Differential pressure measurement is widely used as a method for monitoring the level of closed vessels. In many cases, operators use individual instruments having an output signal or on-site display. On the other hand, a more precise and fault-immune solution may be the measuring arrangement produced by WIKA using two process transmitters in a primary-secondary relationship.
Each level monitoring application using differential pressure measurement is founded on the following principle: For the amount of the contents in the vessel, the differential pressure between the liquid and the gaseous phase is set. Furthermore, the hydrostatic pressure, the specific density of the medium and the vessel geometry are included in the calculation. Typical measuring instruments have two adjacent process connections for the pressures P1 and P2. For level measurement, a capillary must bridge the pronounced distance between your two measuring points.
Measurement with two process transmitters as Primary instrument (right) and Secondary instrument. This measuring arrangement for level monitoring can be utilized flexibly, e.g. for information on the mixing ratio of liquids.
Connection via signal cable
When monitoring the particular level using electronic differential pressure measurement, on the other hand, both measuring points have one process transmitter each. For instance, the WIKA models CPT-2x and IPT-2x are suitable for this. The two transmitters are arranged as Primary instrument and Secondary instrument. They’re connected, purely electrically, via a signal cable and are therefore not vunerable to disturbances. THE PRINCIPAL instrument supplies the Secondary instrument with power. The Secondary instrument is parameterised with a serial interface or through the display on the Primary instrument. Finally, communication occurs via an interior bus.
Level monitoring in a closed tank
Application example
The figure on the left shows a typical application example for level monitoring with electronic differential pressure measurement on a liquid tank. The Secondary instrument measures the pressure P1, in this instance the gas pressure, and transmits it to the Primary instrument. This detects the pressure P2, here the pressure in the liquid column at the bottom of the tank. THE PRINCIPAL instrument calculates the differential pressure from P1 and P2. Utilizing the measured value and considering the other parameters mentioned, it determines the volume in the tank being an indication of the particular level. This value can be output either as an analogue or digital signal to the control room and/or to the display of the principal instrument.
Advantages of the measuring arrangement with two process transmitters
The technique for level monitoring described here has several advantages when compared to a conventional method, for instance, with a differential pressure transmitter:
Higher accuracy and ?turndown?
Torture deliver higher levels of accuracy. In the case of the WIKA models, they are around 0.05 % of the set span. Furthermore, there is the possibility of ?turndown?, the individual setting of a specific measuring span (this is also possible retrospectively with instruments which are already installed.)
Minimised Disoriented via signal cable with electronic differential pressure measurement minimises the temperature effect. On the other hand, the capillaries found in the conventional measuring method are much more susceptible here. This, subsequently, can ultimately affect the measuring result.
Fast commissioning
The electronic measuring arrangement can be commissioned faster. Contrary to a solution with capillaries, it does not require any test run.
Reduced maintenance effort
The maintenance effort is leaner: In case of a fault, only the process transmitter concerned must be replaced. In contrast, with measuring differential pressure with capillaries, the entire measuring arrangement must be replaced.
Note
Further information on the model CPT-2x and IPT-2x process transmitters, which are ideal for electronic differential pressure measurement for level monitoring, can be acquired on the WIKA website.