Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

if

it is

buried,

it

should

be

used with both very conser-

vative

safety

factors

and

considerable caution.

Available

Sizes

and

Thicknesses

The

range

of

available sizes

is

given

in

Table

4-11,

but

consult

the

appropriate ASTM standards

(D

1785

and

D

2241)

to find

exact

sizes,

thicknesses,

and

pressure

ratings.

Wall

thickness design

for PVC

pipe

is

defined

by

two

separate sets

of

nomenclature:

(1)

standard dimen-

sion ratios (SDR)

and (2)

schedules.

The

ratio

of

pipe

outside

diameter

to

wall thickness

is

called

the

"SDR."

For PVC

pipe,

SDR is

calculated

by

dividing

the

aver-

age

outside diameter

of the

pipe

by the

minimum wall

thickness.

The

available thicknesses

are SDR 35

through

SDR 14

(refer

to

ANSI/ASTM

D

2241

for a

complete discussion

of

SDRs

and

corresponding pres-

sure

ratings; refer

to

ANSI/ASTM

D

1785

for a

com-

plete discussion

of the

wall thicknesses

and

pressure

ratings

for the

various schedules

of PVC

pipe).

Joints

of PVC can be

solvent welded,

flanged,

push-on

with

rubber gaskets,

or

threaded. Threads should

be

used

only

for

100-mm

(4-in.) pipe

and

smaller,

and the

thinnest

threaded pipe should

be

Schedule

80

(see

ANSI/ASTM

D

2464). Solvent-welded Schedule

40

pipe

is

stronger than threaded Schedule

80

pipe,

and

solvent-

welded

Schedule

80

pipe

is the

strongest

of

all.

Gaskets

for PVC flanges

should

be

3.2-mm-

(V

8

-in.)-

thick

ethylene propylene rubber (EPR),

full

faced,

with

a

Durometer hardness

of 50 to 70,

Shore

A.

When connecting

PVC flanges to

raised-face metal

flanges,

remove

the

raised

face

on the

connecting

metal

flange to

protect

the PVC flange

from

the

bolt-

ing

moment.

Valve

Pressure Rating

The

maximum allowable working pressure

of PVC

valves

is

even lower than that

of

Schedule

80

threaded

piping. Most

PVC

valves

are

rated

at

1040

kPa

(150

lb/in.

2

)

at a

temperature

of

38

0

C

(10O

0

F).

The

maxi-

mum

recommended pressure

for any flanged

plastic

pipe

system (PVC, CPVC,

PP,

PVDF)

is the

same.

Fittings

Threaded,

flanged, or

solvent-

welded

fittings are

used

in

exposed

and

buried service

for

piping smaller than

100

mm

(4

in.).

Class

125

mechanical joint ductile

or

cast-

iron

fittings

should

be

used

in

buried applications

for

pipes

100 mm (4

in.)

and

larger.

The

adapters must

be

installed

in the

manner prescribed

by the

manufacturer.

Fittings

for PVC

pipe

include

tees,

crosses,

wyes,

reducers,

and

22.5°, 45°,

and 90°

bends.

Criteria

for

Selection

of PVC

Pressure Pipe

Recommended

criteria

for

using

PVC

pressure pipe

is

as

follows:

•

Seventy-five

mm (3

in.)

and

smaller,

exposed

and

buried

service: Schedule

80 per

ASTM

D

1784

and

D

1785. Other ASTM standards

applicable

to PVC

pipe

are D

2464, Schedule

80

Threaded Fittings;

D

2467 Schedule

80

Socket Type Plastic Fittings;

and

D

2564

Solvent Cements

for PVC

Pipe.

Based

on

the

experience

and

observations

of

various engi-

neers,

this pipe should

not be

used

in

applications

in

which

the

operating pressure exceeds

550 kPa (80

lb/in.

2

).

Use

copper

or

steel pipe

for

water service;

use PVC

pipe only

for

chemical service.

• One

hundred

and 150 mm (4 and 6

in.),

exposed

ser-

vice:

Schedule

80 per

ASTM

D

1748

and D

1785.

Use

solvent-welded

—

not

threaded

—

joints.

Based

on

the

experience

and

observations

of

various engineers,

this

pipe should

not be

used

in

applications

in

which

the

operating pressure exceeds

345 kPa (50

lb/in.

2

).

Use PVC

only

for

chemical service piping.

Use

steel

or

ductile iron

for

water piping

and

other services.

• Two

hundred

mm (8

in.)

and

larger,

exposed

service:

PVC

pipe should

not be

used

at all in

such sizes.

• One

hundred

through

300 mm (4

through

12

in.),

buried

service:

AWWA

C900.

Fittings should

be

cast

or

ductile iron.

•

Three

hundred

fifty

through

900 mm

(14

through

36

in.),

buried service:

AWWA

C905.

This standard

may

not be

sufficiently

conservative

in its

pressure

ratings

for

some applications.

For a

pressure class

of

1035

kPa

(150

lb/in.

2

),

consider using

an SDR of

18

for

350-

to

600-mm (14-

to

24-in.) pipe

and an

SDR

of 21 for

750-

to

900-mm (30-

to

36-in.)

pipe.

These

SDR

values

are

based

on a

surge allowance

of

345 kPa (50

lb/in.

2

)

over

the

pipe pressure class,

with

a

safety

factor

of 2.5 for

350-

to

600-mm pipe

and

a

safety

factor

of 2.0 for

750-

to

900-mm pipe.

•

Never

use

plastic pipe

for air or

compressed

gas

service.

4-6. Asbestos Cement

Pipe

(ACP)

Asbestos cement pipe, available

in the

United States

since 1930,

is

made

by

mixing

portland

cement

and

asbestos

fiber

under pressure

and

heating

it to

produce

a

hard, strong,

yet

machinable product. Over

480,000

km

(300,000

mi) of ACP is now in

service

in the

United

States, and, according

to a

mid-1970s

survey, more than

a

third

of

pipe then being installed

was ACP

[8].

In

recent years, attention

has

been

focused

on the

hazards

of

asbestos

in the

environment and, particu-

larly,

in

drinking water.

The

debate continues with

one

set of

experts advising

of the

potential dangers

and a

second

set of

experts claiming that pipes made with

asbestos

do not

result

in

increases

in

asbestos concen-

trations

in the

water. Studies have shown

no

associa-

tion

between water delivered

by ACP and any

general

disease,

but

fear

may be as

important

as

reality,

so

consult

with

owners

and

local health authorities

before deciding whether

to

specify

ACP.

In

January 1986,

the EPA

published

a

proposed reg-

ulation

banning

further

manufacture

and

installation

of

ACP,

but it was

made clear that

the

proposed action

was

based

on the

hazard

of

inhaling asbestos

fibers

during

manufacture

and

installation

of the

pipe

—

not

because

it

contaminated drinking water.

For

awhile

after

October 1987,

the EPA had

been reassessing

the

January

proposal [8],

but the

proposed

ban was

over-

turned

by a

U.S. Federal Appeals court

in

1991.

Con-

sequently,

ACP is

still being manufactured

and

used.

Available

Sizes

and

Thicknesses

Available

sizes

are

given

in

Table

4-11.

Refer

to

ASTM

C296

and

AWWA

401, 402,

and 403 for

thicknesses

and

pressure ratings

and

AWWA

C401

and

C403

for

detailed design procedures.

AWWA

C401-83,

for

100-

to

400-mm

(4- to

16-in.)

pipe,

is

similar

to

AWWA

C403-84

for

450-

to

1050-mm

(18-

to

42-in.) pipe.

The

properties

of

asbestos cement

for

distribution pipe

(AWWA

C400)

and

transmission pipe

(AWWA

C402)

are

identical. Nevertheless, under

AWWA

C403

the

suggested

minimum

safety

factor

is 2.0 for

operating

pressure

and

1

.5

for

external loads, whereas

the

safety

factors

under

AWWA

C402

are 4.0 and

2.5, respec-

tively.

So the

larger pipe

has the

smaller

safety

factors.

Section

4 in

AWWA

C403

justifies

this

difference

on

the

basis that surge pressures

in

large pipe tend

to

be

less than those

in

small pipes.

But

surge pressures

are not

necessarily

a

function

of

pipe diameter (see

Chapters

6 and 7). The

operating conditions, includ-

ing

surge pressures, should

be

evaluated before

the

pipe class

is

selected.

It is the

engineer's prerogative

to

select

which

of the

safety

factors should apply.

Per

AWWA

C400,

safety

factors should

be no

less than

4.0 and 2.5 if no

surge analysis

is

made.

The low

safety

factors given

in

AWWA

C403 should

be

used

only

if all

loads (external, internal,

and

transient)

are

carefully

and

accurately evaluated.

Joints

and

Fittings

The

joints

are

usually push-on, twin-gasketed cou-

plings (Figure 4-10), although mechanical

and

rubber

gasket push-on joints

can be

used

to

connect

ACP to

iron

fittings.

Ductile iron

fittings

conforming

to

ANSI/AWWA

C110/A21.10

are

used with ACP,

and

adapters

are

available

to

connect

ACP to flanged or

mechanical duc-

tile iron

fittings.

Fabricated steel

fittings

with rubber

gasket joints

can

also

be

used.

ACP may be

tapped with

corporation stops, tapping sleeves,

or

service clamps.

4-7.

Reinforced

Concrete Pressure

Pipe

(RCPP)

Reinforced concrete pressure pipe

can be

made

to

meet

special strength requirements

by

using

a

combination

of

a

steel

cylinder

and

steel

cages,

by

using

one or

more steel cages,

or by

prestressing

with spiral rods

(as

shown

in

Figures

4-11

and

4-12

and in

Table

4-12).

A

distressing number

of

failures

of

prestressed

concrete cylinder pipe

(AWWA

C301)

have occurred

in

the

United States within

the

last decade.

The

outer

shell

of

concrete cracks, which allows

the

reinforce-

ment

to

corrode

and

subsequently

fail.

Therefore,

do

not

depend

on

AWWA

specifications

or on

manufac-

turers' assurances,

but do

make

a

careful

analysis

of

internal pressure (including water hammer)

and

exter-

nal

loads. Make certain that

the

tensile strain

in the

outer concrete shell

is low

enough

so

that cracking

either will

not

occur

at all or

will

not

penetrate

to the

steel under

the

worst combination

of

external

and

internal loading. Note again that strength

is not the

issue.

The

concern

is for the

tensile strain

in the

outer-

most concrete. When cracks

do

occur, they appear

to

penetrate

to a

depth

of

about

18 to 25 mm

(0.75

to

1

in.),

so a

clear cover

of at

least

38 mm

(1.5 in.)

should

be

specified. Water hammer must

be

carefully

analyzed

and

controlled. Wire-wrapped, prestressed

concrete

pipe

has the

worst history

of

failure. Rein-

forced

concrete pipe

is

better,

but if any

significant

water hammer

can

occur, ductile iron

or

steel

is

best.

For

salt

or

brackish water, PVC, HDPE,

or

ductile iron

with

cathodic protection should

be

used.

Sizes

and

Joints

Sizes range

from

600 to

3600

mm (24 to 144

in.),

as

shown

in

Table

4-11.

Joints

are

shown

in

Figures

4-11

and

4-12. Joints

for

buried service include

•

Rubber gasket

and

concrete (Figure

4-1

Ia)

•

Rubber gasket

and

steel (Figure

4-1

Ib,

c, d)

•

Lugged rubber gasket

and

steel (Figure

4-12).

Note that rubber gasket

and

concrete joints (Figure

4-11)

should

be

used only

for

pressures less than

380

kPa (55

lb/in.

2

).

The

other joints

can

withstand pres-

sures

up to

2800

kPa

(400

lb/in.

2

),

but

consult manu-

facturers

for

joint pressure ratings.

Wall

Thickness

Design

The

wall thickness design should

be

based

on

both

external

trench loads

and on a

detailed hydraulic anal-

ysis

of the

pumping system, including water hammer

and

surge pressure. Surge pressures should

not be

able

to

induce even hairline cracks

in the

external surface.

A

variety

of

wall thicknesses

and

reinforcing designs

are

available

for

each

pipe

diameter.

Some

of

these types

of

pipe have severe internal

pressure limitations.

The

AWWA

standards

cited

in

Table

4-12

should

be

consulted.

Fittings

Standard

fittings are

tees, crosses,

45°

wyes, eccentric

reducers, concentric reducers,

flange and

mechanical

joint adapters

to

connect concrete pipe

to

steel

or

duc-

tile iron (Figure 4-13),

and

bends

from

7.5°

to 90° in

7.5°

steps. RCPP

can be

tapped

by

drilling

a

hole into

the

pipe

and

then strapping

a

threaded

or flanged

tap-

ping

saddle

to the

pipe. Alternatively,

a

threaded steel

outlet

connection

can be

cast

in the

pipe wall during

manufacture.

Some pipe designers prefer

to use

fabri-

cated steel specials

for fittings and for any

pipe seg-

ment

containing

an

outlet

or

nozzle.

Linings

Pipe that

is

only partly

full

of

sewage

or in

which

air

can

enter

by any

means (temperature changes, vortices

in

the wet

well,

or

leaks) requires protection against

corrosion caused

by

bacterial action. Sometimes, pipe

can

fail

in

only

a few

years. Anaerobic,

sulfate-reduc-

ing

bacteria (such

as

Desulfovibrio)

living

in the

slime

below

the

water surface utilize

the

oxygen

in

sulfate

and

create hydrogen

sulfide,

which escapes

from

the

water

surface

to the

atmosphere above. Aerobic bacte-

ria

(Thiobacillus)

on the

sides

and

soffit

of the

pipe

convert

the

hydrogen

sulfide

to

sulfuric

acid

at a pH of

2 or

even less. Thiobacillus cannot live

in

pipe that

is

always

full,

so it is

important

to

keep force mains

(whether

concrete,

steel,

or

ductile

iron)

full

at all

times. Therefore, where

a

force main terminates

at a

manhole, design

the

connection

so

that

no

part

of the

force

main

is

exposed

to

air. Either connect

the

force

main

up

through

the

bottom

of the

manhole,

or if it

enters

from

the

side, either

(1) set the

invert

of the

downstream sewer above

the

crown

of the

force main

or (2) use

plastic pipe near

the

manhole.

Lining

and

coating systems

for

pipes include vari-

ous

brush-

or

spray-applied epoxies, resins, polyure-

thanes,

and

coal tars. Coatings such

as

coal-tar

epoxies have

a

history

of

poor performance where

hydrogen

sulfide

attack

can

occur.

A

more

effective

(and

more costly) lining system consists

of PVC

liner

sheets (such

as

Ameron

Tee-Lock®)

that

are

made

with

keys

or

ribs projecting

from

one

side

of the

sheet.

The

smooth

PVC

face

is

attached directly

to the

forms

prior

to

casting

the

concrete.

The

ribs project into

the

concrete

and

form

a

mechanical bond with

it. PVC

liners

can be

cast around

the

entire circumference

of

pipe

and the

longitudinal joint heat-fused

to

form

a

360°

liner,

but it is

cheaper

to

line only

the

portion

above low-water

level.

These

liners

offer

maximum

protection

for

concrete pipe

and

manholes subject

to

corrosive environments,

and

they have

a

40-year

record

of

success.

If the

workmanship

and

inspection

of

the

welding

at the

joints

is

good,

the

system will

have

a

long, trouble-free

life.

The

designer's problems

are:

(1)

to

write specifications that ensure workers

are

not

hurried through this important task

and (2) to

ensure that inspection

is of

high caliber.

PVC

liners

are

usually applied only

to

pipes

in

sizes

of 900 mm

(36

in.)

and

larger

and

usually only

to the

upper 270°

of

the

full

circumference. Corrosion-resistant materi-

als

such

as

vitrified

clay pipe, PVC,

or

HDPE should

be

used

for

smaller sewer pipes.

See

Section

25-1

1

for

more discussion

of

concrete protection.

4-8.

Design

of

Piping

The

emphasis

in

this section

is on the

piping within

the

pumping station

(i.e.,

exposed)

and

problems such

as

pipe

thickness,

flange

bolts,

and

pipe

supports.

The

design

of

external

(i.e.,

buried) piping

is

limited

to

generalities

because there

is an

extensive body

of

excellent literature

on

such problems

[9,

1

1-15].

In

selecting

a

pipe size,

be

aware that

it is the

out-

side

—

not

the

inside

—

diameter

that

is fixed. The

inside diameter varies with

the

wall thickness,

whereas

the

outside diameter does not. This

is

true

for

all

sizes

of

ductile iron, copper, brass,

and

plastic

pipe.

It is

also true

for

most steel pipe used

in

pump-

ing

stations.

Exposed

Piping

The

selection

of

pipe size governed

by

hydraulics

is

given

in

Example

3-1.

Other practical considerations

that depend

on

available pipe diameters

and

wall

and

lining

thicknesses

are

discussed

in

Sections

4-3 and

4-4

under "Available Sizes

and

Thicknesses."

Tie

Rods

Mechanical couplings, mechanical joints,

and

push-on

joints (Figures

4-7, 4-8, 4-10,

and

4-11)

must

be

restrained

from

sliding apart either

by

soil friction

(if

the

pipe

is

buried)

or by tie

rods

(if the

pipe

is

exposed)

as

shown

in

Figures

4-3 and

4-12. Always design rods

Table

4-14.

Root

Areas

of

Threaded

Rods

Nominal

rod

diameter

Root

area

of

coarse

thread

mm in.

mm

2

in.

2

10

3

/

8

43.9

0.068

13

V

2

81.3 0.126

16

5

/

8

130.3

0.202

19

3

/

4

194.8

0.302

22

7

/

8

270.3

0.419

25

1

356.1

0.552

28

IV

8

447.1

0.693

31

IV

4

573.5

0.889

35

1%

679.4

1.053

38

IV

2

834.2

1.293

41

l

5

/

8

977.4

1.515

44

l

3

/

4

1125 1.744

47

l

7

/

8

1321 1.048

50

2

1479

2.292

63

2V

2

2397

3.716

75

3

3626

5.672

and

bolts

for

tensile stress

at the net

cross-sectional

area

at the

root

of the

threads (see Table

4-14).

Example

4-1

Design

of Tie

Rods

for a

Sleeve Pipe

Coupling

Problem: Determine

the

number

and

size

of tie

rods required

for a

sleeve coupling

in a

600-mm

(24-in.)

DIP

under

a

maximum static

and

surge pressure

of

1070

kPa

(155

lb/in.

2

).

Solution:

The

outside diameter

of the

pipe

is 655 mm

(25.8

in.);

because

the

pressure acts

on

the

gross

face

area

of the end of the

pipe,

use the OD

(not

the ID) to

compute thrust.

Sl

Units

U.S.

Customary

Units

Pipe

area

=

«W'f

5

?

=

0.337

m

2

Pipe area

=

^*?

= 523

in.

2

Thrust

=

1070

kPax

0.337

m

2

Thrust

=

155

lb/in.

2

x

523

in.

2

= 361 kN =

81,100

Ib

Materials recommended

in

AWWA

Ml 1 [9] are (1)

rods

or

bolts (ASTM

A

193, Grade

B7

or

equivalent with

a

yield stress

of

725,000

kPa

[105,000

lb/in.

2

]),

(2)

welded lugs (ASTM

A

283, Grade

B or

ASTM

A

285, Grade

C or

equivalent with

a

yield stress

of

186,000

kPa

[27,000

lb/in.

2

]

or

ASTM

A 36

steel with

a

yield stress

of

248,000

kPa

[36,000

lb/in.

2

])

(see

the

ASTM standards

for

stresses).

Reduce

the

tensile yield stress

at the

root

of the

threads

by a

safety

factor

of 2 to

obtain

the

allowable

working stress.

The

total root area required

is

then

,

36IkN

nnnm

2

A

81,000

Ib

t

.,

. 2

A

=

362

000

kPa

=

a

°°

10

m

A

=

2

=

L56

m<

362,000

kPa

^

000

lb/in

2

Many

combinations

of rod

sizes

and

numbers

can be

used.

Try

several

and

choose

a

suitable

one. (Note, however, that

the

resulting bending stresses

in the

pipe

shell

should

be

checked.

Using

two

large-diameter rods,

for

example, causes greater bending stresses than using

four

Pipe

Wall

Thickness

The

hoop (circumferential) tensile stress

in

metal

pipe

due to the

working pressure should

not

exceed

50%

of the

yield strength.

The

working pressure plus

surge

pressure

due to

water hammer should

not

exceed

75% of the

yield strength

or the

mill test pres-

sure. Depending

on the

grade

of

steel used,

the

yield

strength

can lie

between

248,000

and

414,000

kPa

(36,000

and

60,000

lb/in.).

The

yield strength

of DIP

is

more

uniform

—

a

minimum

of

290,000

kPa

(42,000

lb/in.

2

).

smaller ones. Sometimes

the

pipe wall thickness must

be

increased

to

reduce

the

stress.) From

Table

4-14

the

root area

of a

22-mm

(

7

/

8

-in.)

bolt

is 270

mm

2

or

0.000270

m

2

(0.419

in.

2

);

thus,

the

number

of

rods required

is

N

=

0.0010

m

2

x

IQ

6

=

3

?

A

,

__

1.56

in.

2

=

3

?

270

mm

2

0.419

in.

2

Use

four

22-mm

rods.

Use

four

7

/g-m.

rods.

Note that

the rod

material

is

high-strength

steel

(ASTM

193

Grade B7).

If

lower strength

steel (carbon

or

stainless) were used,

the

number and/or size

of

rods would

be

quite

different.

The

rods

can be

supported

by

lugs

welded

to the

pipe, which

is

excellent with

steel

pipe

but

can

be

done with ductile iron

pipe

only

if

great care

is

taken

not to

overheat

the

iron.

An

alternate

detail

for

ductile iron pipe

is to

bolt

an

"ear"

(as in

Figure 4-3a)

to the

pipe

flange so

that

the tie

rod

clears both

the

pipe

flange and the

pipe coupling. From

the

DIPRA handbook

[11]

and

man-

ufacturers'

catalogs,

the

critical dimensions

are (1) flange

bolt circle [749

mm

(29.5

in.)],

(2)

number

and

size

of

bolts

[20

bolts

32 mm

(I

1

I

4

in.)

in

diameter],

(3) flange OD

[813

mm

(32.0

in.)],

(4) flange

thickness

[48 mm

(1.88

in.)],

and (5)

coupling

OD

[782

mm

(30.8

in.)].

If

a

clearance between

the rod and flange of,

say,

8 mm

(

5

/

16

in.)

is

chosen,

the rod is

centered

19

mm

(

3

/4

in.)

from

the flange OD. The ear can be

designed (with

an

adequate

safety

factor

applied

to

the

yield stress)

to

withstand

the

calculated force

in the tie

rod,

but a

more sensible design

is to

size

the ear so

that yield stress

is

reached simultaneously

in the ear and tie

rod.

The

bending

moment

in the ear is

equal

to the

lever

arm

times

the

force

in the tie

rod.

M

=

0.019 m(2.7

x

10~

4

m

2

x

7.25

x

10

8

N/m

2

)

M =

0.75

in.(0.419

in.

2

x

105,000

lb/in.

2

)

=

3.720

N • m =

33,000

Ib

• in.

The

formula

for

bending stress

is

Mc M

s

=

-

=

-s

(44)

where

s is

stress

in

newtons

per

square meter (pounds

per

square inch),

M is

moment

in

newton-

meters (pound-inches),

/ is

moment

of

inertia

in

meters

to the

fourth

power (inches

to the

fourth

power),

and S is the

section modulus

in

cubic meters (cubic inches).

For

rectangular cross-

setctions,

S =

bd

2

/6,

where

b is

width

in

meters (inches)

and d is

depth

in

meters (inches). Rear-

ranging

Equation

4-1 and

using

the

yield stress

for A 36

steel gives

,

,2

6

x

3720

N-m

,

,2 6 x

33,000

Ib

• in.

bd

=

bd =

!

—

248,000,000

N/m

36,000

lb/in.

=

9.0OxIO"

5

m

3

= 5.5

in.

3

Let

b = 63 mm =

0.063

m. Let b = 2.5 in.

"^r"""-

"Jg-'-"

Use

63 mm x 38 mm

plate.

Use

2

1

I

2

in. x

IV

2

in.

plate.

The

hoop tensile stress

is

given

by the

equation

5

=

f

(«)

where

s is the

allowable circumferential stress

in

kilo-

pascals (pounds

per

square inch),

p is the

pressure

in

kilopascals (pounds

per

square inch),

D is the

outside

diameter

of the

iron

or

steel cylinder

in

millimeters

(inches),

and t is the

thickness

of the

iron

or

steel cyl-

inder

in

millimeters (inches). (Theoretically,

D

should

be the

inside diameter,

but the

outside diameter

is

con-

servatively specified

in

most

codes,

partly

because

the

ID

is not

known initially.)

The

longitudinal stress

in a

straight pipe

is

half

of the

circumferential stress.

Hangers

and

Supports

or

hangers must carry

the

weight

of the

pipe

and

fluid in

exposed piping systems.

The

location

of

supports

and

hangers depends

on the

pipe

size, joint

systems, piping configurations, location

of

valves

and

fittings,

weight

of

pipe

and

liquid, beam strength

of

the

pipe,

and the

structure available

to

support

the

weight

and all

other static

and

dynamic forces, includ-

ing

expansion

and

contraction.

Supports

or

hangers must also carry

the

lateral

forces

due to

earthquake. Either design vertical sup-

ports

to

resist

the

horizontal force

of

pipe

and fluid or

augment

vertical supports

and

hangers with horizontal

ones. Note that tension

in

wall anchors must resist

the

full

horizontal force. Typical pipe supports

are

shown

in

Figure

4-15

and a few of the

many hangers manu-

factured

are

shown

in

Figure 4-16.

Some

general design precautions

for

pipe support

system design

include

the

following:

•

Because

pump

casing connections

to

piping systems

are

rarely designed

to

support

any

load transmitted

through

the

connection, supports should

be

provided

on

both

the

suction

and

discharge side

of

pumps

to

pre-

vent

pipe loads being transmitted

to the

pump casing.

•

Flexible pipe couplings

are

recommended

at

pump

inlets

and

outlets. They

are

useful

because

(1)

they

are

"forgiving"

and

allow

the

contractor

to

level

the

pump

and

make minor horizontal adjustments;

(2)

properly

selected,

they

isolate

the

pump

from

slight

movements

of the

pipe;

and (3)

they reduce vibra-

tion

and,

to a

limited extent, noise.

• If

expansion

and

contraction

of the

piping could

occur,

the

pipe supports should allow movement.

•

Flexible joints,

such

as

mechanical joints

and

cou-

plings,

must

be

supported

on

both sides because

they

are not

designed

to

transmit loads.

Flexible

couplings

must

be

constrained with lugs

and lug

bolts

to

prevent longitudinal movement caused

by

internal pipeline

pressure.

ANSI

B31.1

describes hanger

and

support spac-

ings

for

steel

pipe with

a

minimum wall thickness

of

standard weight.

A

maximum bending stress

of

16,000

kPa

(2300

lb/in.

2

)

and a

maximum deflection

(sag)

of 2.5 mm

(0.1

in.)

for

pipes

filled

with water

is

assumed

in the

ANSI spacings.

The

spacings given

in

Table

4-15

should

not be

exceeded.

Hanger

rod

sizes should

be

• at

least

9.5 mm

(

3

/

8

in.)

for

pipe

50 mm (2

in.)

and

smaller;

• at

least

13 mm

(

1

I

2

in.)

for

pipe

63 mm

(2

1

I

2

in.)

and

larger.

Design hanger rods

for a

maximum working stress

of

20 to 40% of the

yield stress

at the

root area

of the

threaded ends (see Table

4-14).

If the

hangers

are

embed-

ded in or

supported

by

concrete,

make sure

the

embed-

ment

is as

strong

as the

threaded

rod or

limit

the

allowable

load

to the

specifications

in UBC

Section

2624.

Capsule

anchors containing

a

two-part resin (such

as

Emhart

Molly

Parabond™)

are

superior

to

expansion anchors,

which

use

friction

to

resist being pulled out. Some engi-

neers

do not

trust glued anchors, however, because good

and

dependable workmanship

is

critical. Embedded

eye

bolts should

be

welded closed

in

addition

to

being welded

to

the

reinforcing bars,

as

shown

in

Figure

4-16a

and b.

Hanger

rod

sizing

for

plastic pipe should

be the

same

as for

steel pipe,

but the

spacing

of

hangers should

be as

recommended

by the

plastic pipe

manufacturer.

Spacings

are

typically half those

for

steel

and

ductile iron pipe.

In

addition

to

installing supports

or

hangers

for

straight runs

of

pipe, provide additional hangers

or

supports

at

•

concentrated loads, such

as

valves;

• fittings;

•

both sides

of

nonrigid

joints;

and

•

pipe connected

to

suction

and

discharge casings

of

pumps.

A

wide variety

of

standard hangers

and

supports

is

available

[17].

Combining Equation

4-1 for

stress

due

to

bending

(s =

McII

)

and

bending moment

for

simple

spans

(M =

wL

2

/8)

and

rearranging,

the

equation

for

calculating pipe support spacings based

on

stress (for

straight

pipe

runs) becomes

/O

„

J

Spacing

= L =

/—

(4-3)

A/

WC

where

L is the

spacing

in

meters (inches),

s is the

allowable stress

in

newtons

per

square meter (pounds

Figure

4-15. Pipe

supports,

(a)

Concrete

pipe

support;

(b)

concrete

base

elbow

support;

(c)

steel

pipe

support

base

in

soil;

(d)

steel

pipe

support

on

floor;

(e)

steel valve support

on

floor.

For

floor

supports,

provide similar horizontal

supports

to

resist

seismic forces. Courtesy

of

Wilson

&

Company,

Engineers

&

Architects.

Figure

4-16.

Pipe

hanger

details,

(a)

Swivel

rings

(for pipe that

requires

no

protecting shield);

(b)

clevis

hanger

[(for

flexible

pipe,

add a

shield

(e.g.,

a

similar

pipe

split

in

half) between

the

pipe

and

hanger];

(c)

clevis hanger (use

for

hanging

from

steel

joists);

(d)

adjustable swivel roller (use

for

pipe that

expands

and

contracts).

Courtesy

of

Wilson

&

Company,

Engineers

&

Architects.

Table 4-15. Support Spacing

for

Iron

or

Steel Pipe

per

ANSI

B31.1

Span

Nominal

pipe

size

Water

service

Steam,

gas,

or air

service

mm

in.

m

ft

m

ft

25

1 2.1 7 2.7 9

50 2 3.0 10 4.0 13

75 3 3.7 12 4.6 15

100 4 4.3 14 5.2 17

150

6 5.2 17 6.4 21

200 8 5.8 19 7.3 24

300 12 7.0 23 9.1 30

400 16 8.2 27

10.7

35

500 20 9.1 30

11.9

39

600 24 9.8 32

12.8

42

per

square inch),

/ is the

moment

of

inertia

in

meters

to the

fourth

power (inches

to the

fourth power),

w

is

the

weight

of

pipe

and

water

per

unit length

in

new-

tons

per

meter (pounds

per

inch),

and c is the

distance

from

the

neutral axis

to the

outer

fiber

(OD/2)

in

meters (inches).

/ for

pipe

is

given

by

7

_

TC[(OD)

4

-(/D)

4

]

64

The

equation

of sag for a

uniformly loaded simple

span

is

s

5wL

,,

cx

8

=

384E/

(4

'

5)

and

rearranging

to find L

gives

L

=

(**™*r

(4-6)

\

5w

J

where

E,

the

modulus

of

elasticity,

is

given

in

Table

A-IO.

Be

careful

to use

compatible units.

Example

4-2

Hanger

Rod

Sizing

and

Spacing

Problem:

A

600-mm

(24-in.) steel pipe

of

standard weight

is filled

with water.

The

allowable

tensile

stress (including

the

factor

of

safety)

in the

hanger rods

is

62,000

kPa

or 62 x

10

6

N/m

2

(9000

lb/in.

2

).

Assume that

a

factor

of

safety

of

1.5

is to be

applied

to the

ANSI maximum bend-

ing

stress

in the

pipe,

so

that stress

is

limited

to

1.07

x

10

7

N/m

2

(1500

lb/in.

2

).

Solution: Calculate

/

from

Equation

4-4

(see Table

B-3 or B-4 for

dimensions

and

weight

of

the

pipe barrel).

SI

Units

U.S. Customary

Units

/

=

A[(0.610)

4

-(0.591)

4

]

/ =

^[(24)

4

-(23.25)

4

]

=

8.08

x

10~

4

m

4

=

2070

in.

4

Calculate

w, the

total mass

of

pipe

and

water

in

kilograms

per

meter (pounds

per

foot) (see

Tables

B-3 and B-4 for the

mass

of the

pipe barrel

and for the

inside cross-sectional area).

w

= 141

kg/m

+

0.273

m

2

x

1000

kg/m

3

w =

94.6

Ib/ft

+

2.94

ft

2

x

62.4

lb/ft

3

= 414

kg/m

= 278

lb/ft

=

23.2

lb/in.

Transforming

mass

to

force,

w

= 414

kg/m

?^_N

=

4060

N/m

kg

From Equation

4-3 the

maximum spacing

is

L

=

/

8

x

1.07

x

IQ

7

N/m

2

x

8.08

x

10~

4

m

=

/8 x

1500

lb/in.

2

x

2070

in?

~

V

4060

N/m x

0.305

m

"^

23.2

lb/in.

x 12 in.

=

7.47

m = 299 in. =

24.9

ft

From Equation 4-6,

the

span

for the

allowable

sag of 2.5 mm

(0.10

in.)

is

,

_

("384

x

2.07

x

IQ

11

N/m

2

_

f384

x 2.9 x

IQ

7

lb/in.

2

L

~

(

5

L

~

(

5

_

4

02

c

2070

in

4

x 0.2

in.A

0

'

25

8.08

x

IQ"

4

m

4

x

0.0025

mV-

25

x

23

3

lb/in

J

4060

N/m J

=

9.35

m

=

379

in

'

-

31

'

5

ft

Buried

Piping

External

Loads

Buried

pipes must support external structural loads,

including

the

weight

of the

soil above

the

pipe plus

any

superimposed wheel loads

due to

vehicles

if the

pipeline crosses

a

runway, railway,

or

roadway.

The

two

broad categories

for

external structural design

are

rigid

and flexible

pipe.

Rigid pipe supports external

loads because

of the

strength

of the

pipe itself. Flexi-

ble

pipe distributes

the

external loads

to

surrounding

soil and/or pipeline bedding material. Consider DIP,

steel,

and PVC to be flexible,

whereas

AC and

RCPP

are

rigid. (FRP

is

also rigid,

but

refer

to

Section

4-5

for

a

warning about buried service.)

Supporting

strengths

for flexible

conduits

are

gen-

erally

given

as

loads required

to

produce

a

deflection

expressed

as a

percentage

of the

diameter. Ductile iron

pipe

may be

designed

for

deflections

up to 3% of the

pipe diameter according

to

ANSI

A21.50.

Until

recently,

plastic pipe manufacturers generally agreed

that

deflections

up to 5% of the

diameter were accept-

able. Some manufacturers

now

suggest that

deflec-

tions

up to 7% of the

diameter

are

permissible. Many

engineers, however, believe these values

are

much

too

liberal

and use 2 to 3% for

design.

Pipeline bedding conditions

affect

the

safe

supporting

strength

of

both rigid

and flexible

conduits. Screenings,

silt,

or

other

fine

materials

are

unsuitable

for

stable

pipe-

line bedding

and

should

be

avoided particularly where

the

groundwater

level

may

rise above

the

trench bottom.

Better

systems range

from

(1)

merely shaping

the

trench

bottom

to (T)

using select bedding material

to (3)

sup-

porting

the

pipe

on a

monolithically poured concrete cra-

dle.

A

stable, granular bedding material

can be

achieved

with

a

well-graded crushed stone with

a

maximum parti-

cle

size

of 3/4 in. and

containing

not

less than

95% by

weight

of

material retained

on the No. 8

sieve.

The

design

of

pipe

to

resist external loads

is

involved

because

it

depends

on the

stiffness

of the

pipe,

the

width

and

depth

of

trench,

the

kind

of

bed-

ding,

the

kind

of

soil,

and the

size

of

pipe. Discussions

are

given

in

AWWA

Mil

[9],

AWWA

M9

[13],

the

DIPRA handbook

[11],

by

Spangler [14, 15],

and in

many

other publications.

Thrust

Blocks

Where changes

in flow

direction occur,

the

exposed

pipes must

be

restrained against

the

resultant thrust.

Changes

in flow

direction occur

at all

bends, tees,

plugs, caps,

and

crosses. Joint systems, such

as

flanges,

welds,

and

grooved couplings,

are

designed

to

provide restraint

up to the

manufacturer's rating.

But

if

the

joint system

is

inadequate

to

contain

the

calcu-

lated thrust, external restraint

is

needed.

Thrust calcu-

lations

are

illustrated

in

Example 3-5.

Buried pipes with mechanical

or

push-on joints

require thrust blocking

at

deflections, bends, tees,

plugs,

or

other changes

in flow

direction. Thrust

blocks

are

constructed

of

cast-in-place

concrete

poured

in the

trench during

pipeline

installation.

They

are

designed

to act as

horizontal spread footings that

distribute

the

resultant force

to the

trench wall.

The

required area

of the

thrust block

can be

determined

So

stress,

not

deflection, governs. Although

the

allowable span

from

Table 4-15

is 9.8 m (32 ft)

based

on an

allowable stress

of 15, 900

kPa

(2300

lb/in.

2

),

use a

lesser

spacing, about

7.3 m

(24

ft),

for an

allowable stress

of 10, 400 kPa

(1500

lb/in.

2

)

in

this problem.

The

force

per

hanger

is

F

=

4060

N/m x 7.3 m F =

23.2 lb/in.

x 12

in./ft

x 24 ft

=

30,000

N =

6700

Ib

and

the

required

area

at the

root

of

threads

is

A

=

30,000

N

A

=

6700

Ib

62

x

10

6

N/m

2

9000

lb/in.

2

=

4.8 x

10~

4

m

2

=

480

mm

2

=

0.74

in.

2

From Table

4-14,

use

rods

31

mm

(I

1

I

4

in.)

in

diameter. However,

if the

rods

are to be

anchored

only

by

embedment

in

concrete, note that

UBC

Section 2624 allows

a

maximum load

of

only

14,000

N

(3200

Ib)

per

bolt. That requirement would permit

a

spacing

of

only

T

14,00ON

,..

,

3200

Ib

.,

.

,

f

L

=

4060NM

=

3

'

45m

L

=

278lWft

=1L5ft

Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

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