Petrochemicals and refining
ANSI flanges · process lines
Flanges in high-pressure, high-temperature process lines. Typical combination: spiral-wound gasket + Inconel 718 Belleville on critical bolts.
Flange
washers
Leak prevention in flanged joints exposed to thermal cycling, vibration or gasket relaxation.
Temperature range
−240 °C → +650 °C
Materials
17-7 PH · Inconel 718 · H-13
Engineering support
In-house engineering team since 1974
01
What is a flange washer
A flange washer —also called a pressure disc spring, disc spring or coned-disc washer— is a conical elastic component fitted under the nut or bolt head in a flanged joint to maintain a constant axial load on the gasket.
Unlike flat washers, its frustoconical geometry stores elastic energy that compensates for the loss of preload caused by:
- Bolt relaxation (creep / stress relaxation) at service temperature.
- Differential thermal expansion between flange, bolt and gasket — different expansion coefficients.
- Embedment and creep of the sealing gasket material.
- Vibration and dynamic loads that tend to loosen the connection.
These washers are not bound to a fixed standard DIN 2093 diameter: they are sized specifically for the bolt diameter and the clamping load required on flanges to standards such as ASME B16.5, DIN EN 1092-1 or others. This technical function is known as live loading of the flanged assembly.
PHOTO + DIAGRAM · flange assembly with a Belleville under each nutBelleville washers → marked in red
02
Failure mechanisms · when they are justified
Fitting flange washers is justified when the joint is exposed to any of the following scenarios. The washer acts as a load reserve: when the gasket or the bolt loses effective length, the washer expands and restores the clamping force.
01
Thermal collapse · thermal shock
Flange, bolt and gasket have different expansion coefficients. During a thermal cycle some part loses effective contact and the joint separates momentarily: a leak appears. The pre-compressed Belleville washer expands and restores the clamping force at every instant of the cycle.
02
Mechanical collapse
Fully rigid contact causes two opposing modes: loss of load through gasket settling, or excessive values that break bolts or gaskets. The elasticity of the Belleville absorbs these stresses and keeps the force within the operating range.
03
Bolt relaxation · creep
At service temperature the bolt loses effective length through stress relaxation. The washer —which was pre-compressed— recovers its free height and compensates for that loss without the need for manual re-tightening.
04
Electrical connections · Joule effect
In flanged electrical connections, Joule-effect heating produces cyclic expansion between parts of different materials. The Belleville maintains continuous preload and prevents the hot spots that occur when the connection cannot dissipate the heat generated.
↘ Typical configuration: a pair of Belleville washers under each nut of the electrical terminal, usually in 17-7 PH or Inconel 718 depending on the service temperature.
PHOTO · flanged electrical connection · sectionwashers in red
Terminals connected by a through-bolt flange. The Belleville washers absorb the cyclic expansion from the Joule effect and maintain the contact pressure between surfaces.
— Functional summary
The Belleville washers elastically absorb the stresses that would cause a leak (separation) or failure (overload), keeping the preload within the operating range throughout the entire service cycle.
03
Materials · three alloys for three ranges
Surisa manufactures flange washers in three materials that cover virtually every industrial application. The choice depends on the service temperature, the working atmosphere (corrosion) and the frequency of thermal cycling.
17-7 PH
AISI 631 · 1.4568
up to ~315 °C
Corrosion
Good (precipitation semi-austenitic)
Mechanical
High strength-to-weight ratio. Retains its elastic properties at moderate temperature and behaves stably in cryogenic service.
Application
Flanges in the chemical industry, electrical connections, moderately corrosive environments, cryogenic equipment down to −240 °C.
Inconel 718
NiCr19NbMo · 2.4668
up to ~650 °C
Corrosion
Excellent · oxidation and severe chemical attack
Mechanical
Creep resistance and thermal-fatigue resistance thanks to γ″ precipitation (Ni₃Nb). Retains its yield strength at high temperature.
Application
High-temperature petrochemicals, steam generation, turbines, exhausts, chemical plants with aggressive fluids.
H-13
X40CrMoV5-1 · 1.2344
up to ~540 °C
Corrosion
Limited · not stainless
Mechanical
Cr 5 % hot-work steel. Hardness retained at elevated temperatures. Excellent thermal-fatigue behaviour.
Application
Flanges in high-temperature process lines without corrosive agents. Applications with intense thermal cycling.
The ranges are indicative for preliminary design. The admissible service temperature also depends on the load, the frequency of thermal cycling and the atmosphere.
Let's talk about your project
Tell us about your use case and our engineering team will help you choose the optimal solution.
04
Temperature range · -240 °C → +650 °C
All three materials overlap in the central range of conventional service. The critical difference lies at the extremes: cryogenics at the bottom, high-temperature creep at the top.
Inconel 718 is the only material suitable for continuous service above 550 °C, thanks to its γ″ precipitation. 17-7 PH is the default choice when there is moderate corrosion or cryogenic service. H-13 covers the middle ground in non-corrosive applications where thermal cycling is intense.
For the upper limit of the range by material, we recommend validation with a specific relaxation calculation at the actual service temperature.
FIG · service temperature ranges by material
05
Applications · sectors where they are critical
Flange washers are the standard when a gasket leak has critical consequences (safety, plant shutdown, electrical inefficiency) and the environment makes the loss of initial preload inevitable.
Power generation
Steam · combined cycle
Heat exchangers, turbines and superheated-steam piping where thermal cycling is continuous and gasket failure has critical impact.
Chemical industry
Corrosive environments
Plants with aggressive fluids where stainless steel or a nickel alloy is mandatory. 17-7 PH for moderate corrosion, Inconel 718 for severe environments.
Power electrical connections
Substations · transformers
Flanged connections in industrial switchgear, transformers and substations. The Belleville absorbs the cyclic expansion from the Joule effect and prevents hot spots.
Cryogenics
LNG · air separation · liquid H₂
Equipment down to −240 °C where 17-7 PH stainless and Inconel 718 maintain stable elastic behaviour. Critical in LNG and liquid-hydrogen plants.
Industrial valves and gaskets
Dynamic sealing · flange closure
Flanged closures on valves with a stem, where leakage is a direct function of maintaining the preload on the gasket assembly.
06
How to determine the tightening torque
The tightening torque in a flanged joint with Belleville washers is not a catalogue value: it depends on several coupled factors that must be assessed together.
The main factors are: bolt diameter and class, thread-nut friction coefficient (a function of the lubricant and coating), clamping load required by the gasket and the load-deflection curve of the selected washer. The stack configuration significantly modifies the latter.
The usual procedure is: (1) define the clamping load required on the gasket, (2) select the Belleville washer whose F-s curve delivers that load within its elastic working zone, (3) calculate the bolt torque needed to reach the total preload with the actual friction coefficient.
07
Frequently asked questions
01 What is the difference between a DIN 6796 Belleville washer and a flange washer?
The DIN 6796 are standardised lock washers for structural fasteners, designed to prevent loosening of bolted joints and to work almost flat. Flange washers are pressure disc springs designed to maintain a significant elastic preload throughout the life of the joint, with a load-deflection curve calculated to compensate for thermal and mechanical relaxation. The former are an anti-loosening safety element; the latter are an active load-maintenance element (live loading).
02 Up to what temperature can I use a flange washer?
It depends on the material. 17-7 PH stainless steel maintains stable elastic properties up to around 315 °C. Inconel 718 is the option for continuous service up to around 650 °C thanks to its creep resistance. H-13 tool steel covers the intermediate range in non-corrosive applications up to ~540 °C. Below zero, 17-7 PH and Inconel 718 maintain stable behaviour down to −240 °C. For service at the limit of the temperature range we recommend validation with a specific calculation.
03 Can I replace a spiral-wound gasket with flange washers?
No: they are complementary elements, not alternatives. The gasket remains the primary sealing element. Belleville washers are fitted under the bolt nut to maintain the preload on that gasket when the system undergoes thermal cycling, vibration or relaxation. The combined installation (spiral-wound gasket + Belleville washers on the bolts) is common in critical petrochemical and power-generation flanges.
04 How do I calculate how many Belleville washers I need per bolt and in what orientation?
They are determined from three pieces of data: the clamping load required on the gasket, the maximum deflection expected from expansion and relaxation, and the load-deflection curve of the selected model. Stacking in parallel adds loads; stacking in alternating series adds deflections. Surisa has calculation software and engineers who size the stack according to the specific flange and service conditions; send us the joint data (flange standard, bolt, temperature, gasket) and we will return the recommended configuration.
05 Do Belleville washers loosen or tighten the bolt?
Neither on their own: they keep the axial load constant. When the bolt relaxes with temperature or the assembly settles, the washer —which was pre-compressed— expands, recovering length and restoring the load. They do not act as an anti-rotation lock (unlike toothed washers), but by maintaining the clamping load they keep the bolt out of the friction-loss zone where it could loosen.
06 Does Surisa manufacture special sizes or only catalogue items?
We work with a standard catalogue for fast service (24 h in mainland Spain, 48 h in Europe) and we manufacture custom sizes for non-standard flanges, specific materials or certification requirements. Surisa has manufactured since 1974 and keeps stock in the three main materials to respond to industrial emergencies.
07 Why don't these washers appear in DIN 2093?
DIN 2093 standardises diameters and loads for general-purpose disc springs. Flange washers are sized specifically for the bolt diameter and the clamping load required by the gasket according to ANSI B16.5, DIN EN 1092-1 or other systems — which is why they do not automatically fall on a fixed DIN 2093 Ø, even though they share the geometric principle of the DIN 2092 standard.
Let's talk about your project
Tell us about your use case and our engineering team will help you choose the optimal solution.