Venturi Tube Specifying 2

[Weegie]

Gents, I'm specifying DP transmitters for flow measurement.
The transmitters will be used with orifice plates, venturi tubes and annubars.
My question is this, my 'Rule of Thumb' delta P range/calibration for the orifice transmitters will be 0-100"WC.
As Venturi's are pre-fabricated, is there a 'Rule of Thumb' maximum value for Delta P ? Is there a maximum DP that is calculated which they neversupercede ?
What about Annubars ?
I've no idea as to how they are fabricated but, I'm guessing that both reference pressure holes on the sensing element are variable so, I can assume a DP range of 0-20 or 0-100" WC could be entered into an annubar calc.
As always, your expert opinions & help are greatly appreciated.

Best regards.

 

[roydm]

Weegie,
A typical good quality transmitter will have a turn-down of 10:1 so if you specify one with a maximum span of 150" you will be able to turn it down enough to cover the most cases The venturis I have used had a similar DP to an orifice plate. Annubars, particuarly on gas tend to have a lower DP so you might need to carry a couple of ranges.

Hope this helps

Roy

 

[roydm]

No the Annubar come with a fixed profile so they will have an odd range if you specify an even flow rate. (unlike an orifice they can't be adjusted to give a nice even DP
Roy

 

[JLSeagull]

Avoid dp exceeding 200-inches or below 10-inches unless it is absolutely necessary to go outside this range. Sometimes it is necessary.

For head type flow element calculations such as orifice plates, venturi tubes, Annubar, V-cones etc. my 'Rule of Thumb' would begin with 0-100 "WC. Another rule that especially applies to compressible fluids is that the pressure differential in inches water column should not exceed the operating pressure in psig. Thus the 100-inch rule applies provided that the operating pressure exceeds 100 psig. With high pressures it was common to consider 200-inches. For revamp/expansion projects, sometimes transmitters on existing flow meters were ranged 300 or even 400 inches. The permanent loss must be considered in these cases as pump horsepower can be wasted if just dropping the pressure for the flow measurement. The permanent loss is a function of the beta ratio.

The flow element rules began with mercury manometers for pressure differential measurement. These included a vessel with mercury with head selections equal to 100-inches water, 200 inches water and a few smaller ranges. The Foxboro pneumatic pressure differential transmitters came with range adjustments. The 100-inch capsule could be calibrated a bit higher, perhaps it was near 125-inches before using the next higher range that exceeded 200-inches. A low range model suitable for 15-inches required a different body.

The early electronic dp transmitters ranges were initially available down to about 20-inches. A ring-balance meter was required for draft ranges. The ring balance meters could take much of a day to calibrate. In the late seventies Rosemount offered a draft range transmitter capable of 0-1" or -.5 to +.5". Since then the flexibility made the 0-100 rule rather moot. The accuracy and stability features of the latest transmitters permits selecting one model for a wide range of pressure differentials.

 

[Weegie]

Thanks for the input and help.
Greatly appreciated.