head loss pipe fitting valve

8/19/2019 Head Loss Pipe Fitting Valve

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Pressure Drop in Pipe Fittings and Valves

A Discussion of the Equivalent Length (Le/D), Resistanceoefficient (!) and Valve Flo" oefficient (v) #ethods

Copyright © Harvey Wilson - Katmar SoftwareOctober 2012

$f %ou are loo&ing for a calculator to perfor' pipe siing and pressure drop calculations

please u'p to the AioFlopage*

ontents

• 1 !ntro"#ction

•

2 $ac%gro#n"

• & 'he 'hree (etho"s for (inor )oss *etermination

o &1 'he e+#ivalent length metho" ,)e *.

o &2 'he resistance coefficient ,K. metho"

o && 'he valve flow coefficient ,Cv.

o &/ Comparison of the e+#ivalent length ,)e *. an" the resistance coefficient ,K.

metho"s

&/1 ffect of pipe material

&/2 ffect of fitting sie

&/& ffect of flow regime ,eynol"s 3#mber.

&// ffect of fitting ro#ghness

o &4 Conversions between the resistance coefficient ,K. an" the valve flow coefficient

,Cv.

• / 'he Crane 52 friction factor5 (etho" for *etermining the esistance Coefficient ,K.

•

4 6cc#racy

• 7 Concl#sion

• 8 eferences

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+* $ntroduction

'he siing of pipes for optim#m economy re+#ires that engineers be able to acc#rately calc#late the

flow rates an" press#re "rops in those pipes 'he p#rpose of this "oc#ment is to "isc#ss the vario#s

metho"s available to s#pport these calc#lations 'he foc#s will be on the metho"s for calc#lating

the 'inor losses in pipe siing an" to consi"er in partic#lar the following aspects9

• the a"vantages an" "isa"vantages of each metho"

• eynol"s 3#mber an" the flow regime ,t#rb#lent vs laminar.

• the fitting sie

• the ro#ghness of the fitting

• the ro#ghness of the attache" piping

• converting "ata from one metho" to another

* -ac&ground

Over the years e:cellent progress has been ma"e in "eveloping metho"s for "etermining the press#re

"rop when fl#i"s flow thro#gh straight pipes 6cc#rate pipe siing proce"#res are essential to achieve

an economic optim#m by balancing capital an" r#nning costs !n"#stry has converge" on the *arcy-

Weisbach metho"; which is remar%ably simple consi"ering the scope of applications that it covers

'he *arcy-Weisbach form#la is #s#ally #se" in the following form9

+#ation ,1. e:presses the press#re loss "#e to friction in the pipe as a hea" ,h ). of the flowing fl#i"

'he terms an" "imensions in +#ation ,1. are9

hLhea" of fl#i"; "imension is length

ƒ(oo"y friction factor ,also calle" *arcy-Weisbach friction factor.; "imensionless

Lstraight pipe; "imension is length

Dinsi"e "iameter of pipe; "imension is length

v average fl#i" velocity ,vol#metric flow cross sectional area.; "imension is lengthtime

gacceleration "#e to earthor

losses ca#se" by friction in the straight pipe 'his sit#ation is aggravate" by the fact that these recent


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"evelopments have not filtere" thro#gh to all levels of engineering yet; an" there are many ol"

"oc#ments an" te:ts still aro#n" that #se ol"er an" less acc#rate metho"s 'here is still consi"erable

conf#sion amongst engineers over which are the best metho"s to #se an" even how to #se them

?nfort#nately one of the most wi"ely #se" an" respecte" te:ts; which playe" a ma>or role in

a"vancing the state of the art; has a""e" to this conf#sion by incl#"ing errors an" ba"ly wor"e"

"escriptions ,See section / below.

3evertheless; by employing the c#rrently available %nowle"ge an" e:ercising care the minor losses

can be "etermine" with more than s#fficient acc#racy in all b#t the most critical sit#ations

.* he hree #ethods for #inor Loss Deter'ination

'he & metho"s which are #se" to calc#late the minor losses in pipe siing e:ercises are the e+#ivalent

length ,)e *.; the resistance coefficient ,K. an" the valve flow coefficient ,Cv.; altho#gh the Cv metho"

is almost e:cl#sively #se" for valves 'o f#rther complicate matters; the resistance coefficient ,K.

metho" has several levels of refinement an" when #sing this proce"#re it is important to #n"erstan"

how the K val#e was "etermine" an" its range of applicability 'here are also several "efinitions for C v;

an" these are "isc#sse" below

=or all pipe fittings it is fo#n" that the losses are close to being proportional to the secon" term in

+#ation ,1. 'his term ,v 2 /2g. is %nown as the 5velocity hea"5 $oth the e+#ivalent length ,) e *. an"

the resistance coefficient ,K. metho" are therefore aime" at fin"ing the correct m#ltiplier for the

velocity hea" term

&1 'he e+#ivalent length metho" ,)e *.

'his metho" is base" on the observation that the ma>or losses are also proportional to the velocity

hea" 'he )e * metho" simply increases the m#ltiplying factor in +#ation ,1. ,ie ƒL/D. by a length

of straight pipe ,ie Le. which wo#l" give rise to a press#re "rop e+#ivalent to the losses in the

fittings; hence the name 5e+#ivalent length5 'he m#ltiplying factor therefore becomes ƒ(L+Le )/D

!n the early stages of a "esign when the e:act ro#ting of the pipeline has not been "eci"e"; the

e+#ivalent length can be estimate" as a broa" br#sh allowance li%e 5a"" 14@ to the straight length to

cover the fittings5 However; if the "esign is complete an" a "etaile" ta%e-off of the fittings is available

a more acc#rate calc#lation of the minor losses is possible by #sing e:perimentally "etermine"

e+#ivalent lengths for each of the fittings an" valves

!t has been fo#n" e:perimentally that if the e+#ivalent lengths for a range of sies of a given type of

fitting ,for e:ample; a A0B long ra"i#s ben". are "ivi"e" by the "iameters of the fittings then an

almost constant ratio ,ie )e *. is obtaine" 'his ma%es the tab#lation of e+#ivalent length "ata very

easy; beca#se a single "ata val#e is s#fficient to cover all sies of that fitting Some typical "ata is

shown in the table below for a few fre+#ently #se" fittings9


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Fitting %pe Le/D

ate valve; f#ll open D

$all valve; f#ll bore &

$all valve; re"#ce" bore 24

lobe valve; f#ll open &20

A0B screwe" elbow &0

A0B long ra"i#s ben" 1&

/4B screwe" elbow 17

/4B long ra"i#s ben" 10

Wel"e" 'ee; thr#-r#n 10

Wel"e" 'ee; thr#-branch 70

a0le of Equivalent Lengths for Pipe Fittings

(lean co''ercial steel pipe)

'his "ata is for ill#stration only an" is not inten"e" to be complete Comprehensive tables of

+#ivalent )ength Eal#es for steel an" plastic pipe are available in another of o#r articles

3ote that this fort#ito#s sit#ation of having a constant ) e * for all sies "oes not apply to some fittings

s#ch as entrances an" e:its; an" to fittings s#ch as changes in "iameter an" orifices - both of which

involve more than one bore sie

'he e+#ivalent length metho" can be incorporate" into the *arcy-Weisbach e+#ation an" e:presse" in

mathematical form as9

3ote that the e:pression F,)e *. is also m#ltiplie" by the (oo"y friction factor ƒ; beca#se it is being

treate" >#st as tho#gh it were an a""itional length of the same pipe

'he pipe length; ); in +#ation ,2. is the length of the straight pipe only Some a#thors recommen"

that ) incl#"e the flow "istance thro#gh the fittings b#t this is wrong 'he ,) e *. factor is base" on the

overall press#re "rop thro#gh the fitting an" therefore incl#"es any press#re "rop "#e to the length of

the flow path 'he error is small an" #s#ally well within the tolerance of the "ata; so trying to meas#re

all the flow path lengths is >#st a waste of time; as well as being technically wrong

'he applicability of the e+#ivalent length ,)e *. "ata to the laminar flow regime will be consi"ere" insection &/& below

&2 'he resistance coefficient ,K. metho" ,sometimes calle" the 5loss coefficient5 metho".

'his metho" can be incorporate" into the *arcy-Weisbach e+#ation in a very similar way to what was

"one above for the e+#ivalent length metho" !n this case a "imensionless n#mber ,K. is #se" to

characterise the fitting witho#t lin%ing it to the properties of the pipe 'his gives rise to9

http://www.katmarsoftware.com/articles/pipe-fitting-equivalent-length.htmhttp://www.katmarsoftware.com/articles/pipe-fitting-equivalent-length.htmhttp://www.katmarsoftware.com/articles/pipe-fitting-equivalent-length.htmhttp://www.katmarsoftware.com/articles/pipe-fitting-equivalent-length.htm


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3ote that in this case the s#m of the resistance coefficients ,FK. is not m#ltiplie" by the (oo"y

friction factor ƒ arly collections of resistance coefficient ,K. val#es ,for e:ample the & r" "ition of

Gerry


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!n +#ation ,4. the fitting "iameter ,*. is again "imensional; an" m#st be in inches Gossibly beca#se

of the significant increase in comp#tational comple:ity over the e+#ivalent length ,)e *. an" Crane K

metho"s; the two-K an" three-K metho"s have been slow to achieve m#ch penetration in the piping

"esign worl"; apart from their #se in some high-en" software where the comple:ity is hi""en from the

#ser 6lso; both of these metho"s s#ffere" from typographic errors in their original p#blications an"

some effort is re+#ire" to get reliable "ata to enable their #se; a""ing to the hesitation for pipe

"esigners to a"opt them

'his slow ta%e-#p of the new metho"s is reflecte" in the fact that Hooper


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Pipe 1ieinch

2!!2Value

.2!!2Value

Diff 3(!2.!)

2/ 0271 02&/ 110

&0 024A 022/ 1/4

&7 0248 0218 180

a0le o'paring !2Values for 4ooper 2! and Dar0% .2! #ethods(Values are for std radius 56 deg 0end in tur0ulent flo")

'his table shows that for piping sies between 15 an" 2/5 as typically #se" in process plants the

"ifferences between these two metho"s are small What little e:perimental "ata has been p#blishe"

shows larger variations than the "ifferences between these two metho"s; an" s#ggests that both

these metho"s are slightly conservative

&& 'he valve flow coefficient ,Cv.

6s the name s#ggests; this metho" is pre"ominantly #se" in calc#lations for valves; b#t as will be

seen later in this article it is easy to convert between C v an" resistance coefficient ,K. val#es so it ispossible to "efine a Cv for any fitting

$y "efinition; a valve has a Cv of 1 when a press#re of 1 psi ca#ses a flow of 1 ?S gallon per min#te of

water at 70B= ,ie S I 1. thro#gh the valve Since the press#re "rop thro#gh a valve is proportional

to the s+#are of the flow rate the relationship between Cv; flow rate an" press#re "rop can be

e:presse" as9

'his is a "imensional form#la an" the "imensions m#st be in the following #nitsQvol#metric flow rate in ?S gallon per min#te

ΔP press#re "rop in psi

SGspecific gravity of li+#i" relative to water at 70B=

!n $ritain a similar e:pression is #se" to "efine a C v which is given in terms of $'perial gallons per

min#te; b#t #sing the same #nits for press#re "rop an" S as in the ?S6 reat care has to be ta%en

when #sing Cv val#es from valve man#fact#rers< catalogs to ascertain which basis was #se" in the

"efinition

!n continental #rope valves were tra"itionally rate" with a valve coefficient "esignate" as K v 'his is

also a "imensional form#la an" the #nits are as "efine" below9

Q' vol#metric flow rate in c#bic metres per ho#r

ΔP' press#re "rop in %gfcmJ

SG' specific gravity of li+#i" relative to water at 14BC


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However; an #p"ate" "efinition is also #se" in #rope which has finally bro#ght the valve coefficient

into the mo"ern era with S! ?nits 6t present this "efinition is not wi"ely #se"; b#t as more an" more

contract#al "oc#ments enco#rage the #se of S! ?nits it can be e:pecte" to grow in pop#larity 'his

coefficient is calle" the 56rea Coefficient5 an" is written as 6v !ts "efinition is9

Q" vol#metric flow rate in c#bic metres per secon"

ΔP" press#re "rop in pascal , 3mJ.≡

ρ"ensity of li+#i" in %gm

&/ Comparison of the e+#ivalent length ,)e *. an" the resistance coefficient ,K. metho"s

6s mentione" earlier; both these metho"s #se a m#ltiplier with the velocity hea" term to pre"ict the

press#re "rop thro#gh the fitting 'here is therefore no real "ifference between the two an" provi"e"

that acc#rate characteriing "ata for the fitting is #se"; both metho"s can give e+#ally acc#rate

res#lts

$y comparing +#ations ,2. an" ,&. we can see that the constants for the two metho"s are "irectly

relate" by9

'h#s; in any specific instance where all the fl#i" an" piping "etails are %nown it is possible to get an

e:act conversion between the constants for the two metho"s However; when engineers tal% of

comparing these two metho"s the real +#estions are relate" to how a K val#e or an ) e * val#e

obtaine" #n"er one set of circ#mstances can be employe" #n"er a "ifferent set of circ#mstances

'hese change" circ#mstances relate mainly to pipe material; fitting sie; flow regime ,ie eynol"s

3#mber. an" the ro#ghness of the fitting itself

3.4.1 Effect of ppe !te#$

'he ro#ghness of the piping attache" to the fitting has no infl#ence on the press#re "rop thro#gh the

fitting However; beca#se the e+#ivalent length ,)e *. metho" e:presses the press#re "rop thro#gh

the fitting in terms of the press#re "rop thro#gh the attache" piping; the pipe ro#ghness "oes affect

the length of piping that wo#l" have a press#re "rop e+#ivalent to the fitting 'his is best ill#strate"

with an e:ample9

6 flow rate of 140 ?Sgpm thro#gh a &5 globe valve with a C v of 104 ,?S #nits. wo#l" res#lt in a

press#re "rop of 204 psi ,#sing +#ation ,7.. 'his press#re "rop wo#l" not be affecte" by the

ro#ghness of the pipe attache" to it !f the piping were galvanie" steel with a ro#ghness of 00075

the press#re "rop in the pipe wo#l" be 282 psi per 100 ft 'he length of galvanie" piping that wo#l"

give an e+#ivalent press#re "rop to the valve wo#l" be 84 ft; giving an ) e * ratio of 2A0 !f the piping

were smooth H*G with a ro#ghness of 000025 the press#re "rop in the pipe wo#l" be only 1DA psi


9/14

per 100 ft an" the length of H*G piping that wo#l" give an e+#ivalent press#re "rop to the valve

wo#l" be 10D ft; giving an )e * ratio of /20

!n or"er to be able to #se the e+#ivalent length metho" as given in +#ation ,2. the ) e * val#es #se"

sho#l" strictly be relevant to the ro#ghness of the piping in #se !n practice the "ifferences are often

not important beca#se of the 5minor5 nat#re of the press#re "rop thro#gh the fittings !n the e:ample

given here the "ifference is //@; an" if this applies to the minor loss which is ,say. 14@ of the overall

loss the effective error in the pipeline press#re "rop is only 8@ an" this co#l" well be within the

overall tolerance of the calc#lation

3evertheless; it is best to be aware of how reporte" )e * val#es were obtaine" an" to what piping

they can be applie" ?nfort#nately the )e * val#es liste" in te:ts "o not #s#ally mention the piping

material; b#t in most cases it will be clean commercial steel pipe 'he inability of the e+#ivalent length

metho" to a#tomatically cope with changes in pipe ro#ghness is a "isa"vantage of this metho"

'he resistance coefficient ,K. metho" is totally in"epen"ent of the pipe ro#ghness an" the material of

the attache" piping is irrelevant when this metho" is #se" to calc#late minor losses

3.4.2 Effect of ftt%g &e

!n section &1 it was note" that it has been fo#n" that the )e * ratio remains almost constant for a

range of sies of a given type of fitting On the other han"; it was note" in section &2 that in general

the resistance coefficient ,K. val#es "ecreases with increasing fitting sie =or the relationship of K ƒ I

)e * from +#ation ,A. to apply it m#st mean that /ƒ remains constant; or that an" ƒchange at the

same rate 'his observation was the basis of the Crane K metho" an" is "isc#sse" f#rther in section /

below

When #sing the e+#ivalent length metho"; the ,)e *. ratio is m#ltiplie" by the friction factor an" since

the friction factor "ecreases as the pipe sie increases the term , ƒ)e *. "ecreases accor"ingly 'his

ma%es the e+#ivalent length metho" largely self-correcting for changes in fitting sie an" ma%es it

very s#itable for preliminary or han" calc#lations where #ltimate acc#racy is not the main goal

'he best available metho" available at present to accommo"ate changing pipe sies appears to be*arby


10/14

Contin#ing with the e:ample of the long ra"i#s ben"s; at a eynol"s n#mber of 100 the *arby &-K

metho" pre"icts that both the 25 an" the 205 ) ben"s wo#l" have K val#es of D2 'his is a h#ge

increase over the t#rb#lent flow sit#ation !t sho#l" be remembere" tho#gh that in the laminar flow

regime velocities ten" to be very low; ma%ing the velocity hea" ,v 2 /2g. low an" since the press#re

"rop is calc#late" as the pro"#ct of the K val#e an" the velocity hea"; the effect of the increase in K is

partially offset an" the press#re "rop can be low in absol#te terms

6gain; the e+#ivalent lengths can be calc#late" from these K val#es an" the (oo"y friction factors to

give an ,)e *. ratio an" this t#rns o#t to be 12D for both ben"s 'his small change in the ,)e *. ratio

compare" with those fo#n" in section &/2; "espite s#ch a large change in eynol"s n#mber; f#rther

reinforces the e+#ivalent length metho" as a very #sef#l techni+#e for preliminary an" non-mission

critical calc#lations

'here is another consi"eration of the flow regime that arises o#t of engineering convention; rather

than from f#n"amentals Strictly; the velocity hea" ,the %inetic energy term in the $erno#lli e+#ation.

sho#l" be e:presse" as ,Lv 2 /2g. 'he correction factor; L; is re+#ire" beca#se by convention the

velocity is ta%en as the average velocity ,ie v I flow rate cross sectional area. !n reality ,average

velocity.2 is not e+#al to ,average of v 2. an" the correction factor is #se" to avoi" having to integrate

to get the tr#e average !n t#rb#lent flow L is very close to 1 an" in laminar flow it has a val#e of 2

!t was state" in section 2 above that to calc#late the press#re "rop in straight pipe the velocity hea" is

m#ltiplie" by the factor , ƒL/D. 'here is no L in the *arcy-Weisbach form#la ,+#ation ,1..; so what

"o we "o for laminar flowM 'he answer is that by engineering convention the effect of L is absorbe"

into the friction factor We co#l" incl#"e L an" #se a friction factor that is only half the #s#al val#e; b#t

to %eep the arithmetic easy L is absorbe" into the friction factor; ƒ; an" the velocity hea" is ta%en as

,v 2 /2g.

6 similar thing is "one with the resistance coefficients ,K val#es. for pipe fittings We "efine the K

val#es to incl#"e the val#e of L >#st to %eep the arithmetic easy

'here is one e:ception when it comes to minor losses What is often calle" the 5e:it loss5; b#t which ismore acc#rately the acceleration loss; is the %inetic energy in the stream iss#ing from the "ischarge of

the pipe 'his energy is lost an" is e+#al to one velocity hea" 'here is no way of getting away from it

that here yo# have to #se the correct val#e of L to get the 5e:it loss5 correct 'he only alternative

wo#l" be to "efine it to have a K val#e of 2 in laminar flow; b#t it wo#l" then appear that in laminar

flow yo# lose 2 velocity hea"s

!n practice this is #s#ally not important !n laminar flow the velocity is low eno#gh that one velocity

hea" is insignificant - an" even if "o#ble" with an L val#e of 2; it is still insignificant 'he K val#es of

fittings in laminar flow can go into the h#n"re"s; or even tho#san"s; an" one measly little 20 isn


11/14

'he e+#ivalent length of a long ra"i#s ben" is #s#ally ta%en ,perhaps a bit conservatively. as 17 !f

the overall press#re "rop is e+#ivalent to a pipe length of 17 "iameters; an" the press#re "rop "#e to

the act#al flow path length ,which is affecte" by the ro#ghness. is e+#ivalent to only 24 "iameters

then it can be seen that a small change in the wall friction insi"e the ben" will have a very small effect

on the total press#re "rop !n a higher resistance fitting li%e a globe valve or strainer the effect of the

friction is even less

:perimental wor% on flow in ben"s has shown that the ro#ghness "oes have a meas#rable impact on

the press#re "rop $#t the e:perimental wor% also shows that there are meas#rable "ifferences in the

press#re "rop thro#gh s#ppose"ly i"entical fittings from "ifferent man#fact#rers $eca#se the

"ifferences are small; all the generally accepte" metho"s have ignore" the ro#ghness in the fitting an"

have rather selecte" slightly conservative val#es for ,)e *. an" ,K.

&4 Conversions between the resistance coefficient ,K. an" the valve flow coefficient ,Cv.

!n or"er to be able to convert between K an" Cv val#es it is first necessary to re-arrange +#ations

,&. an" ,7. to be in similar #nits +#ation ,&. is in the form of a hea" of fl#i" while +#ation ,7. is in

press#re terms 'he relation NG I gh can be #se" to bring the two e+#ations into e+#ivalent forms

Similarly; the velocity term in +#ation ,&. can be s#bstit#te" by vol#metric flowarea an" the area

can of co#rse be e:presse" in terms of the pipe "iameter Once all these transformations; an" a few

#nit conversions; have been "one the relationship becomes9

where * is in inches an" Cv is base" on ?S gallons

7* he rane 8 friction factor8 #ethod for Deter'ining the Resistanceoefficient (!)

'here is no "o#bt that the Crane 'G /10 5=low of =l#i"s thro#gh Ealves; =ittings an" Gipe5 man#al has

playe" a ma>or role in the improvement in the +#ality of hy"ra#lic "esigns for piping over the last 8

"eca"es !n pointing o#t some of the wea%nesses of the Crane metho" this section is not aime" at

"etracting from the enormo#s contrib#tion ma"e by Crane; b#t rather to highlight those areas wherethe state of the art has a"vance" in the meantime an" where engineers involve" in pipe flow rate;

pipe siing an" pipe press#re "rop calc#lations can ta%e a"vantage of more acc#rate metho"s now

available

Grior to 1A87; Crane 'G /10 #se" the e+#ivalent length metho" for calc#lating the press#re "rops

thro#gh fittings 'he switch to #sing resistance coefficients ,K. was ma"e beca#se they believe" that

the e+#ivalent length metho" res#lte" in overstate" press#re "rops in the laminar flow regime ,which

is partially tr#e.


12/14

Crane fo#n" that in f#lly t#rb#lent flow con"itions the resistance coefficient ,K. for many fittings varie"

with pipe "iameter at e:actly the same rate at which the friction factor for clean commercial steel pipe

varie" with "iameter 'his is shown in =ig#re 2-1/ of Crane 'G /10 ,1AA1. !n f#lly t#rb#lent flow the

friction factor ƒ0 is a f#nction of P* ,ie ro#ghness"iameter. only; an" since P is fi:e" by the

ass#mption of clean commercial steel pipe ƒ0 becomes a f#nction of pipe sie only Crane never state"

that lower val#es of ƒ0 in larger pipes were the cause of the "ecrease in the resistance factor K; b#t it

is common for people to forget that correlation "oes not imply ca#sation

!t is "iffic#lt to #n"erstan" why; b#t Crane believe" that the resistance factors ,K. that were

"etermine" in this way wo#l" be constant for all flo" rates for a given sie of fitting 'his was a

strange concl#sion to come to beca#se "ata for laminar flow ha" starte" appearing from aro#n" 1A//;

an" by 1A7& it was well eno#gh %nown an" accepte" to be mentione" in the / th "ition of Gerry


13/14

t#rb#lent flow in clean commercial steel pipe; ƒ0 ; to the e+#ivalent lengths of the fittings 'his is

shown in +#ation ,11.9

'his is why the Crane metho" is sometimes calle" the 5two friction factor5 K metho" 'his also

res#lte" in some engineers "eveloping the mis#n"erstan"ing that the ƒ0 friction factor was somehow

"irectly associate" with the fitting; an" beca#se the fitting ha" a friction factor it also ha" a

ro#ghness Qo# will fin" statements li%e 5o !&t %ot ! the f#cto% fcto# fo# ftt%g th the

f#cto% fcto# of ppe5 in the engineering for#ms on the internet; bearing testament to the belief that

fittings somehow have friction factors Crane never inten"e" people to associate friction factors with

fittings; b#t Craneority of in"#strial pipe flow is in the t#rb#lent flow

regime Crane certainly s#ccee"e" in establishing a comprehensive an" acc#rate "esign metho" for

t#rb#lent flow in steel pipe !n mo"ern times with the ever increasing #se of smooth plastic an" high

alloy pipe it is essential that engineers f#lly #n"erstan" the "esign metho"s they #se; an" that they

employ the right metho" for the problem at han" 'he right metho"s are available in the 2-K an" &-K

resistance coefficient metho"s "isc#sse" earlier; an" it is time for the piping "esign worl" to brea%

with the past an" to embrace the new metho"s

9* Accurac%

(#ch of what has been sai" above co#l" be seen to imply that "etermining the press#re losses in pipe

fittings is an e:act science !t is not Eery few so#rces of e+#ivalent length ,) e *. or resistance

coefficient ,K. val#es give acc#racy or #ncertainty limits 6 notable e:ception is the Hy"ra#lic!nstit#te


14/14

6n area that nee"s partic#lar care is #sing generic "ata for proprietary items (any of the "ata tables

incl#"e val#es for proprietary items li%e gate; globe; b#tterfly an" chec% valves; strainers an" the li%e

'he act#al flow "ata can vary very wi"ely an" variations of -40@ to R100@ from generic "ata can be

e:pecte"

:* onclusion

6t some point in the past the e+#ivalent length ,)e *. metho" of "etermining the press#re "rop

thro#gh pipe fittings gaine" the rep#tation of being inacc#rate 'his was +#ite li%ely a res#lt of Crane

"ropping this metho" in favo#r of the resistance coefficient ,K. metho" ecently this attit#"e has

change" in some circles; an" hopef#lly the analysis "one above will help convince more "esign

engineers that the e+#ivalent length ,)e *. metho" is act#ally very #sef#l an" s#fficiently acc#rate in

many sit#ations However; this metho" "oes s#ffer from two serio#s "rawbac%s 'hese are the

necessity of "efining the press#re "rop properties of the fitting in terms of an arbitrary e:ternal factor

,ie the attache" piping. an" the inability of this metho" to cope with entrances; e:its an" fittings

with two characteristic "iameters ,eg changes in "iameter an" orifices. =or these reasons the

resistance coefficient ,K. metho" is the better ro#te to acc#rate an" comprehensive calc#lations

*arby

head loss pipe fitting valve

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