Instrumentation - Flow
June
4, 2006
Introduction
The measurement of flow in an acid plant is
important for the control of the process and for troubleshooting problems that
may occur. In some cases, an approximately flow rate is acceptable for
control purposes so a relatively inexpensive instrument is acceptable. In
other situations, greater accuracy is required which will require a more
sophisticated instrument cost more. There are a variety of flow measuring
devices available but not all are suitable for acid plant service.
Differential Pressure Devices
Flow devices producing a differential pressure signal, have a reputation for
accuracy, simplicity and reliability. Some differential pressure devices
are orifice plates, venturi tube, flow nozzles, annubars, wedge flow meters,
etc. The most common types found in sulphuric acid plant application are
described.
Annubars
An annubar produces a differential pressure (DP) signal proportional to the
square of the flow rate in accordance with Bernoulli's theorem. This
signal has two components; the high pressure (PH) and the low pressure (PL).
The high pressure is produced by impact of the velocity profile on the sensor.
The velocity profile results in a corresponding impact pressure profile.
Multiple sensing ports located on the front of the sensor, sense the impact
pressure profile. Inside the high pressure chamber, the average velocity
pressure is maintained by the proportionality of the sensing port diameters to
the chamber cross-sectional area.
The velocity profile continues around the sensor and creates a low pressure
profile. The low pressure profile is sensed by ports, located downstream and
opposite the high pressure ports. Working on the same principle as the
high pressure side, an average low pressure is maintained in the low pressure
chamber. The resulting differential pressure is transmitted through the
instrument head to a differential pressure flow meter.
Orifice Plates
A concentric, sharp-edged orifice plate is the simplest and least expensive of
the differential head meters. The orifice plate constricts the flow of a
fluid to produce a differential pressure across the plate. The
result is a high pressure upstream and a low pressure downstream that is
proportional to the square of the flow velocity.
The disadvantage of an orifice plate is it usually produces a greater overall
pressure loss than other differential pressure devices. An advantage of
this device is that cost does not increase significantly with pipe size.
Elbow Meters
As a fluid passes through a pipe elbow, the pressure at the outside radius of
the elbow increases due to centrifugal force. If pressure taps are
located at the outside and inside of the elbow, a reproducible measurement can
be made.
Elbow
flow meters are inexpensive and do not introduce any obstructions to the flow
of liquid.
Magnetic Flow Devices
The operating principle of the magnetic flow tube is based on Faraday's law of
electromagnetic induction, which states that a voltage will be induced in a
conductor moving through a magnetic field.
Faraday's Law: E = kBDV
The magnitude of the induced voltage (E) is directly proportional to the
velocity of the conductor (V), conductor width (D) and the strength of the
magnetic field (B).
Field coils placed on opposite sides of the flow tube generate a magnetic field.
As the conductive process liquid moves through the field with average velocity
V, electrodes sense the induced voltage.
The flowmeter must be completely flooded at all times to obtain an accurate
measurement of flow. Installation in vertical pipes where the flow is
going up is ideal. If the flowmeter and piping are drained during
shutdowns, this should be specified to the vendor so appropriate measures can be
taken to avoid damage to the flowmeter due to overheating.
Mass Flow Meters
True mass flowmeters measure the mass flow directly rather than the volumetric
flow and density and then calculate the mass flow from the two measured values.
Coriolis Meters
The Coriolis meter uses an obstructionless U-shaped tube vibrating at its
natural frequency as the fluid passes through it. The tube twists as
result of the flowing fluid and the angular movement of the tube created by
the vibration. Sensors measure the amount of twist and this is
proportional to the mass flow rate through the pipe. Coriolis
meters operate independent of liquid density, pressure and viscosity.
Thermal Meters
Thermal meters are commonly applied to gas streams only where the transfer of
heat to and from them stream is a usual element of the metering process.
Measuring the heat transfer supplies data from which a mass flow rate may be
calculated. Thermal meters operate independent of density, pressure and
viscosity.
Variable Area Devices
Variable area devices, more commonly known as rotameters, are typically made
from a tapered glass tube that is positioned vertically in the fluid flow.
A flow that is the same size as the base of the glass tube rides upward in
relation to the amount of flow. Because the tube is larger in diameter at
the top of the glass than at the bottom, the float resides at the point where
the differential pressure between the upper and lower surfaces balance the
weight of the float. In most cases, the flow rate is read directly from a
scale inscribed in the flow tube but transmitting rotameters are available.
Ultrasonics
Ultrasonic flow meters utilize sound waves to determine the flow rate of fluids.
Pulses from a piezoelectric transducer travel through a moving fluid at the
speed of sound and provide an indication of fluid velocity. Two
different methods are currently employed to establish this velocity measurement;
transit-time and Doppler methods.
Ultrasonic meters have several advantages, including freedom from obstructions
in the pipe and negligible cost sensitivity with respect to pipe diameter.
However, their performance is sensitive to flow conditions.
Transit-Time
The transit-time method uses two opposing transducers which are mounted at a
45o angle to the direction of flow. The speed of sound from the upstream
transducer to the downstream transducer represent the inherent speed of sound
plus a contribution from the fluid velocity. A simultaneous measurement
is taken in the opposite direction which represents the speed of sound minus
the fluid velocity. The difference between these two values is
representative of the fluid velocity which is linearly proportional to the
flow rate.
This method works well where the fluid is free of entrained gas or solids
which can scatter the sound waves traveling between the transducers.
Doppler
This type of ultrasonic meter uses two transducers mounted in the same case on
one side of the pipe. An ultrasonic sound wave of constant frequency is
transmitted into the fluid by one transducer. Solids or bubbles within
the fluid reflect the sound back to the receiver element. The Doppler
principle states that there will be a shift in apparent frequency or
wavelength when there is relative motion between transmitter and receiver.
Within the Doppler flow meter, the relative motion of the reflecting bodies
suspended within the fluid tends to compress the sound into shorter wave
length (high frequency). This new frequency measured at the receiving
element is electronically compared with the transmitted frequency to provide a
frequency difference which is directly proportional to the flow velocity in
the pipe. In contrast to the transit-time method, Doppler ultrasonic
meters required entrained gases or suspended solids within the flow to
function correctly.
Instruments
| Instrument |
Accuracy/Repeatability |
Installation |
References |
Remarks |
| Annubar |
± 1% over
a flow turndown of greater than 10 to 1, independent of Reynold's number.
± 0.1% repeatability |
7 PDU
3 PDD
(single elbow flow disturbances) |
Dieterich
Standard Diamond II Annubar |
Relatively low pressure drop |
| Orifice
Plate |
± 2-4% of
full scale |
10-30 PDU
4-8 PDD
(dependent on flow disturbances and orifice
b ratio) |
|
Relatively high pressure drop |
| Coriolis |
± 0.2% of
rate |
No
requirements for upstream of downstream pipe diameters |
Micro
Motion Model D65
Rosemount Technical Data Sheet 3031 |
Can also
be used to measure density of fluid |
| Magnetic |
± 0.5% of
rate from 1 to 30 ft/s
± 0.005% from low-flow cut-off to 1.0 ft/s |
5 PDU
3 PDD
butterfly valves not recommended immediately upstream or downstream |
Rosemount
Model 8701 |
Grounding
rings required with lined or not conductive process piping. Pipe must be
running full. |
| Elbow |
± 5-10%
of full scale |
30 PDU |
Rosemount
Technical Data Sheet 3031 |
Suitable
for liquid flow only. Typically, there will be insufficient pressure
differential generated in gas service. |
| Thermal |
± 2% of
full scale |
No
requirements for upstream or downstream pipe diameters |
Rosemount
Technical Data Sheet 3031 |
Suitable
for gas flow measurement only. |
| Variable
Area |
± 2% of
full scale |
No
requirements for upstream or downstream pipe diameters |
Rosemount
Technical Data Sheet 3031 |
|
|
Ultrasonic |
Transit-Time: ± 1% of rate
Doppler: ± 3% of rate |
5-30 PDU |
Rosemount
Technical Data Sheet 3031 |
|
Applications
The following is a list of typical acid plant services for flow meters and the
recommended instrument to measure the flow.
| Service |
Instrument |
Remarks |
| Sulphuric
Acid |
Magnetic |
Use if accurate measurement of flow rate is required. Advantage is no
additional pressure losses are introduced. The probe type
magnetic flow meters should not be used. These were replaced by the
flowtube type. |
| Orifice |
Can be used where accuracy is not important (ie. for alarm or interlock
function). Additional pressure drop is introduced to the system.
Material of construction is typically Lewmet (Lewis Pumps). |
| Ultrasonic |
Used primarily as a diagnostic tool for situations where a flowmeter has not
been installed. Suitable only for liquid flow measurement. |
| Elbow |
Can be used where accuracy is not important (ie. for alarm or interlock
function). Does not introduce any additional pressure drop
to the system. |
| Liquid Sulphur |
Wedge |
Proven reliable and
accurate. Diaphragm seals and a custom steam jacket are required. |
| Liquid SO2 |
Coriolis |
Suitable for use in
commerce (i.e. weighing for sales purposes) |
| Process
Gas |
Annubar |
Used to measure flow
at plant inlet, blower discharge and stack. Prone to plugging if not
maintained properly. |
| Thermal |
Should be considered
as a replacement for annubars in location where annubars continually plug. |
| Water |
Annubar |
Used where accuracy
is required. |
| Orifice |
Can be used where
accuracy is not important (ie. for alarm or interlock function).
Additional pressure drop is introduced to the system. |
| Rotameters |
For small flows
rates, such as acid dilution or make-up water service. |
| Steam |
Orifice |
Used in saturated or
superheated steam service. Typically, used as part of a multi-point level
control for the steam drum level. |
