Honeywell RABO G65 Instruction Manual page 7

®
ELSTER RABO Rotary Gas Meter
Pressure test ports are provided for simplified pressure
connections [Figure 9].
! WARNING
Below 45 O F the neoprene core of the Pressure test ports
does not recover its original shape as rapidly as it would
at temperatures above 45
of the probe used
readings, the valves may not close fully and immediately
or the valves may remain slightly open until the operating
temperature is above 45
and pressure are also factors in the rate of valve
closing. For this reason, plugs should not be used in
applications where discharging gas or liquids would
create a hazard. The gasketed cap is supplied to
eliminate the small amount of leakage that may occur
at lower temperatures after probe insertion and
removal. The cap should be left on the plug at all times
and tightened to deter unauthorized removal. Special
care should be taken to assure that readings are taken
in the shortest space of time and that a probe is never
left in a plug for a period of hours or days. Severe
deformation of the valves may occur if the probe is left in
the plug for a long period of time.
PRESSURE TEST
PORTS
Figure 9. Pressure test ports
Typically, when proving a meter, both the meter pressure
and meter differential pressure are used and recorded.
The RABO meter differential pressure ports are not full
bore and most temperature probes will not fit through
into the gas stream. Installing the temperature probe in
the provided thermowell ports is recommended.
The meter volume is determined though a device that
senses the meter revolutions. An optical pickup can be
used on the index proving wheel or an index pulser may
be used.
On-site proving, performed in-line, also requires
connections that allow air flow through the meter to the
prover. This is accomplished by use of pipe Tees
upstream and downstream of the meter. A bypass loop
is recommended for uninterrupted gas supply to the
customer.
7 Honeywell
O F. Therefore, upon removal
for pressure or temperature
O F. Duration of probe insertion
While performing an accuracy test, the meter is tested for
a specified volume of gas at each flow rate test point.
Poor repeatability at a given rate may be caused by a
test volume that is too small. It the meter does not repeat
within acceptable limits (0.1%), try increasing the test
volume and retesting the meter. Most companies have
developed test plans that include the volume of gas for
each size meter at a given flow rate. Consult your Elster
representative for applicable volumes.
b) Differential Pressure Testing
A differential pressure test is not an accuracy test, but it
can be used to gauge the relative performance of the
meter.
Rotary meters are made from solid parts, machined to
tight tolerances with close clearance fits, and the energy
it takes to turn the meter is generally used to overcome
the friction of the bearings and other rotating parts. If
this friction value increases, it will take more energy to
overcome it. This additional energy is measured as an
increased pressure drop across the meter.
The increased friction is caused by contamination of the
measuring chamber and/or bearings.
Contamination in the measuring chamber can cause the
meter components to wear
Contamination in the bearings makes them harder to
turn. Comparing the pressure drop reading when the
meter is new to the reading collected after time allows
the performance to be monitored.
RABO meters are supplied with Pressure test ports
installed in the meter differential pressure ports [Figure
9]. This facilitates differential meter testing. A differential
pressure test is performed using a differential pressure
manometer. The test is to measure the pressure drop
across the meter at a given flow rate, under known
operating conditions at a known date and time.
The meter differential curve of flow rate vs pressure is
not linear. Testing at multiple flow rates is suggested. It
is preferable to test at 3 rates between 25% and 100%
of flow if possible. Differential pressure tests at flow rates
under 25% are hard to interpret because the meter
differential pressure at the lower flow rates is quite small.
The error in measurement is almost as large as the
reading itself.
The meter differential pressure is also a function of line
pressure and increases as line pressure increases.
Testing the meter at the same conditions (line pressure
and flow rates) yields comparable data.
A change in the differential pressure
change in performance. Testing has shown that a 50%
increase in meter differential pressure (at flow rates over
25%), indicates almost a 1.0% change in meter
accuracy. Baseline data must be captured during initial
start-up to which future data can be compared.
If the differential pressure test shows an increase in the
meter pressure drop at a given flow rate of more than
50% from the original value (1.5 x original value), then it
is recommended that the meter be removed and
serviced.
The frequency of deferential testing is at the discretion of
the user.
against each other.
indicates a