NPS 20 Pipe Dimensions

Nominal size

NPS 20

Metric designator

DN 500

Outside diameter

20.000 in (508.0 mm)

Schedules listed

13

Standard weight wall

0.375 in (9.5 mm)

Extra strong wall

0.500 in (12.7 mm)

What NPS 20 is

NPS 20 pipe measures 20.000 in (508.0 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 500, which is likewise a label and not a measurement in millimetres. Whatever the schedule, the outside diameter stays at 20.000 in (508.0 mm): a heavier wall grows inward and takes the difference out of the bore.

A size for large headers, cooling water mains and tank farm transfer lines.

Every schedule in NPS 20

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 20, which between them give 11 different wall thicknesses. The lightest is Schedule 5 at 0.188 in (4.8 mm) and the heaviest is Schedule 160 at 1.969 in (50.0 mm). Going from one to the other takes the bore from 19.624 in (498.4 mm) down to 16.062 in (408.0 mm), which cuts the flow area by 33% and raises the weight from 39.82 to 379.53 lb/ft, a factor of 9.5.

Standard weight equals Schedule 20 in this size. Extra strong equals Schedule 30. 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 20 pipe weighs about 1573 lb (714 kg) empty and 4097 lb (1858 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 20 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 20 a Class 150 flange is 27.50 in (698.5 mm) across and 1.62 in (41.1 mm) thick, held by 20 bolts of 1 1/8 in. A Class 1500 flange for the same pipe is 38.75 in (984.2 mm) across and 7.00 in (177.8 mm) thick, on 16 bolts of 3 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 20 pipe is bored 19.250 in (488.9 mm); ordered for Schedule 160 it is bored 16.062 in (408.0 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 20 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 20 measures 30.00 in (762.0 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 20 in each schedule chart

Adjacent sizes