9 replies to this topic

[Ronin]

NCEES 2008 solutions for #515 and #516 gives conversion of psig to ft as (psi)*(2.31ft/psi).
Can anyone please help explain where this came from?
Thanks.

[adh]

NCEES 2008 solutions for #515 and #516 gives conversion of psig to ft as (psi)*(2.31ft/psi).
Can anyone please help explain where this came from?
Thanks.

Hi Ronin,

One cubic foot of water = 62.4 lbm
One Square foot of area = 144 in^2

So, 62.4 / 144 = 0.433 psi

and, going the other way, 1 / 0.433 = 2.31 feet of head
(144 / 62.4 = 2.31)

Hope this helps.

Edited by adh, 25 October 2010 - 05:18 AM.

[Ronin]

Thanks for the reply.
I was confused why they hadn't converted psig to psia and then realized the Patm terms will cancel out.
I guess I'm delirious now trying to finish the exam in 16 hours!

[adh]

Thanks for the reply.
I was confused why they hadn't converted psig to psia and then realized the Patm terms will cancel out.
I guess I'm delirious now trying to finish the exam in 16 hours!

I'm with ya buddy!
Hang in there!! and Good Luck!

[buick455]

NCEES 2008 solutions for #515 and #516 gives conversion of psig to ft as (psi)*(2.31ft/psi).
Can anyone please help explain where this came from?
Thanks.

Hi Ronin,

One cubic foot of water = 62.4 lbm
One Square foot of area = 144 in^2

So, 62.4 / 144 = 0.433 psi

and, going the other way, 1 / 0.433 = 2.31 feet of head
(144 / 62.4 = 2.31)

Hope this helps.

I am not sure what this is: 1 / 0.433 = 2.31 feet of head
1 of what?

I know what 1 is but my advice is always carry your units through as that can work you out of a jam and avoid costly mistakes:

Following the units for example:

1 ft of head x 62.4 lbs/Ft ^3 = 62.4 lbs/Ft^2

62.4 lbs/Ft^2 / 144 in^2/ft^2 = 0.433 lbs/in^2

so 1 ft of head = 0.433 lbs/in^2 of pressure

1 ft / 0.433 lbs/in^2 = 2.31 ft/ lbs/in^2

Check:

1FT = 2.31 ft/ lbs/in^2 x 0.433 lbs/in^2

[chemengchuck]

It doesn't seem like the difference between lbf, used in pressure, and lbm, used in density, is taken into consideration here.

[tmacier]

Good advice from Buick445 above on units.

I would encourage everyone to list units in all of the calculations, even the simple problems.

This will prove benificial both is seeing if you did something wrong, but also helping to find the answer.

Tim

[chemicalpe]

Units play an important role on the test. Even though I am chemical I had to work lot of pump and head problems specially in english units and had to learn what some of you mentioned in the above posts. I spent whole lot of time at the beginning understand what units were used where and most of the times they just dont make sense but it's part of the test and we all have to learn it. Good Luck.

[Jonhnny123]

The units for head are actually: ft x lbf / lbm. It's almost always just refered to as feet though.

So if you have 1 ft of head (of water), here is how you'd convert to PSI:

Head x Density (water) x Conversion (ft^2 to in^2)

1 [ft x lbf / lbm] * 62.4 [lbm / ft^3] * 1/144 [ft^2 / in^2]

To convert ft to psi multiply by 0.4333
To convert psi to ft multiply by 2.308 (inverse of 0.433)

Remember, it only applies to water. Different fluids will have different conversion factors.

[Trev... P.E.]

Divide psi by SG and it works for anything..

TDH = psi x 2.31 / SG