Pakarab fertilizers limited multan
Download 3.37 Mb. View original pdf
|
- Navigate this page:
- 4.7 Bubble tube/Air purge method
- DIAPHRAGM BOX
| 4.6 Closed Tank Measurement Should the tank be closed and a gas or vapour exists on top of the liquid, the gas pressure must be compensated for. A change in the gas pressure will cause a change in transmitter output. Moreover, the pressure exerted by the gas phase maybe so high that the hydrostatic pressure of the liquid column becomes insignificant. For example, the measured hydrostatic head in a CANDU boiler maybe only three meters (30 kPa) or so, whereas the steam pressure is typically 5 MPa. Compensation can be achieved by applying the gas pressure to both the high and low-pressure sides of the level transmitter. 84 Field Instrumentation GEN Rev) This cover gas pressure is thus used as aback pressure or reference pressure on the LP side of the DP cell. One can also immediately seethe need for the three-valve manifold to protect the DP cell against these pressures. We have Phigh = Pgas + SH Plow = Pgas P = Phigh - Plow = SH The effect of the gas pressure is cancelled and only the pressure due to the hydrostatic head of the liquid is sensed. When the low-pressure impulse line is connected directly to the gas phase above the liquid level, it is called a dry leg. 4.7 Bubble tube/Air purge method In this method, liquid level is determined by measuring the pressure required to force a gas into the liquid at a point beneath the surface. By this method liquid level is obtained without liquid entering the piping or instrument. Clean air or gas in connected through a restriction to a bubble tube immersed a fixed depth in tank. Restriction reduces airflow and builds up pressure in 85 Field Instrumentation GEN Rev) bubble tube until it just balances the fluid pressure at end of bubble tube. Pressure is kept at this value by air bubbles escaping through liquid. Changes in level cause air pressure in bubble tube to buildup or drop. This pressure can be measured by an instrument connected to bubble tube. It is used for corrosive liquids and solid bearing liquids. For processes that react with air, nitrogen can be used as a purge gas. Primary maintenance problems with air purge system are, A plugged tube causes the pressure indicator to read high. A hole in the tube causes the indicator to read low. If the hole is halfway up the tube, the indicator will work correctly from 0 to 50% actually 50% to 100%) of the scale reading, but the reading will not go above 50%. In situation where air purge system is not suitable due to contaminations, liquid purge system is used. Water is mostly used as purge liquid. A pressure regulator and flow restrictor are piped to the bubbler tube. The supply pressure is determined by the range of liquid level to be monitored, and a self-regulating purge meter is normally used. DIAPHRAGM BOX In this method a diaphragm box is suspended from a chain. Diaphragm is filled with air. The instrument that senses pressure changes and relates to level measurement is mounted above vessel. This method is normally used for open vessels. |