Ultrasonic Gas Meters Handbook - Instromet
Ultrasonic Gas Meters Handbook - Instromet
Ultrasonic Gas Meters Handbook - Instromet
HANDBOOK
INTRODUCTION
1. OPERATION
2. PERFORMANCE
3. INSTALLATION
4. DIAGNOSTICS
4.1 AGC
4.2 Number of pulses accepted
4.3 Velocity of sound
4.4 Noise and signal analysis
4.5 Average velocity
5. OUTPUTS
ULTRASONIC GASMETERS
INTRODUCTION
This handbook is one of a series that Instromet has prepared for the
Gas industry. It describes the operating principle, the performance, the
installation and the output facilities of the different types of Instromet
Ultrasonic gasmeters. Comparisons are made with other types of
meters such as orifice plates, turbine meters and rotary piston meters.
Other handbooks in this series deal with Turbine meters, Rotary dis-
placement meters, Gas chromatographs and with complete Gas
Metering Systems.
1. OPERATION
C = √ (K•P/ρ)
For Natural Gas [1] the velocity of sound is approximately equal to:
C = √(k•P/(Z•ρ))
In figure 1 the basic system is shown. On both sides of the pipe, at posi-
tions A and B, transducers are mounted, capable of transmitting and
receiving ultrasonic waves. The acoustic waves are generated as a
beam perpendicular to the surface of the transducer.
The Instromet Ultrasonic gasmeters use a short pulse signal. The form
of this signal, which is really a short burst of a very high frequency (fig-
ure 2), is recognised at the receiving end and the time elapsed since
emission is measured digitally.
Figure 2. Typical form of high frequency pulse
With zero flow, the travel time from A to B (tAB) is equal to the travel time
from B to A (tBA). This is equal to the average travel time for the acoustic
pulses t0:
where L is the length of the acoustic path and C is the speed of sound in
the gas.
tAB = L
C + vm cos (ϕ)
L
tBA =
C - vm cos (ϕ)
where ϕ is the angle between the path A - B and the pipe axis.
When the two acoustic pulses are transmitted at the same time, the speed
of sound is identical for both measurements and can therefore be elimi-
nated, resulting in:
L
vm = (1/tAB - 1/tBA) (1)
2 cos (ϕ)
where v is the averaged flow velocity along the ultrasonic path. It is
m
clear from (1) that the flow meter is truly bidirectional.
Before pulse detection and recognition take place, the received signal
is pre-processed using Automatic Gain Control (AGC) and a filter sec-
tion. The AGC section is used to cope with a wide spectrum of gas den-
sities, pressures and composition. After the pre-processing stage the
pulse is presented to the detection circuitry. In the detection circuit the
signal is digitized and compared with a ‘fingerprint’ of the expected
pulse signal, making it highly immune to other acoustic signals that
might otherwise influence the measurement. The measurement result,
based on the two transmitted pulses, is either:
Only after the received pulse is accepted will the travel time be deter-
mined and used in the calculation of the gas velocity and the speed of
sound. Matching the received signal with its fingerprint not only elimi-
nates spurious signals, it also makes it possible to determine the time
of arrival more accurately. This method results in the highest measure-
ment quality that can presently be achieved.
2. PERFORMANCE
where A denotes the cross-sectional area of the pipe and k the effect
of the velocity profile.
The uncertainty in the values of tAB and tBA is determined by the elec-
tronics. The path length L and angle ϕ, and the surface area A are
determined by the geometry and any uncertainty in these parameters
will result in an uncertainty in the flow rate.
where M, the Mach number of the flow (v/C), must be much smaller
than unity. These relations show that the measured travel times (both
up and down stream) are equal to the mean travel time t0, with a small
correction M • t0/2 depending on the average gas velocity.
The small value of the mean travel time indicates that an ultrasonic
flow meter is capable of measuring with high repetition rates. In surge
control and other applications, where the flow drops from its set point
to its minimum in less than 0.5 s, this high repetition rate is of prime
importance. Typical repetition rates are 10 to 30 Hz but can be set to a
higher value if necessary. The advanced signal processing used in
Instromet Ultrasonic meters make it even possible to measure volume
in pulsating flow with little additional error [4].
The velocity error ∆v is proportional to the time error ∆t and given by:
C2 tan(ϕ)
∆v = ___________ ∆t
4D
As a function of the pipe diameter this results in:
D δv
mm/s inches/s
6” 4,5 0,18
10” 2,7 0,11
20” 1,4 0,054
30” 0,9 0,036
After entering the pipe the velocity profile will gradually accommo-
date itself until it is axisymmetric and fully developed. This would nor-
mally take some 80 D. Most theoretical and experimental work in flow
in pipes is related to fully developed flow.
Several relations have been put forward to describe the velocity profile
in a round pipe. From these relations one can calculate a theoretical
value for k. Instromet's experience has been that the one given by
Rothfus and Monrad [5] in a study for Shell results in an uncertainty of
approximately 1% for a single path meter operated at Re ≥ 105.
Figure 3. Laminar and turbulent flow patterns
As a result the flow pattern will normally not be axisymmetric and may
contain a swirl component. Practical installations may also show pulsat-
ing flow.
In all cases except the case of one single vortex, the presence of swirl
will also mean that there are radial velocity components present, so-
called cross flows (figure 4).
b. Reflected path
For a fully developed flow the uncertainty in the flow rate is mainly
determined by the geometry of the meter and, for low flow rates, the
turbulent fluctuations and the offset error. The flow rate is calculated
from the average velocity over the path assuming a fully developed
velocity profile. If this is not the case the uncertainty in the flow rate
will increase.
Orifice plate flow meters are similarly affected by the velocity profile.
It is reasonable to assume that, if the same installation conditions are
observed as for orifice plates, similar uncertainties can be expected. For
an installation that satisfies ISO 5167 [6] this would give a basic uncer-
tainty of 1% for a ß of 1. As the installation requirements according to
the standard increase with ß, and are only listed up to a value of 0.8,
the uncertainty may still be somewhat higher in practice, even though
the installation conditions of ISO 5167 for ß = 0.8 are fully met.
The P.Sonic is a single reflection, single path meter with a path config-
uration as in figure 5b, mounted in a machined spool piece. In these
meters the distance between the transducers can be accurately con-
trolled and the cross-sectional area of the pipe is known with great
precision. As a result the dimensions contribute very little to the uncertainty.
In a practical installation the velocity profile will mostly differ from the
undisturbed velocity profile due to the actual piping configuration.
The piping may result in:
* pulsations
The multipath Q.Sonic has been designed to eliminate the effect of the
distortions of the velocity profile. The path configuration has been cho-
sen in such a way that it is possible to detect the type of distortion and to
measure its strength (figure 8). Two double reflection triangular
corkscrew paths, one rotating clockwise and the other counter clock-
wise, are used to measure the swirl strength. Three single reflection
paths are used to measure the asymmetry of the flow pattern.
3. INSTALLATION
As discussed earlier the flow profile has a similar effect as for orifice
plates. Present indications are that observing the installation condi-
tions as stipulated in ISO 5167 will result in a basic uncertainty of about
1%. Similarly, if the smaller, bracketed values listed in this standard are
used for the straight lengths preceding the meter, one can expect to
have to add an additional 0.5% arithmetically.
Independent tests carried out by users [7] have confirmed that the
Q.Sonic only needs 10 nominal diameters of straight upstream pipe to
satisfy the requirements of ISO 9951.
The performance of the Q.Sonic for wet gas has been demonstrated in
trials carried out by ARCO British Limited on the Thames platform in
the North Sea [8].
Pressurising and depressurising the meter can be done at any rate. The
meter does not suffer any damage from high gas velocities.
Sizes and ranges are given in table 3 for the P.Sonic and in table 4 for
the Q.Sonic.
Diameter Product Flow range Qmin Qmax Meter body
3 3
[m /h] [m /h] length
4” P.Sonic 1:40 20 800 5D
6” P.Sonic 1:50 30 1600 4D
8” P.Sonic 1:60 50 3000 4D
10” P.Sonic 1:60 60 5000 3D
12” P.Sonic 1:125 65 8000 3D
16” P.Sonic 1:130 90 12000 3D
20” P.Sonic 1:140 130 19000 3D
24” P.Sonic 1:140 200 28000 3D
30” P.Sonic 1:180 250 45000 3D
Table 3. Sizes and ranges for the P.Sonic single path meter
4. DIAGNOSTICS
The fact that all relevant data are available in digital electronic form
allows for sophisticated diagnostic techniques. These diagnostic data
can be accessed on line and used to generate control charts. In this way
any degradation in performance can be detected in an early stage and
remedied. The following diagnostics are available on Instromet
Ultrasonic gasmeters:
4.1 AGC
5. OUTPUTS
[8] P.Robbins: Thames Alpha gas metering ultrasonic meter (USM) trial,
North Sea Flow Measurement Workshop, October 1996, Peebles
Hydro, Scotland.