×
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

Differential (or not) flowmeter?

Differential (or not) flowmeter?

Differential (or not) flowmeter?

(OP)
I am trying to simulate the use of a pressure differential flowmeter with the gas air. The flowmeter has an upstream pressure tapping, a variable orifice and another pressure tapping downstream. The flow goes off to atmosphere. This is not a pressurised flow. Tests to back up my simulatio show that the downstream pressure tapping shows virtually zero, particularly at low flows. Therefore it would appear that its possible to charactetrise the flow simply by the upstream pressure gradient. Is it possible to remove the downstream pressure tappng in this situation? What are the implications of doing this? Thanks in advance for any advice.

RE: Differential (or not) flowmeter?

If you can be confident that you have approximately zero friction after the plate, then dP is upstream psig and static pressure is upstream psia.  You have to know if your upstream pressure is high enough to give you choked flow (if it is then you have a different, and more accurate, calculation to do).

People do this all the time and many are not very careful about the downstream friction and checking for choked flow.  They don't get very good numbers, but they still use them.

David  

 

RE: Differential (or not) flowmeter?

(OP)

>
>I have an accurate inline flowmeter upstream of the upstream pressure
>tapping. After the upstream pressure tapping going downstream is the
>'choke' which is a movable element. The moving element aims to give a
>more linear flow - pressure characteristic. Downstream of the 'choke'
>is the downstream pressure tapping. I am measuring flows up to 100
>L/min with a low differential pressure.
>I am using the accurate inline flowmeter to 'calibrate' the measured
>flow to the mbar(g) pressure measured at the upstream pressure tapping
>only. Although I could calibrate the accurate measured flow to the
>differential pressure across the 'choke' I am trying to understand why
>I cannot simply use the mbar(g) upstream. As an example, the pressure
>levels at say 50 L/min air flow at 20C and local pressure 1013mbar
>would be:
>Upstream: 3 mbar(g)
>Downstream: 0.2 mbar(g).
>I appear to get good repeatabilty by simply using the upstream
>pressure. However, its clear the flow is not choked by comparing the
>ratio of absolute pressures upstream and downstream. The upstream
>pressure is fed to a differential pressure sensor a distance away from
>the flow measurement point.
>
>Therefore I have 2 main questions:
>1) As the downstream pressure is very low compared to the upstream
>pressure, can it be ignored? What are the implications of ignoring it?
>2) Without the downstream pressure tapping connected to the remote
>pressure sensor, will temperature / humidity variations between the 2
>locations lead to inconsistencies in the readings?
>
>Many thanks for any advice offered.

RE: Differential (or not) flowmeter?

are you measuring the pressure at the two locations i.e. P1, P2 then taking the difference to generate the delta-P signal or are you measuring the d/p by a single transmitter?

RE: Differential (or not) flowmeter?

" As the downstream pressure is very low compared to the upstream>pressure, can it be ignored? What are the implications of ignoring it?"

Well, let's take a stab at it, using your numbers.

The measured dP between the taps is 2.8 mbar.  If you ignore the downstream tap, the measured dP would be 3.0 mbar, a 0.2 mbar error.  The flow rate calculated as roughly proportional to the square root of dP, so the ratio of the reported flow rates would be equal to the root of (3.0/2.8), or about 1.035.  So, ignoring the downstream tap would give you about a 3.5% error in flow reading (high).

RE: Differential (or not) flowmeter?

Whether that is high or not just might depend on how much air we are talking about and how much you are paying for that air.  3.5% for natural gas would probably be totally unacceptable.  Then again, since you are only simulating at this point, cost might not make any difference at all, therefore you can judge all by yourself if a 3.5% error is good enough for your purposes, or not.  Lesson? ... all things are relative.

Let your acquaintances be many, but your advisors one in a thousand'  ...  Book of Ecclesiasticus

RE: Differential (or not) flowmeter?

(OP)
Many thanks for all the responses.

I am measuring a single pressure, the upstream pressure and feeding this pressure to the +ve side of a pressure transducer. I am then measuring the flow independently using an 'off the shelf' thermal inline flowmeter positioned upstream of the pressure tapping and plotting actual flow vs upstream pressure. This then gives me a very accurate linear relationship between just the upstream pressure and actual measured flow.
The downstream pressure is, in effect, a constant zero as it is the -ve side of the pressure transducer.
I can then use this relationship to predict the flow at different flow rates.
This is why I was querying the use of the downstream pressure tapping.
How would a temperature / humidity difference between the actual location of the upstream pressure tapping and the -ve side of the pressure transducer located 2m away affect the stability / error?

With thanks

RE: Differential (or not) flowmeter?

"How would a temperature / humidity difference between the actual location of the upstream pressure tapping and the -ve side of the pressure transducer located 2m away affect the stability / error?"

You mean an external change (outside the pipe) or internal?  For external, look at the data sheet for your transducer(s).  For internal, dunno, what is the mechanism causing that change?

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