process instrumentation i module code: eipin1b study ... - Home Mweb
process instrumentation i module code: eipin1b study ... - Home Mweb
process instrumentation i module code: eipin1b study ... - Home Mweb
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
EIPINI Chapter 4: Level Measurement Page 4-5<br />
4.4 DIFFERENTIAL PRESSURE MEASUREMENT OF LEVEL<br />
Perhaps the most frequently used technique of measuring level, is the method of<br />
measuring differential pressure. The primary benefit of this method is that the<br />
equipment measuring the differential pressure, can be externally installed or<br />
retrofitted to an existing vessel. It can also be isolated safely from the <strong>process</strong><br />
using block valves for maintenance and testing.<br />
There are, however, some disadvantages to this method. One vessel penetrations<br />
near the bottom of the vessel is needed, where leak paths could be the cause of<br />
many problems. Measurement errors also occur due to changes in liquid density.<br />
Density variations are caused by temperature changes or change of product. These<br />
variations must always be compensated for, to maintain accurate measurements.<br />
4.4.1 Open Containers<br />
To determine the fluid level H (Figure 4-7) in an open container, it is only necessary to<br />
measure the difference between the pressure at the bottom of the container and<br />
atmospheric pressure. In Figure 4-7 (a), the pressure difference is measured with a<br />
differential pressure transmitter while in Figure 4-7 (b), a u tube manometer is used.<br />
The pressure instruments may be installed with their zero lines level with the bottom<br />
of the container or at a distance z, below the bottom of the container.<br />
P atm<br />
P atm<br />
P atm<br />
H<br />
P atm<br />
H<br />
z<br />
HP<br />
LP<br />
DP<br />
transmitter<br />
Zero line<br />
z<br />
HP<br />
LP<br />
h<br />
U tube<br />
manometer<br />
Figure 4-7 (a)<br />
Figure 4-7 (b)