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Skid Fabrication Companies
Process skid builders pre-assembling pumps, instruments, and pipework for fast on-site installation.
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Water Treatment Skid Fabrication: Design Standards, Materials, and Factory Acceptance Testing
Skid fabrication for water and process treatment integrates pumps, valves, instrumentation, pipework, and control panels onto a structural steel baseframe, delivering a pre-assembled, pre-tested unit that minimises site installation time and commissioning risk. Structural design: skid base frames are designed to BS EN 1993-1-1 (Eurocode 3, structural steel) or AISC 360 for North American projects; standard structural sections (RHS, SHS, I-beams) in S275 or S355 structural steel; skid frame stiffness is designed to limit deflection during lifting (typically 1/360 of span under full process weight); lifting lugs designed and certified by chartered structural engineer (CE marked, BS EN 13155 or BS EN ISO 10855 for offshore). Piping: process pipework on skids is engineered to BS EN 13480 (metallic industrial piping) or ASME B31.3 (process piping) for ASME-code projects; pipe material selection: 316L stainless steel for pharmaceutical, food, and high-purity applications (Ra roughness less than 0.8 um for hygienic pipework, BS EN 10357 tubes, orbital welded); duplex 2205 for seawater or high-chloride service; carbon steel ASTM A106 Grade B for non-corrosive process fluids; PVC/CPVC, PVDF, or GRP for aggressive chemical service. Skid weight: typical water treatment skid 1 to 50 tonnes; offshore process skids to 500 tonnes requiring heavy-lift crane planning; transport split for road transport restricted to 4.3 m width, 27.4 m length, 4.65 m height, and 44 tonnes gross vehicle weight (UK road haulage regulations).
Instrumentation and control integration: process control panels mounted on skid (IP54 minimum, IP65 for washdown environments; BS EN 60529); PLC selection (Siemens S7-1200/1500, Allen-Bradley CompactLogix/ControlLogix, Schneider Modicon); SCADA interface via Profibus, Profinet, Modbus TCP/IP, or OPC-UA; HMI (7 to 21-inch touchscreen, Siemens KTP or Allen-Bradley PanelView); remote access via secure 4G/5G modem or site Ethernet; IEC 62443 cybersecurity requirements for industrial control systems on water infrastructure. Instrumentation: flow meters (electromagnetic, Coriolis, or ultrasonic, to BS EN ISO 4064 or OIML R49 for custody transfer); pressure transmitters (4 to 20 mA, SIL-rated where safety function required, BS EN 61508); analytical instruments (conductivity, pH, ORP, turbidity, dissolved oxygen, chlorine residual) with 4 to 20 mA output or digital HART/Profibus; temperature sensors (PT100 class A, BS EN 60751). Electrical design: BS 7671 (IET Wiring Regulations, 18th Edition) for UK; hazardous area classification per BS EN 60079 and IECEx if process involves flammable or explosive atmospheres; ATEX equipment selection where required; motor starters (DOL, star-delta, or variable frequency drive); power distribution within skid on DIN rail with MCB, MCCB, or fuse protection.
Factory acceptance testing and quality assurance: FAT is conducted at the fabrication facility before dispatch, using the approved ITP (Inspection and Test Plan) witnessed by the client, third-party inspector (DNV, Bureau Veritas, Lloyds Register), or end-user representative. FAT scope: dimensional check against certified GA drawing; hydrostatic pressure test of pipework (1.5 times design pressure for ASME B31.3 Category D; 1.5 times MAWP for PED-classified pressure vessels, BS EN 13480); electrical continuity and insulation resistance test; control system function test (all I/O simulated; alarm and trip testing; SCADA communication verify); instrument calibration certificates verified; paintwork inspection (DFT measurement to specified coating system; BS EN ISO 12944 corrosivity category C2 to C5M). SAT (Site Acceptance Testing): repeat key functional tests after installation on site; production water quality verified against specification; motor rotation check; flow meter verification. Typical UK skid fabricators: WABAG, Veolia, Biwater, Arvia, Fluidity (John Crane Water), Nijhuis Saur Industries (formerly Biothane), and specialist fabrication shops (Hydro X, Keytas, Process Systems).
Frequently Asked Questions
What standards apply to water treatment skid fabrication?
Key standards for water treatment skid fabrication: Structural: BS EN 1993-1-1 (Eurocode 3, structural steel design); AWS D1.1 (structural welding, USA/international); DNV-OS-C401 (fabrication and testing of offshore structures). Pressure vessels: BS EN 13445 (unfired pressure vessels, EU); ASME VIII Division 1 (pressure vessels, ASME code); PED 2014/68/EU (Pressure Equipment Directive, UK retained as PSSR 2000 Pressure Systems Safety Regulations). Pipework: BS EN 13480 (metallic industrial piping); ASME B31.3 (process piping); BS EN 10357 (stainless steel tubes for food, beverage, pharmaceutical). Welding: BS EN ISO 3834 (quality requirements for fusion welding of metallic materials); BS EN ISO 15614-1 (welding procedure qualification for steel); BS EN ISO 9606-1 (qualification of welders); WRAS approval for drinking water contact materials. Electrical: BS 7671 18th Edition (IET Wiring Regulations); BS EN 60079 series (ATEX, hazardous area); IEC 62443 (cybersecurity for industrial control systems). Functional safety: IEC 61511 (safety instrumented systems, SIS) for safety-critical applications; SIL assessment (SIL 1, 2, or 3) for shutdown valves and emergency stop systems on chemical dosing or high-pressure systems. Coatings: BS EN ISO 12944 for paint systems; NACE RP 0188 for holiday testing of coatings. WRAS (Water Regulations Advisory Scheme) approval required for all wetted materials and components in contact with drinking water.
How is a water treatment skid sized and designed?
Water treatment skid sizing and design process: (1) Process design basis: source water quality (flow rate in m3/h, quality parameters, variability); product water quality requirement; operating hours per day and year; chemical dose requirements; backwash or regeneration flows and volumes. (2) Mass balance and hydraulic calculations: flow balances (feed, product, reject, recycle streams); sizing of pumps (Q-H curve; NPSH available vs NPSH required; BS EN ISO 9906 hydraulic performance); sizing of vessels (contact time calculation for DAF - 10 to 20 minutes; filter bed depth and EBCT for carbon contactors; membrane system flux and recovery). (3) Piping and instrument diagram (P&ID): developed to ANSI/ISA 5.1 symbols; reviewed by process engineer, instrument engineer, and control system engineer; issued as IFR (Issued for Review) then IFC (Issued for Construction) after client approval. (4) 3D modelling: Autodesk Plant 3D, AVEVA E3D, or PDMS used for skid layout; minimum clearances: 600 mm on pump seal side; 1,000 mm on valve actuator operating side; 750 mm between parallel pipe runs; headroom for overhead maintenance. (5) Structural sizing: skid weight (operating, hydro-test, and empty weights calculated); skid stiffness check; lifting lug design (4-point lift standard; 2:1 safety factor minimum on each lug at worst-case lift angle 45 degrees). (6) Design review: HAZOP for safety-critical skids; HAZOP report with action register resolved before FAT.
What materials are used in skid pipework for water treatment?
Skid pipework material selection for water treatment: (1) 316L stainless steel: standard for drinking water contact (BS EN 10357, smooth bore, orbital welded to Ra less than 0.8 um hygienic standard); pharmaceutical purified water and WFI systems (passivated to ASTM A380, electropolished interior); food and beverage (EHEDG guidelines; 3-A Sanitary Standards). (2) 304 stainless steel: lower-cost alternative to 316L for non-aggressive water applications; not suitable for chloride-containing water above 200 mg/L Cl- (pitting corrosion risk) or seawater service. (3) Duplex 2205 (UNS S31803): higher corrosion resistance and strength than 316L; selected for seawater, high-chloride brackish water, or sodium hypochlorite service above 2 percent concentration; 20 to 40 percent premium over 316L but eliminates stress corrosion cracking risk. (4) Carbon steel ASTM A106 Grade B / BS EN 10216-2: for service water, cooling water, and raw water where corrosion is managed by chemical treatment or internal coating; hot-dip galvanised (BS EN ISO 1461) or epoxy-coated internally for potable water contact (DWI/WRAS approved coatings). (5) PVDF (polyvinylidene fluoride): chemical resistance to acids, alkalis, and halogens; used in ozone, chlorine, and chemical dosing pipework; rated to 140 degrees C; electrofusion or butt-welded joints. (6) CPVC: lower cost than PVDF; suitable to 93 degrees C; common for chemical dosing pipework up to 200 mm diameter. (7) GRP (glass reinforced plastic, filament wound to BS EN 1120 or AWWA C950): large diameter corrosion-resistant pipework for aggressive chemical service at lower cost than metallic alternatives.
What is a factory acceptance test for a water treatment skid?
A factory acceptance test (FAT) is a formal, witnessed test of a water treatment skid at the fabrication facility before dispatch to site, verifying that the unit meets contract specifications, design documents, and applicable codes. FAT participants: client's project engineer and instrument/control engineer; third-party inspection body (DNV GL, Bureau Veritas, SGS, TUV) if specified in contract; equipment vendors for specialist items (membrane manufacturer, analyser supplier). FAT procedure document: issued by fabricator 4 to 6 weeks before FAT date; client review and approval; specifies each test, acceptance criterion, and hold point (HP - fabricator must wait for client sign-off before proceeding) vs witness point (WP - client should attend but fabricator may proceed if client gives written waiver). Typical FAT checks: dimensional verification (skid footprint, nozzle positions vs certified GA drawing; tolerance plus or minus 5 mm for pipework nozzles); pipe hydrostatic test (1.5 times design pressure, 30 minutes, no visible leaks; BS EN 13480 or ASME B31.3); instrument loop checks (all I/O signals verified from field instrument to PLC/SCADA; simulated signals for 4 to 20 mA transmitters; valve travel and position feedback verified); alarm and trip testing (every high/high and low/low trip function activated and PLC response verified); control logic function test (automatic sequences run through in simulation mode). FAT punch list: deficiencies categorised as A (must fix before dispatch), B (fix before SAT), or C (cosmetic, acceptable on delivery); A-items closed and re-inspected before skid shipped.
A sterile injectables manufacturer in Staffordshire required a replacement purified water skid to BS EN 13480 hygienic standard within a 14-week programme to avoid a planned line shutdown overrun. The previous skid was carbon steel with polyethylene liners that had failed a DWI extraction test, prompting a compliance notice from their site quality team ahead of an MHRA inspection.
A specialist skid fabricator delivered a 316L stainless steel, orbital-welded, double-pass RO plus continuous electrodeionisation (CEDI) skid with Ra less than 0.8 um electropolished wetted surfaces. The P&ID was issued for HAZOP review at week two; FAT was completed at the fabricator's Coventry facility at week 10 using simulated SCADA I/O and witnessed by the client's QA manager and a Lloyds Register inspector. All A-category punch items were closed within 48 hours of FAT.
The skid was delivered and installed on time in week 13 and passed SAT (site acceptance test) including a 72-hour production water quality verification run showing conductivity 0.08 uS/cm and TOC 28 ppb, well within USP Purified Water limits. The MHRA inspection found the installation compliant with GAMP5 documentation requirements. Total skid supply cost was GBP 285,000 against a project budget of GBP 310,000.
Questions to Ask Shortlisted Providers
- 1
What welding qualification records and weld procedure specifications (WPS) will you provide for all process pipework joints?
For pharmaceutical and food-grade 316L pipework, BS EN ISO 9606-1 welder qualification and BS EN ISO 15614-1 WPS are mandatory; without them, the skid will fail MHRA or FDA inspection.
- 2
Is the control system designed to IEC 62443 security levels and does the SCADA interface use an OPC-UA encrypted protocol?
Water infrastructure skids connecting to plant SCADA must comply with NIS Regulations 2018 cybersecurity obligations; an insecure OPC-DA legacy connection will trigger a corrective action during an OT security audit.
- 3
How is the skid hydrostatic test pressure determined and will you provide a traceable pressure gauge calibration certificate?
BS EN 13480 requires 1.5 times maximum allowable operating pressure; using an uncalibrated gauge invalidates the test certificate and creates a PSSR 2000 compliance gap.
- 4
What is your minimum order value for WRAS-approved wetted component substitutions and how do you handle late specification changes?
WRAS substitutions midway through fabrication trigger a re-extraction test and can add 4 to 6 weeks to the programme; the contract must clarify who bears this cost and time.
- 5
Will you provide as-built documentation including a final updated P&ID, instrument calibration certificates, and ATEX declarations of conformity as part of the delivery package?
Without a complete as-built dossier, the client cannot fulfil CDM 2015 Health and Safety File obligations or complete PSSR written scheme of examination for the skid's pressure vessels.
What Drives Cost in This Category
Upgrading from 304 to 316L stainless for chloride-containing water service adds 15 to 25 percent to pipework material cost; specifying duplex 2205 for seawater service doubles the pipework material cost versus 316L.
Orbital welded, internally electropolished pipework (Ra less than 0.8 um) for pharmaceutical applications costs 40 to 60 percent more than standard manual-welded carbon steel pipework; this is non-negotiable for USP Purified Water compliance.
A simple PLC with standalone HMI costs GBP 15,000 to 30,000; a SIL 2-rated safety instrumented system with dual-redundant PLCs, remote SCADA, and IEC 62443 cybersecurity architecture costs GBP 80,000 to 200,000 for a medium-complexity process skid.
A Lloyds Register or Bureau Veritas FAT witness adds GBP 3,000 to 8,000 in inspection fees plus travel; radiographic weld examination (RT, 10 percent spot check) adds GBP 2,000 to 6,000 for a medium-size skid.
Key Regulations & Standards
Water treatment skids containing pressure vessels above the Article 4 PED threshold must have a written scheme of examination (WSE) prepared by a competent person before the skid is brought into use; WSE defines inspection intervals and maximum allowable working pressure.
All wetted materials and components on skids producing water for human consumption must be WRAS-approved (BS 6920 extraction test); WRAS approval is a material-specific and product-specific certification, not a company-level approval.
All fusion welds on process pipework must be made by welders qualified to BS EN ISO 9606-1; welder qualification certificates must be current (typically 2-year renewal with ongoing production test) and provided in the FAT documentation package.
Skids installed in ATEX Zone 1 or Zone 2 classified areas (chemical dosing rooms, biogas handling) must use ATEX-rated electrical equipment (Category 2G or 3G); ATEX declarations of conformity for each item must be included in the as-built dossier.
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