NPS 3/4 Pipe Dimensions

Nominal size

NPS 3/4

Metric designator

DN 20

Outside diameter

1.050 in (26.7 mm)

Schedules listed

9

Standard weight wall

0.113 in (2.9 mm)

Extra strong wall

0.154 in (3.9 mm)

What NPS 3/4 is

NPS 3/4 pipe measures 1.050 in (26.7 mm) across the outside, which is 0.300 in more than the 3/4 in its name suggests. Below NPS 14 the designation is a label inherited from iron pipe sizing and matches neither the outside diameter nor the bore of any schedule exactly. The metric designator is DN 20, which is likewise a label and not a measurement in millimetres. Whatever the schedule, the outside diameter stays at 1.050 in (26.7 mm): a heavier wall grows inward and takes the difference out of the bore.

Many plant piping specifications set NPS 3/4 as the minimum size for vents, drains and branch connections off a process line, because anything smaller is too easily bent or broken off. It is normally socket-welded or threaded.

Every schedule in NPS 3/4

Dimensions are given in inches, with the millimetre equivalent beneath in grey.

Inside diameter is OD − 2t. Weight is calculated for carbon steel as w = 10.69 × t × (OD − t) lb/ft and is the plain-end weight, excluding coatings, linings and fittings. Water-filled weight adds fresh water at 8.3454 lb per US gallon.

Reading the table

B36.10 lists 9 designations in NPS 3/4, which between them give 7 different wall thicknesses. The lightest is Schedule 5 at 0.065 in (1.7 mm) and the heaviest is Double Extra Strong (XXS) at 0.308 in (7.8 mm). Going from one to the other takes the bore from 0.920 in (23.4 mm) down to 0.434 in (11.0 mm), which cuts the flow area by 78% and raises the weight from 0.68 to 2.44 lb/ft, a factor of 3.6.

Standard weight equals Schedule 40 in this size. Extra strong equals Schedule 80. Where two designations share a wall they are the same pipe under two names, and either may appear on a mill certificate. The match is specific to the size, so it is the wall thickness in inches that should go on a requisition, not the designation alone.

For handling and support loads, a 20 ft length of standard weight NPS 3/4 pipe weighs about 23 lb (10 kg) empty and 27 lb (12 kg) full of water. The water-filled figure is the one that governs pipe support spacing and the load during a hydrostatic test, including on lines that will carry gas or vapour in service.

Flow and pressure capacity by schedule

Two questions decide a schedule: how much the bore will carry, and how much pressure the wall will hold. The flow columns give the water flow through each bore at three velocities that bracket ordinary liquid service, from a pump suction line at the low end to a pump discharge line at the high end. The pressure columns are the ASME B31.3 pressure design equation solved for pressure, first on the nominal wall and then on the wall after the 12.5% mill under-tolerance that seamless pipe is allowed.

Flow is velocity × flow area. Pressure is P = 2·S·t / (D − 2·Y·t) with S = 20,000 psi, E = W = 1.0 and Y = 0.4: seamless carbon steel such as A106 Gr B at moderate temperature, with no corrosion allowance and no threading allowance. It is an illustration of the arithmetic, not a design rating. A dash marks a wall of D/6 or more, where that equation does not apply.

The method behind both halves of that table is set out in the pipe sizing guide and the wall thickness guide, each with a worked example.

Flanges for NPS 3/4 pipe

ASME B16.5 sizes a flange from the NPS and the pressure class, never from the schedule, so every flange below fits every wall in the table above. In NPS 3/4 a Class 150 flange is 3.88 in (98.6 mm) across and 0.44 in (11.2 mm) thick, held by 4 bolts of 1/2 in. A Class 2500 flange for the same pipe is 5.50 in (139.7 mm) across and 1.25 in (31.8 mm) thick, on 4 bolts of 3/4 in. Because the bolt circle moves with the class, flanges of two different classes will not bolt together even though both fit the pipe.

Ratings are for A105 carbon steel at 100 °F and fall with temperature. Thickness excludes the raised face. All six flange types share these dimensions.

The one flange dimension that does follow the schedule is the bore of a weld neck flange, which is machined to the inside diameter of the pipe it is welded to. A weld neck flange for standard weight NPS 3/4 pipe is bored 0.824 in (20.9 mm); ordered for Double Extra Strong (XXS) it is bored 0.434 in (11.0 mm). The schedule therefore belongs on every weld neck flange requisition.

Length through hub is how far the flange stands off the joint, which is what a spool drawing needs. It is measured from the flange face, without the raised face, and is tabulated here for Classes 150 and 300.

Buttweld fittings for NPS 3/4 pipe

ASME B16.9 sets the dimensions of a buttweld fitting from the NPS alone, so the figures below hold for every schedule in the size. A long radius 90° elbow in NPS 3/4 measures 1.50 in (38.1 mm) from its centre to each end, which is the figure a piping layout adds for every change of direction. The fitting is still ordered to a schedule, because its wall has to match the pipe at the weld.

The cap is the exception to the rule above: its length is for walls up to a limiting thickness, and a heavier wall takes a longer cap. See the cap page.

Guides and comparisons for this page

NPS 3/4 in each schedule chart

Adjacent sizes