×
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

Pressure measurement in gas piping

Pressure measurement in gas piping

Pressure measurement in gas piping

(OP)
All,
Can anyone recommend a good source of information for designing/locating pressure taps in an existing gas piping system?
I am trying to locate a tap for pressure control purposes in a 12" gas line (MAOP = 800psig). Unfortunately there is not much straight length of pipe available in the area where measurement is required. Ideally I would be looking to install the tap with 10 straight upstream pipe diameters. I am trying to determine if the 5 ft of straight pipe available will provide reliable pressure readings, and if not, how inaccurate the readings will be.
Thanks.

RE: Pressure measurement in gas piping

If you are not measuring differential pressure, do you still need 10 pipe dia straight run?

RE: Pressure measurement in gas piping

Measuring a static pressure is a bit metaphysical.  The pressure at the wall will vary from the pressure in the center of the pipe, but an instrument that can sense the pressure in the center of the pipe will change that pressure through vortex shedding.  I've seen several studies and simulations that all say that for virtually any real-world application, the difference is not within the accuracy of any calculations you are likely to do with the data.

The reason for the straight-pipe requirements in square-edged-orifice gas-measurement is the assumption in the equations that the flow is "fully developed" prior to going through a known restriction.  If the flow is swirling (for example) then it will tend to corkscrew through the orifice and get a significantly reduced dP for a given mass flow rate.

Measuring pressure does not have the same requirements.  In fact, I will always prefferentially take a pressure reading in a dead leg above a gas flow (this keeps condensation from accumulating in the instrument).

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 Plural of "anecdote" is not "data"

RE: Pressure measurement in gas piping

Pressure is generally easy unless you after mm H2O levels of pressure.  Typically the instrument connection is on the top of pipe.  Provide a 3/4-inch or so connection with a block valve.  

The instrument should be above the tap in pressure, especially if the lfuid is condensible.  (If the pressure is fairly high, perhaps about 500 psig, then the potential head impact for a measurement a few feet below the tap is not such an issue.)  If you were measuring steam it would be different.  Steam may exceed the rating of the instrument.  We typically come off the side or lower connection or if from the top add a siphon accessory to trap condensate thus isolate the high temperature from the instrument.

As previously stated, the upstream diameter requirements apply to flow meters, especially head type flow meters.

John

RE: Pressure measurement in gas piping

I've read about flow rectifirers that shorten the 10 straight upstream pipe diameters requirement.Search the web or look at ISO 5167-1:1995 page 8.
m777182

RE: Pressure measurement in gas piping

You do not need any straight length for pressure measurement. Take a pressure tapping (1/2" or 3/4")at any location which is convenient for you. Take the taing from any portion of the 12" pipe ABOVE the horizontal plane through the centre, so that the taping points downwards into the pipe (and any moisture or condensate falls back into the pipe).

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