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Refinery Based Case Study of a Novel Gasket Designed for Use in Problematic Shell and Tube Heat Exchangers B.Sc. Dene Halkyard Flexitallic Ltd, Cleckheaton, United Kingdom
1 Abstract This paper provides a technical overview of a novel gasket design engineered to address sealing issues associated with the operation of shell and tube heat exchangers. It discusses field service experience using the gasket on a UK refi nery. Shell and tube heat exchangers are commonly used items of process equipment in many industries. Depending on how they are operated heat exchangers can present significant technical challenges with regard to achieving leak free sealing performance. Fluctuations in axial load and radial shear effects brought about by thermal transients around the tube sheet girth flanges during start up or normal operation can a have a significant effect on seal performance. Additional factors such as available gasket load imposed by exchanger design; available space, the presence of stress raising nubbins and ease of access and installation can also have a bearing on gasket design. Refinery processes involving shell and tube heat exchangers can be particularly demanding, equipment is often old and can involve elevated temperatures, high pressures and cyclic operation. It is said that every refinery has its problem exchangers. Additional requirements for improved asset efficiency and emissions compliance place ever increasing demands on gasket performance. The novel gasket design offers benefits over traditional gasket styles more commonly used in shell and tube heat exchanger applications. It offers superior long term sealing performance, particularly in bolted connections subject to thermal transients. Refinery field service discussed in the paper reviews sealing performance in a number of problematic exchangers at the tube sheet to channel and tube sheet to shell connections. The heat exchangers, used to process heavy residual
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hydrocarbons are in excess of 30 years old with a history of managed leakage. High temperature, cyclic operation, seasonal fluctuations in product demand and the requirement for regular tube cleansing exacerbates the potential for seal failure. 2 Background
As an international manufacturer of industrial seals and gaskets Flexitallic has been manufacturing and supplying girth, tube-sheet and floating head gaskets for use on shell and tube heat exchangers for many years. As a consequence Flexitallic Applications Engineers are familiar with the apparent idiosyncrasies associated with the sealing performance of individual exchangers. Shell and tube heat exchangers can come in many designs and is a subject beyond the scope of this paper, however, they are broadly categorised on the basis of service or construction. See ref. [1] for additional information on this topic. All shell and tube heat exchangers contain the following main elements: Shell Shell cover Tubes Channel Channel cover Tube-sheet Baffles Nozzles
• • • • • • • •
A schematic of a sectional view of typical shell and tube heat exchanger with a removable channel cover and stationary tube-sheet, floating head with backing ring and removable shell cover is shown in figure 1.0. The location of girth, tube-sheet and floating head gaskets is indicated in red and by the presence of a circled red number. Floating head
Shell
Tubes
Channel cover
Nozzle
4
3 2 5
Shell cover
Figure 1.0: Heat Exchanger Schematic
Baffles
Tube-sheet Channel
1
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Gasket positions indicated by the presence of red circled numbers are generally named in accordance with the following notation: • • • • •
Location 1 – Channel cover or lid gasket Location 2 – Channel to tube-sheet gasket Location 3 – Shell to tube-sheet gasket Location 4 – Floating head gasket Location 5 – Shell cover gasket
Nozzles are pipe connections that are used to connect the exchanger to process piping consequently they utilise standard flanges and gaskets such as those defined in ASME B16.5 and ASME B16.20 respectively. However girth i.e. channel cover, tube-sheet, shell cover and floating head connections are usually dimensionally bespoke and gaskets are designed accordingly. Flange face configuration of bespoke connections is generally recess to spigot (male to female) in line with TEMA guidelines (see ref. [1]). In spigot to recess face configurations the gasket, when insitu, is confined on the outer diameter. This kind of arrangement can give rise to gasket width limitations particularly on floating head connections that can have an impact on gasket selection. For multi-pass exchangers the direction or channelling of process fluid is controlled by the use of partition plates on the tube side of the exchanger. This necessitates the use of gaskets with the incorporation of pass bars. The number and location of pass bars is dictated by the direction and complexity of the process flow on the tube side of the exchanger. The pressure differential across the pass bars is usually significantly lower than that between the shell and tube side of the exchanger or between process media and the atmosphere. Depending of gasket style pass bars can be integrated i.e. part of the primary seal, or fixed in position by welding. An area of particular interest is the gasketed connections located across the tubesheet. The tube-sheet is the barrier that separates the two process fluids i.e. the shell and the tube fluids, between which heat transfer takes place. As a consequence gaskets at these locations can be, depending on operational conditions, subject to changes in both axial and radial load. In a fixed tube-sheet arrangement, as shown in figure 1, there is a gasket located on both the shell and channel side of the tube-sheet. In such cases it is generally recommended to use the same gasket style in both locations. It is regular practice to use common bolting across the tube-sheet to effect compression of both the shell to tube-sheet and channel to tube-sheet gaskets. A schematic of this arrangement is shown in figure 2.
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Bolting
Shell flange
Channel flange
Channel/tubesheet gasket
Shell/tubesheet gasket Tube sheet
Figure 2: Tube-sheet - Bolting and Gasket Ar rangement 3 Current Gasket Technology Various styles of gasket are used in shell and tube heat exchangers. The following short overview represents the most commonly used styles encountered in the Oil and Gas industry. 3.1 Metal Jacket Gaskets
The traditional gasket style used for the sealing of non-standard heat exchanger connections in the Oil and Gas Industry has for many years been based around metal jacketed technology. Metal jacketed gaskets, as the name implies, are comprised of a thin metal jacket, typically 0.4mm formed around a soft compressible core. Common jacket materials include soft iron, carbon and austenitic stainless steels in combination with core materials such as graphite foil, millboards and fibre reinforced sheet gasket materials. Figure 3 shows a typical section through a double metal jacketed gasket. Lid Metal jacket
Core Box Figure 3: Double Metal Jacketed Gasket - Section
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Other metal jacketed styles are sometimes used incorporating surface corrugations, internal metal components or of single jacket construction. Metal jacketed gaskets have several inherent weaknesses with regard to creating and maintaining a seal namely; they require a relatively high unit stress to form a seal because of the requirement for metal deformation (stress raising nubbins are sometimes used to mitigate this). Successful metal to metal sealing also requires relatively smooth and blemish free flange sealing faces, this can be particularly problematic on older equipment. They possess very poor recovery characteristics; as a consequence sealing performance under load fluctuations can be compromised. Resistance to radial shear effects has been shown to be poor under RAdial Shear Tightness (RAST) testing. 3.2 Corrugated Metal Gasket Corrugated metal gaskets (CMG) are also used in used in heat exchanger applications. They are comprised of a relatively thin metal, typically 0.4 mm thick, core that has been concentrically corrugated and faced with a soft facing material. Corrugation pitch and height can vary depending on the manufacturer and gasket geometry. A typical material combination would be austenitic stainless steel in combination with a graphite based facing material. See figure. 4. Facing material
Metal core Figure 4: Corrugated Metal Gasket – Section This style of gasket offers some benefit over double jacket gaskets with regard to reduced seating stress and flange surface requirements. It is generally not suitable for use in narrow land width sealing applications and may not be suitable for cyclic service because of resilience limitations. 3.3 Spiral Wound Gaskets Spiral wound gaskets (SWG) have been used in heat exchanger service for many years and special TEMA type designs have been developed to assist in gasket Installation. SWG are manufactured by spirally winding alternative wraps of a thin strip of a metal (wire) and non-metallic sealing material (filler) around a mandrel. The strips are formed into a chevron profile during the winding process, resistance spot welding the wire at the start and termination of the winding process results in a
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sealing element. Gasket resilience and stiffness can be altered to suit the specific application. Handling and installation of larger gaskets can present difficulties and care may need to be exercised to prevent the gasket from ‘springing’ if twisted or mishandled. Over compression of the sealing element is normally prevented by the incorporation of a solid metallic inner compression ring of predefined thickness. The requirement for compression control can limit the use of SWG, especially in applications where space is limited, such as floating head connections. Typical materials of construction used in hydrocarbon service are graphite in combination with austenitic stainless steel. SWG offer significant advantages over metal jacketed gaskets in several key areas. Namely, lower seating stress requirements, improved resistance to radial shear effects and an increased tolerance to sealing face surface finish. Figure 5 shows a section through a spiral wound gasket modified for heat exchanger service. Inner compression ring
Sealing element
Outer location nose
Figure 5: Spiral Wound Gasket - Section 3.4 Kammprofile Gaskets
Kammprofile gaskets are comprised of a serrated solid metal core with soft nonmetallic facing material bonded to each face. Serration profile, facing material thickness and density should be carefully controlled as it can have a bearing on sealing performance. As with SWG, for hydrocarbon service typical materials of construction are stainless steel and flexible graphite. Benefits offered over other gasket styles are low stress sealing, their ability to accommodate flange face defects, use of narrow sealing widths, high load bearing capability and ease of handling and installation - even for very large gaskets. A typical schematic of a sectioned kammprofile gasket is shown in figure 6.
Metal core
Facing material
Serrations Figure 6: Kammprofile Gasket - Section
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4 Novel Gasket Design 4.1 Design and Construction Over many years of field service experience the shortcomings of traditional gasket styles in problematic heat exchanger applications is well known to Flexitallic engineers. With the objective to address such issues, extensive research and development work was undertaken that resulted in the production of a novel gasket, herein known as the Change gasket. The Change gasket has been designed to possess the optimum combination of both stiffness and resilience and as a consequence be capable of effecting a high performance seal under conditions of long term cyclic service; such as those routinely encountered in demanding shell and tube heat exchanger sealing applications. The preferred manufacturing approach involved producing a gasket from a wire strip; thus offering both maximum manufacturing flexibility and cost savings with regard to material utilisation. The strip is spirally wound however, the strip profile and thickness differs significantly from that used in the manufacture of SWG. See figure 7. The wire strip used in the manufacture of the Change gasket approximately 5 times thicker than that used in the production of SWG. As a consequence significant modification was required to standard SWG production machinery to enable manufacture.
Change
Spiral Wound
Figure 7: Gasket Wire Profiles Upon winding, the unique profile and increased thickness of the wire results in a stiff yet resilient interlocked structure with greatly improved handling characteristics compared to that of standard SWG. The resulting stiffer construction can in many instances negate the requirement to control gasket compression allowing its use in applications where space may be limited. A soft filler material, such as graphite, is incorporated into the gasket during the winding process. Once wound the resulting profile of the gasket sealing faces bears resemblance to that of a kammprofile gasket. Both sealing faces may be subsequently covered with a layer of soft facing material, providing low stress sealing and good conformance on poor flange faces. A schematic of a sectioned profile of the gasket in given in figure 8.
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Metallic winding strip
Serrations
Figure 8: Change Gasket - Section One of the main operational challenges in manufacturing the gasket was how to permanently fix the winding strip at the start and termination of the winding process. Conventional resistance spot welding could not be used because of the increased wire thickness. Weld integrity in an unconfined spiral wound seal arrangement such as this plays a crucial role in maintaining seal integrity. To this end various welding techniques were investigated. Following extensive trials, high power, precision laser welding was selected. Laser welding involves creating a localised, high precision, deeply penetrating high energy plasma reducing. Due to the localised nature of the technique thermal stresses are kept to a minimum resulting in a reduced heat affected zone compared with traditional welding methods. No additional welding material is involved, the result is a high strength weld capable of maintaining gasket integrity when exposed to fluctuating high load. See figure 9 for weld detail. Section View
Plan View
Figure 9: Change Gasket Weld Detail 4.2 Laboratory Testing
During the development programme the Change gasket was subjected to the usual battery of both standard and non-standard laboratory testing. ASME VIII (ref.[2], PVRC ROTT (ref. [3]) and EN13555 (ref. [4]) testing was undertaken to assess sealing behaviour under assembly and dynamic loading conditions and to generate the relevant gasket constants. However, to assess long term multi-cyclic load
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conditions and attempt to accurately simulate shell and tube heat exchanger conditions, particularly at the tube sheet gasket positions, testing was undertaken using a modified extended version of the Shell Thermal Cycle test. This test is part of the Shell Type Approval Testing requirement for semi-metallic gaskets. In addition to testing the Change gasket benchmark testing was also undertaken on all the previously reviewed gasket styles. 4.2.1 Extended Shell Thermal Cycle Testing The Shell thermal cycle test involves subjecting a standard gasketed bolted connection to a series of thermal cycles. Leakage performance is determined by a series of pressure decay tests carried out at defined stages during the test. The test rig consists of a pair of ASME B16.5 NB 4”raised face flanges incorporated into a welded pressure vessel that is fitted with an internal heating element. The test gasket is stressed via hydraulic tensioning of eight ASTM A193 B16 stud bolts. The induced bolt stress is 290 N/mm2. The test medium is nitrogen. A schematic of the test rig is given in figure 10. The test regime was as follows: a) Assemble and pressurise the test rig to 51 bar b) Undertake room temperature pressure decay test over 1 hour c) Depressurise and heat up the test rig to 320oC at 2oC/min. d) Re-pressurise to 33 bar followed by pressure decay test over 1 hour e) Allow to cool In the standard test parts c) through e) are repeated a total of 3 times. Test pass or fail is determined by pressure testing; the maximum allowable pressure drop either prior to and/or after the final thermal cycle is 1 bar (14.5 psi) over a one hour period. Leakage rates higher than this result in test failure. Each thermal cycle takes approximately 24 hours to complete. Extended benchmark testing of the all the gasket styles involved increasing the number of thermal cycles from three to twenty four or until test failure. The number of cycles and test temperature was selected after consultation with industry engineers; it being regarded as being representative of a typical process temperature and number of process trips, or thermal excursions an exchanger may be subjected to between scheduled maintenance outages. Graphical test results are shown in figure 11. Under standard Shell test conditions i.e. three thermal cycles, all gasket styles are compliant. However test modification to increase the number of thermal cycles highlighted some significant differences in sealing performance.
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To vent/gas supply
Pressure isolation valve
Test gasket
Thermal insulation Heating element
Figure 10: Shell Test Rig – Schematic 4.2.2 Extended Thermal Shell Cycling - Test Results With the exception of the corrugated metal gasket and the Change gasket all other gasket styles; kammprofile, double jacketed gasket and SWG gave similar levels of performance, all failing to meet or exceed the maximum allowable pressure drop requirement of 1 bar/hour after exposure to between 14 to 16 thermal cycles. The corrugated metal style gasket performed the worst failing pressure test after 5 thermal cycles. It can be seen from the data that the Change gasket significantly out performed all other gasket styles and was the only gasket able to meet the requirements of the test. After the completion of all 24 cycles the measured pressure drop was 0.07 bar (1 psi). The test was subsequently repeated and the results proved to be consistent. The test data clearly demonstrates that, under the described test regime, the Change gasket offers significantly improved sealing performance under thermal cycling conditions compared to traditional gasket styles.
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Figure 11: Pressure v Thermal Cycle Number Legend: DJ = Double jacketed gasket CMG = Corrugated metal gasket CGI = Spiral wound gasket Kamm = Kammprofile gasket Change = Change gasket Red broken line ( ) = Failure point (1 bar/hr pressure drop) 4.2.3 Radial Shear Testing An important consideration when designing a gasket for heat exchanger applications is the possibly of the gasket being exposed to radial shear. This can occur due to differences in radial expansion when the tube sheet is exposed to a thermal gradient resulting from different temperatures on the shell and tube-side of the exchanger. Radial shear can result in gasket damage and seal failure. Work carried out in the past has shown that certain gasket styles are superior to others at resisting radial shear. See ref [5]. A test protocol has been developed by Yarmouth Research in the US, see ref. [6]. The test involves exposing a gasket located in a tongue and groove flange arrangement to a thermal differential induced by heating one of the flanges to 300oC while cooling the other using water. The minimum amount of radial shear the test gasket is exposed to during the test is 0.51 mm. The test is comprised of exposing a
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test gasket to 20, 40, 60, 80 and a 100 thermal cycles. Following each radial shear test the assembly is pressure tested at room temperature with nitrogen at 40 bar on an hourly basis over the next 4 hours. In addition to leakage performance bolt relaxation is recorded at the start and finish of each test. This is an important indicator of seal performance as loss of strain energy in the connection is primary reason for seal failure. SWG have been shown to perform well in radial shear testing and they typically give rise to an average bolt relaxation value in the order of 25%. Average bolt relaxation as measured for the Change gasket was 15% with no significant leakage observed, see ref. [6]. 4.2.4 Sealing on Nubbins Depending on the age and original gasket style designed for the heat exchanger it is not uncommon to for girth flanges to have stress raising nubbins incorporated into the bottom of the recess of the mating flange. See figure 12. The primary objective is to raise the unit stress on the gasket thus improving sealing efficiency on assembly. Nubbins raised sections in the bottom of the flange, are generally used in connections where solid metal or double jacketed gaskets are used. Their presence can preclude the use of SWG and kammprofile sealing technology as they can result in gasket damage that can compromise seal integrity. As a consequence nubbin removal is normally recommended when using SWG or kammprofile gaskets, adding additional cost and time to maintenance programmes. Spigot flange
Gasket
Recessed flange Nubbin Figure 12: Flange and Gasket Arrangement with Nubbin - Section Sealing performance of the Change gasket has been assessed in a flanged connection with nubbins present. A specially developed test fixture was used in which a 0.4mm x 3mm (high x wide) nubbin was machined. A test gasket was installed and the test fixture pressurised using nitrogen gas, across a range pressures from 17 to 103 bar (250 to 1500 psi). The test pressure was held for 5 minutes during which the test rig was submerged in water and subjected to a bubble test. Leakage performance at two gasket stresses was investigated. The tabulated test data is shown in table 1. Post test visual inspection of the sectioned gasket was carried to assess gasket damage. The gasket remained intact with only minor
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deformation. Several Change gaskets have now been successfully used in connections in which stress raising nubbins are present. Table 1: Change Gasket Leakage Performance with Nubbins Internal Pressure N2(bar)
Leak rate (bubbles/min)@ Gasket stress 103 N/mm2
Leak rate (bubbles/min)@ Gasket stress 206 N/mm2
17.2
0
0
34.5
0
1
51.7
0
1
68.8
0
0
86.0
0
0
103.4
0
0
5
Field Service
Since the development and introduction of the Change gasket many have been supplied and are currently being used in non-standard connections in shell and tube heat exchangers around the world. The following is an account of the Change gasket being used in a problematic application at Total’s Lindsey Oil Refinery (LOR) located at Immingham in the UK. 5.1 Background
Flexitallic Application Engineers were consulted following repeated leakage issues on two banks of shell and tube heat exchangers on the visbreaker unit. The visbreaking process is designed to reduce the viscosity and pour point of the heavier hydrocarbon fractions resulting from vacuum distillation (VD) of crude oil. Visbreaking is one of several cracking processes used on modern complex oil refineries, see ref. [6]. The commercial success of any refining operation depends to a great extent on the ability of the refinery to process a range of crude oil qualities and ensure optimum product yield is aligned with market demand. This generally means having the flexibility to convert higher molecular weight hydrocarbon fractions to lower weight fractions with the correct molecular structure. Visbreaking is a relatively mild thermal cracking procedure compared to other commonly used processes. Mild refers to the extent of the cracking reaction rather than the absolute temperatures at which it takes place. The extent of the reaction is controlled by
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quenching the products immediately after leaving the reactor. Two technologies are commonly employed, furnace (or coil) and soaker (or drum) processes. In the former the visbreaking reaction takes place directly in the furnace coils while in the latter it occurs away from the furnace in a specially designed unit known as a soaker. Soaker units operate at relatively low temperatures and use longer residence times compared with coil units. A simplified schematic of the soaker visbreaking process is shown in figure 13. The main benefits of the soaker process are reduced energy requirements and increased run times because of reduced reactor coking rates however, the decoking process itself can be more complex due to soaker design. Coking, or fouling, is an important consideration in refinery processes as it can have a significant impact on plant efficiency. Carbon deposits have a negative effect on both heat transfer efficiency and feedstock and product throughput.
Figure 13: Soaker Visbreaking Process - Simplified Schematic 5.2 Application Details
The visbreaking operation used on the refinery is based on soaker technology. Prior to visbreaking the virgin feedstock (VD residues) is pre-heated via a bank of four shell and tube heat exchangers. The exchangers were commissioned around 33 years ago and have history of ‘managed leakage’. Over the years the girth flanges have been re-machined to rectify flange distortion and corrosion issues and remain within corrosion allowance limits. Mechanical clamps and injectable sealants have been employed on a temporary basis on a number of occasions to mitigate leakage at the tube-sheet connections and allow continued running of the visbreaker unit in between planned shutdown programmes. Several different gasket styles have been previously evaluated with some noted differences in performance, however, none have given the required level of service.
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The exchangers have been designed to ASME VIII Div. 1 TEMA R pressure vessel code and categorised as TEMA Class AES. The flanges do not contain stress raising nubbins. Test, design and operating conditions are given in table 2.
Table 2: Test, Design and Operating Conditions Shell Side
Tube Side
Design Pressure (barg)
38
36
Test Pressure (barg)
57
56
Design Temperature (oC)
370
390
Op. Temp Outlet (oC)
335
283
Op. Temp Inlet (oC)
192
365
Media
VD Residue
Visbreaker Residue
As is common, for reasons of economy visbreaker residue is used as the heating medium on the tube-side of the exchanger. The negative impact of this is regular coking or fouling of the internals of the exchanger bundle. Two banks of exchangers are used to pre-heat feedstock, however only one bank is in operation at any one time. This allows routine maintenance to be carried out without shutting down the visbreaker unit. Each bank of 4 heat exchangers consists of 2 x 2 vertically aligned units designated 91E1 - A, B, E, F and 91E1 - C, D, G, H. Each of the two banks is connected in series. Photographs of the units in operation are given in figure 14.
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Figure 14 Visbreaker Preheater Shell and Tube Heat Exchangers
Because of the nature of the feedstock and pre-heat medium (visbreaker residues) fouling or coking of the exchangers, particularly on the tube side is commonplace. For operational efficiency reasons cleaning or de-coking is undertaken on a regular basis. Cleaning requires taking the units off line. The cleaning regime employed depends on the extent and location of coking. If coking is limited to the tube-side of the exchanger cleaning involves removal of the channel cover and ‘reverse jet’ cleaning the tube bundle using ultra-high pressure water. In this instance the tube sheet gaskets are not removed and remain under compressive load. However the act of taking the unit off line subjects the tube sheet gaskets to significant thermal shock. Subsequently some time in the future; typically after a 2 to 3 month period the tube sheet gaskets are subject to additional thermal shock when bringing the unit back on line and up to operational temperature. Coking on the shell side of the exchanger is less problematic and requires less frequent attention however, shell side cleaning requires the removal of the shell cover i.e. the tube bundle is fully removed, and removal of the floating head to allow full ‘through jet’ cleaning. In this case the connection across the tube-sheet is broken and new gaskets installed on re-assembly of the exchanger. The frequency of cleaning (de-coking) is primarily determined by the nature of the VD feedstock (crude) and process conditions. Over the last few years and increasingly as we go into the future the ability of a refinery to process a wide range of heavier and dirtier crude feed stocks is a key factor to economic success. However as can be seen in this example it can place increasing demands on gasketed flanged connections. Changes in product demand patterns have also, over the last few years exacerbated sealing issues on the pre-heater exchangers. The need for bitumen a key raw material used in the production of road surfaces has increased and, as consequence has become an economically viable refinery product. This has had a direct impact on
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the production of visbroken distillates as VD heavies are an important source of bitumen. The demand pattern for bitumen is seasonal and summer months in Europe generally gives rise to an increase in demand, providing economic incentive to maximise the production of VD heavies. At such times the visbreaker unit is in put into bypass mode reducing the inlet and outlet temperatures on both the shell and tube side of the exchanger in the order of 100 oC. The frequency of placing the visbreaker unit to bypass mode depends on product demand patterns. Typically the exchangers are in bypass mode 12 days per calendar month during summer. This of course leads to additional non-cleaning related thermal transients resulting in additional stress fluctuations across the tube-sheet connection. It estimated that the average number of thermal cycles the exchangers are exposed to because of cleaning and/or standby mode operation is two per month.
5.3 Gasket Selection
Over recent years many different gasket styles have been evaluated. Originally the exchangers were supplied with double metal jacketed gaskets. Following leakage issues spiral wound gaskets were installed and more recently kammprofile gaskets have been used. Both spiral wound and kammprofile gaskets proved to offer advantages over jacketed gaskets however all gasket styles failed with regard to providing long term i.e. greater than shell side cleaning interval requirements, leak free performance at the tube sheet gasket locations. In light of gasket history and operational considerations Change gaskets were designed for both the channel/tube sheet and shell/tube sheet locations. Because of the relatively large size of the connection, in excess of 1.5 m diameter and the available space both gaskets were fitted with solid inner rings. The channel tube sheet gasket was fabricated with internally welded double jacketed pass bars. See figure 15 for gasket details.
Figure 15: Change Gasket Channel to Tube Sheet
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Gasket materials of construction were as follows: Winding, inner ring and jacket material: UNS31603 (SS316L, WN1.4404) Filler and facing material: High purity graphite (Flexicarb HT) • •
Bolting and tightness calculations were carried out in accordance with ASME VIII Appendix II and PVRC Convenient method (Draft). ASME VIII appendix II calculations indicated code compliance with operational conditions requiring a bolt area of 47076 mm2 (Am1) with a corresponding available bolt area (Ab) of 54387 mm2. PVRC Convenient calculations indicated a T3 tightness class at a residual bolt stress of 210 MPa (30.5 ksi) with an equivalent gasket stress of 110 MPa (15.9 ksi). The gaskets were installed using best practice assembly techniques. Hydraulic torque was used to induce bolt stress during flange assembly. Target bolt stress on installation was 310 MPa (45 ksi) with an equivalent assembly gasket stress of 162 MPa (23.5 ksi). Flange calculations were carried out to ensure flange stresses and bending moments were kept within acceptable ASME/Taylor Forge design limits. 5.4 Sealing Performance At the writing of this paper the Change gaskets were installed 14 months ago (May 2013) on units 91-E1 A, B, E and F and 8 months ago (October 2013) on exchangers 91-E1 C, D, G and H. Throughout these periods the visbreaker preheaters have been in constant use; processing visbreaker feed stock or in standby mode, depending on product demand profile. Since installation there has been no requirement to undertake shell side cleaning so the tube-sheet connection has not been opened and the gaskets have not been replaced. However, tube side cleaning has been carried out on a number of occasions. The total number of thermal cycles to which the tube-sheet connection has been subjected is in the order of 28 for units A through D and 16 for units F through G. Regular visual monitoring of the connections has shown no incidence of leakage across any of the 16, previously problematic tube-sheet connections. Discussions with plant engineering personnel suggest that if using conventional gasket technology that an incidence of leakage would have been extremely likely over this time period and that the adoption of Change gasket technology represents a significant improvement in the maintenance of joint integrity on these problematic units.
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6 Conclusions Under the refinery field conditions described the Change gasket offers a significant advantage over traditional gasket designs namely; SWG, jacketed and kammprofile gaskets, in heat exchanger tube-sheet connections subject to thermal transients. Under the laboratory test conditions described the Change gasket is able to accommodate fluctuations in both axial and radial load to a much greater degree, maintaining a high integrity seal when benchmark tested against other gasket styles. The Change gasket represents a fundamentally new design concept offering the optimum balance of resilience; usually associated with spiral wound gaskets and stiffness; usually associated with kammprofile gaskets, over the typical stress range(s) generated in bolted connections. 7 References [1]
Standards of the Tubular Exchanger Manufacturers Association (TEMA) 9th Edition (2007).
[2]
ASME VIII Div 1 (2010) Appendix II Rules for bolted flanged connections with ring type gaskets
[3]
PVRC ROTT Draft 10.01 Standard test method for gaskets constants for bolted joint design (April 2001)
[4]
BS EN13555 2014 Flanges and their Joints. Gasket Parameters and test procedures relevant to the design rules for gasketed circular fl ange connections.
[5]
Heat exchanger gaskets radial shear testing. Viega, Kavanagh and Reeves. PVP2008-61121
[6]
Yarmouth Research and Technology. Report/Project No. 2120450 2013
[7]
William L Leffler. “Petroleum Refining in Nontechnical Language” 4th Edition (2008).
[8]
Russ Currie “Change Gasket – Overview and Recent Developments” PVP2013-97050 ASME Pressure Vessels and Piping Conference. Paris 2013
8 Acknowledgements I would like to express my special thanks to Mr Kevin Wallace, plant engineer on Totals Lindsey Oil Refinery for his support in producing this paper.
