Overview – Skid as Integrated Process Module
A process skid (skid-mounted equipment or modular unit) is an integrated assembly of process equipment, piping, valves, instrumentation, and control systems mounted on a common structural steel base frame or within a containerized enclosure . The skid is designed as a functionally complete, self-contained unit that is fully prefabricated, assembled, and tested at the manufacturer's facility prior to shipment .
Skid-mounted construction enables modularization of industrial processes, reducing field installation work to positioning the skid, securing it to foundations, and connecting utility and process interfaces . This product is designed and manufactured per ASME Section VIII Division 1, GB/T 25198, PED 97/23/EC, or CE/CPR requirements, with material selection and welding per applicable pressure equipment directives .
Skid Classification – Structural Types
Open-Frame Skid
- Construction: Structural steel channel or I-beam base frame (open structure)
- Equipment: Mounted directly on the frame; piping and instrumentation exposed
- Application: Indoor installations, moderate environments, where visual inspection and maintenance access are required
- Advantages: Lower weight, easier access for maintenance, lower cost
- Limitations: Less protection from weather and corrosive atmospheres; requires additional shelter in outdoor service
Containerized Skid (Enclosed)
- Construction: Equipment housed within a shipping container or custom enclosure
- Equipment: Fully enclosed with access doors and ventilation
- Application: Outdoor installations, harsh environments (offshore, desert, arctic), rapid deployment
- Advantages: Weather protection, temperature control, vandalism prevention, transport-ready
- Limitations: Higher cost, limited access for maintenance, increased weight
Skid Classification – Functional Types
| Type |
Primary Function |
Typical Components |
Application |
| Pump skid |
Fluid transfer / boosting |
Pumps (centrifugal, positive displacement), suction diffusers, discharge headers, VFDs, check valves |
Water injection, chemical dosing, oil transfer |
| Compressor skid |
Gas compression |
Compressors, suction scrubbers, aftercoolers, pulsation dampeners |
Gas boosting, vapor recovery, LNG |
| Chemical injection skid |
Additive dosing |
Storage tanks, metering pumps, flow meters, check valves, control panel |
Corrosion inhibition, scale prevention, hydration suppression |
| Heat exchanger skid |
Heat transfer |
Shell & tube or plate heat exchangers, piping, temperature control valves |
Process heating/cooling, waste heat recovery |
| Separation skid |
Phase separation |
Separator vessels, level controls, dump valves, mist eliminators |
Oil-water-gas separation, condensate knockout |
| Metering skid |
Flow measurement |
Coriolis/ultrasonic meters, flow straighteners, pressure/temperature sensors |
Custody transfer, process monitoring |
| Gas conditioning skid |
Gas treatment |
Filters, coalescers, heaters, pressure regulators |
Dew point control, solids removal |
Structural Design – Base Frame Parameters
Base Frame Material
- Carbon steel: A572 Gr.50 or equivalent – standard structural frame
- Stainless steel: 304L / 316L – for corrosive environments or food-grade service
- Aluminum: 5-series or 6-series – for weight reduction (e.g., mobile refueling skids)
Frame Dimensions (Typical)
| Parameter |
Range |
Notes |
| Length |
2.0m – 12.0m |
Limited by transport and lifting capacity |
| Width |
1.5m – 4.5m |
Road transport limit: 4.5m (standard) |
| Height (equipment) |
2.0m – 6.0m |
Transport height limit: 4.0m (highway) / 6.0m (special) |
| Frame beam height |
150mm – 400mm |
Selected per equipment load and span |
| Lifting lug rating |
2× skid weight (minimum) |
Per ASME / design safety factor |
| Fork pocket dimensions |
Standard or custom |
For forklift handling during installation |
Equipment Layout – Design Principles
Per modularization design practice, skid layout follows these principles :
- Vertical arrangement: Tall equipment placed at skid center; shorter equipment and valves located at perimeter for maintenance access
- Weight distribution: Heavy equipment positioned over structural supports; center of gravity within 1/3 of skid footprint
- Maintenance access: Minimum walkway width ≥ 900mm; overhead clearance ≥ 2,150mm
- Piping routing: Process piping minimized for compactness; flanged connections at skid boundaries for field tie-in
- Instrumentation access: Transmitters and control valves accessible from walkways or platforms
Piping System – Materials and Fittings
Pipe Materials
| Service Temperature |
Material |
Specification |
| -20°C to +425°C |
Carbon steel 20# / SA-106 Gr.B |
GB 8163 / ASTM A106 |
| -196°C to +600°C |
Stainless 304L / 316L |
ASTM A312 / GB/T 14976 |
| -40°C to +280°C |
Duplex 2205 |
ASTM A789 |
| Elevated temperature (> 425°C) |
Alloy (1.25Cr-0.5Mo / 2.25Cr-1Mo) |
ASTM A335 |
Pipe Fittings and Flanges
- Fittings: Seamless per GB/T 12459 or ASME B16.9
- Flanges: Weld neck or slip-on per ASME B16.5 / HG/T 20615 (Class 150 – 2500)
- Flange sealing face: Raised face (RF) – standard; RTJ for high pressure or hydrogen service
- Gaskets: Spiral wound (SS + graphite) with inner/outer rings; PTFE for instrumentation
- Bolting: Stud bolts per HG/T 20634; material 35CrMo with 30CrMo nuts, zinc-plated
Welding and Inspection
- Pipe welding: GTAW (root pass) + SMAW or SAW (fill) – per ASME Section IX
- Weld examination:
- All butt welds: 100% RT (X-ray) per JB/T 4730 / ASME V – Grade II acceptance
- Fillet welds: 100% MT or PT – Grade I acceptance
- Post-weld treatment: Pickling and passivation for stainless steel; PWHT per UCS-56 for carbon steel > 38mm thickness
Skid Integration – Typical Equipment Roster
A process skid may integrate the following equipment (selection based on process function) :
Rotating Equipment
- Pumps: Centrifugal (ANSI / API 610) or positive displacement (gear, diaphragm, peristaltic)
- Compressors: Reciprocating, screw, or centrifugal (API 618 / 619)
- Drivers: Electric motors (TEFC, explosion-proof per NEC / ATEX), diesel engines, or pneumatic
- Variable frequency drives (VFDs) for speed control
Static Equipment
- Pressure vessels: Separators, filters, coalescers, surge drums (per ASME VIII-1)
- Heat exchangers: Shell & tube, plate-and-frame, or finned tube (per TEMA)
- Storage tanks: Atmospheric or pressurized (vertical/horizontal)
- Piping manifolds: Suction and discharge headers with block and check valves
Instrumentation and Control
- Flow meters: Coriolis, ultrasonic, magnetic, or turbine
- Pressure transmitters (PT) and temperature transmitters (TT)
- Level transmitters (LT) and level switches (LS) on vessels
- Control valves (CV) with pneumatic or electric actuators
- Safety instruments: Pressure relief valves (PRV), emergency shutdown (ESD)
- Programmable logic controller (PLC) or distributed control system (DCS) interface
- Control panel: Explosion-proof enclosure (NEMA 4X / IP65)
Piping and Valves
- Isolation valves: Ball valves (full port) and gate valves
- Check valves: Swing or spring-loaded – prevent backflow
- Instrument isolation: Needle valves (316SS) for pressure/temperature tap connections
- Drain and vent: Ball valve + needle valve combinations for safe venting
Fabrication Process – Skid Assembly Sequence
- Structural frame fabrication: Base frame welded from structural steel; drilling for equipment mounting and lifting lugs
- Equipment placement: Major equipment (vessels, pumps, compressors) positioned and bolted/welded to frame
- Piping installation: Process and utility piping installed per isometric drawings; flanged connections at skid perimeter
- Instrumentation installation: Transmitters, control valves, and sensors mounted; tubing and wiring routed
- Electrical installation: Motors, VFDs, control panels, and junction boxes wired; cabling in conduits or cable trays
- Insulation and cladding: Thermal insulation applied to hot/cold piping; aluminum cladding or jacketing
- Surface treatment: Sandblasting (Sa2.5 grade) per GB/T 8923; primer (zinc-rich epoxy), intermediate, and polyurethane finish coat; minimum dry film thickness ≥ 250μm for carbon steel structures
- Testing and pre-commissioning: Hydrostatic testing, pneumatic leak testing, instrumentation loop checks, and functional testing
Shop Testing – Factory Acceptance Test (FAT)
Per skid manufacturing practice, FAT is conducted at the fabricator's facility prior to shipment :
| Test Type |
Procedure |
Acceptance Criteria |
| Hydrostatic test (piping and vessels) |
1.3 × design pressure per ASME UG-99; hold ≥ 30 minutes |
Zero pressure drop; no visible leakage |
| Pneumatic leak test |
0.6 MPa air or nitrogen; soap bubble application |
No bubbles at joints; leakage ≤ 1×10⁻⁵ Pa·m³/s |
| Instrument loop test |
Simulated 4-20mA signals; verify PLC/DCS readout |
±0.5% of span accuracy |
| Functional test |
Operate pumps/compressors at minimum and normal speed |
Mechanical integrity; vibration within limits |
| Electrical continuity test |
Megger test for insulation resistance |
≥ 1 MΩ at 500V |
| Painting inspection |
Dry film thickness gauge – 5 random points |
≥ 250μm (CS structure); ≥ 100μm (galvanized structure) |
Transport and Lifting Considerations
Skid dimensions and weight are constrained by transport and lifting equipment capabilities :
- Road transport: Width ≤ 4.5m; Height ≤ 4.0m (standard); Length ≤ 12.0m (standard)
- Shipping container: 20ft (6.1m) or 40ft (12.2m) for containerized skids
- Lifting: Skid mass ≤ 30,000kg for single-crane lift; heavier units split into modules
- Lifting lugs: Designed per ASME BTH-1; rated at 2× skid weight
- Transport fixings: Clamp or bolt skid to transport frame; protect instruments with covers
Field Installation Requirements
Upon arrival at site, the following installation steps are required :
- Foundation preparation: Reinforced concrete foundation with anchor bolts; flatness ≤ 1mm/m
- Skid positioning: Crane lifting onto foundation; align with anchor bolt template
- Leveling: Shim plates to achieve levelness ≤ 1mm/m (longitudinal) and ≤ 1.5mm/m (transverse)
- Securing: Tighten foundation bolts to specified torque
- Field connections: Flanged piping tie-in to interconnecting lines (field welding for non-flanged)
- Electrical tie-in: Connect power supply and DCS/PLC communication cables
- Final commissioning: Site acceptance test (SAT) – repeat FAT at site with process fluids
Application Profiles – Representative Skid Types
Gas Conditioning Skid (Upstream / Midstream)
- Service: Remove liquids and particulates from natural gas prior to compression or metering
- Components: Filter coalescer (gas/liquid separation), heater (dew point control), pressure regulator, instrumentation
- Design conditions: Operating pressure 0.6 – 3.5 MPa; operating temperature -19°C to +60°C
- Piping material: Carbon steel 20#; SS 304 for instrumentation tubing
- Special features: Electric heat tracing on pressure regulator to prevent freeze-up
Chemical Injection Skid (Offshore / Onshore)
- Service: Metered injection of corrosion inhibitors, anti-scalants, or hydrate inhibitors
- Components: Six 1,000-gallon storage tanks (316SS, sloped bottom); six metering pumps (Waukesha or equivalent); flow meters, check valves; pneumatic control system
- Dimensions: 7,400 × 1,250 × 3,500 mm; weight ~10,000 kg (empty)
- Frame material: A572 GR50 square tube with drip pan for spill containment
- Paint system: Sandblasting, zinc primer, polyurethane finish
Pump Skid (HVAC / Process Cooling)
- Service: Chilled water circulation or cooling water distribution
- Components: End-suction pumps (2 to 5 units; 10–200 kW), suction diffusers, triple-duty valves, common discharge/suction headers, pump controller
- Operating range: System head 3–6 bar; flow up to 380 lps
- Header size: 150–450mm; skid length 2.1–4.9m (depending on pump count)
- Optional components: VFDs, plate heat exchangers, expansion tanks, chemical pot feeders
CO₂ Recovery Skid (Process)
- Service: Recover CO₂ tail gas; reduce footprint and construction cost
- Integration: Vertical oil separators, water-cooled shell & tube exchangers (layered arrangement)
- Piping material: S30408 (low-temperature), carbon steel 20# (ambient/hot service)
- Performance comparison vs. conventional construction:
- Total investment reduced 72.0% (from ¥3,500万 to ¥980万)
- Civil cost reduced 74.0%
- Construction area reduced 75.2% (from 1,700m² to 422m²)
- Construction period reduced 50.0% (from 6 months to 3 months)
- Production capacity and product quality unchanged
Documentation per Shipment
- Material test certificates (EN 10204 3.1 or 3.2) – structural steel, piping, vessels, fittings, flanges, instruments
- Pressure vessel design report and ASME U-stamp data report (if applicable)
- Piping isometric drawings and weld maps
- P&ID (piping and instrumentation diagram) – as-built
- Electrical schematic diagrams and cable routing
- Hydrostatic test report (with pressure and time records)
- Pneumatic leak test report
- NDE reports (RT, PT, MT, UT) – per JB/T 4730 or ASME V
- Instrument calibration certificates (with traceability)
- Safety valve set pressure certificate and calibration report
- Painting inspection report (dry film thickness, adhesion test)
- Factory acceptance test (FAT) procedure and results
- Operation and maintenance manual (including lubrication schedule, start-up/shutdown procedure)
- Packing list and shipping dimensions
- Lifting procedure and center of gravity report
Selection Checklist – Process Skid
- Process function – Pumping / Compression / Injection / Separation / Metering / Conditioning / Heat transfer
- Process fluid – name, composition, density, viscosity, corrosivity, impurities
- Flow rate or capacity – design operating range (minimum, normal, maximum)
- Operating pressure – inlet and outlet
- Operating temperature – inlet and outlet
- Design pressure and temperature – per ASME/GB code
- Material selection – Carbon steel / 304L / 316L / Duplex / Alloy (based on fluid corrosivity and temperature)
- Equipment type – Pump (centrifugal / positive displacement) / Compressor (recip / screw / centrifugal) / Vessel (vertical / horizontal)
- Control requirements – Manual / Local PLC / DCS interface (protocol: Modbus / Profibus / HART)
- Electrical classification – Explosion-proof (NEC Class 1 Div 1/2 or ATEX Zone 1/2) / Non-hazardous
- Environmental conditions – Indoor / Outdoor / Offshore / Desert / Arctic (affects enclosure and heating/cooling)
- Transport constraints – Maximum dimensions and weight for road/ship transport
- Codes and standards – ASME VIII-1 / GB/T 25198 / PED / CE/CPR / GOST / DOSH
- Skid type – Open-frame / Containerized
- Special features – Heat tracing, insulation, chemical injection, sampling ports
Design Limitation Statement
The process skid is subject to the following limitations:
- It is rated only for the design pressure and temperature specified on the datasheet; operation outside these limits voids the design
- Field modification of piping or equipment after site installation requires re-rating and re-testing per original code
- Skid must be installed on a level foundation of adequate bearing capacity (≥ 100 kN/m² typical); unlevel installation causes misalignment and seal/shaft failure in rotating equipment
- Not suitable for processes requiring frequent layout changes – modifications to skid piping are constrained by existing frame and equipment placement
- Not suitable for highly erosive service (solids content > 2% by weight) without erosion-resistant linings or reducers
- Skid performance is verified only for the process conditions specified at FAT; changes in fluid composition or flow rate may affect pump/compressor curve and control tuning
