Like all mechanical flowmeters, the Neptune Type
S meter is essentially 8 a fluid motor driving a gear train whose output
is a number roller bank
displaying the output shaft speed as flow rate (volume).
The motor provides a known number of revolutions per gallon, however,
like all fluid motors, this number will vary somewhat with changes in
typical Type S, 5/8 meter has about 47 revolutions per gallon,
design in a 2" size meter has 2.56 rev/gal. when calibrated
with water. If you use a fluid with a higher viscosity, the number
per gallon will increase.
The motor produces revolutions based on fluid velocity and it's
characteristics, but the register must display a number in engineering
units of Gallons,
Pounds, Cubic Feet, etc. To accomplish this, two gear trains
are used. The first gear train is located directly on the output
shaft of the motor
and provides a "course" reduction to reduce the number of revolutions
per gallon to roughly what the register is going to display. A second
gear train provides a "fine" adjustment through what are called
"change gears". The change gears are selected at the factory
to make the meter read correctly on water. If the customer knows what
his fluid viscosity is, this change gear ratio is then adjusted per a
set of curves, to the expected operating condition.
A PD meter can accommodate a wide range of viscosity variations,
for example, a 2" meter will exhibit about a 1% shift in
going from 500 to 2000 Cp, after undergoing a initial shift of
about 7% in going from water to
500 Cp. Accordingly, adjusting the change gear ratio is not a
If the customer reports that the meter register is incorrect, it can be
field adjusted fairly easily. We need to know:
a. The exact error, i.e. "meter is reading high by 27# in a batch
Key points: - is the register reading high or low?
- what is the error in what size volume?
b. The change gear ratio. The change gears are stamped with the
number of teeth and they are always located next to a stamped "S" or
"R". The "S" means stuffing box gear. The "R"
means register. We need to know the number of teeth on both the "R"
and "S" gears.
Location of these gears:
On a 157 non-resettable register, you can see them through the lens.
On a 600 resettable register, remove the small plate on the top and you
will see them just inside the cover.
On a 800 batching or resettable register, it is a bit more involved:
a. Remove the valve linkage ... easy way, pull out the cotter pin at
the valve arm.
b. Loosen the two square head bolts at the two lower corners on the
front of the register face. Don't try to remove them, just back them
off 8-10 turns.
c. Lift off the register and turn it over. You will see a plate with
the two gears, again next to the stamped "R" and "S".
Call us and give us this information. We will:
a. Calculate the error as a percent
b. Apply this % as a correction to the existing change gear ratio.
c. Send you gears marked "R" and "S" to meet
this new ratio.
The new change gears are only as accurate as the customers determination
of his error. Press the customer for an accurate measurement.
Why the change gears have a little plate across the middle:
The obvious answer is that the plate slides into the split shaft and locks
it into place.
The less obvious answer is that the plate serves as a "mechanical
fuse". The working chamber of a positive displacement meter
with direct shaft coupling, like the Neptune Type S, produces
an amazing amount
of force. Should something jam the register gear train, this
plate will shear, minimizing damage to the register. The customer
will complain that
the meter seems to be working but the register is not turning.
A sheared off plate is easy to spot ... the ends of the plate
are crimped into the
face of the gear. If the crimp marks do not line up with the
split in the shaft, the plate is broken off.
Sometimes the plate will shear and the register is perfectly
all right. The only time this will happen is when the meter is
asked to accelerate rapidly
and the inertial mass of the gear train in the register cannot
respond fast enough. The "fuse" blows. We have seen this happen at unloading
stations where a large hose is connected to a tanker and the operator
does not allow some time for gravity to fill the hose. He turns on his
pump and the resulting high velocity air coming out of the hose hits the
meter, the working chamber responds, but the register can't move fast
enough and the "fuse" pops.
In this case, he will also sometimes crack the disk ... when the solid
fluid hits the rapidly spinning disk, the shock is tremendous.
Excessive speed of the disk or piston with air can also cause "nose
burn". This relates to the wear placed on the mating surface
of the disk or piston on the divider plate.