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What Is FIP in Plumbing? A Completed Guide 2025

Update on 2025/11/26

Introduction

In hydraulic systems, industrial process piping, and OEM fluid power applications, accurate specification of threaded connections is non-negotiable. One of the most frequently referenced — and occasionally misunderstood — designations is FIP (Female Iron Pipe).

FIP refers to female tapered pipe threads conforming to the ANSI/ASME B1.20.1 NPT (National Pipe Taper) standard. These are internal threads designed to accept male NPT or NPTF threaded components, providing a pressure-tight mechanical seal when properly assembled with thread sealant or anaerobic compounds.

Although the term originated with iron and steel pipe, modern FIP threads appear across a wide range of materials including 316/316L stainless steel, carbon steel, brass, and specialty alloys — materials routinely specified in hydraulic power units, test stands, refineries, and high-purity gas delivery systems.

For procurement engineers and system designers, understanding the precise meaning of “FIP” eliminates costly mismatches between drawings, BOMs, and delivered components.

FIP vs MIP: Critical Distinctions in Industrial Specifications

Parameter	FIP (Female Iron Pipe)	MIP (Male Iron Pipe)
Thread location	Internal (inside fitting)	External (outside nipple or adapter)
Mates with	MIP or MNPT external threads	FIP or FNPT internal threads
Typical components	Female threaded elbows, tees, couplings, valves, gauges	Threaded nipples, plugs, male adapters
Pressure rating impact	Same rating as mating male thread when correctly sealed	Same rating as mating female thread

In hydraulic manifolds and valve assemblies, correct pairing of FIP and MIP ends is essential for maintaining rated burst pressures and avoiding cross-threading during assembly.

FIP vs NPT vs NPTF: Clearing Up Decades of Industry Confusion

This is where most catalog descriptions fall short.

NPT (ANSI/ASME B1.20.1) – Standard tapered pipe thread requiring thread sealant (PTFE tape or pipe dope)
NPTF (SAE J476, also called Dryseal) – Modified taper with tighter root/crest tolerances; achieves metal-to-metal seal without sealant in many cases
FIP/FNPT – Trade terminology indicating the female version of the above standards

In practice:

When a drawing simply says “1/2" FIP”, North American manufacturers interpret it as female NPT
When Class 2500–6000 psi hydraulic or fuel systems are involved, engineers typically specify NPTF female (still commonly labeled as “FIP” in catalogs) for superior sealing and galling resistance

Leading hydraulic fitting manufacturer, including HGW Hydraulic, offer both standard NPT and SAE-certified NPTF female threads in 304/316 stainless and A105 carbon steel to meet diverse application requirements.

FIP Thread Standards, Taper Angles, and Complete Dimension Reference

All FIP threads sold in North America follow one of two standards:

Standard	Full Name	Taper Angle	Seal Method	Typical Max Pressure (316SS, Class 3000)
NPT	National Pipe Taper (ASME B1.20.1)	1°47′	Requires sealant	4000–5000 psi (275–345 bar)
NPTF	National Pipe Taper Fuel (SAE J476)	1°47′	Dryseal – often no sealant	6000–10,000 psi (414–690 bar)

Complete FIP / NPT / NPTF Size & Thread Data Table (most competitors only give 3–4 sizes; we give all common sizes that procurement actually uses)

Nominal Size	Threads Per Inch (TPI)	Thread OD Male (in)	Thread ID Female (in)	Pitch Diameter at End (E1)	Max Working Pressure 316SS Class 3000 (psi)
1/16″	27	0.3125	0.276	0.281	9,000
1/8″	27	0.405	0.369	0.374	6,500
1/4″	18	0.540	0.492	0.502	6,500
3/8″	18	0.675	0.627	0.637	5,500
1/2″	14	0.840	0.782	0.792	5,500
3/4″	14	1.050	0.992	1.002	5,000
1″	11.5	1.315	1.243	1.256	4,500
1-1/4″	11.5	1.660	1.583	1.596	4,000
1-1/2″	11.5	1.900	1.822	1.835	4,000
2″	11.5	2.375	2.285	2.298	3,500
2-1/2″	8	2.875	2.761	2.786	4,000
3″	8	3.500	3.376	3.401	3,800

Data source: ASME B1.20.1-2013 & SAE J476; pressure values based on HGW Hydraulic internal testing + third-party burst reports.

How to Positively Identify FIP Threads in the Field or Incoming Inspection

Procurement and QC departments use these four methods (in order of speed):

1. Thread Gauge (Go/No-Go)

Fastest and most reliable. HGW Hydraulic includes calibrated NPT/NPTF plug gauges with every 500+ piece order.

2. Visual + Tactile Check

FIP = internal threads only
Taper is visible: the thread crests get progressively smaller toward the face
NPTF crests appear slightly crushed/truncated compared to NPT

3. Optical Comparator or Thread Profile Projector

Required for ISO 9001 / IATF 16949 incoming inspection when full traceability is mandatory.

4. Pitch Diameter Measurement with Thread Wires

Used when customer drawing specifies “NPTF Class 2” tolerance.

FIP Materials Selection for Industrial & Hydraulic Applications

Material	Common Grades	Max Temp	Corrosion Resistance	Typical Applications	Lead Content	Price Index
Carbon Steel A105	Forged	425°C	Poor (needs plating)	General hydraulic, oil & gas	<0.35%	1.0×
316/316L Stainless	ASTM A182 F316/316L	540°C	Excellent	Food, pharma, offshore, CNG	<0.03%	4.2×
Duplex 2205 / 2507	ASTM A182 F51/F53	600°C	Outstanding	Desalination, high-chloride service	<0.03%	7.5×
Brass CA377 / C37700	Forged	150°C	Good	Compressed air, low-pressure water	2.5–3.7%	2.1×
Monel 400 / K500	ASTM SB564	480°C	Superb in seawater	Marine hydraulic, subsea control lines	Nil	12×+
6-Moly (AL-6XN, 254SMO)	UNS N08367	600°C	Extreme pitting resist	Chlor-alkali, flue gas desulfurization	Nil	15×+

HGW Hydraulic standard stock program maintains over 180 SKUs of 316/316L Class 3000 and 6000 FIP fittings with EN 10204 3.1 certificates ready for immediate dispatch.

FIP in High-Pressure Hydraulic Systems: Why Serious Applications Move to NPTF (Dryseal)

Standard NPT was never designed for 700 bar mobile hydraulics or 99.999% helium leak-tight instrument lines.

NPTF advantages in real-world hydraulic service:

Interfering crest/root contact creates primary metal-to-metal seal
Reduced sealant contamination risk (critical in oxygen and hydrogen systems)
Galling resistance increased 300–400% in stainless steel
Tested to 4× working pressure without leakage in HGW laboratory

Case study: A North American OEM of 700-bar hydraulic test benches switched from NPT to NPTF female ports after experiencing 4% field leakage rate. Post-switch failure rate dropped to 0.03% across 42,000 manifolds.

FIP Application Scenarios in Industrial & Hydraulic Systems (With Real Project References)

Application	Typical Size	Material Preference	Pressure/Temp	Why FIP is Specified
Hydraulic power unit manifold ports	1/4″–1″	316SS NPTF Class 6000	400–700 bar, –40~120°C	Compact footprint, field-replaceable valves
Instrument air headers	1/8″–1/2″	316SS or Brass	10–20 bar	Legacy drawings, millions of existing FIP take-offs
CNG dispensing stations	1/4″–1″	316L NPTF	250–350 bar	Fast make/break during dispenser maintenance
Offshore firewater deluge valves	1″–2″	Duplex 2205	15–25 bar seawater	Corrosion resistance + compatibility with existing carbon-steel piping
High-purity nitrogen purge panels	1/4″–1/2″	Electropolished 316L	5–150 bar	No PTFE tape allowed → NPTF Dryseal required
Mobile hydraulic test benches	1/4″–3/4″	Carbon steel Cr13 coating	700 bar cyclic	Cost-effective + proven fatigue life with NPTF

Comprehensive Compatibility Matrix: FIP with Global Thread Standards

Female Thread	Male Thread Counterpart	Seal Method	Compatibility Notes	Typical Cross Reference
FIP (NPT)	NPT, NPTF, NPSM	Sealant required	100% compatible	—
FIP (NPTF)	NPTF only	Dryseal primary	NPT male acceptable but loses Dryseal benefit	SAE AS71051
FIP (NPT)	BSPT (R)	Sealant	Limited (1–2 turns engagement only) → high leak risk → not recommended	—
FIP (NPT)	BSPP (G)	Washer or O-ring	Not compatible without special BSPP male with 60° cone + bonded washer	—
FIP (NPT)	JIC 37°	Never	Completely different flare → catastrophic failure if forced	—
FIP (NPT)	SAE ORB (O-ring Boss)	Never	Different thread form and seal method	—
FIP (NPT)	DIN 3852 Form A/B	Sealant	Metric taper different → not interchangeable	—
FIP (NPT)	Metric ISO 6149	O-ring	Not compatible	—

Rule of thumb for procurement: Never mix imperial tapered with metric or parallel standards without an explicitly tested adapter.

Top 5 Costly Mistakes When Specifying or Purchasing FIP Fittings

1. Accepting “NPT” when the system actually demands NPTF Dryseal

→ Result: field leakage in oxygen-clean or high-cycle applications

→ Cost: $80,000–$350,000 per shutdown incident

2. Confusing nominal size with actual thread OD

→ 1/2″ FIP is NOT 0.500″ OD (actual ≈0.840″) → wrong hole drilled in manifolds

3. Ordering Schedule 40 fittings for 500+ bar service

→ Class 3000 or 6000 forged fittings are mandatory above 250 bar

4. Missing material traceability requirements

→ EN 10204 3.1/3.2 certificates + positive material identification (PMI) now mandatory in most offshore and pharma projects

5. Assuming all 316/316L is the same

→ Low-carbon 316L + sulfur 0.008–0.015% is required for orbital welding and galling resistance

HGW Hydraulic eliminates these risks with lot-level traceability, standard 3.1 certificates, and pre-shipment PMI on every stainless order.

Professional Selection Guidelines: When to Specify FIP and When to Move On

Scenario	Recommended Thread	Reason
Pressure ≤250 bar & cost critical	Standard NPT (FIP)	Lowest cost, widely available
Pressure 300–700 bar or >5000 cycles	NPTF Dryseal	Superior fatigue & sealing performance
Clean hydraulic (NAS 6 or better)	SAE ORB or JIC preferred	NPT/F generates particles during assembly
Oxygen, hydrogen, or explosive decompression	NPTF + Monel or no tape	PTFE tape prohibited
Future maintenance by non-specialized crews	FIP/NPTF	Most field technicians carry NPT tools
Space-constrained manifolds	FIP/NPTF	Smallest envelope for given flow compared to flange or ORB

Sealing Best Practices for FIP/NPTF Threads in Critical Service

Application Class	Recommended Sealant	Wraps / Coats	Torque (ft-lb) for 316SS Class 3000	Notes
General hydraulic	PTFE tape (yellow, high-density)	2–3 wraps	25–35 (1/2″)	Start second thread, clockwise, no fraying
High-purity gas	Loctite 577 or 567 anaerobic	360° coverage	Hand-tight + 1.5 turns	No tape contamination
Oxygen service	PFA tape or Swagelok-approved oxygen paste	1.5 wraps	Controlled torque only	BAM or EIGA approved only
Steam ≤150 psi	Graphite/nickel paste	3 coats	40–50 (1/2″)	High-temp anti-seize
NPTF Dryseal	None or light hydraulic oil only	—	Hand-tight + 1–2 turns	Over-torquing destroys interference seal

HGW Hydraulic includes torque specification sheets with every Class 3000/6000 order.

Frequently Asked Questions from Procurement & Engineering Teams

Q1: Is “FIP” still an acceptable callout on 2025 engineering drawings?

A: Yes. In North America, “FIP” or “FNPT” remains universally understood as female tapered pipe thread per ASME B1.20.1. For new designs requiring Dryseal performance, specify “FIP-NPTF” or “FNPTF” plus SAE J476 reference.

Q2: What is the real pressure rating of a 1/2″ 316SS Class 3000 FIP fitting?

A: Typical working pressure is 5500–6000 psi (380–414 bar) with 4:1 safety factor. Burst exceeds 20,000 psi when assembled with matching NPTF male and proper sealant. Full test reports available from HGW Hydraulic on request.

Q3: Do you offer low-lead or lead-free FIP fittings for potable water or food contact?

A: Yes. All HGW 316/316L fittings contain <0.03% Pb and comply with NSF/ANSI 372 and FDA requirements. Brass CA377 is limited to non-potable compressed air and hydraulic service.

Q4: What is the MOQ and standard lead time for custom 2507 Duplex FIP fittings?

A: Standard MOQ is 100 pieces per size. Stock 316SS Class 3000 items ship same day; Duplex and Monel orders typically 4–6 weeks ex-works with EN 10204 3.2 + NORSOK M-650 certification available.

Q5: Can FIP ports be orbital-welded after machining?

A: Yes, provided 316L with controlled sulfur 0.008–0.015% is used and wall thickness meets ASME B31.3. HGW Hydraulic supplies weld-ready extended-leg FIP fittings specifically for automated orbital welding.

Q6: Are your FIP threads inspected 100% with go/no-go gauges?

A: Every production lot undergoes 100% go/no-go gauging plus sample pitch-diameter and taper inspection per ASME B1.20.1. Gauge certificates traceable to NIST are included with shipments over 500 pieces.

Conclusion

Specifying the correct female tapered pipe thread eliminates one of the most common sources of leakage, delays, and field rework in hydraulic and industrial piping systems. While the term “FIP” is simple, the underlying standards, material selection, pressure ratings, and sealing practices directly impact system reliability, total cost of ownership, and safety.

When performance, traceability, and delivery certainty matter, partner with a manufacturer who treats FIP threads as a precision component—not a commodity.

HGW Hydraulic maintains one of North America’s largest inventories of certified Class 3000 and 6000 316/316L and Duplex FIP fittings, with full material certificates, PMI reports, and 48-hour quotations as standard.

Need compliant, high-performance FIP solutions delivered on time? Contact our engineering sales team at sales@hgwhydraulic.com or visit www.hgw-hydraulic.com for drawings, stock list, and live inventory.

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