Total System Head

7/17/2019 Total System Head

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SUBJECT : Calculating the total system head in USCS units 7-1:

USCS stands for "United States Customary System Units" as opposed to the SI

(Le Syst`eme International d`Units) or metric units that have been adopted by the

International standards Organiation (ISO)!

It turn out that "head" is a very convenient term in the pumping business!

Capacity is measured in gallons per minute and each gallon of li#uid has $eight

so $e can easily calculate the pounds per minute being pumped! %ead or height

is measured in feet so if $e multiply these t$o together $e get foot& pounds per

minute $hich converts directly to $or' at the rate of foot pounds per

minute e#uals one horsepo$er!

*ressure is not as convenient a term because the amount of pressure that the

pump $ill deliver depends upon the $eight (specific gravity) of the li#uid being

pumped and the specific gravity changes $ith temperature type of fluid andfluid concentration!

If you $ill refer to +I, - you should get a clear picture of $hat is meant by

static head! .ote that $e al$ays measure from the center line of the pump to the

highest li#uid level

/o calculate head accurately $e must calculate the total head on both the suction

and discharge sides of the pump! In addition to the static head $e $ill learn that

there is a head caused by resistance in the piping fittings and valves called

friction head and a head caused by any pressure that might be acting on the

li#uid in the tan's including atmospheric pressure called " surface pressure

head"!



Once $e 'no$ these heads $e $ill then subtract the suction head from the

discharge head and the amount remaining $ill be the amount of head that the

pump must be able to generate at the rated flo$! %ere is ho$ it loo's in a

formula0

System head 1 total discharge head & total suction head

% 1 hd & hs

/he total discharge head is made from three separate heads0

hd 1 hsd 2 hpd 2 hfd

• hd 1 total discharge head

• hsd 1 discharge static head

• hpd 1 discharge surface pressure head

• hfd 1 discharge friction head

/he total suction head also consists of three separate heads

hs 1 hss 2 hps & hfs

• hs 1 total suction head

• hss 1 suction static head

• hps 1 suction surface pressure head

• hfs 1 suction friction head

3s $e ma'e these calculations you must sure that all calculations are made in

either "feet of li#uid gauge" or "feet of li#uid absolute"! In case you have

forgotten "absolute means that you have added atmospheric pressure (head) to

the gauge reading!

.o$ $e $ill ma'e some actual calculations0

+igure 45 demonstrates that the discharge head is still measured to the li#uid

level but you $ill note that it is belo$ the ma6imum height of the piping!

3lthough the pump must deliver enough head to get up to this ma6imum piping

height it $ill not have to continue to deliver this head $hen the pump is running

because of the "siphon effect"! /here is of course a ma6imum siphon effect! It is



derived from0 -7!8 psi (atmospheric pressure) 6 5!- feet 9 psi 1 !7 feet

ma6imum siphon effect!

:e $ill begin $ith the total suction head calculation

-! /he suction head is negative because the li#uid level in the suction tan' is

belo$ the centerline of the pump0

hss 1 & ; feet

5! /he suction tan' is open so the suction surface pressure e#uals atmospheric

pressure 0

hps 1 feet gauge

! <ou $ill not have to calculate the suction friction head I $ill tell you it is0

hfs 1 7 feet at rated flo$

7! /he total suction head is a gauge value because atmosphere $as given as

hs 1 hss 2 hps & hfs 1 &; 2 & 7 1 &- feet of li#uid gauge at rated flo$

/he total discharge head calculation

-! /he static discharge head is0

hsd 1 -5= feet



5! /he discharge tan' is also open to atmospheric pressure thus0

hpd 1 feet gauge

! I $ill give you the discharge friction head as0

hfd 1 5= feet at rated flo$

7! /he total discharge head is0

hd 1 hsd 2 hpd 2 hfd 1 -5= 2 2 5= 1 -= feet of li#uid gauge at rated flo$

/he total system head calculation0

% 1 hd & hs 1 -= & (&-)1 -; feet of li#uid at rated flo$

.ote0 did you notice that $hen $e subtracted a minus number (&-) from a

positive number (-=) $e ended up $ith a positive -; because $henever you

subtract minus numbers it is the same as adding them> If you have trouble $ith

this concept you can learn more about it from a mathematics boo'!

Our ne6t e6ample involves a fe$ more calculations but you should be able to

handle them! In this e6ample $e are going to learn ho$ to handle a vacuum

application! *ipe friction numbers are ta'en from the %ydraulic Institute

?ngineering @ata Aoo'! <ou can get a copy of this publication from your library

if you $ant to see the actual charts! I have some of this information in the chartsection of this $eb site!

http://www.mcnallyinstitute.com/Charts/friction_6.html






Specifications0

-! /ransferring - gpm! $ea' acid from the vacuum receiver to the storage

tan'

5! Specific ,ravity & !B

! Discosity & e#ual to $ater

7! *iping & 3ll ;" Schedule 7 steel pipe

=! @ischarge piping rises 7 feet vertically above the pump centerline and then

runs 7 feet horiontally! /here is one BE flanged elbo$ in this line

;! Suction piping has a s#uare edge inlet four feet of pipe one gate valve and

one BE flanged elbo$ all of $hich are ;" in diameter!

8! /he minimum level in the vacuum receiver is = feet above the pumpcenterline!

! /he pressure on top of the li#uid in the vacuum receiver is 5 inches of

mercury vacuum!

/o calculate suction surface pressure use one of the follo$ing formulas0



• inches of mercury 6 -!-9 specific gravity 1 feet of li#uid

• pounds per s#uare inch 6 5!-9specific gravity 1 feet of li#uid

• Fillimeters of mercury 9 (55!7 6 specific gravity) 1 feet of li#uid

.o$ that you have all of the necessary information $e $ill begin by dividing thesystem into t$o different sections using the pump as the dividing line!

/otal suction head calculation

-! /he suction side of the system sho$s a minimum static head of = feet above

suction centerline! /herefore the static suction head is0

hss 1 = feet

5! Using the first conversion formula the suction surface pressure is0

hps 1 &5 %g 6 -!-9 !B 1 &5!-5 feet gauge

! /he suction friction head hfs e#uals the sum of all the friction losses in the

suction line! +riction loss in ;" pipe at - gpm from table -= of the %ydraulic

Institute ?ngineering @ata Aoo' is ;!-8 feet per - feet of pipe!

in 7 feet of pipe friction loss 1 79- 6 ;!-8 1 ! feet

+riction loss coefficients (G factors) for the inlet elbo$ and valve can be added

together and multiplied by the velocity head0

+I//I., G +HOF /3AL?

;" S#uare edge inlet != 5 (a)

;" B flanged elbo$ !5B 5 (a)

;" ,ate valve !-- 5 (b)

/otal coefficient G 1 !B

/otal friction loss on the suction side is0

hfs 1 ! 2 -!8 1 5! feet at - gpm!

7! /he total suction head then becomes0

hs 1 hss 2 hps & hfs 1 = 2 (&5!-5) & 5! 1 &5!-5 feet gauge at - gpm!



/otal discharge head calculation

-! Static discharge head 1 hsd 1 7 feet

5! @ischarge surface pressure 1 hpd 1 feet gauge

! @ischarge friction head 1 hfd 1 sum of the follo$ing losses 0

+riction loss in ;" pipe at - gpm! from table -= is ;!-8 feet per hundred feet

of pipe!

In 77 feet of pipe the friction loss 1 779- 6 ;!-8 1 58!5 feet

+riction loss in ;" elbo$0

from table 5 (a) G 1 5B

from table -= D595g 1 -!B5 at - gpm!

+riction loss 1 G D595g 1 !5B 6 -!B5 1 !; feet

/he friction loss in the sudden enlargement at the end of the discharge line is

called the e6it loss! In systems of this type $here the area of the discharge tan' is

very large in comparison to the area of the discharge pipe the loss e#uals D595g

as sho$n in table 5 (b)!

+riction loss at e6it 1 D595g 1 -!B feet

/he discharge friction head is the sum of the above losses that is0

hfd 1 58!5 2 !; 2 -!B 1 5B!8 feet at - gpm!

7! /he total discharge head then becomes0

hd 1 hsd 2 hpd 2 hfd 1 7 2 2 5B!8 1 ;B!8 feet gauge at - gpm!

c! /otal system head calculation0

% 1 hd & hs 1 ;B!8 & (&5!5) 1 B!B feet at - gpm!

Our ne6t e6ample $ill be the same as the one $e ust finished e6cept! that there

is an additional - feet of pipe and another BE flanged elbo$ in the vertical leg!



/he total suction head $ill be the same as in the previous e6ample! /a'e a loo' at

figure 4 7

.othing has changed on the suction side of the pump so the total suction head

$ill remain the same0

hs 1 &5!-5 feet gauge at - gpm!

/otal discharge head calculation

-! /he static discharge head "hsd" $ill change from 7 feet to feet since the

highest li#uid surface in the discharge is no$ only feet above the pump

centerline!(/his value is based on the assumption that the vertical leg in the

discharge tan' is full of li#uid and that as this li#uid falls it $ill tend to pull the

li#uid up and over the loop in the pipe line! /his arrangement is called a siphon

leg)!

5! /he discharge surface pressure is unchanged0

hpd 1 feet

! /he friction loss in the discharge pipe $ill be increased by the additional -

feet of pipe and the additional elbo$!

In - feet of pipe the friction loss 1 -9- 6 ;!-8 1 !; feet



/he friction loss in the additional elbo$ 1 !; feet

/he friction head $ill then increase as follo$s0

hfd 1 5B!8 2 !; 2 !; 1 !B feet at - gpm!

/he total discharge head becomes0

hd 1 hsd 2 hpd 2 hfd

1 2 2 !B

1 ;!B feet gauge at - gpm!

=! /otal system head calculation

% 1 hd & hs 1 ;!B & (&5!-5) 1 - feet at - gpm!

+or our last e6ample $e $ill loo' at gauges! /a'e a loo' at +I, =0

Specifications0

• Capacity & gpm!

• Specific gravity & -!

• Discosity & Similar to $ater

• *iping & inch suction 5 inch discharge

• 3tmospheric pressure & -7!8 psi!



@ivide the heads into t$o sections again0

/he discharge gauge head corrected to the centerline of the pump in feet of

li#uid absolute is found by adding the atmospheric pressure to the gauge reading

to get absolute pressure and then converting to absolute head0

hgd 1 (- 2 -7!8) 6 5!- 9 (-! Specific ,ravity) 2 7 1 5;-!- feet absolute

.ote the 7 foot head correction to the pump centerline!

/he discharge velocity head at gpm! is found in table B of the %ydraulic

Institute ?ngineering @ata Aoo'

hvd 1 -5! feet at gpm!

/he suction gauge reading is in absolute terms so it needs only to be converted tofeet of li#uid absolute!

hgs 1 7 6 5! 9 -! 25 1 8! feet absolute

.ote the 5 foot head correction to the pump centerline!

/he suction velocity head at gpm! is found in table -- of the *ipe +riction

Fanual0

hvs 1 5!; feet at gpm!

/he total system head developed by the pump 10

% 1 (hgd 2 hvd ) & ( hgs 2 hvs ) 1 (5;-!- 2 -5!) & (8! 2 5!;)1 -B!55 feet

absolute at gpm!

For information about my C !ith o"er #$$ Seal %

&um' Sub(ects e)'lained* clic' here

Lin' to the Fc .ally home page

http://www.mcnallyinstitute.com/Book.html/cd_rom_adv.htm

http://www.mcnallyinstitute.com/index.html

http://www.mcnallyinstitute.com/Book.html/cd_rom_adv.htm

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Total System Head

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