Bellows Seal

What is a Bellows Seal

A bellows seal is a thin-wall metal bellows, an accordion-shaped tube of convolutions, used as the dynamic seal between a moving shaft or stem and a stationary housing. The convolutions let the bellows extend and compress along its axis, absorbing the motion of the moving part, while the unbroken metal wall presents no joint, gap, or rubbing interface for the medium to leak through. This is the defining difference from gland packing or a pusher mechanical seal, both of which rely on a sliding contact between a soft packing or an elastomer and the moving part, and both of which therefore wear and eventually leak.

Two distinct devices share the bellows seal name because they share this physics. The first is the bellows seal valve, in which a welded metal bellows is fixed at one end to the rising valve stem and at the other to the bonnet. As the stem strokes to open and close the valve, the bellows simply stretches and shortens, and the process fluid is contained entirely inside the bellows and body, never reaching the stem packing or the atmosphere. The second is the metal bellows mechanical seal, in which an edge-welded bellows replaces both the coil springs and the dynamic secondary O-ring of a rotating pump or compressor seal, pressing the seal faces together while flexing to follow shaft runout and thermal growth without any elastomer.

In a bellows seal valve, the bellows is the primary stem seal, but it is never the only one. Because a bellows is a fatigue component with a finite life, codes require two secondary seals: a stem backseat that mechanically isolates the upper chamber when the valve is fully open, and a graphite gland packing loaded by a balanced gland. If the bellows ever fails, this packing contains the medium and holds emissions near packing level until the valve can be serviced. The primary seal carries the zero-leakage claim; the secondary seals carry the safety margin.

The industrial driver for bellows seals is fugitive emission control. A conventional gland-packed valve leaks small but continuous amounts of vapour through the dynamic packing, and the leak rate rises steeply for low-viscosity, high-permeability media: heat-transfer oil, for example, permeates packing on the order of fifty times faster than steam, and gases such as chlorine, hydrogen, ammonia, and phosgene are both highly mobile and acutely hazardous. For these media a bellows stem seal removes the leak path entirely, which is why bellows seal valves are standard in chemical, petrochemical, fine-chemical, heat-transfer-oil, and nuclear service, and why rotating bellows seals dominate hot-hydrocarbon and hot-water pump duty.

The trade-offs are equally fundamental. A bellows tolerates axial motion but is weak in torsion and limited in stroke, so valve designs must convert handwheel rotation into pure linear stem travel and must keep the stroke to roughly a quarter of the bellows free length. The thin bellows wall, typically a few tenths of a millimetre, derates corrosion allowance and pressure capacity compared with a solid stem, and the welded multi-ply construction raises unit cost well above a packed valve or a pusher seal. A bellows seal is therefore an engineered answer to a specific emission or temperature problem, not a default upgrade for every line.

Bellows Seal Types and Classification

Bellows seals split first by the motion they seal, then by the valve or machine they serve. Confusing the two families is the most common specification error, because a stem bellows and a rotating bellows look similar but obey opposite rules: one must never rotate, the other does nothing but rotate. The table below maps the main classes, their motion, and their typical hosts.

Bellows seal globe valves are the most common form. The linear rising stem suits the multi-turn globe trim perfectly, and the bellows handles the full lift of the disc. They serve both manual isolation and, with an actuator and a contoured plug, throttling control duty. Body forms follow EN 13709 (steel globe valves with bolted bonnet) and EN 1902 for nominal pressure designations such as PN16 and PN40, with parallel ASME B16.34 classes for North American projects.

Bellows seal gate valves use the same rising-stem bellows around a wedge or parallel gate. Gate valves give a straight-through, low-pressure-drop bore, which suits viscous or fouling media such as heat-transfer oil and polymer melt, where the bellows eliminates the packing leak that hot oil would otherwise drive. Both globe and gate bellows valves are rising-stem, non-rotating-stem designs by necessity.

Bellows-stem rotary valves apply the principle to quarter-turn ball and plug valves, where the bellows must accommodate a rotating shaft rather than a sliding stem. These are less common and use specialised bellows-stem geometries, but they extend zero-emission stem sealing to the quarter-turn valves preferred for fast, tight shut-off.

Rotating and stationary metal bellows mechanical seals are the pump-side family. In the rotating-bellows arrangement the bellows assembly turns with the shaft, which is mechanically simpler and most common; in the stationary-bellows arrangement the bellows is fixed in the housing and the mating ring rotates, which is preferred at high surface speeds where a rotating bellows would be thrown out of balance. Both are pusherless and elastomer-free, which is exactly why they tolerate temperatures and chemistries that defeat a spring-and-O-ring pusher seal.

Forming and Ply Technologies

How the convolutions are made, and how many metal layers form each wall, governs stroke, pressure capacity, fatigue life, and cost. Two forming methods and two ply philosophies cover almost the entire market. The table below compares the four practical combinations on the metrics that matter for selection.

Hydroformed (mechanically formed) bellows start as a thin, seamless drawn tube. Internal hydraulic pressure, sometimes with a mechanical roll, expands the tube into rounded convolutions. The result is smooth, seamless, and inexpensive, with no welds along the convolution wall to initiate cracks. Hydroformed bellows dominate valve stem seals, where the stroke is modest and the rounded profile resists the static pressure of the line. Their limitation is stroke: a rounded convolution can flex only so far per unit length before the wall over-strains, so a long-stroke or highly flexible application needs the welded form.

Edge-welded bellows are assembled from individually stamped diaphragm plates, each a flat washer with a profiled face. Plates are welded in pairs at their inner diameters to make a convolution, then convolutions are welded at their outer diameters to build the bellows core, which is finally welded to end fittings. The deep, nested diaphragm shape gives far more stroke and flexibility per unit length than a hydroformed tube, which is why virtually every rotating metal bellows mechanical seal uses edge-welded construction: the seal must flex continuously to follow shaft runout and thermal growth with low spring rate. The penalty is cost and the many welds, every one of which must be defect-free.

A persistent myth holds that welded bellows are inherently less reliable than formed ones. Per MSS SP-117, an edge-welded bellows that is correctly designed and qualified to the standard should achieve the same service life as a hydroformed bellows. The reliability gap, where it exists, comes from inadequate weld quality or from applying a bellows outside its rated stroke and pressure, not from the forming method itself. This is why the qualification cycle test in MSS SP-117 is run at the maximum rated pressure: it proves the assembly, welds included, at its worst case.

Ply count trades flexibility against strength. A single-ply bellows is the most flexible and lowest cost, but its thin wall limits pressure. A multi-ply or double-ply bellows nests two or more thin layers so the assembly gains pressure capacity and start-up torque strength without becoming as stiff as a single thick wall would be. Double-ply edge-welded bellows are the standard answer for high-pressure rotating seals, and multi-ply hydroformed bellows extend valve stem seals to higher PN classes. A common bellows wall thickness for these thin plies is on the order of 0.1 to 0.3 mm per layer, which is precisely why corrosion allowance must be treated far more conservatively than on a solid valve stem.

Bellows Alloys, Media and Standards

The bellows is a wetted component with a wall a fraction of a millimetre thick, so alloy choice is even more critical than for a solid valve trim. A mismatch does not cause slow surface attack; it can perforate the seal and release the medium. Common bellows alloys, in rising order of corrosion and temperature capability, are austenitic stainless 321 and 316Ti, precipitation-hardening AM350, the nickel alloys Inconel 625 and Inconel 718, and the high-nickel chemical alloys Hastelloy C-276 and Alloy 20.

321 and 316Ti stainless are titanium-stabilised austenitic grades that resist sensitisation when welded, which matters for the many welds of an edge-welded bellows. They are the default for water, steam, condensate, light hydrocarbons, and general chemical service, and they cover the bulk of bellows seal valve duty in the moderate temperature band. Like all 300-series stainless, they are vulnerable to chloride stress corrosion cracking, so chloride-bearing media push the selection up the list.

AM350 is a precipitation-hardening stainless that, after heat treatment, develops high tensile strength and hardness. This fatigue strength is why AM350 is a favoured plate material for both valve stem bellows that must survive thousands of strokes and for rotating mechanical seal bellows; John Crane, for example, offers edge-welded bellows in heat-treated AM350 as an option alongside Alloy 718. Inconel 625 and Inconel 718 are nickel-chromium superalloys that hold strength and oxidation resistance to high temperature and resist chloride media; Inconel 718 in particular is the workhorse plate material for high-temperature mechanical seal bellows, and Inconel 625 is a standard choice for hot chlorine and hydrogen valve service and for TA-Luft fugitive-emission duty.

Hastelloy C-276 and Alloy 20 handle the aggressive acids that defeat stainless: C-276 for wet chlorine, hydrochloric acid, and ferric chloride; Alloy 20 for hot sulfuric acid. They appear as bellows plate options on chemical-duty seals such as the John Crane Type 680 (Alloy 20 bellows) and the EagleBurgmann MFL family (Hastelloy C-276 bellows). The table below is a quick lookup for matching medium to bellows alloy; it is for initial selection only, and the manufacturer corrosion chart at the real concentration, temperature, and velocity must always be confirmed before purchase.

Three standard families govern bellows seals. MSS SP-117 covers the design, materials, fabrication, installation, qualification, examination, testing, and shipment of metal bellows assemblies for manual and automated on-off globe and gate valves, including the helium leak test and the maximum-pressure cycle qualification. ISO 15848-1 is the fugitive-emission type test for the whole valve stem seal and body joints, classifying tightness as Class A (helium leak no greater than 1.0E-5 mg per second per metre of stem perimeter), Class B, or Class C, with endurance classes CO1, CO2, and CO3. German TA-Luft is the clean-air regulation that bellows seal valves satisfy by reaching the strictest tightness class, and API 624 / 641 are the parallel North American fugitive-emission type tests. Valve bodies additionally follow EN 13709 and EN 1902 (PN designations) or ASME B16.34.

Key Specification Parameters

A bellows seal data sheet can list dozens of lines, but eight parameters drive the selection decision: nominal pressure rating, temperature range, cycle (fatigue) life, stroke-to-free-length limit, tightness class, bellows alloy and ply, secondary seal arrangement, and end or face configuration. Each is explained below, with typical values verified from manufacturer literature and standards.

Nominal pressure rating for bellows seal valves is given in PN classes per EN 1902, most commonly PN16 and PN40, with the bellows qualified to the body rating. A representative DIN bellows seal globe valve to EN 13709 is offered in PN16 and PN40 bodies, and per EN 12266-1 the body passes a shell strength test at 1.5 times nominal pressure while the bellows and stem seal pass a tightness test at 1.1 times nominal pressure at room temperature. For rotating bellows mechanical seals the limit is expressed as a pressure-velocity (PV) envelope rather than a single PN figure, and double-ply construction raises the pressure ceiling.

Temperature range is one of the strongest reasons to choose a bellows. Standard DIN bellows seal globe valves cover roughly minus 10 to plus 300 or 350 degrees Celsius depending on body material, and high-performance versions extend to minus 45 up to plus 650 degrees Celsius or above with the right alloy. Rotating metal bellows mechanical seals reach far higher than elastomer pusher seals: the John Crane Type 609 high-temperature seal is rated to about 425 degrees Celsius (800 degrees Fahrenheit), and EagleBurgmann MFLWT bellows seals cover roughly minus 40 to plus 400 degrees Celsius, with cryogenic MFLCT variants down to about minus 100 degrees Celsius. Because the bellows carries no elastomer, the upper limit is set by the alloy and the seal faces, not by an O-ring.

Cycle (fatigue) life is the headline durability figure for a valve bellows. A well-designed bellows seal valve is rated for more than 10,000 full open-close cycles, several times the switching life of a gland-packed valve, with the qualification cycle test run under maximum rated pressure per MSS SP-117 and minimum life cycles set by that standard. Stroke-to-free-length limit is the parameter that protects that life: as a rule the bellows stroke should not exceed about 25 percent of its free convolution length, and exceeding this ratio sharply shortens fatigue life, more so at high temperature and pressure. A selection that quietly over-strokes the bellows will pass the factory test and fail in the field.

Tightness class and emission test define what zero-leakage actually means on paper. The strongest claim is helium mass-spectrometer leak tightness, and against ISO 15848-1 a bellows seal valve typically reaches Class A, the lowest tightness class, with a helium leak rate no greater than 1.0E-5 mg per second per metre of stem perimeter. The relevant secondary seal arrangement should be stated explicitly: a compliant bellows seal valve has two backup seals, the stem backseat and the balanced-gland graphite packing, and the data sheet should name both.

The remaining parameters are configuration choices the buyer must specify:

• Bellows alloy and ply: 321, 316Ti, AM350, Inconel 625, Inconel 718, Hastelloy C-276, or Alloy 20, in single, multi, or double ply, matched to medium and pressure per Chapter 4.
• Forming method: hydroformed for valve stems, edge-welded for rotating seals and long-stroke duty, per Chapter 3.
• End / face configuration: for valves, butt-weld, flanged (EN 1092 / ASME B16.5), or threaded ends; for mechanical seals, the seal-face pairing such as carbon-graphite against silicon carbide.
• Stem-rotation prevention: a sleeve-nut or yoke-nut mechanism that converts handwheel rotation into pure linear stem travel, mandatory on every bellows seal valve.
• Actuation: manual handwheel, pneumatic or electric actuator; an actuator changes the cycle profile and must respect the same stroke limit.

Selection Decision Factors

To turn the preceding chapters into a specific model, follow the decision sequence below. Most bellows seal failures trace not to a bad part but to a premature decision at the wrong level, usually choosing the device before confirming the motion, or sizing the stroke before confirming the bellows free length. These steps double as an RFQ template.

1. Confirm the sealed motion first: linear stem (globe or gate bellows seal valve), quarter-turn (bellows-stem rotary valve), or continuous rotation (metal bellows mechanical seal). This single choice fixes the entire downstream specification, and getting it wrong invalidates every later step.
2. Justify the bellows against the alternative: a bellows is warranted when the medium is toxic, flammable, carcinogenic, regulated, high-permeability (heat-transfer oil), or under vacuum, or when temperature exceeds what an elastomer seal tolerates. If the medium is benign and low cycle, gland packing or a pusher seal is the correct, cheaper answer.
3. Set pressure and temperature: choose the PN class (PN16, PN40, or higher per EN 1902 / EN 13709) or the PV envelope for rotating seals, and the temperature band. Confirm the alloy supports the upper temperature; bellows valves reach plus 300 to plus 650 degrees Celsius and bellows pump seals reach about plus 425 degrees Celsius with the right alloy.
4. Match bellows alloy to medium: 321 or 316Ti for water, steam, and light hydrocarbons; Inconel 625 for hot chlorine, hydrogen, and TA-Luft duty; Hastelloy C-276 for wet chlorine and HCl; Alloy 20 for hot sulfuric acid. Derate corrosion allowance for the thin bellows wall and verify the manufacturer corrosion chart.
5. Verify the stroke-to-free-length ratio: confirm the bellows stroke stays at or below about 25 percent of free convolution length for the chosen valve travel. Over-stroking is the leading cause of premature field fatigue and is invisible on a factory leak test.
6. Specify forming, ply, and secondary seals: hydroformed or edge-welded, single or multi or double ply per pressure and stroke, and confirm the valve includes both the stem backseat and the balanced-gland graphite backup packing.
7. Require the emission and qualification evidence: ISO 15848-1 tightness Class A (or API 624 / 641 for North American projects) and TA-Luft compliance where regulated, plus MSS SP-117 qualification with helium leak test and maximum-pressure cycle test report for the bellows assembly.
8. Confirm actuation and total cost of ownership: manual handwheel with sleeve-nut, or pneumatic / electric actuator with the same stroke limit. Weigh the higher bellows purchase price against the avoided fugitive-emission penalties, fire risk, and packing-maintenance labour over the service life.

One dimension that buyers routinely underweight is manufacturer serviceability: availability of replacement bellows cartridges, documented helium leak-test certificates per shipment, alloy traceability for the thin bellows wall, and field support for actuator integration. A bellows seal protects against exactly the releases that shut a plant down, so a supplier who can prove qualification and resupply the part on a known lead time is worth more than a marginal price saving. Established makers of bellows seal valves and rotating metal bellows mechanical seals include John Crane (Type 609, 680), EagleBurgmann (MFL, MFLWT, MFLCT series), Flowserve, Velan, Conval, and TLV, all of which publish qualification data and maintain spare-parts and service networks.