For RMS/PRMS quotations that can be compared with each other, the supplier needs at least: minimum and maximum inlet pressure with units and a clear gauge/absolute statement; required outlet pressure and acceptable deviation; the maximum permitted operating pressure of the downstream system; minimum, normal and maximum flow rate together with the reference temperature and absolute pressure it is expressed at; gas composition and contaminant information; inlet gas and ambient temperature ranges; number of streams and expected redundancy; and the purpose of measurement. On top of these come the preferred metering technology, volume conversion and telemetry, responsibility for odorization, the hazardous area classification, site conditions, the network operator's specification, required documents and tests, and the scope boundary.
This list applies to pressure reduction and metering stations fed from a higher-pressure line and serving a specific consumer or delivery point. At city gate stations between transmission and distribution, the responsibility boundary between operators and odorization carry more weight, and where the network operator has its own specification, that specification takes precedence. This article is not about how a station is arranged; it is about how design and quotations drift apart when data is missing.
What does missing data turn into at bid evaluation?
When a value is missing from the RFQ, suppliers do not stop quoting; they fill the gap with their own assumption. One takes the pipeline design pressure as the maximum inlet pressure, another uses the stated normal operating pressure. One sizes the streams for today's flow, another adds a capacity margin. One includes a heater, another notes that it will be priced separately if needed. The price difference then reflects different stations being priced, not a difference in quality or efficiency.
Three outcomes are typical: technical clarification rounds get longer; the lowest price often belongs to the most optimistic assumption; and when assumptions clash with project data after award, regulator, meter or heater changes and extra cost follow. The purpose of an RFQ is not just to collect prices but to make every supplier work from the same design input.
Which missing data leaves which decision open?
Missing data, the design decision it leaves open and how quotations diverge
Missing or unclear data | Design decision left open | How quotations diverge |
Inlet pressure given as a single value | Whether the regulator has enough capacity at minimum inlet pressure; pressure ratio and noise at maximum pressure; pipe and flange pressure class | Different regulator sizes and pressure classes; some offers risk insufficient capacity at low inlet pressure |
Outlet pressure given, but no deviation and no downstream maximum permitted pressure | Expected regulator control accuracy; the pressure band where the monitor and slam-shut functions must fit | Different regulator classes and pilot arrangements; safety layers vary from offer to offer |
Single flow value with no reference conditions | Stream capacity, meter range, number of streams | One supplier proposes a single stream with a large meter, another parallel streams of different capacity; meter sizes do not match |
Gas composition and contaminants unknown | Filter or separator type, materials, cooling assessment, compressibility method for volume conversion | Some offers include only a dry gas filter, others a liquid separator; conversion built on different assumptions |
No inlet gas or ambient temperature | Heating need and heat duty; equipment temperature range; enclosure or thermal insulation | Offers with and without a heater end up side by side |
Redundancy not defined | Number of regulating and metering streams; how supply and measurement continue during maintenance | Single-stream and 2×100% arrangements on the same bid list; most of the price gap comes from here |
Purpose of measurement not stated | Meter and converter class, verification, records, bypass rule | Process monitoring and billing measurement priced under the same line item |
Why must flow data state its reference conditions?
Natural gas flow is usually given as volume converted to reference conditions, but the temperature and pressure behind a standard or normal cubic meter are not the same in every contract. The abstract of ISO 13443 itself notes that the multiplicity of reference conditions causes confusion and that overlooking the difference can have serious consequences, particularly in custody transfer. The meter, however, sees gas at metering conditions, that is, at the pressure and temperature where it is installed; conversion to reference conditions is done in a volume conversion device or flow computer using pressure, temperature and gas composition. The RFQ should therefore state flow together with the reference temperature and reference absolute pressure, and say whether operating pressures are gauge (bar(g)) or absolute (bar(a)).
Example scenario: same gas, different numbers
The educational calculation below is simplified so that its assumptions stay visible: the gas is treated as ideal (compressibility ignored), reference conditions are 15 °C and 1.01325 bar(a), and operating temperature is also 15 °C. Real sizing uses the compressibility factor and actual operating temperature; these results are not design values.
Educational calculation: effect of reference conditions and pressure units on meter volume
Case | Calculation | Result |
10 000 m³/h at reference conditions; meter on the 20 bar(g) inlet side | 10 000 × 1.01325 / 21.01325 | ≈ 482 m³/h actual volume |
Same gas; meter on the 4 bar(g) outlet side | 10 000 × 1.01325 / 5.01325 | ≈ 2 021 m³/h (about 4.2 times) |
20 bar gauge pressure mistaken for absolute | 10 000 × 1.01325 / 20 | ≈ 507 m³/h (about 5% deviation) |
Gas stated as 10 000 m³/h at 0 °C reference, expressed at 15 °C reference | 10 000 × 288.15 / 273.15 | ≈ 10 549 m³/h (about 5.5% difference) |
The table shows two things. Whether the meter sits upstream or downstream of regulation can change the actual volume for the same quantity of gas several times over. A small ambiguity in units or reference conditions creates a difference of a few percent, which often explains why meter sizes and ranges in competing offers do not match.
RFQ data sheet
The form is laid out so that the owner or EPC team can fill it in and attach it to the request for quotation. Unknown fields should not be left blank: mark them as unknown and ask the supplier to state in the offer which assumption it used for each one.
RMS/PRMS RFQ data sheet
Data | Unit / format | Why it is needed | Effect if missing |
Station role and consumer served | Text: plant inlet, district supply, power plant, delivery point | Sets scope and the expected continuity of supply | Station type and redundancy are interpreted differently |
Inlet pressure: minimum / normal / maximum | bar(g) or bar(a) stated explicitly; plus pipeline design pressure | Regulator capacity is checked at minimum inlet pressure, pressure ratio and noise at maximum; pressure class | Regulator size and pressure class change from offer to offer |
Outlet pressure and acceptable deviation | bar(g); ± value or permitted band | Regulator control accuracy and pilot selection | Regulation looser than needed, or unnecessarily tight, gets priced |
Maximum permitted operating pressure downstream | bar(g) | Upper limit for slam-shut and relief functions | Safety chain undefined; offers cannot be compared |
Flow: minimum / normal / maximum | m³/h together with reference temperature (°C) and reference absolute pressure; or kg/h | Stream capacity, meter range, number of streams | Meter size and stream arrangement do not match; low flow may fall outside the meter range |
Seasonal profile and future capacity | Summer/winter split; target year and expected maximum flow | Expansion margin, reserved stream space or a stream added later | One offer is sized for today, another for the future |
Gas composition and quality | Mol % analysis or network quality range; relative density, calorific value | Volume conversion, cooling assessment, materials | Conversion and heating assessment built on different assumptions |
Dust, liquids and water content | Known contaminants; water dew point data if available | Filter or separator type; hydrate risk on cooling | Filtration scope and liquid handling capacity are not equal |
Inlet gas temperature | °C, minimum / maximum | Assessment of cooling across the pressure drop and of heating need | Offers with and without a heater get compared |
Ambient temperature and climate | °C, minimum / maximum; snow, wind, coastal exposure | Equipment temperature range, enclosure, thermal insulation, panel climate control | Open-air versus enclosed layout remains unclear |
Number of streams and redundancy | E.g. regulation 2×100%, metering 1+1; whether active/monitor regulators and a slam-shut function are required | Continuity of supply and measurement during maintenance | Most of the price gap comes from different arrangements |
Purpose of measurement | Process monitoring / custody transfer for billing / measurement under legal metrology | Meter and converter class, verification, records, bypass rule | Systems with different requirements priced under the same line item |
Preferred metering technology | Open if no preference; if any, the reason (existing infrastructure, spares) | Meter type, straight runs, position relative to the regulator | Offers arrive with different technologies and cannot be compared directly |
Volume conversion, flow computer, telemetry | Converter or flow computer; compressibility method; communication protocol and SCADA interface | Calculation chain and data transfer | Conversion device or communications may fall out of scope |
Odorization | Required or not, whose responsibility, at the station or not | Scope for equipment and odorant storage | One offer includes it, another does not |
Hazardous area classification | Project document: classification drawing or report prepared with a method such as IEC 60079-10-1 | Selection of electrical equipment and panels | Equipment protection type left to assumption |
Site and layout | Plot plan, allocated area, inlet/outlet direction, elevation, access | Skid dimensions, enclosure, transport | Layout changes later; piping and foundation work grows |
Network operator specification | Document title and revision | Mandatory arrangement, material and documentation requirements | A non-compliant offer gets revised later |
Required documents and tests | List: calculations, material certificates, pressure and function tests, acceptance test expectations | Documentation and test scope affect cost | Gaps surface at delivery |
Scope boundary | Flange points, foundations, cabling, commissioning, training | Puts offers on the same scope | The visible price gap comes from scope differences |
Why are temperature and gas quality separate rows?
Gas cools when its pressure is reduced (the Joule-Thomson effect). How much it cools depends on the pressure drop, inlet temperature and gas composition; a method for calculating the Joule-Thomson coefficient for use in natural gas metering is specified in ISO 20765-5. A rough estimate based on a single fixed coefficient is therefore not a reliable basis for comparing offers; the heating decision is made from the outlet temperature calculated for the lowest inlet temperature and largest pressure drop. If the gas contains water, cooling can also create conditions for hydrate formation. Ambient temperature is a separate input: the catalog abstracts of the European product standards for regulators and safety shut-off devices (EN 334, EN 14382) define their scope from −20 °C to +60 °C; if site conditions can fall outside this range, equipment selection and the need for an enclosure are assessed separately.
What to ask suppliers so that offers become comparable
· List of assumptions: the value used for every field marked unknown in the form.
· List of deviations: every deviation from the specification and the form, with the reason.
· Sizing summary per stream: regulator capacity at minimum inlet pressure and maximum flow; the meter range compared with the required flow range.
· Regulator performance in the same terms: regulator data sheets may state control accuracy and lock-up pressure as classes, and a class may depend on set pressure and pilot; ask for the set pressure and pilot each statement applies to.
· Definition of safety functions: which stream has active/monitor regulators and a slam-shut function. Regulators are not always designed as safety devices; overpressure protection should be shown as a separate function.
· Basis of the heating decision: the outlet temperature calculated for the worst case and the method used.
· Metering system statement: the class or uncertainty of the whole system including conversion, not just meter accuracy. EN 1776 and OIML R 140 define requirements for the measuring system separately from the meter.
· Scope matrix: each item marked included or excluded against the scope boundary.
Limits of the form
· The form does not replace design or safety review. Slam-shut and relief settings and the number of protection layers are set per project from the downstream maximum permitted pressure and risk assessment; this article does not propose set values.
· Whether a metering point falls under legal metrology, and which approval or verification it needs, depends on national legislation and the contract; the form only asks for the purpose of measurement.
· Where the network operator has a specification, preferences in the form should not contradict it; in case of conflict, the specification governs.
· The obligation, location and verification of odorization are set by national regulation and agreements between operators. According to the catalog abstract of the pressure regulating station standard EN 12186, auxiliary facilities such as odorization are outside its scope, so odorization should be defined as a separate specification item.
· For gases other than conventional network gas, such as hydrogen blends or raw/sour gas, the scope of the standards should be checked separately; the current abstract of EN 1776, for example, excludes raw/sour gas and certain hydrogen blend ranges.
Related pages
· Gas pressure reduction and metering stations (RMS/PRMS): Station layout, safety chain and stream redundancy are covered here.
· Gas heating, pressure regulating and metering skids: Where the heater sits when temperature data shows a heating need.
· City gate stations: Stations where odorization and the boundary between operators come to the fore.
· Orifice or ultrasonic for natural gas metering?: To assess the metering technology preference in the form against your scenario.
· Flow computer or PLC?: For volume conversion and traceability of billing data.