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
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
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.
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.
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.
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.
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.