When you request quotations for a truck loading skid, define the boundaries before the equipment list. Every line item needs one clear answer: is it included in the scope of supply, excluded, priced as a separate option, or an interface where the two parties must hand data to each other? For interface lines, also state who is responsible and what crosses the boundary: flange size and pressure class, signals, power, records or approvals. Without this, bids that carry the same name price different work and cannot be compared.
The approach applies to a new skid, a bay refurbishment or a conversion to custody transfer. The matrix below is a template; buyer and contractor decide together, under project conditions, who keeps each item. Safety functions are covered here only at the level of function and interface: setpoints, layout and operating instructions come from the project risk assessment and an expert design review. Legal metrology requirements vary by country and contract.
Why the word «skid» produces different bids
One supplier may mean only the metering and control unit made up of strainer, gas eliminator, meter and control valve; another the bay equipment including pump and loading arm; a third may include automation and commissioning. Industry guidance separates these layers too: the Energy Institute HM 9 guideline states that it covers the gantry measurement system, not the design of a complete loading system or safe-operation requirements such as electrical, static electricity and spillage prevention. API MPMS Chapter 6.2A addresses minimum requirements for the design, selection and operation of truck and rail metering systems. A metering-focused skid bid therefore does not mean that the safety and site interfaces are resolved.
Expensive surprises usually sit at the boundaries: the spool between skid and loading arm, pump start and permissive signals, ownership of the ground verification unit, proving connections and who files for inspection.
Four classes: included, excluded, option, interface
· Included (I): Designed and supplied by the contractor and covered by the bid price. The governing specification and standard are written on the same line.
· Excluded (E): Supplied by the buyer or another contractor. Excluding an item does not remove responsibility; state who sits on the other side of the boundary.
· Option (O): Priced separately and possibly added later, for example additive injection, spare parts or training. The technical definition of an option must be as clear as the base scope.
· Interface (X): A boundary where the parties must give each other data, signals, power or approval. For each interface, write the responsible party, the content of the data and the delivery date.
An item can be both excluded and an interface: the pump may stay with the buyer while its start permissive and running feedback are wired to the skid control system. Showing this in the matrix moves the «we understood it differently» debate from commissioning to the bid stage.
Scope matrix template
The table below is a blank template; no line represents a default scope of supply. Enter I, E, O or X in the «Class» column; more than one mark is possible. Enter buyer, contractor or third party in the «Responsible» column. The last column lists the interface data that must be settled before bidding.
Truck loading skid RFQ scope matrix (template for the buyer to complete)
Item | Class (I/E/O/X) | Responsible | Interface data to define at the boundary |
Metering and control skid (strainer, gas eliminator, meter, secondary measurements, control valve) | Product properties, flow range, measurement purpose; inlet/outlet terminal points (size, pressure class, material); weight and anchor points | ||
Interconnecting piping (pump to skid, skid to loading arm) | Terminal points and flange data, piping specification, supports; responsibility for pressure test and cleaning; condition keeping the line between meter and transfer point full | ||
Pump or common supply header | Duty point and supply pressure range; start, stop and status signals; where the pump interlock logic resides; motor power supply | ||
Loading arm or hose (top/bottom) | Fleet adapter and coupler type, flow per arm, arm position signal, location of the transfer point, vapor connection | ||
Civil works, foundation, canopy, platform and access | Skid location and loads, anchoring detail, owner of platform and fall protection, drainage and spill collection | ||
Power supply, cabling and hazardous area | Supply voltage and source, cable entries, hazardous area classification drawing, equipment protection type, equipotential bonding | ||
Truck ground verification interface | Owner of the verification unit; permissive signal type and receiving system; risk assessment defining the safe state on loss of permissive | ||
Truck compartment overfill prevention interface | Sensor types and socket connection used by the fleet, owner of the monitoring unit, permissive and fault signals | ||
Emergency shutdown (ESD) interface | Owner of the ESD logic, incoming and outgoing signals, target state of valve and pump, safety function classification | ||
Vapor return and vapor recovery | Vapor connection on the arm, terminal point of the vapor line, owner of the recovery or combustion unit, back pressure data | ||
Automation: batch controller, PLC, TAS, ERP | System of record, protocol and signal list, load order and result fields, local operation rule when communication is lost | ||
Meter proving: prover or master meter connection | Proving method, connection and isolation valves, owner of the prover, access for a portable prover | ||
Legal metrology, approval and inspection | Applicable regulation and accuracy class, who files the approval, coordination of initial verification and sealing, parameter access rights | ||
Additive injection or blending | Number of components, ratio and recording requirement, device performing the control | ||
Testing and acceptance (FAT, SAT, SIT) | Test location and attendees, whether external interfaces are real or simulated, acceptance criteria and signing authority | ||
Installation, commissioning, training, spares, maintenance | Boundary of site services, product and truck availability for commissioning, spare parts list, maintenance scope | ||
Documentation and conformity file | Document list and language, certificates, party responsible for the assembly under hazardous-area legislation, as-built drawings and calibration records |
Metrological boundary: where is the transfer point?
Treating the measuring system as identical to the skid boundary is a common mistake. OIML R 117-1 states that a meter by itself is not a measuring system; the smallest possible measuring system includes a meter, a transfer point and a hydraulic path whose characteristics must be taken into account. The transfer point is where the liquid is defined as delivered, and in delivery systems it lies downstream of the meter. In «empty hose» systems it sits upstream of the hose; in «full hose» systems it is a closing device at or near the hose end, and the term hose includes rigid pipework.
The RFQ consequence: even if the loading arm is excluded from the skid scope, when the transfer point is at the arm outlet the arm and the connecting pipe are part of the measuring system's hydraulic path, so the arm supplier, the piping contractor and the party responsible for the measuring system must agree on the same data. R 117-1 also lists the gas elimination device, filter, pump, flow control device, vapor return and correction devices among the components often needed for correct operation; some of them may lie outside the skid.
Accuracy expectations should likewise be written at system level. The MID Annex MI-005 places truck loading and unloading systems in accuracy class 0.5, and in OIML R 117-1 systems not stated otherwise in the table default to class 0.5. In this class the maximum permissible error is 0.5 % for the complete measuring system and 0.3 % for the meter; the margin covers gas elimination, conversion, installation and calculation. Liquefied gases under pressure and LNG fall into wider classes. Asking only for meter accuracy in an RFQ leaves the owner of system performance undefined.
Safety interfaces: define the function and the boundary, not the design
Truck ground verification, compartment overfill prevention and emergency shutdown are links in the loading permissive chain; each usually acts on the control valve and pump through a signal from a device outside the skid. The RFQ should not specify how these functions are designed but answer the following:
· Who owns the function: which party supplies and documents the permissive unit, the ESD logic and the connected field devices?
· Which signal crosses the boundary: signal type, receiving system and how a fault is reported.
· What safe state is expected on loss of permissive, and which study defined it: HAZOP, risk assessment or safety requirements specification.
· If the function is treated as a safety instrumented system, which party holds the IEC 61511 lifecycle documents?
Compartment overfill prevention is not the same scope as storage tank overfill protection: API Std 2350 addresses storage tanks, while in Europe the requirements for overfill prevention systems on the tank vehicle side are set by EN 13922. Request the fleet's sensor types as tank-vehicle data in the RFQ. Principles such as grounding of conductors and reduction of charging are covered by guidance such as IEC TS 60079-32-1 and API RP 2003; which measures are needed depends on the product and the risk assessment.
If the skid will be used in a hazardous area, one more conformity question arises. The European Commission guidelines to Directive 2014/34/EU (ATEX) state that equipment electrically and mechanically interconnected into a complete functional assembly is a product under the directive, and that the party placing it on the market becomes the manufacturer of that assembly. The assembly manufacturer may presume conformity of compliant components but must assess the additional ignition hazards created by the combination. For EU projects the RFQ should ask who carries this assessment; a single device certificate does not cover the whole skid.
Automation interface: where is the record of truth kept?
The boundary between batch controller, PLC, terminal automation system (TAS) and ERP decides which system holds the master measurement record. Together with the signal list and protocol, state where the load order comes from, which fields return the per-compartment result, who may change metrological parameters, whether the bay runs in local mode when communication is lost, and how records are synchronized afterwards.
Proving, inspection and test boundaries
On custody transfer skids the proving method changes the design: the location of prover or master meter connections, leak-tightness checks of isolation valves and access for a portable prover must be defined early. EI HM 11 gives good-practice guidance for proving gantry metering systems and notes that local conditions, regulations and contract conditions may take precedence. Who supplies the prover and who coordinates initial verification and sealing should be separate lines.
The same logic applies to acceptance testing. IEC 62381:2024 defines requirements and checklists for the factory acceptance test (FAT), an optional factory integration test (FIT), the site acceptance test (SAT) and the site integration test (SIT) for process automation systems, and gives the parties a means to agree on test scope and responsibilities. The RFQ should state where each test takes place, whether external interfaces such as the pump, TAS and permissive units are real or simulated, and who signs acceptance.
Documents to attach to the RFQ
1. Product list with density, viscosity, temperature and vapor pressure ranges for each product
2. P&ID or block flow diagram with the scope boundary marked, plus a terminal point list
3. Loading bay layout, space allocated to the skid and access routes
4. Hazardous area classification drawing (prepared to IEC 60079-10-1 or the project standard)
5. Fleet data: loading method, adapter and vapor coupler type, overfill sensor types, number and volume of compartments
6. Pump and supply line data: duty point, supply pressure range, whether a common header is used
7. Control system architecture: existing or planned PLC, TAS and ERP, protocols and a draft signal list
8. Measurement purpose, applicable regulation, sales unit and preferred proving method
9. Risk assessment or HAZOP outputs and safety requirements for the safety functions
10. Project specifications: piping, materials, painting, electrical, instrumentation and cabling
11. Testing and acceptance expectations, deliverable document list and document language
12. Expectations for site services, training, spare parts and maintenance
Example scenario: how one line is filled in
Example scenario (fictional): A terminal keeps its existing pump and bottom loading arm and wants a new metering and control skid. The pump line becomes «E + X»: the pump stays with the buyer, but its start permissive and feedback will be wired to the skid control; the interface data requested are the duty point, the signal type of the motor control circuit and where the interlock logic resides. The loading arm line also becomes «E + X»; if the transfer point is at the arm outlet, the arm's internal volume and the condition keeping it full enter the measuring system documentation.
Where the matrix is not enough
· LPG and other liquefied gases under pressure: Keeping the product liquid in the meter, the effect of the vapor return line on measurement and a different accuracy class require additional interface lines.
· LNG: Cryogenic temperature, cool-down and material selection change the interfaces; R 117-1 treats LNG measuring systems in a separate accuracy class.
· Heated and heavy products: HM 9 excludes heated and heavy products; heating, insulation and trace heating interfaces must be added.
· Conversions of existing bays: If the matrix is filled in without a site survey of existing equipment, old cabling, grounding and permissive hardware remain hidden scope.
· Contract and regulation: The matrix does not replace the contract, legal metrology decisions or the HAZOP; national regulations and contract terms take precedence.
Clarifying the scope with TLY Enerji
Depending on project scope, TLY Enerji provides system design, product supply, engineering, installation, commissioning and maintenance services for fuel and LNG truck loading and unloading facilities, and design, supply and engineering support for LPG loading and unloading stations. Which lines of the matrix fall within TLY Enerji's scope is agreed with the buyer on the basis of the completed matrix and the technical specification.
Related pages
· Truck loading systems: Process flow and components of the loading chain the matrix lines refer to.
· Custody transfer truck loading skids: Technical background for the proving, sealing and record lines.
· Automated truck loading terminals: How batch controller, TAS and ERP layers are handled at terminal scale.
· Coriolis or positive displacement meter for truck loading?: The meter technology decision to make before filling in the metering skid line.
· Flow computer or PLC for billing data: Which device keeps the measurement calculation and record at the automation interface.