NPS 18 Pipe Dimensions

Nominal size

NPS 18

Metric designator

DN 450

Outside diameter

18.000 in (457.2 mm)

Schedules listed

13

Standard weight wall

0.375 in (9.5 mm)

Extra strong wall

0.500 in (12.7 mm)

What NPS 18 is

NPS 18 pipe measures 18.000 in (457.2 mm) across the outside, so here the size designation and the outside diameter in inches are the same number. That holds from NPS 14 upward and for no size below it. The metric designator is DN 450, which is likewise a label and not a measurement in millimetres. Whatever the schedule, the outside diameter stays at 18.000 in (457.2 mm): a heavier wall grows inward and takes the difference out of the bore.

A main header and large transfer line size.

Every schedule in NPS 18

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 13 designations in NPS 18, which between them give 13 different wall thicknesses. The lightest is Schedule 5 at 0.165 in (4.2 mm) and the heaviest is Schedule 160 at 1.781 in (45.2 mm). Going from one to the other takes the bore from 17.670 in (448.8 mm) down to 14.438 in (366.7 mm), which cuts the flow area by 33% and raises the weight from 31.46 to 308.79 lb/ft, a factor of 9.8.

Standard weight does not coincide with any numbered schedule in this size. Extra strong does not coincide with a numbered schedule here. 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 18 pipe weighs about 1413 lb (641 kg) empty and 3439 lb (1560 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.

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 18 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 18 a Class 150 flange is 25.00 in (635.0 mm) across and 1.50 in (38.1 mm) thick, held by 16 bolts of 1 1/8 in. A Class 1500 flange for the same pipe is 36.00 in (914.4 mm) across and 6.38 in (162.1 mm) thick, on 16 bolts of 2 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 18 pipe is bored 17.250 in (438.1 mm); ordered for Schedule 160 it is bored 14.438 in (366.7 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 18 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 18 measures 27.00 in (685.8 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 18 in each schedule chart

Adjacent sizes