Metal bellows seals span roughly -250°C to over 500°C in published vendor figures, while elastomer bellows seals are typically constrained to about -50°C to 200°C, a gap of more than 300°C on the hot end that dictates the choice once process temperature crosses the 200°C line [S2][S6][S8].
The decision is not simply "hot service equals metal"; it is driven by the elastomer's glass-transition and thermal-degradation thresholds, by alloy selection on the metal side, and by whether the application is also chemically aggressive or cryogenic. This map lines the two technologies up against temperature, pressure, chemical and cost criteria so the specifier can pick without overpaying or under-engineering.
Temperature Windows: Hard Numbers From Each Side
Elastomer bellows seals built on FKM (Viton), EPDM or nitrile compounds hold a working window of roughly -50°C to 200°C, with practical upper limits near 150°C for EPDM and 200°C for FKM before hardness drift, set loss and extrusion failures begin [S2]. A second vendor reference puts the elastomer seal ceiling closer to 200°C even with high-temperature grades, after which the rubber element hardens, loses elasticity and starts to leak [S9].
Metal bellows seals shift that window dramatically. The most conservative published figure is -200°C to 400°C depending on alloy [S2]; mid-range vendor literature quotes -250°C to over 500°C [S6]; and edge-welded metal bellows are specified for -212°C to +425°C service when built from corrosion-resistant alloys [S4]. Specialised Inconel-constructed metal bellows seals reach 600°C on the high side and remain serviceable for cryogenic duty on the low side [S8]. The data anchors a clean rule of thumb: above 200°C, specify metal; below -50°C, specify metal; between those bounds, either may work but elastomer is cheaper.
A useful internal reference for the elastomer side of this comparison is the polyurethane elastomer behaviour page, which documents hardness, glass-transition and chemical-resistance patterns that frame why generic rubber compounds fail near 200°C.
Alloy Selection Drives the Metal-Bellows Upper Bound
Not all metal bellows are equal. The material set typically includes stainless steel (300 series), AM350, Inconel 625/718, Hastelloy and titanium, and the published temperature ceiling moves with each step up the alloy ladder. Standard stainless constructions plateau near 400°C, AM350 pushes into the 425°C band typical of edge-welded designs [S4], and Inconel extends that ceiling to 600°C in vendor data [S8].
The other metal-side variable is construction type. Edge-welded bellows use individually stamped diaphragms welded into a multi-convolution core, giving both higher burst pressure and the broadest temperature range [S4]. Formed (cold-formed, hydroformed or electroformed) bellows are cheaper to mass-produce but show "much narrower" mechanical and temperature limits, which is why they are rarely specified in oil-and-gas, cryogenic or power-generation service [S4]. For context on how these alloys are produced and what controls their purity, see the metal powder processing page.
The driving engineering fact: the metal bellows replaces the dynamic O-ring or wedge as the secondary seal, so once the bellows alloy is rated for the process temperature, the seal no longer has a soft polymer that can melt, extrude or chemically attack.
Comparison Matrix: Metal vs Elastomer Bellows Seals
Stacking the two designs against the criteria that actually drive a pump or compressor seal decision yields a clear separation. Temperature capability favours metal across the full published range; chemical resistance favours metal where the process is aggressive, but tracks the elastomer compound on the polymer side; pressure capability favours metal up to several hundred bar; dynamic elastomers are absent in metal and present in elastomer designs; initial cost is lower for elastomer, lifecycle cost is lower for metal in hot or cyclic service; and installation complexity is easier on the elastomer side because no metal-bellows handling care is needed [S1][S2][S5].
Three selection rules follow directly: (1) process temperature above 200°C or below -50°C forces metal; (2) strong oxidisers, hot oils, hydrocarbons or solvents above 150°C force metal because elastomers swell and lose hardness; (3) clean water, mild chemicals below 120°C and cost-sensitive OEM builds favour elastomer [S1][S2][S5]. A compact reference for the elastomer side of the comparison is the bellows seal type page, which lays out the construction variants behind these decision points.
Failure Modes Above 200°C: Why Elastomer Seals Quit
Elastomer bellows seals fail by a small, predictable set of mechanisms once the temperature window is exceeded. Above about 149°C (300°F) the rubber element starts to harden, lose flexibility and crack; at sustained high temperature, chemical degradation and thermal ageing set in, and accelerated wear follows [S9]. The pusher-seal family that competes directly with bellows designs is also limited by the same elastomer secondary seals, so even non-bellows elastomer-Secondary designs cap out near the same ceiling [S10].
Metal bellows seals avoid this failure path by removing the dynamic elastomer entirely. Because there is no soft polymer in the spring/secondary-seal path, the design "maintains consistent sealing performance under thermal stress" and is generally the preferred choice for high-temperature and chemically demanding service [S5]. This is also why hot-oil circuits, thermal-fluid heaters, steam boiler feed pumps and cryogenic LNG pumps converge on metal-bellows designs: the failure mode that takes out an elastomer seal simply does not exist in the metal design [S1][S3][S5].
Where Each Design Earns Its Slot
Metal bellows seals are the default for high-temperature pumps and compressors, hot oil and thermal-fluid service, steam, hot chemical reactors, vacuum service, and cryogenic applications where elastomer brittleness would otherwise be a problem [S1][S2][S3]. Elastomer bellows seals remain the default for clean-water HVAC pumps, light chemical service, general industrial OEM builds and any application where the process fluid is benign, the temperature stays inside roughly -50°C to 150°C, and the lowest first cost matters [S2][S5].
For cryogenic duty specifically, the published edge-welded metal-bellows range starts at -212°C and -350°F, well below the glass-transition of any common elastomer [S4]. The same edge-welded construction carries up to 425°C, covering the majority of refinery and chemical-plant hot services [S4]. If the seal will see repeated thermal cycling, the metal design also avoids the compression-set and hardness-drift penalties that accumulate in rubber secondary seals.
Decision Signals To Watch On The Next Spec
Two trackable signals tell you when to switch from elastomer to metal on a given pump. First, measure the seal-chamber temperature at steady state, not just at the suction line: a 20-30°C rise across the chamber is common in hot-oil service and is what pushes the elastomer past its limit. Second, confirm the alloy ceiling against the actual process temperature plus a 25°C margin, not against the seal catalogue maximum; AM350 at 400°C and Inconel at 600°C behave very differently in a thermal upset, and that margin is what protects the seal during a process excursion [S4][S8].
For non-metallic seal materials and the elastomer families that bracket this comparison, the metal material reference page summarises the alloy-property baselines used in the published temperature ceilings. A related engineering comparison that often shows up alongside bellows-seal decisions is covered in ENB vs DCPD vs VNB: choosing the right EPDM third monomer, where the elastomer-side temperature ceiling is broken down by polymer architecture.