We have been using Krohne radar level gauges since 1993: in batch reactors, high-pressure LPG spheres, and in decomposition vessels. For us, it is fair to say that the radar level gauge has proved to be one of the most cost-effective solutions to liquid level measurement. It is certainly our first option for any new level control applications. If you only look at the purchase price, it is true that the initial cost is higher than for a simple pressure level transmitter. But with the pressure transmitter, you have to take account of the on-costs of the instrument stand, extra valving, manifolds, tubing and fittings. The radar level gauge of course is non-contact and, in our experience, generally zero maintenance as well: whilst the pressure transmitter needs continuing calibration and checking of the HP/LP legs for blocking and odd-legging.
Even on the simplest level transmitters, these additional maintenance procedures bring its overall cost up to that of a radar level gauge in about three years. If you add downtime to this equation, the breakeven point is less than one year. With more complicated level measurement problems - such as wet leg or purge systems, chemical seals and displacer systems - the radar system is much less expensive, both in terms of initial purchase price and in running costs. And it has better accuracy and reliability. Our early experience with radar level taught us two things. Firstly, it worked well as a straightforward level detector in situations where other methods had always failed: making polymer in a stirred batch reactor. Secondly, and somewhat unexpectedly, it became a multi-purpose quality control tool as well.
Initially a Krohne BM70 was specified simply as a high/low level detector to ensure the reactor was not over-filled, and that it was empty after draining out. However, by comparing recorder charts we noticed that the repeatability of the level measurement was extremely accurate - to within a few millimetres per batch. Such dependable performance opens up many new uses for the radar level gauge.
* It becomes a temperature control check, since heating cooling causes the reactor contents to rise and fall by predictable amounts.
* To calibrate the batch meters themselves: the three liquid charges could be checked, saving on extra batch meters and only calibrating them on failure.
* It checks on the addition of the catalyst: since subsequent polymerisation causes the reactor contents to fall by a known amount.
* It checks on the addition of other chemicals, which disturb the signal when they are poured in and interrupt the beam.
* Operators can make sure that the stirrer is working, since its action causes surface oscillations, which, in turn, result in a noisy band on the level signal.
All in all it is a bit like having a CCTV inside the reactor. It has gained wide acceptance with the operators, who find it a real comfort factor. Its cost is minute in comparison to the benefits: indeed one batch of material which we saved early on in the programme would have represented a bigger cost outlay than all the Krohne units installed on the site.
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