Fixed Ultrasonic Gas Leak
Detection
Standards
Certification
Education & Training
Publishing
Conferences & Exhibits
Edward Naranjo
Emerson Process Management,
Rosemount Analytical
Presenter
• Edward Naranjo
– Director of Marketing, Flame and Gas
– Ph.D. Chemical Engineering from the University of California,
Santa Barbara; B.S. Chemical Engineering, Caltech
– ISA Orange County Section president
– ISA Process Measurement and Control Division (PMCD)
Director-elect
2
Content
•
•
•
•
•
•
•
Introduction
Detection Coverage
Detector Placement
Allocation
Commissioning
Maintenance
Conclusion
3
Ultrasonic Gas Leak Detection
• UGLD is a detection method
used to establish the presence
of high pressure leaks
– Well suited for open, ventilated
areas
– Does not require transport of gas
to the sensor
– Provides 360° coverage
Ultrasonic Gas Leak Detection (Continued)
Because of its principle of operation, UGLD can
serve as an additional and complementary means
for mitigating the risk of fire and explosions1, 2
1
2
HSE. 2004. Fire and Explosion Strategy, Issue 1. Hazardous Installations Directorate, Offshore Division.
http://www.hse.gov.uk/offshore/strategy/fgdetect.htm. Downloaded October 22, 2010.
HSE. 2007. Acoustic Leak Detection. Hazardous Installations Directorate, HID Semi Permanent Circular
SPC/TECH/OSD/05. http://www.hse.gov.uk/foi/internalops/hid/spc/spctosd05.htm. Downloaded October 22, 2010.
Safety in Diversity
• All sensing technologies are vulnerable under
certain conditions, resulting in poor detection
efficiency
– In a Health and Safety Executive (HSE) study,
traditional fixed gas detectors accounted for 62% of
all detected gas releases over nine years3, 4
– Remaining 38% of releases were mainly detected by
personnel
3
4
HSE Offshore Technology Report – OTO 1999 079, Offshore Hydrocarbon Release Statistics, January 2000.
http://www.hse.gov.uk/research/otohtm/1999/oto99079.htm. Downloaded October 18, 2010.
HSE Offshore Technology Report – OTO 2000 112, Offshore Hydrocarbon Release Statistics, 2000.
http://www.hse.gov.uk/research/otohtm/2000/oto00112.htm. Downloaded October 22, 2010.
Safety in Diversity (Continued)
• Diverse detection methods offer the best
safeguard against fire and explosion hazards
– Having few common failures increases likelihood
of detection on demand
Challenges for UGLD Use
• Despite UGLD’s wider acceptance, the allocation,
installation, commissioning, and maintenance of
ultrasonic gas leak detectors remains poorly understood
– No regulatory standards and few corporate codes of practice5,6
– No guidelines on the optimal combination of point IR, open path
IR detectors, and ultrasonic gas leak detectors for particular
applications7
– Correspondence between a hazard defined by gas concentration
and a noise level is a persistent question
5
6
7
BP. 2009. GP 30-85. Fire and Gas Detection.
Shell. 2002. DEP 32.30.20.11-Gen. Fire, Gas and Smoke Detection Systems.
HSE. 2004. Fire and Explosion Strategy, Issue 1. Hazardous Installations Directorate, Offshore Division.
Measurement Principle
Standards
Certification
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Publishing
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Gas Leak Detection from Ultrasound
Pressurised Gas Leak Frequency Range
Ultrasonic
Audible
| 0 to 25 kHz |
25 to 100 kHz
|
Leak Rate
• Molecules from escaping gas produce a mixture of
audible and ultrasonic noise known as broadband sound
– Leak rate (or mass flow rate) is a measure of hazard severity
– Directly proportional to the cross sectional area of the orifice and
the pressure inside the vessel8, 9
p1
A
8
9
p2
 max  Ap1 M
m
RT
 
 1
  1
 2


   1
Cp
Cv
Bird, R. B., Stewart, W. E., and Lightfoot, E. N. 1960. Transport Phenomena. New York: John Wiley & Sons. Ch. 15.
Whitaker, S. 1986. Introduction to Fluid Mechanics. Malabar, FL: Robert E. Krieger Publishing Co. Ch. 10.
Minimum Pressure
• High pressure systems must be pressurized to a
minimum of 145 psi (10 bar) for UGLDs to detect
a leak
– At these pressures, the ultrasound generated by a
leak is greater than background ultrasound emissions
SPL (dB)
Alarm threshold
Safety margin
Ultrasonic background noise
time (s)
Minimum Pressure (Continued)
• Minimum pressure that supports generation of ultrasound
is approximately 30 psi (2 bar) as given by

 p2 
 2   1

  

p


1

 1  crit 
– For air ( = 1.4), the critical pressure ratio is 0.53
 15 
p1  
  30 psi (2 bar)
0
.
53


Typical levels of ultrasonic background noise prevent
use of ultrasonic detectors at such low pressures
13
Sound Pressure Level (SPL)
120
110
100
SPL (dB)
• SPL varies according
to the sound source’s
power dWs/dt,
distance to the leak r,
and room constant S:10
90
80
70
60
50
 W
SPL  10 log s12
 10

RT 
W s 
m
M
0  1
10

  10 log 1  4 
2

S
 2r

40
0
5
10
15
20
25
Distance from Source (m)
SPL vs. distance. Methane: 0.1 kg/s, p = 47 bar (682 psi),
d = 4 mm; (■) ethylene: 0.1 kg/s, p = 37 bar stance for
methane and ethylene leaks. () (537 psi), d = 4 mm.
Ambient background SPL ≈ 40 dB.
Raichel, D. R. 2006. The Science and Applications of Acoustics, second edition. New York: Springer.
14
Leak Detection Range
• The detection range of an ultrasonic gas leak detector
depends on the leak rate and ultrasonic background
noise
– Leak rate governs the ultrasonic noise level generated by the
leak
– In practice, most industrial plants have ultrasonic background
noise levels of approximately 55 dB
15
Detection Coverage
16
Leak rate = 0.1 kg/s (methane) (ea. 4 mm leak at 46 bar)
Coverage is reduced for both lower gas pressure or higher background noise
Installation,
Commissioning, and
Maintenance
Standards
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Installation
– Avoid shadowing or
blockage by installing
detectors where sound
paths are unobstructed by
large solid structures
1 - 2 meters
• Detectors are installed at
heights of 1 - 2 m above
potential leak spots
Detector Placement
• Placement of ultrasonic gas
leak detectors is a function of
three factors:
– Location of potential gas leaks
– Acceptable level of risk (minimum
leak rate to be detected)
– Extraneous sources of ultrasound
• Likely sources of gas plumes:
–
–
–
–
–
–
Valves
Weld joints
Flanges
Gaskets
Vessels
Well bays
Gas leak detector placed near potential
source of leaks.
Detector Placement (Continued)
• Minimum leak rates define the sensitivity required of
ultrasonic gas leak detectors
• A categorization of leak rates by the Health Safety
Executive (HSE) provides guidance on what’s an
acceptable level of risk11:
Either…
…Or
And
Category
Quantity
Released (kg)
Mass Flow Rate
(kg/s)
Duration
(min)
Gas Cloud
Volume
(m3)
Minor
< 1 kg
< 0.1 kg/s
< 2 min
< 10
Significant
1 - 300
0.1 – 1.0 kg/s
2 – 5 min
10 – 3,000
Major
> 300 kg
> 1.0 kg/s
> 5 min
> 3,000
11
HSE Report. 2001. OSD Hydrocarbon Release Reduction Campaign, Report on the Hydrocarbon Release
Investigation Project ~ 1/4/2000 to 31/3/2001.
20
Commissioning
• Commissioning consists of two steps:
– Test detection system with test unit
– Verify system performance with leak simulation equipment
21
Leak Simulation
Detector
Nozzle
• Leak simulations are performed around the expected
perimeter of coverage to verify detection range
– Ensure detection is unaffected by blockage and acoustic reflections
22
Leak Simulation (Continued)
• Leak simulations are performed with inert gases
– Similar molecular weight and specific heat ratios as target gases
No
Gas
Molecular
Weight
(g/mol)
1
Methane
16.04
1.32 (13)
2
Nitrogen
28.02
1.404 (12)
3
Hydrogen
2.02
1.410 (12)
4
Helium
4.00
1.667 (12)
5
Air
28.97
1.40 (13)
110
100
Specific
Heat
Ratio
SPL (dB)
90
80
70
60
50
0
5
10
15
20
Distance from Sound Source (m)
25
UGLD Response to Inert and Combustible Gas Leaks. () Methane,
() Nitrogen, () Hydrogen, () Helium. Leak Rate = 0.01 kg/s.
Measurement Error =  3 dB.
12
13
Atkins, P. W. 1986. Physical Chemistry, 3rd edition. New York: W. H. Freeman and Company.
Engineering Toolbox. http://www.engineeringtoolbox.com/specific-heat-ratio-d_608.html. Downloaded October 25, 2010.
Maintenance
• Regular visual
checks
– Blockages on
windscreen
– Visual damage to unit
• Testing by means of
portable test tool
Detector Selection
Standards
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Detector Selection
• Ultrasonic gas leak detectors should be
sufficiently sensitive to detect gas leaks at predefined levels
– Alarm threshold values may need to be adjusted if
new sources of ultrasound are introduced close to
detectors
• Detectors should be equipped with electronic
high pass filters to screen audible spectrum of
broadband sound
– Filtering background noise interference enhances
immunity to false signals
Detector Selection (Continued)
• Instruments equipped with fail to safe features
enhance operational confidence
– Example: Acoustic self check periodically verifies
integrity of electronic circuitry and operation of
acoustic sensor
• Detectors should support analog output and
two-way digital communication to integrate to
emergency shutdown systems (ESDs)
Conclusions
• Ultrasonic gas leak detectors offer an additional
and complementary means of detecting gas
leaks
– Share few common failure modes with conventional
gas sensors
– Improve detection rate
• Guidelines for UGLD allocation are simple
– 360 coverage lends itself well for calculating the
number and location of detectors in a room
– Detection radius is influenced by ultrasonic
background noise, minimum discharge rate to be
detected, and ultrasonic interferences
Conclusions (Continued)
• Results from several mapping surveys suggests
certain process areas have similar levels of
background ultrasound
– Enable users to estimate detection coverage based
on collected data
• Examples of UGLD allocation in offshore and
onshore facilities were presented
– Classification of spaces into very low noise, low noise,
and high noise prove useful for deciding on
appropriate alarm level and detection coverage
– Further fine tuning is possible by conducting a
mapping survey
Conclusions (Continued)
• Through commissioning users can verify leak
detectors respond to leaks at a specified radius
– Functional test is a close proxy of a hazard scenario
• A minimum amount of maintenance must be
carried out with ultrasonic gas leak detectors
– Visual inspection
– Testing
Conclusions (Continued)
• When choosing a detector, users are advised to
consider features that improve reliability and
integration to ESDs
– Fail to safe features and calibrators can ensure
devices are operational at all times
– Analog output and two-way digital communication are
increasingly essential for the deployment of safety
systems