Most flange quotations that end badly do not fail on price. They fail because something was left out of the inquiry. A stainless steel flange is a dimensional part tied to a specific standard, and a supplier will quote what is written — or the cheapest interpretation of it. When the bolt circle does not match the mating valve, or the certificate shows a grade the piping specification never allowed, the argument is expensive and the schedule has already moved.
This guide sets out the twelve details that should be fixed before a stainless steel flange inquiry leaves your desk, for buyers and project engineers sourcing to overseas specifications. It assumes nothing about a supplier’s stock, certification or capacity: those are questions for the quotation.
Why an incomplete inquiry costs more than a slow one
A supplier reading “100 pcs stainless steel flange 6 inch 150 lb” must choose a type, standard, facing, grade, bore, finish and documentation package. Each choice changes the price and production route. If the inquiry does not fix them, the quotation fixes them at the cheapest end — rarely the one the piping specification intended.
The reverse is also true. A buyer who specifies all twelve items receives comparable quotations, because every supplier prices the same object — and the technical discussion moves to the points that genuinely need negotiating.
The five dimensional details
1. Flange type
Name the type explicitly: weld neck, slip-on, blind, plate, lap joint, threaded, socket weld, orifice or reducing. Type sets the weld detail, bore, hub and often the facing. “Flange” on its own is not a type.
2. Dimensional standard and series
State the standard, and the edition where the project fixes one. ASME B16.5 covers flanges from NPS 1/2 through NPS 24, ASME B16.47 covers larger sizes, EN 1092-1 covers PN-rated flanges, and JIS B2220 covers the Japanese series. These systems are not interchangeable. A Class 150 flange and a PN 16 flange are both “about 150” in conversation and different in thickness, bolt circle and bolt size. If the project works to a company specification, send its number rather than a summary of it.
3. Size, schedule and bore
Give the nominal size (NPS or DN) and the pipe wall schedule. On a weld neck flange the bore has to match the pipe inside diameter; on a slip-on flange it has to clear the pipe outside diameter. Where the pipe is non-standard, give the actual outside diameter and wall thickness. Size alone does not define a bore, and a flange bored for the wrong schedule is a scrap part.
4. Pressure class and design temperature
A class is a pressure–temperature rating, not a single pressure number: the same Class 150 flange has a very different allowable pressure at ambient temperature and at several hundred degrees. Give the design pressure and temperature from the line list and let the supplier confirm the rating against the applicable standard.
5. Facing and face finish
Specify raised face (RF), flat face (FF) or ring joint (RTJ); for RTJ, give the groove type and ring number. Flat face flanges are normally used with full-face gaskets and should not be mated to a raised face without checking the joint design. Finish is a separate requirement: a machined raised face is usually called out as an Ra value — commonly 3.2–6.3 µm for a smooth finish — and it has to suit the gasket. MSS SP-6 covers finishes for contact faces. Too rough damages a soft gasket; too smooth may fail to bite. Agree finish and gasket together.
The four material and evidence details
6. Material grade and product specification
Write the full designation, not the trade shorthand. “ASTM A182 F316L” is a specification; “316L” is a family. The same applies to duplex grades such as F51 (UNS S31803) or F53 (UNS S32750), and to plate flanges cut from ASTM A240 plate. ASTM A182/A182M fixes chemistry, mechanical properties, heat treatment and marking for forged flanges — which is why the product specification belongs next to the grade. If dual certification is required, say so in the inquiry.
7. Delivery condition and heat treatment
Austenitic and duplex flanges are normally supplied solution annealed. If the service calls for post-weld heat treatment, hardness limits, or compliance with NACE MR0175 / ISO 15156 for sour service, put it in the inquiry. These requirements change material selection and testing, and they cannot be retro-fitted to a forging that has already been made and marked.
8. Inspection, testing and documentation
Say which documents you need and to what standard: an EN 10204 3.1 certificate, positive material identification, a dimensional report, hardness survey or non-destructive testing such as dye penetrant. If a third-party inspector must witness testing, name the body and the scope. A vague request for “all certificates” produces a folder that satisfies nobody and delays the shipment.
9. Marking and traceability
Flanges are marked with size, class, material grade, the manufacturer’s symbol and a heat number. Ask how the marking is applied, whether the heat number traces back to the certificate, and whether each case carries a tag matching the packing list. Where traceability is a project requirement, this is part of the deliverable.
The three commercial details
10. Quantity, spares and supply scope
State pieces by size and type, then say whether gaskets, bolting or insulation kits are in scope. Add the spares allowance if the project has one, and state whether partial shipments are acceptable. A quotation that quietly includes gaskets is as misleading as one that quietly excludes them.
11. Packaging and shipping marks
For export, specify seaworthy packing, rust-preventive treatment and shipping marks. Face protection matters: a raised face that arrives with handling damage is a rejected flange, whatever the certificate says.
12. Delivery terms and required date
Give the Incoterm, the destination port or site, and the date the material is needed on site rather than the date you would like it shipped. Ask for the basis of the quoted date and flag any item that depends on raw material availability. A date without a basis is a hope.
The twelve-point inquiry checklist
| Item | What a clear line looks like |
|---|---|
| 1. Type | Weld neck, raised face |
| 2. Standard | ASME B16.5, edition per project specification |
| 3. Size and bore | NPS 6, Sch 40, bore matched to pipe ID |
| 4. Class and conditions | Class 150 at design pressure and design temperature |
| 5. Facing and finish | RF, 3.2–6.3 µm Ra, for a spiral-wound gasket |
| 6. Material | ASTM A182 F316L, dual certified if required |
| 7. Condition | Solution annealed; sour service per NACE MR0175 if applicable |
| 8. Inspection | EN 10204 3.1 certificate plus PMI, scope stated |
| 9. Marking | Heat number stencilled; tag to match packing list |
| 10. Quantity | Installed count plus agreed spares; accessories in or out |
| 11. Packing | Seaworthy cases, faces protected, marks defined |
| 12. Delivery | Incoterm, destination, required-on-site date |
What a usable quotation should return
A quotation that answers the checklist is short and comparable. It should state the standard and class being quoted, flag deviations instead of quietly substituting, give the material grade with its product specification and heat treatment, show the weight per piece, list the documents included and those that cost extra, describe the packing, and give validity, Incoterm and the basis of the delivery date. A quotation that does not name the standard it quotes against is not a quotation; it is an indication.
Four mistakes that account for most re-quotes
- Leaving the facing to the supplier. RF and FF flanges are different parts with different gasket requirements, and RTJ flanges need a groove that cannot be added later.
- Treating class and PN as equivalent. They are separate rating systems with different dimensions. Substitution changes the joint, not just the label.
- Ordering “SS316” without a product specification. Chemistry, heat treatment, marking and the certificate all follow from it.
- Raising inspection after production. Test scope, witness points and documentation should be agreed before the material is cut, not when the goods are ready to ship.
Frequently asked questions
Is ASME Class 150 the same as PN 16?
No. They are different rating systems and differ in thickness, bolt circle, bolt size and facing dimensions. A PN 16 flange should not be substituted without a dimensional check against the mating part and approval under the applicable piping code.
Can flanges be produced to my own drawing?
Many suppliers can machine to a customer drawing, but it must define material specification, dimensions and tolerances, facing and finish, marking and inspection. Ask for a drawing review and written approval before production, and keep the approved revision as the inspection reference.
What is the difference between F316 and F316L?
Both are austenitic grades under ASTM A182 and differ essentially in carbon content: the “L” grade is low carbon, which reduces the risk of sensitisation in welded joints, at slightly lower strength. Whether it is required depends on the welding procedure, the service environment and the governing code, so the choice belongs to the piping specification.
Do I need an EN 10204 3.1 certificate?
That depends on the end user, the project specification and the jurisdiction. A 3.1 certificate is an inspection certificate signed by the manufacturer’s authorised inspection representative, independent of the manufacturing department. If the project requires it, state it in the inquiry: it affects price and cannot be produced retrospectively.
Which detail is most often missing?
In our experience, the facing and its finish, then the bore or schedule and the material product specification. Each is cheap to state at the inquiry stage and expensive to correct after delivery.
Conclusion
A stainless steel flange inquiry is a short technical document, not a message. Twelve lines turn a vague request into something a supplier can price once and a buyer can compare. The effort is minutes; the cost of omitting any one of them is usually measured in weeks.
This article is general technical information for procurement planning, not a substitute for the applicable code, the project specification or the judgement of a qualified engineer. Final selection of type, material, rating, facing and inspection regime must be confirmed against the governing standard and the service conditions by the responsible engineering party.
Ready to price a line? Send the twelve points with your drawing or line list and we will confirm the specification before quoting. Contact our flange team with your inquiry, or review the pipe flange range for the types most often requested. For selection background, see weld neck vs slip-on flanges, the blind flange guide and 304 vs 316 stainless steel flanges.