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This section discusses the mechanical design of shell and tube heat exchangers and their components. Emphasis is placed on company practices which differ from industry standards. Contents: Design Pressure and Temperature Bundle Design Channel and Shell Design heat exchanger gasket heat exchanger insulation
I.D., 1/16" tk Typically no ribs for solid metal design Many closures are designed with nubbins and clad gaskets Be sure to look at both a clad with no nubbin, and a solid metal with no nubbin before nubbin is removed Gasket material must be softer than nubbin material Too thin and nubbin will cut gasket Too thick and gasket will not deform correctly and provide good seal Somewhat determined by gasket material Retorque Leaks are usually from damaged nubbins and require nubbins to be inspected and repaired Chevron Corporation Heat Exchanger and Cooling Tower Manual Not necessary for seating the gasket Torque if leaker only 500-38 Composition Asbestos 500 Shell and Tube Exchanger Component Design Considerations March 1994 Fig 500-15 Gasket Design Considerations (3 of 3) Heat Exchanger and Cooling Tower Manual 500 Shell and Tube Exchanger Component Design Considerations The following are recommended for asbestos substitute gasket materials: Use flexible graphite sheet, graphite-filled spiral-wound gaskets or double-jacketed gaskets for all hydrocarbon and steam services Use reinforced elastomer bound sheet gaskets for AWSI Class 150/300°F maximum water service Use PTFE (“Teflon”) filled spiral-wound gaskets (first choice) or sheet gaskets (second choice) in sulfuric acid service See Figure 500-16 for other chemical service recommendations Fig 500-16 Suitability of Materials in Sheet- or Spiral-Wound Gaskets Note (1) (2) Service (3) PTFE Suitable? Graphite(4) Suitable? H2SO4 Yes No Alloy 20 HNO3 Yes No T-304 SS HF Yes Yes Monel H3PO4 Yes No T-316 SS HCI Yes Yes Hastelloy C Metal for foil reinforcement of graphite sheet gaskets, or for windings in SWG (1) This table gives conservative recommendations for materials resistant up to at least 200°F Please consult with the Materials and Engineering Analysis Division when selecting gaskets for a new chemical service (2) All concentrations (3) PTFE (Teflon) is a suitable replacement material for “Blue African” asbestos for all the chemical services listed above PTFE sheet gaskets are not fire safe, whereas PTFE filled spiral-wound gaskets (SWG) are often considered fire safe Thus the first choice for most acid applications will be Teflon-filled SWG (4) Flexible graphite is “fire safe” and suitable for most chemical services except those that are highly oxidizing, such as nitric acid or concentrated sulfuric acid Double-Jacketed Gaskets (See Figure 500-17) Double-jacketed gaskets have greater compressibility and resilience than solidmetal gaskets Even compression is achieved by the use of the overlapped jacket on the inside and outside diameters Double-jacketed gaskets are generally reliable; however, they are much less forgiving to gasket surface alignment or surface condition problems than composition asbestos Spiral-Wound Gaskets (See Figure 500-18) Spiral-wound gaskets provide the best sealing capabilities However, they tolerate less flange face misalignment and require more care in handling than either composition asbestos or double-jacketed gaskets They are custom-designed to meet the compression requirement of body flange bolting Spiral-wound gaskets are fully seated when the flanges are pulled up snugly against the compressing guide ring This ring also prevents gasket crushing by over-tightening of bolts Chevron Corporation 500-39 March 1994 500 Shell and Tube Exchanger Component Design Considerations Heat Exchanger and Cooling Tower Manual Fig 500-17 Double-Jacketed Gasket Fig 500-18 Spiral-Wound Gasket Solid-Metal Gaskets These gaskets, are prone to leakage and are no longer recommended Solid-metal gaskets come in many shapes They have good strength and are resistant to corrosion They are effective at higher temperature and pressures than the other types of gaskets Solid-metal gaskets require an excellent seating surface condition and alignment They have been used with nubbin-seating surfaces A nubbin is a very small (1/4 inch wide) seating surface on the face of the flange Because the nubbin is small, less force is needed to seat the gasket March 1994 500-40 Chevron Corporation Heat Exchanger and Cooling Tower Manual 500 Shell and Tube Exchanger Component Design Considerations 550 Insulation 551 Reasons for Insulating Exchanger shells, channels, and flanges are insulated for the following reasons: • To minimize heat loss and consequently save fuel Obviously, insulating may not be appropriate in “cooling services,” such as for cooling water exchangers • To protect personnel working where surfaces are over 140°F Exchangers which are not readily accessible need not necessarily be insulated to protect personnel Exchanger shells which are accessible but should not be insulated may use alternative means to protect personnel, such as guard posts and signs 552 Types of Insulation Calcium silicate, fiber glass, and mineral wool are the common types of insulation used on exchangers Calcium silicate is generally preferred, especially in areas of high-foot traffic or where flammability is a concern See Section 100 of the Insulation and Refractory Manual for more information on the types of insulation available See IRM-MS-1381 for installation requirements 553 Weatherjacketing In general, 3/16 inch pitch cross-crimped aluminum weather jacketing should be used on exchanger shells, and flat aluminum or mastic weather jacketing used on exchanger heads See Section 100, Model Specification IRM-MS-1381, and Standard Drawing GD-N99785 in the Insulation and Refractory Manual for more detailed information 554 Flange Insulation In general, body flanges over 100°F should be insulated for the following reasons: • To save heat • To protect personnel • To prevent large thermal gradients across the flange during inclement weather conditions Large thermal gradients across the flange can cause distortion of the flange and ultimately cause the flange to leak If the flanges and insulation are improperly designed, the following problems may occur: • Chevron Corporation If flange and bolts are not of similar materials, differential thermal expansion at the operating temperature can unseat the gasket or cause the bolts or flange to yield 500-41 March 1994 500 Shell and Tube Exchanger Component Design Considerations Heat Exchanger and Cooling Tower Manual • The bolts will relax and stretch if the internal operating temperature is above the creep stress limit of the bolts This problem can cause leakage and possibly auto-ignition because of the high temperatures • Improperly designed insulation will “soak up” leakage and may cause autoignition To prevent these problems, the following criteria should be used for design and insulation of flanges: • The flanges and bolts should be of similar material, i.e., B7 or B16 studs for carbon steel or low alloy flanges • Flange and bolt materials should be designed for the maximum internal design temperature and corrosive nature of the process fluid In other words, the stress in flange and bolt material must be kept below the creep stress limits at the maximum internal design temperature Temperature limits for commonly used studs are as follows: Less than 750°F A193 B7 750°F to 950°F A193 B16 Above 950°F Consult CRTC’s Heat Exchanger specialists, Fuels and Processing Unit on a case-by-case basis • Use insulation covers designed for safe leakage See Section 100 of the Insulation and Refractory Manual and Model Specification IRM-MS-4197 for the design of leak-safe, removable insulation covers • Apply insulation when the flange is cold (after hydrotest and before startup) to minimize startup stresses Insulation may be temporarily removed after startup to inspect for leaks A practical problem in the plants is the mixing of B7 and B16 studs, especially during plant turnarounds when a great deal of bolting and unbolting is being done If a location cannot guarantee that these studs can be totally segregated, then another option is to leave flanges over 750°F uninsulated with a weathercover over the flange to protect against wind and rain March 1994 500-42 Chevron Corporation ... Corporation Heat Exchanger and Cooling Tower Manual 500 Shell and Tube Exchanger Component Design Considerations 550 Insulation 551 Reasons for Insulating Exchanger shells, channels, and flanges are... flange to yield 500-41 March 1994 500 Shell and Tube Exchanger Component Design Considerations Heat Exchanger and Cooling Tower Manual • The bolts will relax and stretch if the internal operating...Heat Exchanger and Cooling Tower Manual 500 Shell and Tube Exchanger Component Design Considerations The following are recommended for asbestos