by Jason Schoenbaechler • Sep 8, 2018
A venturi allows you to measure flow by comparing 2 different pressure values. These pressures are measured at points we call "taps". The tap is a hole in the wall of the venturi, the "High Pressure Tap" is located near the inlet of the venturi and the "Low Pressure Tap" is in the throat of the venturi.
As the diameter of the venturi gets smaller the fluid must accelerate to maintain the same rate. This acceleration causes the pressure to drop creating the differential pressure or DP. The DP is usually measured in inches of water column.
Tap Locations on BIF Universal Venturi Tube
According to dictionary.com Static Pressure is "the pressure exerted by a fluid that is not moving or flowing.". A "True" venturi meter measures static pressure at both the high and low-pressure taps. Practically, this means that the pressure is measured in a location where the flow is not changing and is moving parallel to the pipe wall.<
For our purposes, this means that the pressure taps need to be in a straight section of pipe.
A corner tap is exactly what it sounds like. It's a tap that is placed in or near the corner of a meter. This may be done for a variety of reasons. The 2 most common are to reduce the laying length of a meter, or to make installation or manufacturing easier.
On a full body venturi like the BIF model 20181, a corner tap would be located where the inlet section meets the cone. A more common place to see a corner tap used is on an insert style meter like the BIF model 20182. On an insert the high-pressure tap is located on the center holding flange facing directly into the flow.
The effects of upstream disturbances to the flow are reduced with a static tap.
The corner tap is more likely to sense the effect of a non-uniform flow. This causes a higher uncertainty in the flow rate of the meter. This also means that when a corner tap is used more straight pipe is needed upstream of the meter to guarantee it's accuracy. You can compare the requirements of static and corner taps in the BIF Installation Effects charts here.
There are times when using a corner tap is advantageous.
The first is with insert venturi, in this case it's much easier to install the insert with the corner tap in the flange. The alternative is to install a static tap in the pipe upstream of the insert. While this can be done, and BIF can supply a kit for installing a static tap, it complicates the installation process.
BIF Stainless Steel Insert Venturi with Corner Tap
The second is in applications where you would like to amplify the differential pressure. You can see in the computer simulation below that the pressure increases in the corner. If you have a low head driving the flow, a corner tap may allow you to increase the DP without an increased permeant pressure loss.
CFD simulation of pressure in BIF Universal Venturi Tube
The venturi coefficient or "C" value is used in the calculation to convert the DP to a flow rate. As you see in the illustration above, the pressure measured at a corner tap is different than the pressure measured at a static tap. This causes a change to the C value.
Some venturi manufacturers like BIF publish 2 different C values, one for static taps and one for corner taps. Others only publish a static tap value and use a correction factor for corner tap applications. In either case this change is there to account for the difference in the pressure measured at the high-pressure tap.
This basic explanation of the difference between static and corner taps should help you when specifying your next venturi meter. If you have additional questions or need help please contact your local BIF sales representative.