# Problem in Pipe Design

conditions. Check this steel-wall thickness against

the external load due to 10 feet of fill.

STEP 2 EXTERNAL LOAD DESIGN

A . MODIFIED IOWA FORMULA

Prior to checking the anticipated horizontal

deflection of the pipe, the designer must evaluate

and determine the component parts to be used in the

modified Iowa formula.

1. DEAD LOAD, We

(Check for the maximum fill height.)

Fill Height:

H = 10 ft

Soil Unit Weight:

w = 120 pcf

Pipe O.D.:

BC = 49.5 in.

Earth Load

We = 10 (120) 49.5/12

= 4950 lb/ft of pipe

= 413 lb/in. of pipe

For fill heights greater than 8 feet, the HS-20 live

load may be neglected; therefore, Wl = 0, meaning

W = We

2. PIPE STIFFNESS

Modulus of Elasticity of Steel:

Es = 30,000,000 psi

Modulus of Elasticity of Mortar:

El = 4,000,000 psi

Cement Mortar Lining Thickness:

Tl = 0.5 in.

Let Il = moment of inertia of the cement mortar lining

= ( 0 . 5 )3/ 1 2

= 0.0104 in.4/in. of pipe

Is = the moment of inertia of the steel cylinder

= ( 0 . 2 3 6 )3/ 1 2

= 0.0011 in.4/in. of pipe

Pipe stiffness:

E I = 30,000,000 (0.0011)+ 4,000,000 (0.0104)

= 74,600 lb-in.

DESIGN EXAMPLE

Design a 48-inch nominal I.D. pipeline for an operating

pressure of 200 psi and a transient pressure of 80 psi.

Additionally, this pipeline will be field pressure tested

to 250 psi. The pipe will have fill heights of 5-10 feet

and the pipe zone will consist of course-grained soils

with little or no fines, compacted to 90% of Standard

Proctor. Shop applied-cement mortar lining and tape

wrap coating will provide the corrosion protection.

STEP 1 INTERNAL PRESSURE

Nominal Pipe Size:

D = 48 in.

Steel Wall O.D.:

O . D . = 49.5 in.

Working Pressure:

Pw = 200 psi

Transient Pressure

Pt = 80 psi

Field Test Pressure:

Pf = 250 psi

Let t = Steel Wall Thickness, in.

Fs = Allowable Unit Steel Stress, psi

= 21,000 psi when P = Pw

= 31,500 psi when P = Pw + Pt or P = Pf

(based on ASTM A139 Grade C material

with minimum specified yield of the steel

= 42,000 psi)

A . WORKING PRESSURE CONDITION

4 9 . 5 ( 2 0 0 )

t = ÐÐÐÐÐÐÐÐÐÐÐÐÐÐ = 0.236 in.

2 ( 2 1 , 0 0 0 )

B. TRANSIENT PRESSURE CONDITION

49.5(200+ 80)

t = ÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐ = 0.220 in.

2 ( 3 1 , 5 0 0 )

C. FIELD TEST PRESSURE CONDITION

4 9 . 5 ( 2 5 0 )

t = ÐÐÐÐÐÐÐÐÐÐÐÐÐÐ = 0.196 in.

2 ( 3 1 , 5 0 0 )

The pipe designer must select the condition that

results in the maximum steel thickness. In this design

example, condition A for working pressure governs.

SAMPLE PROBLEM

1

C . EXTERNAL PRESSURE (VACUUM)

1 . Above ground pipelines subjected to vacuum

conditions require a wall thickness designed to

resist collapse. For such pipe, where collapse

pressure is less than 581 psi and t / D is greater

than 43, the collapse pressure is determined by the

Stewart formula:

w h e r e P c = collapse pressure

D = the pipe outside diameter in inches

t = the pipe wall thickness in inches

The Stewart Formula accounts for variation in wall

thickness (t), out-of-roundness, and other

manufacturing tolerances.

2 . Buried pipelines in granular soil with a friction

angle greater than 25¡, where ring deflection is

not less than 5%, will not collapse due to vacuum.

Shown below is a graph showing actual vacuum vs.

ring flexibility for different soils.1

PIPE: D = 10% RING DEFLECTION DUE TO

INSTALLATION

SOIL: UNSATURATED -- COHESIONLESS

H/D = 0.43

UNIT WEIGHT OF DRY SOIL = 15 kN/m3

(100 LB/FT3)

t

Pc=50,200,000 ÐÐ

D

solve for t=

t=D(Pc/50,200,000)

[ ]3

1/3

2

1 From ASCE paper by Watkins/Tupac, 1994, Ò V a c u u m

Design of Welded Steel Pipe in Buried Poor SoilÓ

If the water table is above the top of the pipe,

the soil will not liquefy if density of the

embedment is 90% Standard Proctor (ASTM D698 or

A A S H T O T-99). The height of water table, h, above

ground surface, must be added to the internal

vacuum. The worst case is an empty pipe with the

water table above ground...

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