Carbon Steel Threaded Flanges

Samples, Third-Party Inspection and Test Records

Sample pieces can be produced before the batch runs, so that dimensions, bevel profile and fit-up can be checked on your side first. Sample cost and delivery time are advised with the quotation, based on size and material.

Buyer-nominated inspection is supported. SGS, BV, Intertek or your own inspector can witness dimensional checking, marking, coating and container loading at the production site, and we provide the documents and site access needed for the visit. Where a visit is not practical, the same checks can be followed through a live video walkthrough. Third-party inspection fees are normally carried by the buyer.

Non-destructive testing and hydrostatic test records are available on request and can be shipped with the order, together with the EN 10204 3.1 material certificate, the heat number and marking list, dimensional records, packing list and certificate of origin. Tell us at enquiry stage which records your project requires, and they are included in the quotation.

Threaded vs Weld Neck

Criterion	Threaded (TH)	Weld Neck (WN)
Installation	Screwed onto pipe thread, no welding	Butt welded, full penetration
Hot Work Permit	Not required	Required
Welder Qualification	Not required	Required (ASME Section IX)
Pressure Class Range	Class 150 and Class 300	Class 150 - Class 2500
Thermal Cycling	Not recommended, thread loosens	Recommended
Vibration Service	Not recommended	Recommended
High Temperature	Not recommended	Suitable
Leak Tightness	Lower, depends on thread sealant and make-up	Higher, inspectable butt weld
Pipe Schedule Required	Not required	Required at quotation stage
Typical Applications	Firewater, instrument air, nitrogen, water utilities	Oil & gas, refinery, high-pressure steam, sour
Assemble / Disassemble	Can be broken and remade	Cut out and reweld

Selection Logic

Buyer FAQ

Q1: Why are threaded flanges mainly supplied in Class 150 and Class 300?

A1: ASME B16.5 dimensions threaded flanges from Class 150 to Class 2500. Class 150 and Class 300 cover almost all threaded service, and these two classes are what we supply as standard. Above these classes, the threaded connection is not the governing standard detail — the joint relies on the pipe thread for load transfer, and the thread cut removes material from the pipe wall, which limits the pressure capability. For Class 600 and above, or for any hydrocarbon, high-temperature, or cyclic service, weld neck flanges are the specified connection. If your piping class calls for a threaded flange above Class 300, please share the specification and we will confirm what can be supplied.

Q2: Do threaded flanges require welding?

A2: No. That is the main reason to specify them. The flange bore is machined with a tapered pipe thread and is screwed directly onto the threaded pipe end. Installation needs no welder, no weld procedure, no weld inspection, and no hot work permit. This makes threaded flanges the standard choice in operating plants, fuel handling areas, offshore platforms, and any location where hot work is restricted or where a welding crew is not available. A thread sealant or PTFE tape is used on the pipe thread during assembly.

Q3: Is the thread NPT or BSPT? Can the two be mixed?

A3: Standard thread is NPT to ASME B1.20.1. BSPT to ISO 7-1 can be supplied on request. NPT and BSPT must never be mixed on the same joint. They differ in thread angle, pitch, and thread form: NPT has a 60-degree thread angle, BSPT has 55 degrees. A BSPT male thread will not seal correctly in an NPT female thread, and forcing the connection damages both components and produces a joint that leaks under pressure. Please state the required thread standard clearly on your enquiry, and match it to the pipe thread standard used on your project.

Q4: Can threaded flanges be used on high-temperature lines?

A4: Threaded flanges are not recommended for high-temperature service or thermal cycling. The failure mode is well known: repeated heating and cooling causes the pipe and the flange to expand and contract at different rates, which relaxes the thread make-up and opens a leak path. Threaded joints are intended for ambient-temperature, non-cyclic service. If your line operates at elevated temperature or sees regular start-up and shutdown cycles, specify weld neck flanges. Please share your operating temperature and cycle frequency and we will confirm the recommendation.

Q5: Are threaded flanges prone to leakage? How is the joint sealed?

A5: A threaded joint relies on the thread make-up and a thread sealant, so it does not have the inherent leak tightness of a welded butt joint. In low-pressure, ambient-temperature, static service, a properly made-up threaded joint with the correct sealant performs reliably and is accepted practice in firewater, instrument air, and utility service. The leak risk rises with pressure, temperature, cycling, and vibration. If your service is flammable, toxic, or high-pressure, a welded joint is the correct specification. Pressure testing after assembly is always recommended for threaded systems.

Q6: When is a threaded flange the right choice over a welded flange?

A6: Three situations: where hot work is prohibited or a permit cannot be issued, such as an operating plant or a fuel handling area; where no qualified welder or welding equipment is available on site; and where the line is low-pressure, ambient-temperature utility service and the welding cost is not justified. Firewater systems, instrument air and nitrogen headers, and temporary or skid-mounted piping are typical examples. Outside these situations, a welded connection is generally the safer and more durable specification.

Q7: Can threaded flanges be used for natural gas service?

A7: Threaded connections are used in natural gas distribution, but their use is governed by your local code and by the gas utility or operator specification. Most codes restrict threaded joints in gas service by pressure, by location, and by whether the piping is concealed or exposed. Above-ground, low-pressure, accessible installations are the common case. Please confirm your applicable code and pressure limit before specifying, and state the service and design pressure on your enquiry so the appropriate flange and thread standard can be supplied.

Q8: Are threaded flanges available in large diameters?

A8: ASME B16.5 dimensions threaded flanges up to NPS 24 inch, but in practice threaded flanges are used mainly in smaller sizes. As diameter increases, making up a threaded joint becomes progressively harder: the torque required to seat the threads rises sharply, and the risk of galling and of an improperly seated joint increases. For larger diameters, a welded flange is both more practical and more reliable. If your project requires a large-diameter threaded flange, please share the size and service and we will advise on feasibility before quotation.

Q9: Do I need to specify pipe schedule for threaded flanges?

A9: No. The flange bore is threaded, so there is no bore to match to your pipe inside diameter, and no pipe schedule is required for the flange itself. You need to specify nominal pipe size, pressure class, facing type, and thread standard. Note that the pipe you thread must have sufficient wall thickness to accept the thread cut — this is a pipe selection matter governed by your piping code, not by the flange. Please confirm the pipe schedule against your code requirement for threaded connections.

Q10: What is the HS Code and tariff for carbon steel threaded flanges imported into Oman and the UAE?

A10: Carbon steel threaded flanges are classified under HS Code 7307.91 (tube or pipe fittings of iron or steel — flanges of iron or steel, other than stainless). GCC Common External Tariff is 5% on CIF value for goods imported into Oman and the UAE. Standard import documentation includes Commercial Invoice, Packing List, Bill of Lading, Certificate of Origin (Form A or applicable FTA), MTC 3.1, and Fumigation Certificate for wooden packaging. Since 2026, electronic Bayan (e-Bayan) declaration is mandatory for Oman customs clearance; UAE importers file through Dubai Customs or the relevant emirate portal. We can provide full export documentation support per your request.

Q11: What sizes and pressure ratings are available for carbon steel threaded flanges?

A11: NPS 1/2 inch to 24 inch. Class 150 to Class 2500 under ASME B16.5; Class 150 and Class 300 are supplied as standard. Sizes and ratings outside this range can be reviewed against your drawing. The exact range confirmed for your order is stated in the quotation.

Q12: How is the wall thickness determined for carbon steel threaded flanges?

A12: Flange dimensions, including hub and thickness, are fixed by the ASME standard for the size and pressure class you specify. They are not chosen separately. The matching pipe schedule is confirmed at quotation stage.

Q13: What is the lead time for carbon steel threaded flanges?

A13: Standard sizes in regular material normally take 7-15 days. Non-standard sizes, special materials and custom orders take longer. The exact lead time for this product is given in the specification table on this page and is confirmed in writing with your quotation. Sea freight to the Gulf is additional and depends on the destination port. All lead times are subject to the production schedule at the time of order.

Q14: Is there a minimum order quantity for carbon steel threaded flanges?

A14: There is no fixed minimum. Standard items can ship from a single pallet, and custom production is normally quoted at full container load. Small trial orders and mixed-size orders are accepted. Send your size list and we will confirm what can be combined in one shipment.

Q15: How are carbon steel threaded flanges packed and marked?

A15: Flanges and fittings are packed in plywood cases or on plywood pallets with anti-rust treatment, strapped and stretch-wrapped for sea freight. Each piece carries the heat number, and the material, size, standard and pressure class or schedule are marked on the item or on the case label. Shipping marks are printed to your specification, and a fumigation certificate is provided where wooden packaging is used.

Q16: Can you produce carbon steel threaded flanges to our drawing or to non-standard specifications?

A16: Yes. Send the drawing, material, quantity and the applicable standard, and we will confirm feasibility and quote. Non-standard angles, non-standard schedules, special materials and special dimensions are produced against your drawing. The details we need to quote are listed on our custom service page. The details we need to quote are listed on our custom service page.

Engineer's Note

The following technical guidance reflects the perspective of a senior piping engineer with extensive experience in industrial piping system design.

Threaded flanges solve a specific problem — assembling a flanged joint without hot work — and they should be specified for that reason, not simply to avoid welding cost. The following factors should drive your specification:

1. The Real Reason to Specify Threaded

• Hot work prohibited or permit not obtainable: operating plants, fuel handling areas, offshore platforms
• No qualified welder or welding equipment available
• Low-pressure, ambient-temperature utility service where welding cost is not justified
• If none of these apply, a welded joint is normally the better engineering choice

2. Pressure and Temperature Limits

• ASME B16.5 dimensions threaded flanges from Class 150 to Class 2500
• Intended for ambient-temperature, non-cyclic service
• Not for Class 600 and above, not for high temperature, not for thermal cycling

3. Thermal Cycling Is the Dominant Failure Mode

• Differential expansion between pipe and flange relaxes the thread make-up
• Repeated heating and cooling opens a leak path even in a joint that was tight at assembly
• If the line sees start-up and shutdown cycles, specify a welded connection

4. Thread Standard and Compatibility

• NPT to ASME B1.20.1, 60-degree thread angle
• BSPT to ISO 7-1, 55-degree thread angle, available on request
• The two must never be mixed: different angle, different pitch, different thread form
• State the thread standard on the enquiry and match it to the project pipe thread standard

5. Vibration and Support

• Threaded joints are not suitable for vibrating lines such as pump discharges and reciprocating compressor piping
• Vibration loosens the thread make-up over time
• Provide independent pipe support; do not rely on the threaded joint for alignment or load

6. Sealing and Testing

• Use a thread sealant compatible with the fluid, temperature, and pressure
• Threaded joints do not have the inherent leak tightness of a butt weld
• Always pressure test threaded systems after assembly

7. Common Mistakes to Avoid

• Using threaded flanges to avoid welding cost on hydrocarbon or flammable lines
• Specifying threaded joints on hot or cycling lines
• Mixing NPT and BSPT on the same joint
• Using threaded flanges on vibrating equipment connections
• Assuming a large-diameter threaded joint can be made up reliably on site

For project-specific engineering support, please share your service conditions (size, class, temperature, fluid, cycling vs static, vibration, thread standard, and any hot work restriction) and we will review the specification and provide a written recommendation.

Frequently Asked Questions

Why are threaded flanges mainly supplied in Class 150 and Class 300?

ASME B16.5 dimensions threaded flanges from Class 150 to Class 2500. Class 150 and Class 300 cover almost all threaded service, and these two classes are what we supply as standard. Above these classes, the threaded connection is not the governing standard detail — the joint relies on the pipe thread for load transfer, and the thread cut removes material from the pipe wall, which limits the pressure capability. For Class 600 and above, or for any hydrocarbon, high-temperature, or cyclic service, weld neck flanges are the specified connection. If your piping class calls for a threaded flange above Class 300, please share the specification and we will confirm what can be supplied.

Do threaded flanges require welding?

No. That is the main reason to specify them. The flange bore is machined with a tapered pipe thread and is screwed directly onto the threaded pipe end. Installation needs no welder, no weld procedure, no weld inspection, and no hot work permit. This makes threaded flanges the standard choice in operating plants, fuel handling areas, offshore platforms, and any location where hot work is restricted or where a welding crew is not available. A thread sealant or PTFE tape is used on the pipe thread during assembly.

Is the thread NPT or BSPT? Can the two be mixed?

Standard thread is NPT to ASME B1.20.1. BSPT to ISO 7-1 can be supplied on request. NPT and BSPT must never be mixed on the same joint. They differ in thread angle, pitch, and thread form: NPT has a 60-degree thread angle, BSPT has 55 degrees. A BSPT male thread will not seal correctly in an NPT female thread, and forcing the connection damages both components and produces a joint that leaks under pressure. Please state the required thread standard clearly on your enquiry, and match it to the pipe thread standard used on your project.

Can threaded flanges be used on high-temperature lines?

Threaded flanges are not recommended for high-temperature service or thermal cycling. The failure mode is well known: repeated heating and cooling causes the pipe and the flange to expand and contract at different rates, which relaxes the thread make-up and opens a leak path. Threaded joints are intended for ambient-temperature, non-cyclic service. If your line operates at elevated temperature or sees regular start-up and shutdown cycles, specify weld neck flanges. Please share your operating temperature and cycle frequency and we will confirm the recommendation.

Are threaded flanges prone to leakage? How is the joint sealed?

A threaded joint relies on the thread make-up and a thread sealant, so it does not have the inherent leak tightness of a welded butt joint. In low-pressure, ambient-temperature, static service, a properly made-up threaded joint with the correct sealant performs reliably and is accepted practice in firewater, instrument air, and utility service. The leak risk rises with pressure, temperature, cycling, and vibration. If your service is flammable, toxic, or high-pressure, a welded joint is the correct specification. Pressure testing after assembly is always recommended for threaded systems.

When is a threaded flange the right choice over a welded flange?

Three situations: where hot work is prohibited or a permit cannot be issued, such as an operating plant or a fuel handling area; where no qualified welder or welding equipment is available on site; and where the line is low-pressure, ambient-temperature utility service and the welding cost is not justified. Firewater systems, instrument air and nitrogen headers, and temporary or skid-mounted piping are typical examples. Outside these situations, a welded connection is generally the safer and more durable specification.

Can threaded flanges be used for natural gas service?

Threaded connections are used in natural gas distribution, but their use is governed by your local code and by the gas utility or operator specification. Most codes restrict threaded joints in gas service by pressure, by location, and by whether the piping is concealed or exposed. Above-ground, low-pressure, accessible installations are the common case. Please confirm your applicable code and pressure limit before specifying, and state the service and design pressure on your enquiry so the appropriate flange and thread standard can be supplied.

Are threaded flanges available in large diameters?

ASME B16.5 dimensions threaded flanges up to NPS 24 inch, but in practice threaded flanges are used mainly in smaller sizes. As diameter increases, making up a threaded joint becomes progressively harder: the torque required to seat the threads rises sharply, and the risk of galling and of an improperly seated joint increases. For larger diameters, a welded flange is both more practical and more reliable. If your project requires a large-diameter threaded flange, please share the size and service and we will advise on feasibility before quotation.

Do I need to specify pipe schedule for threaded flanges?

No. The flange bore is threaded, so there is no bore to match to your pipe inside diameter, and no pipe schedule is required for the flange itself. You need to specify nominal pipe size, pressure class, facing type, and thread standard. Note that the pipe you thread must have sufficient wall thickness to accept the thread cut — this is a pipe selection matter governed by your piping code, not by the flange. Please confirm the pipe schedule against your code requirement for threaded connections.

What is the HS Code and tariff for carbon steel threaded flanges imported into Oman and the UAE?

Carbon steel threaded flanges are classified under HS Code 7307.91 (tube or pipe fittings of iron or steel — flanges of iron or steel, other than stainless). GCC Common External Tariff is 5% on CIF value for goods imported into Oman and the UAE. Standard import documentation includes Commercial Invoice, Packing List, Bill of Lading, Certificate of Origin (Form A or applicable FTA), MTC 3.1, and Fumigation Certificate for wooden packaging. Since 2026, electronic Bayan (e-Bayan) declaration is mandatory for Oman customs clearance; UAE importers file through Dubai Customs or the relevant emirate portal. We can provide full export documentation support per your request.

What sizes and pressure ratings are available for carbon steel threaded flanges?

NPS 1/2 inch to 24 inch. Class 150 to Class 2500 under ASME B16.5; Class 150 and Class 300 are supplied as standard. Sizes and ratings outside this range can be reviewed against your drawing. The exact range confirmed for your order is stated in the quotation.

How is the wall thickness determined for carbon steel threaded flanges?

Flange dimensions, including hub and thickness, are fixed by the ASME standard for the size and pressure class you specify. They are not chosen separately. The matching pipe schedule is confirmed at quotation stage.

What is the lead time for carbon steel threaded flanges?

Standard sizes in regular material normally take 7-15 days. Non-standard sizes, special materials and custom orders take longer. The exact lead time for this product is given in the specification table on this page and is confirmed in writing with your quotation. Sea freight to the Gulf is additional and depends on the destination port. All lead times are subject to the production schedule at the time of order.

Is there a minimum order quantity for carbon steel threaded flanges?

There is no fixed minimum. Standard items can ship from a single pallet, and custom production is normally quoted at full container load. Small trial orders and mixed-size orders are accepted. Send your size list and we will confirm what can be combined in one shipment.

How are carbon steel threaded flanges packed and marked?

Flanges and fittings are packed in plywood cases or on plywood pallets with anti-rust treatment, strapped and stretch-wrapped for sea freight. Each piece carries the heat number, and the material, size, standard and pressure class or schedule are marked on the item or on the case label. Shipping marks are printed to your specification, and a fumigation certificate is provided where wooden packaging is used.

Can you produce carbon steel threaded flanges to our drawing or to non-standard specifications?

Yes. Send the drawing, material, quantity and the applicable standard, and we will confirm feasibility and quote. Non-standard angles, non-standard schedules, special materials and special dimensions are produced against your drawing. The details we need to quote are listed on our custom service page.

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