.

Employing a gasket for this purpose will in-turn eliminate corrosion related risks, expenses, and downtime. Achieving this necessitates satisfying multiple components.

1. Maintain fire-safety and use of a semi-metallic primary sealing component for seal integrity and blow-out resistance.

2. To eliminate all gaps & crevices, the entire gasket, from the primary sealing element’s OD inward, must maintain a seal against the entire flange surface to the bore. It must also be maintained through the natural flange bending/rotation associated with RF flanges.

3. Gasket technologies that eliminate crevices have more gasket area to compress. Available bolt load must be adequate while also addressing flange rotation concerns. Class 150 piping systems in particular demand low seating stress components, of which a spiral wound is not.

4. The inner ring must not loosen or fall off during shipping, handling and installation even in cold weather. (Origins of this requirement are virgin PTFE inner rings shrinking in cold weather and falling out.)

Solution Significant developments have been made to gasket sealing technology for HF alkylation units over the

last 5-10 years culminating in the introduction and successful use of a gasket called the Change-HF.

Change-HF

The Change-HF gasket successfully combines two innovative sealing technologies – a Change gasket as the primary outer seal and the use of substantially thick, highly compressive PTFE at the inner ring acting as an interior seal and corrosion barrier. The Change-HF design is illustrated in Figures 4 & 5 below with additional component information to follow.

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Figure 4. Change-HF Cross-sectional illustration Figure 5. Cross-sectional design of the Change-HF

The Change Gasket (Primary Seal)

The primary seal of the Change-HF sitting just inside the raised faced OD is a Change gasket. A Change gasket is a semi-metallic gasket introduced in 2012 and a technological advancement meant to further reduce bolted joint leaks and emissions while overcoming existing gasket limitations. The Change gasket design combines aspects of both a traditional spiral wound (SW) and a kammprofile, improving upon both using reinvented winding wire approximately five times thicker than traditional spiral wound wire. The new heavy gauge metal wire is formed with a functional edge that simulates the serration profile of a grooved metal gasket. The heavy wire is wound like a SW incorporating filler material and is held together via a unique and optimized laser welding process. Stronger, thicker wire combined with laser welding offers significantly improved handling over a SW. The Change gasket is finished with layers of facing similar to a kammprofile. Features are depicted in Figures 6 thru 9 below.

Figure 6. Change gasket cross sectional cutaway

Figure 7. Heavy gauge winding wire & formed edge

Figure 8. Wire’s top edge beneath facing

Figure 9. Laser weld at outer diameter

The Change gasket has been an excellent replacement for a spiral wound where handling and seating stress are concerns (Y of 6,400 versus 10,000 psi for Change and SW respectively). Furthermore, Change is not plagued by a kammprofile’s shortcomings of inflexibility and poor recovery.

The heavy gauge Change wire is energized when compressed but does not plastically deform like SW wire. The result is enhanced usable recovery, or recovery in a live joint. The Change gasket’s spring-like behavior allows it to recover more than other semi-metallic types. Its elasticity makes it ideal for thermal cycling and has also exhibited tremendous benefits when used in piping systems. Class 150 piping systems are

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weaker, have less available bolt load, and are more susceptible to bending loads and pipe strain. Change gaskets are more flexible and forgiving than both kamms and spirals allowing for headache-free sealing in challenging piping systems. For test data on the Change gasket, refer to Appendix A.

Specific to the Change-HF, Monel is chosen for the wire metallurgy and graphite as the soft sealing component. Monel alleviates any material degradation concerns and graphite satisfies the important fire safe requirement set by refineries. The outer ring is painted carbon steel. Details on the specialized inner ring are described next.

Interior Corrosion Barrier & Secondary Seal

The inner ring and secondary seal’s function is to eliminate all gaps and crevices to the bore if one wants to maintain original flange integrity. This can only be accomplished by maintaining inner ring gasket-to-flange contact and sealing through the natural flange bending & rotation that occurs during installation. Used in this manner, the inner ring adds sealing area, especially one taken the bore. There must be enough bolt load to compress both the primary and secondary seals’ full surface area without exceeding bolt and flange limitations for the standard ASME B16.5 Class 150 and 300 HF Alky systems. Note that achieving adequate gasket stress may require high bolt load and therefore result in more flange rotation. The inner ring must maintain contact though that natural raised face flange bending.

To address the matter of rotation and bolt load limited flanges like Class 150, the Change-HF utilizes the low seating stress properties of a highly conformable PTFE called Sigma 600. Sigma 600 is comprised of 100% PTFE fibers restructured and fully sintered to reduce cold flow even at elevated temperatures. Like other types of highly compressible PTFE, 60-70% compression is required to optimally densify the material upon compression.

To achieve this optimum compression, the inner ring is multi-layered. Before explaining the inner ring design, first note that bolt load is naturally concentrated towards the exterior primary seal, not at the inner ring. Once the inner PTFE is compressed to the Change gasket’s compressed thickness of ~0.123”, no additional compression will occur. Although 6mm or ¼” thick PTFE is readily available, it would not be adequately compressed before limiting on the Change seal. Consequently, the Change-HF inner ring has a layer of 1/8” Sigma 600 above and below a 1/32” Monel core. That combination was specifically chosen to achieve sufficient compression of the PTFE, nominally 63%. This level of conformability also allows the inner to maintain contact to flange through its natural bending.

The Change-HF inner ring can also seal against some existing corrosion as long as the corrosion does not extend to the ID of the Change wire and is not too deep. Figure 10 illustrates how the design can resist further flange deterioration.

Figure 10. Inner ring sealing corroded flange.

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Regarding inner ring security, the inner ring will not fall out because the Change is wound and welded directly to it. The Sigma 600 is secured to the inner core with 3M77 spray adhesive. 3M77 is NOT water based and will NOT introduce water or moisture into the process. Note that because the Change-HF inner ring is substantially thicker than a typical gasket, one should take care to adequately spread the flanges on installation so the gasket can be placed without being hindered by the raised face.

Although the Change-HF inner ring is described as being ‘designed to the bore’ for simplicity’s sake, the ID is actually 1/32” diametrically larger than the bore for NPS ½ to 24” and 1/16” larger for NPS > 24” to prevent intrusion. Pipe Schedule or bore dimension is required to finalize the design. For ease of identification in operating plants with multiple bores, pipe schedule is stenciled on the outer ring as depicted in Figure 11. Figure 11. Outer ring stenciling includes pipe schedule.

Multiple pipe schedules within an HF Alky unit is common. If one chooses to standardize on the largest schedule, smaller bore flanges could see flange surface corrosion up to the ID of the inner ring. End users must weigh the potential risk of reduced flange integrity versus protrusion from installing a small bore gasket in a large bore flange.

Reduced Load Requirements

A key feature of the Change-HF is the low load required to compress the entire surface area of the primary and secondary seals. This is particularly relevant to HF alkylation systems designed to Class 150 which is already load challenged. The Change gasket’s minimum seating stress, Y, is 6400 psi. Through compression testing, it was determined that 1900 psi is required to compress the two 1/8” Sigma 600 layers down to the primary Change seal. Once the Change seal is engaged, all additional load is applied to the Change gasket portion. When calculating gasket stress, allocate 1900 psi for the inner ring and the remainder applies to the primary Change seal.

Table 1 indicates the bolt stress required to achieve the designated Change gasket stress for NPS ½” through 24” Class 150 and 300. The 1900 psi stress required for the inner PTFE is included.

Not only can minimum seating stress easily be satisfied on all sizes, but one can even achieve 50% additional Change gasket stress on all Class 150

Table 1. Bolt Stress to achieve stated CHANGE Gasket Stress

NPS Class 150 Class 300

Y* 50% over Y* Y* 50% over Y* 1/2 7,317 10,253 7,317 10,253 3/4 11,794 16,703 7,357 10,419 1 14,091 19,720 8,789 12,300 1 1/4 20,761 28,475 12,950 17,762 1 1/2 26,279 35,396 12,564 17,505 2 25,460 34,792 12,730 17,396

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gaskets without exceeding 50 ksi bolt stress. These values are even more significant when considering that HF Alky units use B7M bolts that have a minimum yield strength of 80,000 psi – much lower than a B7’s 105 ksi.

Lower seating stress components also leave a wider margin to accommodate installation error when targeting higher gasket and bolt stresses especially for Class 300 systems.

(Values represent an inner ring for schedule 80 pipe. Area of the wound portion does not include the wire bead.)

2 1/2 31,045 41,931 10,382 14,023 3 38,194 48,546 18,625 26,248 4 28,821 37,374 22,106 29,839 5 26,700 35,233 26,700 35,233 6 31,573 39,765 23,803 31,223 8 39,727 47,577 24,196 31,600 10 28,724 36,261 20,992 28,569 12 34,965 42,151 22,361 30,631 14 31,835 39,229 20,749 28,582 16 31,448 39,038 21,342 29,508 18 31,773 39,395 22,702 31,024 20 30,112 37,750 23,887 31,878 24 31,815 39,823 21,520 28,769 *Y is the gasket minimum seating stress. 6,400 psi for Change.

Conclusion

The Change-HF gasket succeeds in satisfying all the gasket related challenges of an HF alkylation plant. The Change gasket primary seal of Monel and graphite provides fire safety and seal integrity. The multi-layer inner ring eliminates all gaps & crevices to the bore while maintaining a seal against the flange surface through natural flange bending & rotation. The low seating stress components of both the primary seal and inner ring easily allow sealing to the bore of a load-challenged Class 150 system and leave a wider margin to accommodate installation error when targeting higher gasket and bolt stresses especially for Class 300 systems. And finally, with welding and adhesion, the inner ring stays secure through shipping, handling and installation even in cold weather.

Since its launch in 2015, the Change-HF has eliminated corrosion, clamps and future repair in multiple users’ HF Alkylation units. References available upon request.

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APPENDIX A

Supplemental Change Gasket Information

Change gasket recovery surpasses that of commonly used semi-metallic gaskets. Table A1 includes compression and recovery values for a Change compared to a spiral wound gasket (SWG) and kammprofile.

Table A1. Semi-metallic gasket compression and recovery.

Gasket Type % Compression (18,000 psi gasket stress)

% Recovery (of Compression)

% Recovery (of Initial Thickness)

SWG w/inner 30 26 7.8

Kammprofile 25 6 1.5

Change 30 34 10.2 Even more significant, because the Change wire is thicker through the center, it does not undergo plastic deformation upon compression. Unlike the behavior of 0.007” thick standard spiral wound wire under compression, Change wire remains in its elastic phase. Compression energizes the Change wire maximizing its usable recovery in a live joint. Figures A1 and A2 below depict Change and standard spiral wound wire under compression. Red indicates plastic deformation (not to scale).

Figure A1. Spiral Wound wire compressed Figure A2. Change wire under compression

Thermal Cycling Testing

The sealing industry and some major end users have developed series of tests to determine gasket sealing characteristics and constants. Such tests can be used internally at a gasket manufacturer as part of a quality assurance program or for product development. Certain tests are also specified by the end user for type approval testing. One such test used for over 20 years for gasket prequalification is the widely known Shell Thermal Cycle test. An excellent way to assess a gasket’s useable recovery is to perform this thermal cycle testing.

This end user designed rig and test depicted in Figures A3 and A4 employs two NPS 4 inch ASME B16.5 Class 300 raised face flanges, fitted with internal heating elements. Internal heating best simulates the real

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world compared to moving a blind flange assembly in and out of an oven to heat and cool. Gasket compression is achieved through hydraulically tensioning standard ASTM A193 B16 UNC bolts to a pre-set nominal stress of 290 MPa (42 ksi). Test pressures and temperatures are kept within the B16.5 Class service envelope. An initial room temperature assembly screening test is carried out in which the assembly is pressurized with nitrogen to 51 bar (740 psi). The maximum allowable pressure drop after 1 hour is 1 bar (14.5 psi). The assembly is then subjected to thermal cycling.

Figure A3. Thermal cycle test rig schematic Figure A4. Thermal cycle test rig photograph

To compare Change gasket performance against commonly used semi-metallic gaskets, a well known

oil & gas refiner suggested a test with 24 thermal cycles. The requested test was to simulate the potential temperature excursions of a moderately efficient refinery between major outages (approx. every 4-5 years) with no bolted joint re-torque. The assembly is purged and heated to 320°C (608°F) at a rate of 2°C/minute followed by pressurization with nitrogen to 33 bar (478 psi). The assembly is then isolated and left at temperature for 1 hour after which the pressure is recorded. The rig is allowed to cool to room temperature before the next thermal cycle begins. Each thermal cycle takes approximately 24 hours to complete and is run without stoppage until completion of the cycles. Maximum allowable pressure drop after 24 cycles is 1 bar.

The gaskets were fabricated in house to Flexitallic Engineering Standards with the exception of the graphite faced corrugated metal gasket (CMG) which was purchased through a distributor. In all cases, the materials of construction were 300 series stainless steel and graphite. Both SWs had inner rings. Results are shown in Figure A5. The Change gasket lost only 1.5 psig total over the 24 cycle duration out-performing all other types tested.

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Figure A5. Thermal Cycle Test Results - 24 cycles to 320°C (608°F)