Piping FAQ

NPS (Nominal Pipe Size) is the North American size designation in inches. NB (Nominal Bore) is the same idea used in India and the UK. DN (Diamètre Nominal) is the metric designation in millimetres. They name the same sizes: NPS 4 = 4" NB = DN 100. None of them is the actual outside diameter for sizes up to NPS 12.

A 4" (NPS 4, DN 100) pipe to ASME B36.10M has an outside diameter of 114.3 mm (4.500 in) for every schedule. Only the wall thickness and bore change with the schedule.

From NPS ⅛ to NPS 12 the nominal size is a historical bore reference, so the OD is larger than the number (a 2" pipe is 60.3 mm OD). From NPS 14 upward, the OD in inches equals the NPS: a 14" pipe is 355.6 mm OD.

A schedule number (Sch 10, 40, 80, 160) sets the wall thickness for a given pipe size. The same schedule gives a different thickness at each size. Sch 40 is 3.91 mm at 2" but 8.18 mm at 8".

Yes for NPS ⅛ to NPS 10, where STD and Sch 40 have the same wall thickness. From NPS 12 upward STD stays at 9.53 mm (0.375 in), while Sch 40 keeps getting thicker.

Up to NPS 8 they are identical. From NPS 10 upward XS is fixed at 12.70 mm (0.500 in), while Sch 80 is thicker.

Sch 40S comes from ASME B36.19M for stainless steel pipe; Sch 40 comes from ASME B36.10M for carbon and alloy steel. They match up to NPS 10. At NPS 12, 40S is 9.53 mm and Sch 40 is 10.31 mm.

Weight in kg/m = (OD − WT) × WT × 0.02466, with OD and wall thickness WT in mm. For stainless steel multiply by about 1.014. Example: 6" Sch 40 (168.3 × 7.11 mm) = 28.26 kg/m.

Seamless pipe to ASTM A106 / A53 / A312 is usually allowed −12.5 % on nominal wall thickness. Design codes such as ASME B31.3 add this tolerance back when choosing the schedule.

IS 1239 (Part 1) covers steel tubes from 15 to 150 NB in three classes. Light has a yellow colour band, medium blue and heavy red. A 50 NB tube is 60.3 mm OD with walls of 2.9 mm (L), 3.6 mm (M) and 4.5 mm (H).

IS 1239 covers small-bore ERW tubes from 15 to 150 NB. IS 3589 covers larger welded steel pipes from 168.3 mm to 2540 mm OD, mainly for water, sewage and slurry lines.

Seamless pipe is pierced from a solid billet and has no weld. ERW pipe is formed from coil and joined by electric resistance welding without filler metal. LSAW and SSAW pipes are submerged-arc welded from plate or coil for large diameters.

Weld neck (WN), slip-on (SO), blind (BL), socket weld (SW), threaded (THD), lap joint (LJ) and plate flanges. Special types include orifice, long weld neck, reducing flanges and spectacle blinds.

Use weld neck flanges for high pressure, high or cycling temperature, and lines with vibration or fatigue loading. The tapered hub and full-penetration butt weld spread stress smoothly. Slip-on flanges are cheaper and easier to align, but their fatigue life is roughly one third of a WN.

The ASME class is a pressure-rating designation, not a pressure in psi. The allowable pressure depends on the material group and temperature. A Class 150 A105 flange is rated 19.6 bar at 38 °C but 13.8 bar at 200 °C.

No. A Class 150 carbon-steel flange is rated about 285 psi (19.6 bar) at ambient temperature, and about 170 psi (11.7 bar) at 260 °C. The "150" is a class number.

Raised face (RF) has a raised sealing ring: 1.6 mm high for Class 150–300 and 6.4 mm for Class 400 and above. Flat face (FF) is flat across the whole face and is used against cast-iron equipment. Ring type joint (RTJ) has a machined groove for a metal ring gasket in high-pressure service.

PCD (pitch circle diameter, or bolt circle) is the diameter through the centres of the bolt holes. With an even number of holes, measure from the far edge of one hole to the far edge of the opposite hole, then subtract one hole diameter.

Measure the outside diameter, the PCD, the number of bolt holes, the hole diameter and the thickness. Compare them with ASME B16.5, B16.47, EN 1092-1 and BS 10 tables. The Identify a flange tool in Teshi PipeData does this matching for you.

ASME B16.5 covers flanges from NPS ½ to NPS 24. ASME B16.47 covers large-diameter flanges from NPS 26 to NPS 60, in two non-interchangeable series. Series A (MSS SP-44) is heavier, with fewer and larger bolts; Series B (API 605) is more compact.

No. Series A and Series B have different outside diameters, bolt circles and bolt counts for most sizes and classes. Always state the series on the purchase order.

Table D and Table E come from British Standard BS 10 and are still widely used in India for water, steam and utility lines. Table E is heavier than Table D. Each table fixes the flange OD, PCD, bolt count and bolt size.

IS 6392 is the Indian Standard for steel pipe flanges, rated by PN (nominal pressure in bar) from PN 2.5 to PN 40. Its OD, PCD and drilling closely follow the older ISO / DIN PN flange series used in EN 1092-1.

PN 10 and PN 16 have the same drilling up to DN 150. From DN 200 the PN 16 flange has more bolts or larger bolts and a bigger OD and PCD, so they no longer mate. The PN number is the approximate rating in bar at ambient temperature.

A spectacle blind (figure-8 blank) is a plate with one solid paddle and one open ring joined by a web. It is installed between two flanges and rotated to isolate or open the line. Dimensions follow ASME B16.48.

ASTM A105 (often specified as A105N, normalised) for forged carbon-steel flanges. ASTM A350 LF2 is used for low-temperature service down to −46 °C, and IS 2062 or ASTM A516 Gr 70 plate for plate flanges.

A serrated (concentric or spiral) finish of 3.2–6.3 µm Ra (125–250 µin AARH) is standard for spiral-wound and compressed fibre gaskets.

A long radius (LR) elbow has a centre-line radius of 1.5 × NPS and is the default choice. A short radius (SR) elbow has a radius of 1.0 × NPS. SR elbows save space but add pressure drop and are not suitable for pigging.

For ASME B16.9 90° LR elbows the centre-to-end dimension A is 1.5 × NPS in inches. For example, a 6" elbow is 229 mm (9 in) and a 4" elbow is 152 mm (6 in).

A concentric reducer keeps both ends on the same centre line. An eccentric reducer has one flat side. Eccentric reducers go on pump suctions with the flat side on top, so air pockets cannot form.

A bend whose centre-line radius is 3 or 5 times the nominal pipe size. They are usually made by induction bending and supplied with tangent lengths. 5D bends allow pigging in pipelines.

Butt-weld fittings (ASME B16.9) are welded end to end with a bevel, and are used for all sizes. Socket-weld fittings (ASME B16.11) take the pipe into a socket and are fillet-welded. They are used for small bore, usually NPS 2 and below.

They are pressure classes for ASME B16.11 forged fittings. Socket-weld Class 3000 matches Sch 80 pipe and Class 6000 matches Sch 160 pipe. For threaded fittings, Class 3000 matches Sch 160 and Class 6000 matches XXS.

ASME B31.3 recommends about 1.6 mm (1/16") between the pipe end and the socket bottom before welding. Without the gap, weld shrinkage and thermal expansion can crack the fillet weld.

ASTM A234 WPB for carbon steel and ASTM A420 WPL6 for low temperature. ASTM A403 WP304/316 is used for stainless steel, and A234 WP11/WP22/WP91 for chrome-moly service.

A stub end is a short pipe piece with a flared lap. It is used with a loose lap-joint flange. This saves cost in stainless or alloy lines, because only the stub end touches the fluid, and makes bolt-hole alignment easy.

A weldolet (MSS SP-97) is a forged branch outlet butt-welded onto a run pipe. It replaces a reducing tee for branches up to about half the run size. Sockolets and threadolets are the socket-weld and threaded versions.

IS 1538 covers grey cast iron fittings, and IS 9523 ductile iron fittings, for pressure pipes carrying water, gas and sewage. They include socket and flanged bends, tees, tapers, collars and flanged spigots, made to match IS 1536 / IS 8329 pipes.

ASTM A106 is seamless carbon-steel pipe for high-temperature service and is the usual choice for refinery and power piping. ASTM A53 covers seamless and welded (ERW) pipe for general use: water, air, gas and low-pressure steam.

SS 316 contains 2–3 % molybdenum, which SS 304 does not. The molybdenum gives much better resistance to chlorides, pitting and many chemicals. Both have about 205 MPa minimum yield strength.

L grades have low carbon (≤ 0.030–0.035 %) to avoid sensitisation and intergranular corrosion after welding. H grades have controlled higher carbon (0.04–0.10 %) for strength at high temperatures.

Duplex grades such as UNS S31803 / S32205 (2205) are about half austenite and half ferrite. That gives roughly twice the yield strength of 316 and high resistance to chloride stress-corrosion cracking. Super duplex S32750 (2507) is used for seawater.

Both are carbon-steel forgings with similar chemistry. A350 LF2 is impact-tested at −46 °C for low-temperature service, while A105 is for ambient and high temperatures.

ASTM A335 P91 (9Cr-1Mo-V) is a creep-strength-enhanced ferritic steel for main steam and reheat lines in supercritical power plants. It needs strict control of pre-heat, PWHT and hardness.

The closest equivalents are P250GH (1.0460) to EN 10222-2, the older DIN C22.8, and JIS S25C. Always compare chemistry and mechanical properties for critical service.

ASTM A193 B7 is the standard chrome-moly alloy stud bolt for flanged joints, with 860 MPa minimum tensile strength up to 2½" diameter. ASTM A194 2H is the matching heavy-hex nut. For low temperature, A320 L7 studs and A194 Gr 4/7 nuts are used.

Find the ASME B16.5 material group (for example 1.1 for A105, 2.2 for F316). Then read the rating for the class at the design temperature from Table 2, interpolating between temperatures. The P-T tool in Teshi PipeData does this automatically.

t = P·D / (2·(S·E·W + P·Y)), where P is the design pressure, D the OD, S the allowable stress, E the joint factor, W the weld-strength factor and Y the coefficient (0.4 below 482 °C). Add the corrosion allowance, then divide by 0.875 for the 12.5 % mill tolerance. Order the next schedule up.

Under ASME B31.3 the hydrostatic test pressure is 1.5 × design pressure × (ST/S). ST and S are the allowable stresses at test and design temperature, with the ratio capped at 6.5. The pneumatic test is 1.1 × design pressure.

Typical design velocities are 1–3 m/s for water service lines, 0.6–1.5 m/s at pump suctions and 1.5–3 m/s at discharges. Saturated steam runs at 20–40 m/s. Higher velocity raises pressure drop, noise and erosion.

Carbon steel expands about 12 × 10⁻⁶ per °C, so 100 m of pipe heated by 100 °C grows about 120 mm. Austenitic stainless steel expands about 17 × 10⁻⁶ /°C, roughly 40 % more.

Post-weld heat treatment relieves residual welding stress and tempers the weld zone. Codes require it above certain wall thicknesses (for example over 19 mm in carbon steel under B31.3), and for most chrome-moly alloys such as P11, P22 and P91.

A 3.1 inspection certificate is issued by the manufacturer. It gives the actual test results for the supplied heat or lot, and is validated by the manufacturer's inspection department, which is independent of production.

A 3.1 certificate is validated by the manufacturer alone. A 3.2 certificate is additionally witnessed and validated by the purchaser's representative or a third-party inspector such as Lloyd's, BV, TÜV, SGS or EIL.

IBR (Indian Boiler Regulations, 1950) approval is mandatory for pipes, fittings and flanges used in steam service in India. IBR material is supplied with Form III-A / III-C certificates verified by the Inspecting Authority. Teshi is an IBR-approved manufacturer.

Positive material identification (PMI) uses handheld XRF or OES analysers to confirm the alloy chemistry of each piece. It is commonly specified for alloy and stainless material in refineries.

It is the standard for materials in sour (H₂S-containing) oil and gas service. It limits hardness (for example ≤ 22 HRC for carbon steel) and controls chemistry to prevent sulphide stress cracking.

Common dimensional standards:
- Pipe: ASME B36.10M and B36.19M, API 5L, IS 1239 / 3589.
- Fittings: B16.9 (butt-weld), B16.11 (forged), MSS SP-97 (olets), IS 1538 / 9523 (cast and ductile iron).
- Flanges: B16.5, B16.47, B16.36 (orifice), B16.48 (line blanks), EN 1092-1, IS 6392, BS 10.
- Gaskets: B16.20 and B16.21.
- Valve face-to-face: B16.10.

Teshi manufactures and supplies forged and plate flanges of all types up to 6500 NB, butt-weld and forged fittings, pipes, fabricated bends and spools, vessels and critical fabrications. Sand blasting and epoxy coating are done in-house. Materials cover carbon, alloy, stainless and duplex steel.

Tap Send inquiry to Teshi on any page of Teshi PipeData, or build a list of items and send them together. You can also email info@teshigroup.com or WhatsApp +91 78900 09110 with your BOQ. Teshi replies within one working day.

Yes. Material is supplied with EN 10204 3.1 mill test certificates as standard. 3.2 certificates with third-party inspection (TPI), and IBR certification, are available on request.

Yes. Teshi supplies EPC contractors and plants across India and exports worldwide. Send your delivery location with the inquiry for a delivered price.

Yes. Teshi machines flanges and rings to customer drawings, including large-diameter flanges up to 6500 NB, plate flanges to IS 6392 / IS 1538 / BS 10 and AWWA, and special facings.