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Orifice Plate Flowmeter - Converting Square Root To Flowrate


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#1 whotobe

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Posted 04 November 2012 - 10:48 AM

Evening all,

A flowmeter I'm looking at (cooling water/glycol) is giving me units of "Square Root".

How can I go about converting this to a useful set of units, i.e. m3/hr or similar.

What process data and governing equations would I require for this calculation - can anyone point me in the direction of a useful document which will explain the procedure, or give a fairly high level explanation of the operation of an orifice plate which delivers values in units of square root?

I have the orifice spec sheet with the orifice diameter. The inlet pressure I can estimate but I'm not sure I'd be able to read off the immediate downstream pressure.

Any help would be greatly appreciated.

#2 gunley

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Posted 04 November 2012 - 06:37 PM

The general equation for calculating orifice is Bernoulli's. How can you operate orifice without reading the pressure from downstream ?

#3 breizh

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Posted 04 November 2012 - 06:51 PM

whotobe ,

This document should help you :

http://www.mcnallyin...-html/13-12.htm

Breizh

#4 whotobe

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Posted 05 November 2012 - 01:16 PM

The general equation for calculating orifice is Bernoulli's. How can you operate orifice without reading the pressure from downstream ?


The issue I'm having is that the orifice plate flow-meter is a dP meter displaying the flow rate as "square root". There isn't a visible (local) PI on the orifice plate outlet, so I only know the inlet Pressure from upstream equipment.

Can anyone really explain he physcial context of "square root". I understand that Q (vol flow rate) is directly proportional to the square root of the pressure drop. Does the value displayed by "square root" on the indicator represent sqrt(head/density) ?

Assuming that the V equation is V = Cd x sqrt(h/rho) where Cd is the co-efficient of discharge (or a constant k).

Once I have the velocity, calculating the vol flow rate is elementary, however to get velocity I need to understand what is practically meant by square root. Theoretically it's the square root of the pressure difference?

Any help would be great. Bernoulli's equation goes a long way in explaining the equations / derivations but I'm looking for information on how to use practical site data from a square root indicator to calculate the flow rate.

If I can get an understanding of what square root actually tells me, then I can estimate K (Cd) and get V, therefore calculate Q.

#5 S.AHMAD

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Posted 05 November 2012 - 10:34 PM

1. If you know the design flowrate at maximum flow that is at the maximum DP cell range (e.g 100 kPa) then you can calculate the flowrate by following equation if DPnew is known from the DP cell reading/indicator.
2. Qnew = Qold. SQRT(DPnew/DPold)
3. Since the flowrate is proportional to square root of pressure differential across the orifice
4. However, if the flowrate is far from the design then the above equation is not that accurate. You need to use the flow equation which is related to beta ratio and hence Reynold's no.

#6 faleh

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Posted 06 November 2012 - 05:13 AM

Hi whotobe

Mr S.AHMAD is right. But you should know that this equation is rule-of-Thump equation just for site. As you know the Cd (Coefficient of discharge) is vary with reynolds No. , operating density and so on. Do you have any Idea about the Sqr root ? What kind of information you get by this Sqr root? As Mr S.AHMAD mentiioned or As Below?

Q(mass)= N* Cd * Ev * Y * d_or^2 * (Roh*Dp)^0.5
Y=1 for Liq

For further information you can see API-MPMS 14.3.1¬4




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