×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Converting Feet Gas Column

Converting Feet Gas Column

Converting Feet Gas Column

(OP)
I recieved an old document with pressure listed in "Feet Gas Column".  Does anyone know what this is or how to convert it to inches water column?

RE: Converting Feet Gas Column

It is really ugly.  Static head of a gas varies with both static pressure at the top and the column height, and it is not linear.

The equation used most often in Oil & Gas for figuring bottom-hole pressure with a gas column is:

P(bottom) = P(top)* exp((0.01875*SG*depth)/T/Z)

With:
Pressure in psia
Temperature in Rankine
SG relative to air
depth is column height in feet

[Note:  since atmospheric pressure is a significant number of feet of gas column it is important to get it right at the top.  Predicting gauge pressure at the bottom is a really tricky thing since on well-magnitude problems your elevation can be thousands of ft lower than surface pressure, for a monometer problem this shouldn't be a problem]

The result is in psia and can be converted to ft of pure water at 60F by dividing the answer by 0.433 psi/ft.

This equation tracks well on wellbore-magnitude problems (i.e. hundreds of feet), I'm not sure how well it would work on monometer-magnitude problems.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

RE: Converting Feet Gas Column

Can't the gas be assumed to be of constant density when the pressure involved is no more than what can be reasonably measured using inches of water?

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Converting Feet Gas Column

Probably, but I'd be pretty careful how big a column that I assumed constant density on.  At zero psig and 60F the density of air is 0.0607 lbm/ft^3.  At 10 psi (43 ft of water) it is 0.1126 lbm/ft^3 (85% increase).  That seems like a lot, but it may not be.

David

RE: Converting Feet Gas Column

But 43 ft is a long way for "water.  (TG for Hg)  If you limit max dP to 1 meter or so (that's pretty long for a glass tube) it shouldn't be too much more than 0.5 psi for a full scale measurement, so effectively a typical measurement range dP should be pretty accurate (admitting that's a relative term).

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources