Errata HI ANSI 9.8

[stanier]

Hi All,

Has anyone any knowledge of there being an error in the formula in this standard (9.8.7.3)?

The formula given is S = 1.0D + 2.3(Q/0.785*D^2)/(g*D)^0.5)*D

where Q is in L/s;D in metres;g 9.81m/s

For Q=750 L/s D= 1m I get 702m which is incredibly high?

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[ione]

Yes there's an error. It should be Q expressed im m^3/s.

If you enter your data in the formula reported in HI ANSI 9.8 but for imperial units:

S = 1.0*D + 2.3((12*0,409*Q/D^2)/(12*g*D)^0.5)*D

with:

S in inches
D in inches
Q in USgpm
g = 32,3 ft/s^2

you'll get S = 67 inch = 1.7 m (which is what you'd get entering Q in m^3/s in the formula for metric system

 

[stanier]

ione,

Thanks for that. I wonder if ANSI/HI know? The second formula in 9.8.7.3 works with Q in L/s.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[ione]

The first formula they’ve provided is not correct from a dimensional point of view. A simple dimensional analysis of the units involved shows that Q must be expressed in m^3/s.

The second formula on the other hand works with Q in l/s, because if you take a look at it you'll notice that the term 1/1069 at denominator is given by:

2.3/(785*(9,81)^0.5)

While in the first formula they’ve reported 0.785, which clearly exhibits a shift of a 10^3 factor (from l/s to m^3/s).

Said that I sincerely don’t know whether at the HI are aware of this.