Basic design

Turbine flow meters have a free spinning rotor whose velocity is directly proportional to the axial velocity (impact force) of the fluid stream. The turbine essentially transfers axial velocity into rotational velocity. This rotational velocity is then sensed with a pickup which pulses once for each time a rotor blade passes under it.

Helix versus turbine meters

We offer both helix and turbine meters. They are totally different meters, but because they are both called turbines, there is a natural assumption of equivalence.

If the meter has an integral, mechanical register and the rotor has the appearance of a low pitch screw with a section cut out and a shaft inserted in the middle ... it is probably a helix meter.

If the meter is electronic and has a hub with short blades sticking out ... it is probably a turbine.

Helix are bulk water meters, also called Woltmann's. In our product line you can find these under AMCO Water Metering (formerly ABB-Kent Meters), Aaliant (formerly Hersey), Neptune Technologies, Hays Fluid Products, and the closely allied "propeller" meter by McCrometer.

Turbine manufacturer's we offer are Blancett, Sponsler, and Great Plains.

In general, if you want a Rate of Flow indication or analog output signal, try to use a turbine. If you only want to totalize, and the materials and fluids are compatible, use a helix (bulk water) meter.

The pickups (which detect the rotor blades spinning motion)

There are three type of pickups in common usage: magnetic inductive, magnetic reluctance and modulated carrier. The pickups screw into the meter body and are spaced in close proximity to the tips of the rotor blades.

The magnetic inductive pickup uses a magnet imbedded in each turbine rotor blade. The pickup is essentially a wire coil through which the magnetic field passes, producing a voltage pulse. These are not commonly used, although we occasionally see this design on large bulk water meters as a method of developing a higher speed pulse output.

The magnetic reluctance type is the most commonly used. This type contains a magnet in the pickup with a spool of wire around it. When the rotor blade passes under the pickup, the blade "collapses" the magnetic field causing it to move through the wire spool, thus creating a pulse output.

The modulated carrier type of pickup is used for applications where the drag created by the magnet would affect the operation of the meter. This is frequently seen on gas turbine meters running on low molecular weight gases.

The modulated carrier pickup uses a small oscillator running at a fixed frequency. As the rotor blades pass through the electromagnetic field created by the oscillator, it shifts the carrier frequency. This frequency shift is demodulated out, amplified and conditioned, eventually coming out as a square wave pulse of constant amplitude, with a frequency directly proportional to the rotational velocity of the rotor. This design does not load down the rotor like the magnet based designs.

Quadrature or Bi-directional turbine meters

By installing two pickups at precise angles to each other, depending on the number of blades, you can have both pickups responding to what, electrically speaking, appears to be the same blade, but with pulse outputs offset from each other. With forward flow, i.e. clockwise rotation of the rotor, pickup A produces a voltage pulse with 20-30 degrees of phase angle ahead of the voltage pulse from pickup B. With reverse flow, pickup B produces a pulse before pickup A.

If your display has a "quadrature" input, you connect both pickups and the display will typically give you forward flow, reverse flow and net flow in a selected direction.

Some amplifiers have a "sense" signal, which tells you which direction the flow is, and cuts off the output from the "backwards" flow pickup. This requires two displays. This option is seldom sold, the quadrature design is more powerful and less complicated.

Bearing selection

Bearings are ball or sleeve (journal) types.

Ball bearings are commonly found in fluids such as cryogenics, fuels and oils. Special design ball bearings are used in clean non-corrosive gases such as air, ammonia and the perfect gases.

Sleeve bearings introduce considerably more drag than ball bearings which tends to limit the range and linearity of the turbine. Sleeve bearings are available in a much wider selection of materials than ball bearings and thus are widely used for chemical compatibility reasons.

Pivot bearings are primarily used in insertion type meters and tangentially shot meters such as multijet and single jet designs.

There are companies which offer a single choice - sleeve bearings in tungsten carbide. Tungsten carbide is a very hard material, exceeded only by diamonds and a few ceramics. It has excellent wear properties and if properly applied will almost never wear out. On the negative side, it needs a lubricating fluid. Using tungsten carbide bearings on low lubricity fluids means losing the bottom end of the performance curve.


Accuracy considerations

A turbine meter designed for process control work has a typical linearity of +/- 0.5% on liquids and +/- 1.0% on gases. Repeatability's are excellent, +/- 0.05% for liquids and +/- 0.10% for gases.

Stating the accuracy becomes something of a problem because the inherent error of the test equipment becomes very significant compared to the error of the turbine meter. Measurement uncertainty of a liquid is easily in the 0.15 to 0.25% range on a test bench, and this varies with the Reynolds number at which the test is performed. Accordingly, it is common not to see accuracy statements in the literature of precision turbines.

There is another class of meter which is designed for rigorous flow conditions, typically as one might find in crude oil. These meters have heavy rotors and shafts and can take lots of abuse. However, the extra mass introduced to make the product so rugged increases the drag, the inertial response times, and produces a meter of limited performance. These meters are typically 1-1.5% accuracy meters, have high pressure drops, always have tungsten carbide bearings and lower costs associated with larger production runs of standardized designs.

Viscosity considerations

The design condition for turbine meters is the range of flow where the driving forces on the rotor are equal to the drag, or retarding, forces, and the rotor speed is linear and directly proportional to flow velocity.

Turbine meters are viscosity sensitive. As the viscosity increases, drag forces on the rotor blades increase, the low flow response drops off and the linearity of the meter begins to suffer. Our basic rule of thumb is to never go above 20 cP in a 1" meter. As the flow rate increases and larger meters are required, the rotor sensitivity to viscosity changes is lessened.

If your situation dictates a turbine on a high viscosity fluid, we can provide a linearizer and electronically "pull-up" the droop in the response curve and produce a flat output curve. This works because turbines are repeatable, even when being run at higher viscosity's. Some turbine meter companies proclaim very large turndowns with their meters, but this is based on using a linearizer as part of the package. Our position is that we'd prefer to use a meter which can handle the viscosity, which would typically be a oval gear meter with the same 1/2% of rate accuracy.

When you have a low viscosity, low lubricity fluid such as alcohol, MEK, acetone, etc. a turbine is a very good choice. The potential problem is in the application. If the customer wants to batch this fluid, this dictates an explosion proof housing on the batcher which is expensive. On applications requiring local indication or signal outputs, the standard Sponsler housing is all you need.

Fluid/Gas cleanliness considerations

Turbines are used on gas flows and work very well. However, when you get into dirty gas carrying small particulates or material that will coat, you are better off using a vortex shedder (see our Endress & Hauser shedder)

Sponsler precision turbines have sharp edges on the blades in the rotor assembly. If you have particulate matter and put a ding in the leading edge you will experience a shift in accuracy.

Maintenance

There is little to maintain in a turbine meter. If a fluid deposits residues which can build up in the bearings, or if debris has been caught up, a lower than normal output reading is the indication of this type of problem.

On examination the rotor should spin freely. Blowing into a small meter should cause the rotor to spin, as it slows down and comes to a stop, it will rock slightly as the rotor blade lines up with the magnet in the pickup. If the blade stops abruptly it is time to clean out the bearings. If you can avoid disassembly of the meter by using a solvent to clean the bearings, by all means do so. If your meter has graphite bearing, exercise extra caution. Graphite bearing break easily and it is worthwhile buying a spare set of bearings before disassembly of the meter.

Disassembly techniques vary, in general the turbine assembly is held in place by end supports, which are locked in place with a variety of techniques. On the turbine we offer, the Blancett meter uses a retaining ring which comes out easily with a ring tool. The Great Plains meter uses a pressed in ring, which can be pried out, straightened and reused - although a new ring would be a better choice. The Sponsler turbine uses three cones to position the rotor shaft. In the body, you will find 3 very slight radiuses through which these cones slip. Once inserted, they are rotated to a self locking position. To disassemble, make up a tool to rotate the cones without bending them, slip them out and the rotor will come out. In all cases observe the rotor carefully and be sure that it is reassembled facing the same way it came out.