Pete’s Plugs
®
are
provided for simplified pressure connections
[Figure 9]. 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.
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 against each other. 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
Pete’s Plugs
®
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 indicates a 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.
Figure 9.
Pete’s Plugs
®
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.
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.
PETE’S PLUGS
®
RABO
®
Rotary Gas Meter
06 Elster Instromet
