Tapping of Orifice Plates

There are some differences in calculations as long as I remember. Refer to ISO 5167 for the proper procedures.

Hi,

Yes definitely,as there is a change in the cross section of the pipe.i.e in the vena contracta area is the area of minimum cross section further after vena contracta the fluid coming out expands to fill in the pipe,further in case of orifice meter there will be abrupt change in the cross section after vena contracta..

Hope it is useful!!!!

Rgds

Arun

It would affect the reading,because you sized based on flange tapping and calibrated the transmitter accordingly.

Now if you change the tapping to vena contracta ,for the same flow, the differential pressure measured is different for flange tapping and vena contracta tapping.

See ISO or MPMS standard and you will see different discharge co-efficient for flange,vena contracta, and D and D/2 tappings

Orifice flange taps facilitate pressure sensing immediately before and after the orifice plate, because the vena-contractor does not occur immediately at the outlet of the orifice plate the differential pressures measured for particular flow rates will vary from flange tapped installations.

Regarding the second question; the further downstream you go from the orifice plate the closer measured pressure will be to it's optimum value (i.e. upstream pressure - irrecoverable pressure loss which is inherrent with all differential pressure type flow elements). This means that the measurable dp range for particular flows will be less than flange taps, therefore the calibrated dp range of the transmitter will need to be adjusted accordingly. This can have adverse effects on accuracy of the flow indication.

As per standards( ref: handbook of petroleum and natural gas engineering vol-2 pg 803)

Flange taps are located 1in upstream and 1inch down stream of the orifice .

Pipe taps are 2.5 times ID for upstream and 8 pipe ID for down stream.

Venacontracta taps are one ID upstream and down stream is located at plane of vena contracta.

Corner taps located immediately adjacent to upstream and down stream faces of the orifice .

from above standards we can see that all taps are not located at same plane of vena contracta hence your reading should differ ,if the taps are only changed .