Flow element max. allowable differential pressure
Flow element max. allowable differential pressure
(OP)
What is the rule to decide and fill in the max. allowable differential pressure in the Process Datasheet For Flow Elements?
Thanks.
Thanks.





RE: Flow element max. allowable differential pressure
Fifty years ago the mercury manometers were mostly ranged 0-100 inches water column. That has been the throw-down starting point since.
However, if low pressure and compressible than one guide is the maximum inches water differential corresponds to the maximum operating pressure. Thus, if the operating pressure is 20 psig, start with 20 inches water. You don't want the permanent loss to be a large percentage of the inlet pressure.
For high pressure applications you can use higher differentials. Where necessary people increase to 200 inches water. That is typically the maximum for a new design. One might use still higher differential on a plant modification.
Other type flow meters exist too.
RE: Flow element max. allowable differential pressure
RE: Flow element max. allowable differential pressure
On liquids, you limits are how much force you can exert on a plate. Again, AGA 3 publishes that limit. If you want to go higher, you can use a a thicker plate. I've seen as high as 1000" of H2O used.
RE: Flow element max. allowable differential pressure
I like to use 200" W.C. as my "throw-down number".
Admittedly, this is on the high side.
I quickly will drop to 100" or even 30" W.C. when 200" might not be available.
I don't go as quickly down below 30" W.C., but will do so if pushed.
I consider 10" W.C. to be close to a practical minimum for an orifice plate dP flow measurement. It is too easy to introduce errors into your measurement at lower dP.
The above numbers are for the range of the dP transmitter, not the operating dP.
RE: Flow element max. allowable differential pressure
RE: Flow element max. allowable differential pressure
Actually, things are more complicated than that. I've seen vendor's literature that states incredible accuracy for their instruments. In fact, a transmitter on a test bench under ideal conditions does have a very high level of accuracy. Field instruments set and then neglected for long periods of time don't do nearly as well. There are many potential sources of unexpected errors as well. If you're measuring a vapor flowrate using an orifice, for example, you might get a small slug of condensate that forms in your instrument's impulse line. Capillary atractive forces could easily hold it in place, and it may deaden the transmitter's response. With a large differential, this might go unnoticed, but with a small differential it could cause an appreciable error.
As you might infer, I believe that it is much better to think of instrument errors in terms of +/- some value and NOT a % of span.
For what it's worth,
Doug
RE: Flow element max. allowable differential pressure
RE: Flow element max. allowable differential pressure
RE: Flow element max. allowable differential pressure
The coefficient of discharge will be less than .5%. The DP transmitter and pressure transmitter will be less than .25% each (including calibration and drift errors.) The square root sum of the squares of all parts of the metering will be less than .75% for a typical meter
RE: Flow element max. allowable differential pressure
RE: Flow element max. allowable differential pressure
WRONG.
You will not get very good accuracy no matter what Fisher Rosemont, Yokagawa, Honeywell, or anyone else says. You will have moisture and scale and unexpected contaminants in every part of your sensing system. The installers will not have done all that you expected. Connections will have leaks and obstructions. Valves may even be inadvertently closed. The sensors will drift and change with time. The fluid's pressure, temperature, and composition will differ from what you have calibrated it for. Your velocity pattern will not be fully developed. Erosion, corrosion, deposition, and maybe even plate bending will alter the geometry of the orifice. All of the above will vary with time. I think you're self dillusional if you tell me you can measure fluid flowrates in industrial settings within fractions of a percent in accuracy. And the smaller the range of the physical parameter you are measuring (dP in this case), the greater the error as a percentage of the measurement. I know you can turn a screw on an old Foxboro dP cell and change the range of the transmitter to nearly anything you'd like. The diaphragm remains the same and is no more sensitive because the range is changed. Why would modern electronic instruments behave differently?
RE: Flow element max. allowable differential pressure
RE: Flow element max. allowable differential pressure
RE: Flow element max. allowable differential pressure
RE: Flow element max. allowable differential pressure
RE: Flow element max. allowable differential pressure