flow splitter for coriolis flow meter


Thread Starter

Kirill St.

Why do some companies use the double tube concept in the coriolis meter construction and almost no company that produce single tube coriolis flow meter?

Is there is some problem in manufacturing single tube coriolis flow meter? In addition the flow splitter in double tube concept is very sophisticated to design and produce. Is there some theoretical database that helps to design this kind of splitters?
In the beginning there was the MicroMotion single "U" shaped tube.
This vibrated at mains frequency (vulnerable to noise) and required a very stable base as a reference for the tube vibration and twist.
They quickly realised the way to resolve the problems with this approach was to vibrate the tube not against a fixed mass (the installation) but against a second identical tube.

The base idea was very sound but splitting the flow wasn't the only way forward. It involves halving the flow velocity through the tubes or reducing tube diameters to maintain flow velocity.

Also, the flow splitter design becomes critical to achieving an equal split between the two flow streams.

However, there is another way forward. This recognises that mass flow is constant along the length of the tube. In this design the tube is a double loop with the first loop vibrated against the second loop.
This means a better flow path due to the tube shape; maintains the flow rate through the tube, the tubes are more flexible and free form weld failures.
In short, this was, at that time, a better solution.

Note that tube shape is important. The more complex shapes may be more difficult to manufacture but can deliver a high degree of flexibility without sacrificing tube wall thickness unnecessarily. Flexibility is the key to good signal to noise ratios. (and remember that back then electronics were very basic. WE didn't have computers let alone microprocessors for example). And the low operating frequency meant they operated right at the maximum vulnerability to pipe noise. This meant signal to noise ratio was very important.

To many, the EXAC single tube design was technically superior.
MicroMotion engaged in a patent suit against them. This was never actually resolved because Rosemount, who owned MicroMotion, bought out Fisher who owned Exac.

If this were purely a technical decision, many believe the D type twin tube should have been discontinued in favour of the single tube Exac design but the D type had the greater commercial footprint and the greater brand recognition so the EXAC was discontinued and with it, its possible development path.

With the money spent since then on coriolis meter design and development who can say where they would have ended up?

As it was the D type was in any event a step on on the twin tube development path. MicroMotion might have succumbed to the same temptation other manufacturers of other products fall victim to, to continue with the existing design.

It may be that another product influenced the matter. Neptune produced the M Dot twin tube coriolis meter.
This has the coathanger shape tube layout which again is very flexible and this was thought by many a much superior meter. Possibly its tube design influenced the development of the Elite mass flowmeter.

The experimentation with straight tube meters was more of a problem.
Khrone produced a twin straight tube mass meter. But the problem with straight tubes is that they are far less flexible than bent tubes with far less movement to detect. This meant that the Khrone design used very small bore tubes to give high enough velocity to give sufficient signal.

Next was Solartron.
Solartron adapted its single straight tube density meter to measure mass flow.

In this case the tube is vibrated at against external nodal masses clamped to the tube at either end. The tube has a triple ply stainless steel bellows at each end to isolate it from temperature stresses and noise though because it operates at 800-1200Hz it is largely immune to extraneous noise.

This meter was the first single straight tube mass meter and the first to use a two wire IS signal.

But it was a late comer in the market and while its density performance was excellent as expected (it is still density meter hardware) mass flow performance was less than conventional twin tube and there were some issues with air that needed resolving. The mass flow performance meant it was exclusively competing in a niche where twin tubes were unsuitable and not competing in the general applications.

Had this meter continued in production, had it benefited from the R&D and investment twin tubes had enjoyed it would, and could, have proven very very successful. Note that this was a full bore single straight tube. It had a lot of advantages but just not enough at the time to be considered worth continuing with.

A very bad decision if you ask me.

So one of the major problems for straight single tube coriolis meters is that hey are late arrivals and it would take a major investment and effort to catch up with twin tubes so they can compete on an equal basis and not simply settle for the applications twin tube cant handle.

It must be recognised that the twin tube is the result of that historic decision to scarp the Exac.

There are a great many commercial decisions that shape the market. The interval between twin tubes setting out on their development path and the single tubes that followed Exac is such that twin tubes had the commercial high ground.

Without the equivalent investment we have no way to know whether single or twin was the best development path, nor if twin tube is reaching limits single tube would not reach.

And not all single tubes are as advantageous as others.
Many of the problems of the twin tube such as entrained free gas pockets would be as much a problem for the Exac double loop single tube as for the parallel twin tube meters since this is a condition where mass flow can vary along the length of the tube and the successive coils of the Exac would become as unbalanced as in the twin tube. But perhaps not such a problem for the Solartron style sensor? there is no imbalance to worry about and the other issues with gas might be resolvable?