Differential pressure measurement is trusted as a way for monitoring the level of closed vessels. Oftentimes, operators use individual instruments with an output signal or on-site display. In contrast, a far more precise and fault-immune solution is the measuring arrangement developed 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 level of the contents in the vessel, the differential pressure between the liquid and the gaseous phase is determined. In addition, the hydrostatic pressure, the precise 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 info on the mixing ratio of liquids.
Connection via signal cable
When monitoring the 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. Both transmitters are arranged as Primary instrument and Secondary instrument. They are connected, purely electrically, via a signal cable and so are therefore not vunerable to disturbances. THE PRINCIPAL instrument supplies the Secondary instrument with power. The Secondary instrument is parameterised via a serial interface or through the display on the principal instrument. Finally, communication occurs via an internal bus.
Level monitoring in a closed tank
Application example
The figure on the left shows an average application example for level monitoring with electronic differential pressure measurement on a liquid tank. Mammoth measures the pressure P1, in this case the gas pressure, and transmits it to the Primary instrument. This detects the pressure P2, here the pressure in the liquid column in the bottom of the tank. THE PRINCIPAL instrument calculates the differential pressure from P1 and P2. Utilizing the measured value and taking into account the other parameters mentioned, it determines the quantity in the tank as an indication of the level. This value can be output either as an analogue or digital signal to the control room and/or to the display of the Primary instrument.
Benefits of the measuring arrangement with two process transmitters
The technique for level monitoring described here has several advantages in comparison with a conventional method, for example, with a differential pressure transmitter:
Higher accuracy and ?turndown?
Process transmitters deliver higher levels of accuracy. Regarding the WIKA models, they are around 0.05 % of the set span. In addition, there is the possibility of ?turndown?, the average person setting of a specific measuring span (that is also possible retrospectively with instruments which are already installed.)
Minimised temperature effect
Transmission via signal cable with electronic differential pressure measurement minimises the temperature effect. In contrast, the capillaries found in the conventional measuring method are much more susceptible here. This, in turn, can ultimately affect the measuring result.
Fast commissioning
The electronic measuring arrangement could be commissioned faster. Unlike a solution with capillaries, it generally does not require any test run.
Reduced maintenance effort
The maintenance effort is leaner: In the event of a fault, only the procedure transmitter concerned has to be replaced. In contrast, with measuring differential pressure with capillaries, the entire measuring arrangement should 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.