Orifice Plate Flow Meters - Measurable Range

[SKahle89]

All,

I have an orifice plate flow meter installed to track N2 usage throughout my plant. The issue is that the meter (the transmitter) goes full scale when a downstream solenoid gets stuck open. Obviously I need to rescale the transmitter.

My question is, how do I know what the true measurable range is for my orifice plate; over what range are the orifice plate calculations valid? Theoretically, zero pressure loss = zero flow, and 100% pressure loss = infinite flow. Clearly, that isn't actually the case. Based on pipe size and orifice size, how do I know what the "range" of the actual orifice plate is?

-SK

 

[BigInch]

An orifice plate meter has a practical turndown ratio of 3:1. This means that if an orifice meter with a design flow rate of 200,000 m³ per day is installed, the flow range that the meter can measure accurately will be between 100,000 m³ per day and 300,000 m³ per day.

as reported here,

http://en.wikipedia.org/wiki/Turndown_ratio

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SKahle89]

BigInch,

Thanks, exactly what I wanted to know.

-SK

 

[JLSeagull]

With the newest very highly accurate dp transmitters I regard the turndown about 5 or 6 to 1 for the same accuracy expected twenty years ago.

The range is associated with the bore of the plate, the pipe inside diamter and the transmitter range. Without changing the plate, the maximum flow rate is normally extended by extending the differential pressure at 100% flow. You need orifice sizing software to calculate the bore. However, if the bore is set associated with a given differential pressure an maximum flow range, you can use a square root relationship to determine the maximum range obtained by increasing the pressure differential.

Volumetric Flow (Q = A • v)
Square root relationship between the output and flow rate for head type Flowmeter
10 % dp output= 31.6 % flow rate
25 % dp output= 50 % flow rate
50 % dp output= 70.7 % flow rate
100 % dp output= 100 % flow rate

?P = constant x ? x Q2
Fluid density affects the measurement. Not needed to recalibrate if the density used did not change.
Pressure drop is proportional to the square of the flow rate.
Double the flow… quadruple (four times) the differential
Q = constant • (?P/?)½
Flow rate is proportional to the square root of the differential pressure

...

 

[CJKruger]

SKahle89, you state "and 100% pressure loss = infinite flow", which is a bit misleading.

At 100% of "maximum pressure drop" the flow rate will be what is called "meter maximum".

The "maximum pressure drop" is your dP cell range (often 100 inH2O). So, by using the orifice equations you can calculate the "meter max" flow based on a dP=100 inH2O and the orifice size.

If you cannot find the equations, let us know.

 

[SKahle89]

CJKruger and all,

My transmitter is scalable from 150"H2O-1000"H20 or so. What I was getting at with my orignial question is that that at some point the sqrt method for calculating flow falls apart (ceases to correlate accurately). So, I can easily go rescale my transmitter and control software to read within range, but I want to make sure that the reading is accurate for the given orifice, or if I need to install a new plate.

 

[CJKruger]

SKahle89, the sqrt relationship is accurate as long as you have subsonic flow through your orifice.

 

[JLSeagull]

For nitrogen (compressible fluids) a throw-down rule of thumb is that the pressure differential in inches of water should not exceed the inlet pressure in psi_. If your nitrogen pressure is above 200 psig I would not be concerned with a pressure differential of 200 inches of water. The absolute maximum is 1000-inches H2O due to plate deflection in addition to the transmitter range capability. Also, it costs money to compress a gas just to drop it across an orifice. However, only part of the drop is permanent depending upon the beta ratio, etc.

 

[dcasto]

An orifice plate has a practicle turndown of 16 to 1 with stacked DP's With stacke DP's you can end up withless than .5% error in the delta P portion of the equation. We used 2 stacked DP's 0 to 50 and 0 to 250 inches and get 6 to 1. With 3 DP's 1000,200,40 16 to 1. We'd have less than .5% error.

If you use a single 0 to 1000 transmitter, at 15% flow (6 to 1), your DP is 28 inches +/1 1.5 inches, so your measurement is +/-2.5% or so.

Remember, your transmitter is +/-.15% of span!

 

[hacksaw]

what is the upstream pressure, plate bore, and line I.D., and normal d/p?

rangeability is or can be more than just the square root transformation esp when you go to high ranges

 

[JLSeagull]

If the existing transmitter is more than a dozen years old the accuracy could be 0.25% for a Rosemount 1151 or even 0.5% span for a Foxboro. The newest transmitters are available in classic mode at 0.06% span or ultra at 0.025% span.