Flange joints are central components in pipeline and plant engineering. They enable detachable, tight, and controllably load-bearing connections between pipes, fittings, vessels, and machines. In demolition, deconstruction, and repair, flange joints are encountered daily: in industrial plants, on tanks and containers, in utility corridors, and in tunnel structures. For tasks such as safely loosening, separating, or exposing such connections, precise procedures and suitable tools are required depending on the situation — from concrete pulverizers for removing concrete at embedments to steel shears or a cutting torch when worn bolts, thick-walled flanges, or flange-adjacent steel components must be separated. When electric or pneumatic operation on site is not economical or safe, compact hydraulic power units for mobile tools provide the required energy.
Definition: What is meant by a flange joint
A flange joint is a detachable, force- and form-fit connection of two components using matching flanges that are clamped together by bolts and nuts. Tightness is achieved by deliberately applying a bolt preload that loads the seal between the sealing faces. Flange joints are standardized worldwide (e.g., to common European and American standards) and are selected according to nominal pressure rating, temperature range, nominal size, sealing face geometry, and material. They are demountable, allow inspection and replacement of components, and are therefore widespread in pipework, pump, compressor, and vessel connections.
Design and components of a flange joint
The classic build-up includes:
- Flanges with bolt pattern (bolt circle), sealing faces (e.g., smooth, raised face, or tongue-and-groove) and a hub or collar suited to the application (e.g., weld neck flange, lap joint flange, blind flange).
- Bolts, nuts, and washers whose strength class, coating, and length must match the thickness of the flanges, the seal, and the nominal pressure rating.
- Seal (soft-material based, fiber-reinforced, PTFE, spiral wound, metallic), sized according to sealing face geometry, medium, temperature, and pressure.
- Surfaces with defined roughness, since surface condition significantly influences sealing performance.
In existing installations, flange joints may be embedded in concrete or mortar (e.g., at foundations, penetrations, brackets). For exposing such areas during deconstruction, concrete pulverizers are often used to remove concrete cover and make anchor plates accessible. In heavily reinforced areas or massive foundation pedestals, concrete splitters as well as stone-splitting cylinders are considered to induce controlled cracks and separate components without blasting.
Flange types, sealing principles, and standards
Flange types include weld neck flanges, lap joint flanges (with stub end), slip-on and threaded flanges, as well as blind flanges. Sealing faces range from smooth faces to raised faces and tongue-and-groove to ring grooves (for metallic ring gaskets). The selection of the seal follows the sealing principle:
- Soft-material/fiber and PTFE seals for broad application ranges, good conformability, limited surface pressure.
- Spiral wound seals for higher pressures and temperatures, robust sealing performance with suitable preload.
- Metallic seals (e.g., Ring Joint) for high loads, demanding surface quality, and defined clamping.
Standards define pressure series (e.g., PN classes) or pressure ratings, dimensions, material requirements, sealing face forms, and test conditions. For planning, assembly, and testing, guidelines on bolt tightening, lubrication, tightening patterns, and documentation are also relevant.
Design: preload, tightness, and tightening patterns
The tightness of a flange joint results from the balance of bolt preload, gasket surface pressure, and operating loads (pressure, temperature, vibration). The design considers:
- Bolt forces and tightening factor (influence of friction, lubrication, surface condition).
- Gasket characteristics (required minimum surface pressure, permissible maximum pressure, recovery).
- Thermal effects (embedment, relaxation, temperature cycles) and operating loads (bending moments from pipeline loads).
Preparations
- Clean and inspect sealing faces; professionally rework if damaged.
- Check the seal (type, dimensions, medium, temperature); do not reuse old seals.
- Select bolts/nuts according to strength class and length; lightly lubricate threads and bearing surfaces (if permitted).
Stepwise tightening
- Align components, center the seal, insert bolts finger-tight.
- Tighten in a cross pattern in at least three stages (e.g., 30/60/100% of target value).
- Check runout and perform a final pass using a torque or angle-controlled method.
Documentation
- Document tightening parameters, lubricant, sequence, and date.
- If required, perform controlled retightening after a temperature or pressure cycle.
Assembly and disassembly practice in deconstruction
In concrete demolition and special deconstruction as well as in gutting works and cutting, flange joints often have to be loosened, separated, or secured. The approach depends on the condition of the bolts, accessibility, and the environment (e.g., ATEX zone, potentially explosive media, residual contents in tanks):
Disassembly on pipelines and equipment
- Depressurize and purge the system, secure it, and release any remaining energies.
- Heat corroded or seized bolts in a targeted way only if safe; alternatively, cold-cut. Steel shears or Multi Cutters can cut bolts, nuts, and studs without the sparks associated with thermal cutting.
- For flanges with welded nozzles, cut close to the flange if needed; a cutting torch is suitable for thick-walled vessel openings and flange welds when large cut-outs are required.
- Support loads, attach lifting devices, and secure the flange halves against uncontrolled springing open (consider residual compression of the seal).
Exposing flange-related embedments in concrete
- Expose embedments, brackets, or penetrations step by step with concrete pulverizers; separate reinforcement in a controlled manner (e.g., with combination shears or steel shears).
- Divide massive foundations that carry flange supports into blocks using concrete splitters to keep vibrations low.
- Hydraulic power packs supply energy for mobile tools; selection depends on output, hose lengths, and workplace conditions.
Typical failure modes and causes of damage
- Under- or overtightening leads to leakage or gasket damage (blowout, extrusion).
- Incorrect seal selection (medium, temperature, sealing face form) promotes early failure.
- Damaged sealing faces (grooves, notches, corrosion) reduce the effective sealing area.
- Embedment/relaxation due to soft seals or coatings reduce preload.
- Misalignment/assembly stresses from pipeline loads add additional stress to bolts and seal.
- Corrosion (crevice, pitting, or galvanic) weakens bolts and flange areas; in deconstruction this complicates loosening and may require cold-cutting methods.
Testing and proof of tightness
After assembly, flange joints are visually inspected (gasket protrusion, bolt projection, uniformity) and the tightening parameters documented. Tightness tests are performed hydrostatically or pneumatically depending on the rule set. Leak detection can be performed with suitable test media and measurement methods. Tests must be carried out with appropriate protective measures; special caution and reliable isolation are required for pressure tests. The information is general; the applicable standards and operating instructions must be observed.
Materials, surfaces, and corrosion protection
Flanges and bolts are often made of unalloyed or stainless steels; alloyed materials are common for special media. Coatings and galvanic separation layers reduce corrosion. Sealing faces require defined roughness; improper grinding or blasting impairs sealing function. In deconstruction, a clean cut line facilitates material separation: cut edges produced with steel shears on flanges, studs, and sections are easy to handle and support construction waste separation and recycling.
Safety and environmental protection for work on flange joints
- Residual media hazard: drain, flush, and degas before work; document release.
- Avoid sparks and ignition sources; cold-cutting methods are often advantageous in sensitive areas.
- Load handling: control flange gaps, secure components, and be mindful of pinch and shear points.
- Minimize dust and noise; when removing concrete with concrete pulverizers, provide protective measures for personnel and surroundings.
- Work permits, hazard analysis, and briefing must be prepared before starting; information is general and does not replace company-specific requirements.
Application contexts in construction, industry, and infrastructure
In gutting works and cutting of buildings, flange joints occur on heating, ventilation, and media lines. In the concrete demolition and special deconstruction of industrial plants, large nominal sizes, thick-walled flanges, and complex embedments are to be expected. In rock excavation and tunnel construction, flange joints are widespread on pump systems, ventilation lines, and shotcrete systems; access is often restricted, which is why compact, hydraulically driven cutting and gripping tools are practical. In natural stone extraction, flanges appear on processing and conveying equipment; maintenance and replacement require clean disassembly. For special operations (e.g., tanks and vessels with flange-mounted internals), a cutting torch, steel shears, and Multi Cutters are just as applicable as concrete pulverizers for removing foundation areas.
Practice-oriented workflows
Assembly sequence in repair
- Identification: nominal size, pressure rating, material, sealing face form, seal data.
- Preparation: cleaning, inspection, replacement of damaged components.
- Assembly: alignment, stepwise tightening in a cross pattern, documentation.
- Testing: tightness test and, if applicable, pressure test, release.
Disassembly sequence in deconstruction
- Secure: depressurization, medium-free condition, electrical/thermal releases.
- Loosen: undo bolts in an orderly manner; cold-cut stuck fasteners (steel shears/Multi Cutters) or, if permitted, apply controlled heating.
- Separate: cut close to the flange, section components; use a cutting torch for large openings.
- Expose: concrete pulverizers for concrete removal; concrete splitters on massive components to minimize vibrations.
Notes on quality assurance
- Use only matching seals and new bolt sets; check markings.
- Apply lubricant and tightening procedures consistently; avoid mixed practices.
- Visually inspect sealing faces and cut edges after separation work to rule out consequential damage or cracks.
- Keep measuring and testing equipment calibrated; document measured values traceably.