Flange Pressure Rating Fundamentals

1. ASME B16.5 Flange Classes

ASME B16.5 defines seven standard pressure classes. The class number is not the pressure rating—it's a dimensionless designation. Actual ratings depend on material and temperature.

Class	Rating @ 100°F (Group 1.1)	Typical Use
150	285 psig	Low-pressure water, HVAC, process utilities
300	740 psig	Steam systems, gas distribution
400	990 psig	Intermediate service (less common)
600	1,480 psig	High-pressure gas, oil pipelines, refineries
900	2,220 psig	Injection wells, high-pressure process
1500	3,705 psig	Extreme pressure applications
2500	6,170 psig	Ultra-high pressure, specialty systems

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Key point: A Class 150 flange is rated for 285 psig at ambient—not 150 psig. The class number is historical nomenclature, not a pressure value.

Physical comparison of flange classes by size and thickness. — Physical comparison of flange classes by size, thickness, bolt count, and bolt circle.

Physical Differences by Class

Higher classes have thicker walls, more bolts, larger bolt diameters, and larger bolt circles. Flange facing also varies: raised face (RF) is standard for most classes; ring-type joint (RTJ) is used for Class 600+ in critical service.

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Not interchangeable: Flanges of different classes cannot mate—bolt patterns, facing dimensions, and thicknesses differ. Always match classes across a joint.

2. Pressure-Temperature Ratings

Material strength decreases at elevated temperatures. ASME B16.5 provides pressure-temperature (P-T) tables showing maximum allowable working pressure at each temperature for every material group and class.

Pressure-temperature derating curve for flange ratings.

Carbon Steel P-T Ratings (Group 1.1)

Values per ASME B16.5 Table 2-1.1 for ASTM A105, A216 WCB, A350 LF2:

Temp (°F)	Cl 150	Cl 300	Cl 400	Cl 600	Cl 900	Cl 1500	Cl 2500
-20 to 100	285	740	990	1,480	2,220	3,705	6,170
200	260	675	900	1,350	2,025	3,375	5,625
300	230	655	875	1,315	1,970	3,280	5,470
400	200	635	845	1,270	1,900	3,170	5,280
500	170	600	800	1,200	1,795	2,995	4,990
600	140	550	730	1,095	1,640	2,735	4,560
700	110	535	710	1,065	1,600	2,665	4,440
800	80	410	550	825	1,235	2,060	3,430
850	65	270	355	535	805	1,340	2,230
900	50	170	230	345	515	860	1,430
950	35	105	140	205	310	515	860
1000	20	50	70	105	155	260	430

All values in psig. Source: ANSI/ASME B16.5 Table 2-1.1

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At 850°F, Class 600 drops to 535 psig—only 36% of its ambient rating. Temperature derating is dramatic above 800°F due to creep effects.

Interpolation

For temperatures between tabulated values, linear interpolation is permitted per ASME B16.5:

Linear Interpolation Formula

P_T = P₁ + (P₂ − P₁) × (T − T₁) / (T₂ − T₁)

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No class interpolation: You may interpolate between temperatures within a class, but you cannot interpolate between classes. Always round up to the next class if design pressure exceeds a class rating.

3. Material Groups

ASME B16.5 organizes materials into groups with similar mechanical properties. Each group has its own P-T rating table. Groups 1.x are carbon/low-alloy steels; Groups 2.x are stainless and high-alloy steels; Groups 3.x are non-ferrous alloys.

Group	Material	Typical Specs	Max Temp
1.1	Carbon Steel	A105, A216 WCB, A350 LF2	1,000°F
1.2	Low-Temp Carbon	A350 LF3, LF6	1,000°F
1.5	C-½Mo	A182 F1	1,000°F
1.7	1¼Cr-½Mo	A182 F11	1,100°F
1.9	2¼Cr-1Mo	A182 F22	1,100°F
1.10	3½Ni	A350 LF3	1,000°F
2.1	18Cr-8Ni (304)	A182 F304, F304H	1,500°F
2.2	16Cr-12Ni-2Mo (316)	A182 F316, F316H	1,500°F
2.3	18Cr-10Ni-Ti (321)	A182 F321, F321H	1,500°F
2.5	Ni-Cu (Monel)	B564 N04400	900°F

Material selection flowchart for ASME B16.5 flange classes.

When to Use Higher Groups

High temperature (>800°F): Cr-Mo alloys (Groups 1.7, 1.9) resist creep better than carbon steel
Corrosive service: 304/316 stainless (Groups 2.1, 2.2) for acids, chlorides, sour gas
Cryogenic (<-50°F): Austenitic stainless (Group 2.1+) retains toughness at low temps
Marine/seawater: Monel or duplex stainless for pitting resistance

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Cost trade-off: Stainless flanges cost 3–5× more than carbon steel but may allow a lower class at high temperatures, potentially offsetting material cost.

4. Flange Class Selection

Proper class selection ensures safe operation while avoiding over-specification that increases cost and weight.

Selection Steps

1. Determine design pressure (P_d) and design temperature (T_d)
2. Select material group based on corrosion and temperature needs
3. Apply safety factor: P_req = P_d × 1.1 to 1.25
4. Look up P-T table for material group at T_d
5. Select lowest class where rated pressure ≥ P_req

Selection Examples

Application	Design Conditions	Class	Material
Water main	150 psig, 80°F	150	A105 (rated 285 psig)
Gas gathering	600 psig, 150°F	600	A105 (rated 1,430 psig)
Steam header	400 psig, 600°F	600	A105 (rated 1,095 psig @ 600°F)
Sour gas	900 psig, 200°F	900	F316 (corrosion + pressure)
High-temp process	300 psig, 900°F	600	F22 Cr-Mo (rated 555 psig @ 900°F)
LNG cryogenic	150 psig, -260°F	300	F304 SS (cryogenic toughness)

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Account for transients: Include margin for pressure surges, relief valve accumulation (10% overpressure), temperature excursions during upsets, and corrosion allowance.

5. Cyclic Service Derating

Repeated pressure or thermal cycling causes fatigue. ASME B31.3 defines "severe cyclic conditions" when stress exceeds 80% of allowable and cycles exceed 7,000. For such service, apply derating or enhanced construction.

Cyclic Service Classification

Service Type	Cycle Count	Derating Factor	Notes
Normal	≤ 1,000	1.00	Standard design
Moderate Cyclic	1,000 – 7,000	0.80	Enhanced inspection recommended
Severe Cyclic	> 7,000	0.67	Per B31.3 Para. 300.2
High-Cycle Fatigue	> 100,000	≤ 0.50	Use B31.3 Appendix W analysis

Derated Pressure

P_derated = P_rated × Derating Factor

Example: Class 600 at 400°F is rated 1,270 psig. For severe cyclic service (>7,000 cycles):

P_derated = 1,270 × 0.67 = 851 psig

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Cycle sources: Daily startups/shutdowns, batch operations, pressure swings from compressors, thermal cycling from intermittent flow, and vibration from rotating equipment.

Fatigue Analysis per ASME B31.3 Appendix W

For detailed fatigue assessment in severe cyclic service:

Fatigue Life Calculation: S_a = (P_max - P_min) / 2 × Stress Intensity Factor N = (C / S_a)^m Where: S_a = Alternating stress amplitude (psi) N = Number of cycles to failure C, m = Material constants from fatigue curve (S-N curve) Design criterion: Expected cycles × Safety Factor ≤ N Typical values: For carbon steel at room temperature: C ≈ 3.0 × 10⁶ psi m ≈ 3.0 (for N < 10,000 cycles) m ≈ 5.0 (for N > 10,000 cycles) Safety Factor = 2 to 20 (depending on consequences of failure)

Thermal Cycling Effects

Temperature cycles cause differential expansion and stress:

ΔT Cycle Range	Thermal Stress Level	Design Action
< 100°F	Low	Standard design adequate
100-200°F	Moderate	Consider expansion loops, flexibility analysis
200-400°F	High	Detailed flexibility analysis required per ASME B31.3
> 400°F	Severe	Spring supports, expansion joints, or special materials

Enhanced Inspection for Cyclic Service

Baseline inspection: Ultrasonic thickness measurements at installation
Periodic inspection: Every 1-5 years depending on cycle count and stress level
Focus areas: Flanges, welds, branch connections, areas of constraint
Inspection methods: UT, liquid penetrant (PT), magnetic particle (MT), visual
Acceptance criteria: Any crack indication = immediate evaluation/repair

6. Large Diameter Flanges (ASME B16.47)

For pipe sizes NPS 26 and larger, ASME B16.47 covers two series of flanges: Series A (comparable to API 605) and Series B (comparable to ASME B16.5 scaling).

B16.47 Series Comparison

Feature	Series A	Series B
Size range	NPS 26 - 60	NPS 26 - 60
Bolt pattern	API 605 based	B16.5 extrapolation
Pressure classes	75, 150, 300, 400, 600, 900	75, 150, 300, 400, 600, 900
Weight	Lighter (fewer/smaller bolts)	Heavier (more/larger bolts)
Industry preference	Common in oil/gas pipelines	Less common, higher strength margin

Class 300 Large Flange Ratings (Group 1.1)

Size (NPS)	Bolts (Series A)	Bolts (Series B)	Bolt Size	Weight (lb) Series A
26	28 × 1¼"	32 × 1¼"	1¼"	750
30	28 × 1½"	32 × 1½"	1½"	950
36	32 × 1½"	40 × 1½"	1½"	1,250
42	36 × 1¾"	44 × 1¾"	1¾"	1,850
48	44 × 1¾"	52 × 1¾"	1¾"	2,450
60	52 × 2"	60 × 2"	2"	3,800

Weights are approximate for weld neck flanges, carbon steel

Pressure-Temperature Ratings

B16.47 flanges use the same P-T tables as B16.5 for material groups. Temperature derating is identical.

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Series compatibility: Series A and Series B flanges are NOT interchangeable—different bolt circles, hole sizes, and bolt counts. Always specify series when ordering large diameter flanges.

7. Valve Pressure Ratings (ASME B16.34)

ASME B16.34 specifies pressure-temperature ratings for valves. Valve ratings are coordinated with flange ratings to ensure compatible joints.

Valve Rating Consistency

Valves must be rated for the same or higher class as the connecting flanges:

Design Rule: Valve Class ≥ Pipe Flange Class at design temperature Example: Pipeline: Class 600 flanges at 400°F Valves: Must be Class 600 or higher, rated for 1,270 psig @ 400°F Common error: Selecting Class 600 valve rated for ambient (1,480 psig) but used at 600°F (where Class 600 rating drops to 1,095 psig) → Underdesigned!

Valve Body Material Groups

B16.34 uses same material groupings as B16.5:

Group	Valve Body Material	ASTM Spec	Application
1.1	Carbon steel (cast)	A216 WCB	General service, -20°F to 1,000°F
1.2	Carbon steel (forged)	A105	Small valves, threaded connections
1.9	2¼Cr-1Mo	A217 WC9	High temp service, up to 1,100°F
2.1	CF8 (304 cast SS)	A351 CF8	Corrosion resistance, -425°F to 1,500°F
2.2	CF8M (316 cast SS)	A351 CF8M	Superior corrosion resistance

Pressure Testing of Valves

Valves are hydrostatically tested before shipment per B16.34:

Test Type	Test Pressure	Duration	Acceptance Criteria
Shell test	1.5 × rated pressure at 100°F	Per B16.34 Table 8	No visible leaks
Seat test (gate/globe)	1.1 × rated pressure	Per manufacturer	No visible leaks
Seat test (check)	Per B16.34 Table 10	Per manufacturer	Max leakage per API 598
Seat test (ball)	1.1 × rated pressure	Per manufacturer	Zero leakage (API 6D)

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Critical point: Valve pressure rating applies to body strength. Seat tightness is a separate specification covered by API 598 (general valves) and API 6D (pipeline valves). Seat leakage classifications range from Class I (most stringent, zero visible leakage) to Class VI (50 ml/hr per inch of seat diameter for soft-seated valves).

8. Gasket Selection for Flange Ratings

Gasket material must be compatible with flange rating, service temperature, and fluid. Incorrect gasket selection is a leading cause of flange joint leakage.

Gasket Types by Pressure Class

Flange Class	Typical Gasket Type	Material	Temp Range
150-300	Full-face or ring	Compressed fiber, rubber, graphite	-100°F to 750°F
400-600	Spiral-wound with centering ring	Graphite filler, 316 SS winding	-325°F to 1,000°F
900-1500	Spiral-wound or metal	Graphite or corrugated metal	-450°F to 1,200°F
2500	Ring joint (RTJ)	Soft iron, 304 SS, Monel	-450°F to 1,000°F

Gasket Factor (m) and Seating Stress (y)

ASME B16.5 Appendix S provides gasket factors for bolt load calculations:

Gasket Type	m Factor	y (psi)	Service Limitation
Rubber, flat	0.50	0	Low pressure only
Compressed fiber	1.25	1,600	≤ 400°F
Graphite (flexible)	1.75	4,500	Up to 750°F
Spiral-wound (graphite)	3.00	10,000	Up to 1,000°F
Metal (corrugated)	3.75	14,500	High temp/pressure
Ring joint (RTJ)	5.50	18,000	Highest ratings

Bolt Load Calculation

Required bolt load to seat and maintain gasket seal:

Bolt Load for Gasket Seating: W = π b G y Where: W = Minimum bolt load (lb) b = Effective gasket width (in) G = Gasket diameter (mean) (in) y = Gasket seating stress (psi) Operating Bolt Load: W = H + H_p H = π G b m P (gasket compression) H_p = π G² P / 4 (internal pressure force) Where: P = Internal pressure (psig) m = Gasket factor Bolt stress check: S_bolt = W / (n × A_bolt) ≤ S_allow Where: n = Number of bolts A_bolt = Root area of each bolt (in²) S_allow = Allowable bolt stress (psi)

Torque Requirements

Proper bolt torque is critical for gasket sealing:

Bolt Size	Class 150 (ft-lb)	Class 300 (ft-lb)	Class 600 (ft-lb)	Class 900+ (ft-lb)
½"	15	20	25	Hydraulic tensioning
⅝"	30	40	50	Hydraulic tensioning
¾"	50	65	85	Hydraulic tensioning
⅞"	75	100	130	Hydraulic tensioning
1"	110	145	190	Hydraulic tensioning
1⅛"	150	200	260	Hydraulic tensioning
1¼"	200	265	350	Hydraulic tensioning

Values are approximate for lubricated B7 studs. Actual values depend on gasket type, material, and assembly procedure. For Class 900+, hydraulic tensioning strongly recommended.

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Torque procedure: Use cross-pattern tightening (star pattern), three passes at 30%, 60%, and 100% of target torque. Never tighten bolts sequentially around the flange—this causes uneven gasket compression and potential leakage or flange warping.

9. Common Design and Selection Errors

Flange rating errors lead to leaks, failures, and safety incidents. Avoid these frequent mistakes:

Top 10 Flange Rating Errors

Error	Consequence	Prevention
Using class number as pressure rating	Class 150 ≠ 150 psi! Leads to under-design	Always reference P-T tables for actual rating
Ignoring temperature derating	High-temp service exceeds rating, flange fails	Check rating at design temperature, not ambient
Mixing flange classes	Class 300 won't mate with Class 600 (different bolt patterns)	Verify all flanges in joint are same class
Wrong material group	Group 1.1 rating used for Group 2.1 material	Match material group to actual flange material spec
Inadequate gasket for service	Gasket blows out, leakage, fire/explosion risk	Select gasket rated for pressure, temp, and fluid
Forgetting surge pressure	Water hammer or valve closure exceeds rating	Add 10-25% margin for transients
B16.47 Series A/B mix	Bolt holes don't align, cannot assemble	Specify series explicitly on drawings
Insufficient bolt torque	Gasket not properly seated, leaks under pressure	Use torque wrench, follow tightening procedure
Excessive bolt torque	Gasket crushed, bolt yielding, flange warping	Don't exceed specified torque, use lubricant
Cyclic service not considered	Fatigue cracking in flanges or bolts	Apply derating factors for >7,000 cycles

Real-World Failure Case Studies

Case 1: Class Confusion Leads to Leak Incident: Steam header flange joint leaked during startup Design intent: 450 psig @ 600°F steam service Engineer specified: Class 300 flanges (thinking 300 psi) Actual rating: Class 300 @ 600°F = 550 psig (from P-T table) Margin: Only 550 - 450 = 100 psi (22% margin, inadequate) Root cause: Surge pressure during startup (~550 psi) exceeded rating. Gasket blew out, steam release, personnel injury. Correction: Replace with Class 600 flanges (rated 1,095 psig @ 600°F = adequate margin)

Case 2: Material Group Confusion Incident: Sour gas pipeline flange leaked H₂S Design: 900 psig @ 150°F, sour service (H₂S > 50 ppm) Material: ASTM A350 LF2 (low-temp carbon steel, Group 1.2) Class selected: Class 600 Engineer's error: Used Group 1.1 P-T table by mistake Group 1.1 rating @ 150°F = 1,430 psig (appeared adequate) Actual rating (Group 1.2 at 150°F): 1,295 psig Still adequate BUT: Wrong material for sour service! NACE MR0175 requirement: Hardness < 22 HRC for sour service A350 LF2 can exceed hardness limit → sulfide stress cracking (SSC) Correction: Replace with Group 2.2 (F316) stainless steel flanges, rated for sour service and adequate pressure

Design Review Checklist

Verify these items before finalizing flange selection:

☐ Pressure rating at design temperature verified from P-T table
☐ Surge/transient pressure margin included (10-25%)
☐ Material group matches actual flange material specification
☐ Corrosion/environmental compatibility checked (sour, chloride, cryogenic)
☐ All flanges in joint are same class
☐ B16.47 series (A or B) specified if NPS ≥ 26
☐ Gasket type and material suitable for P, T, and fluid
☐ Bolt material specified (typically ASTM A193 B7 or B7M)
☐ Torque values calculated or specified
☐ Cyclic service evaluation if >1,000 cycles expected
☐ Low-temperature impact testing required? (ASME B31.3 Table A-1)
☐ Hydrotest pressure specified (typically 1.5× design pressure)

10. Bolt and Stud Material Selection

Flange bolts (studs + nuts) must be compatible with flange rating, temperature, and environment. ASTM A193/A194 specifications cover bolting materials.

Standard Bolt Materials by Service

Service Condition	Stud Material (ASTM A193)	Nut Material (ASTM A194)	Temperature Range
General service	B7 (Cr-Mo steel)	2H (carbon steel)	-20°F to 1,000°F
Low temperature	B7M (Cr-Mo, impact tested)	2HM	-50°F to 1,000°F
Corrosive service	B8 (304 SS)	8 (304 SS)	-325°F to 1,500°F
Sour/corrosive	B8M (316 SS)	8M (316 SS)	-325°F to 1,500°F
High strength (Class 2500)	B16 (Cr-Mo-V)	7	-20°F to 1,100°F

Allowable Bolt Stress by Temperature

Bolt stress decreases at elevated temperature, similar to flange derating:

Temperature (°F)	A193 B7 (ksi)	A193 B8 (ksi)	A193 B8M (ksi)
-20 to 100	25.0	20.0	20.0
200	25.0	18.3	18.3
400	25.0	15.8	16.7
600	23.3	13.8	14.6
800	17.5	11.7	12.9
1,000	10.0	9.5	11.3

Bolt Lubrication

Proper lubrication critical for achieving correct preload:

Never-seize compound: Most common, nickel or copper-based, prevents galling
Teflon tape on threads: Allowed for gas service, easier disassembly
Graphite-based: High-temperature service (>800°F)
Molybdenum disulfide (moly): High-load applications

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Torque vs. tension: Torque is a poor proxy for bolt tension (actual clamping force). 80% of applied torque overcomes friction; only 20% creates tension. For critical joints (Class 900+), use hydraulic tensioners that directly measure and apply bolt load.

Cold Weather Bolting Precautions

Bolting below -20°F requires special considerations:

Impact testing: Use B7M (not B7) for temperatures below -50°F
Warm bolts before tightening: Heat to 50-70°F to reduce brittleness
Reduced torque rate: Tighten slower to avoid sudden loading
Re-torque after warmup: Thermal expansion may loosen bolts after system warms

Reference Standards

ASME B16.5	Pipe Flanges and Flanged Fittings (NPS ½ – 24)
ASME B16.47	Large Diameter Steel Flanges (NPS 26 – 60)
ASME B16.34	Valves—Flanged, Threaded, and Welding End
ASME B31.3	Process Piping (design and cyclic service requirements)
ASME B31.8	Gas Transmission and Distribution Piping Systems
ASME Sec II-D	Material Properties and Allowable Stresses
ASTM A193/A194	Alloy-Steel and Stainless Steel Bolting Materials
API 598	Valve Inspection and Testing
API 6D	Pipeline Valves (specification and testing)
NACE MR0175/ISO 15156	Petroleum and Natural Gas Industries—Materials for Use in H₂S Environments

Flange Pressure-Temperature Ratings

150 – 2500

1.1 – 3.19

-20°F to 1000°F

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