Flow, level & control
between the taps on the separation stack is filled only with the lighter liquid, the differential pressure is minimum value or the lowest range value (LRV) of the transmitter. When it is filled with the heavier liquid, the differential pressure is at its maximum value, or the upper range value (URV) of the transmitter. Although this technique overcomes many of
the problems associated with the previously described methods, particularly with respect to corrosion, it does have one main drawback. The small difference in both the specific gravity of the two liquids and the distance between the taps on the separation stack results in a very small differential pressure span. In many cases, the size of this span is often
lower than the recommended minimum span for most remote seal transmitters. One way of overcoming this problem is to
use remote seals and transmitters which are sensitive enough to detect very low span changes. An example is ABB’s own remote-seal based 2600T interface level transmitter, which has been specifically designed for use at very low differential pressures. These transmitters use a remote seal
with a highly sensitive diaphragm available with a range of fill fluids for a variety of applications. Protection against leakage of the fill fluid is ensured by an all-welded construction, which offers a significantly extended service life than seals utilising a conventional gasket or thread construction, particularly in vacuum applications.
ApplicAtion exAmple
A chemical plant wanted an interface level transmitter for use in a chemically aggressive hydrocarbon reprocessing application. In this application, a mixture of process
hydrocarbons cleaned from the plant’s tanks and reactors, and water used for cleaning the
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reactors, was piped into a holding tank where it was allowed to settle. The customer wanted to be able to pump the
hydrocarbon back into the process for reclamation without also pumping any of the water. In designing a
solution, several obstacles had to be overcome. Firstly, the application involved a very low differential pressure span impossible for most remote seal transmitters to measure. A second challenge was the
location of the application, which was subject to considerable swings in ambient temperature. Such inconsistent conditions can often pose a potential problem when measuring very small pressure differentials. To solve this problem, the entire
transmitter, with remote seals connected, would have to be temperature characterised together in an environmental chamber. A microprocessor-based ABB 2600T
draft range differential pressure transmitter was installed because of its small upper range limit, suitable for the close requirements of the application. The temperature characterisation data from the environmental chamber was stored in the transmitter’s memory. The transmitter’s on- board temperature sensors monitor the ambient temperature. Accurate pressure measurement
is ensured by the transmitter’s microprocessor, which compares the data from the environmental chamber with the ambient temperature conditions and
adjusts the transmitter’s output accordingly. A major concern at the outset was the risk
of any pressure imbalance inside the capillary system due to changes in ambient temperature, which would cause the fill fluid to expand or contract. The effect of this potential change was calculated under laboratory conditions, with the uncertainty of the system being predicted to be less than 0.5 per cent of span. Since this new interface level
transmitter was installed, the interface level control has greatly improved. The customer has also reported that downtime has been eliminated, saving over £30,000 per year on the cost of maintenance alone. Before this, monthly maintenance was required to
clean the previously installed buoyancy transmitter system to prevent shutdowns. Despite this, the instruments would frequently foul up anyway, resulting in the process being shut down.
SummAry
Selecting the right solution for an interface level measurement application requires consideration of many factors, including accuracy, aggressiveness of the application media and the level of maintenance deemed acceptable for the application. Opting for a remote seal differential
transmitter system provides an ideal solution for aggressive applications and can help to eliminate maintenance whilst delivering greatly enhanced measurement accuracy.
ABB
www.abb.com/measurement October 2021 Instrumentation Monthly
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