Why Fugitive Emission Testing (FET)?. 1. To check performance ... application under the guidelines of local/ national ..
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E M I S S I O N C O N T RO L
Keywords: FE • Oil & Gas • Testing
Effective solutions to beat Fugitive Emission The industry is seeing an increasing trend in the demand for stricter emission control norms mandated by international, national and local organizations all over the world. Hence a thorough understanding of the Fugitive Emission subject is becoming extremely important. Fugitive Emission is unintended or irregular release of pollutants, hazardous fluids, gases or vapors into the environment, through any static or dynamic joints of pressurized equipment such as pressure vessels, pipelines, valves etc. and not from usual sources such as chimneys, stacks and vents.
3) ISO 15848-2: Procedure for Fugitive Emission- Production Acceptance Test of Valves. 4) ANSI/ISA – S93.00.01-1999: Standard Method for Evaluation of External Leakage of Manual & Automated On-Off Valves. 5) API 622, 23 rd edition: Oct. 2011: Type Testing of Process Valve Packing for (Downstream Segment) Fugitive Emissions. One of the frequently used standards by the industry is ISO 15848-1:2006. Excerpts from ISO-15848-1-2006:
By Narayan Patil, Virgo Valves & Controls Ltd.
a) •
E
missions of Benzene from oil refineries & chemical plants cause a long term health risk to workers and local communities or the release of Hydrogen Sulfide (H2S), often referred as “sewer gas” at various concentrations have following effects on humans as: • • • •
10 ppm = Eye irritation 100 ppm = Coughing, loss of smell above 15 min of exposure 500~700 ppm = Loss of consciousness leading to fatality 800 ppm = Rapid loss of consciousness, severely affecting respiration causing death
Another example is emissions of Sulphur Dioxide to the atmosphere, & if reacts with the water vapor enhances acidity. This results in the acidification of receiving waters and consequent damage to ecosystems, especially lakes and forests which inturn has adverse effect on human health Why Fugitive Emission Testing (FET)? 1. To check performance level of On-Off & Control valves with regards to emissions. 2. To minimize and control leaks at process facilities & carry out regular leak detection and repair activities. 3. Routine inspections, using appropriate techniques can be used to identify leaks and estimate leak rate in order to decide on appropriate corrective action.
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FET Techniques, Standards, and Codes: Various international standards and codes are available for selecting the Fugitive emission testing methodology and acceptance criteria. It is therefore the responsibility of end user to select appropriate code for an intended application under the guidelines of local/ national statutory body. Frequently used standards/codes are: 1) ASME Section V, Article 10: a) Appendix IV: - Detector Probe Technique. b) Appendix V: - Tracer Probe Technique. c) Appendix IX: - Hood Technique. 2) ISO 15848-1: Procedure for Fugitive Emission – Classification System and Qualification Procedures for Type Testing Of Valves.
Scope: Limited for application in volatile air pollutants and hazardous fluids. • Use to detect external leakage of Valve Stems and Body joint(s). • Valve selection from standard production at random. b) Type Test Conditions: • The valve has been tested and accepted as per ISO 5208 or any applicable standard prior to FET. • Application of any protective coating is not permitted prior to test. • Valve shall be clean & dry & if required, stem packing may be changed before test. c) Test Fluid & Parameters: • Test fluid = Helium gas (97% minimum purity by volume). • Test pressure = Rated pressure at test temperature w.r.t. given shell material in relevant standard.
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D – Guidelines for achieving Tightness Class •
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Tightness Class A typically with bellow seals or equivalent stem (shaft) sealing system for quarter turn valves (measured only with helium using vacuum method). Tightness Class B with PTFE based packings or elastomeric Seals. Tightness Class C with Flexible Graphite based packings.
FET challenges as per ISO 15848
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Test temperature = Mechanical cycling at both room temperature and at selected test temperature. d) Valve Performance Criteria: The valve performance criterion is based on combination of following: A) Tightness class, B) Endurance class, and C) Temperature class.
A – Tightness Class It defines allowable leakage. However, leakage values are calculated considering Helium as test fluid. Above Tightness Class is defined for Only Valve Stem sealing system, and allowable leakage from body seals: ≤ 50 ppmv. Three types of tightness class are: • Tightness Class A: Allowable leakage rate = 1.76 x 10^-8 atm. cc/sec/mm stem dia. • Tightness Class B: Allowable leakage rate = 1.76 x 10^-6 atm. cc/sec/mm stem dia. • Tightness Class C: Allowable leakage rate = 1.76 x 10^-4 atm. cc/sec/mm stem dia.
•
thermal cycle i.e. 1500 no. of total mechanical cycles having three thermal cycles. C03: Addition to CO2 of 1000 Mechanical cycles with one thermal cycle i.e. 2500 no. of total mechanical cycles having four thermal cycles.
C – Temperature Class Primarily five classes are defined as below (alternative temperature classes shall be subject to agreement between purchaser & manufacturer):
During FET, the valve needs to pass through required mechanical cycles under different temperature range depending on application. Temperature range can be anywhere between (-) 196 deg. C to 400 deg. C. For applications where temperature is between (-) 196 deg. C to 200 deg. C, FET with stringent acceptance criteria is relatively easier to achieve as there are different seal materials available in Elastomers and Plastomers which can be used as primary seal(s) in addition to graphite seal(s). FET becomes more critical for high temperature service applications (temp > 200 deg. C) because of the limited choices available for sealing materials like graphite and/or special metallic seals. Also in many cases, because of various factors governing a specific
(t-196ºC)
(t-46ºC)
(tRT)
(t200ºC)
(t400ºC)
-196ºC
-46ºC
Room temperature, ºC
200ºC
400ºC
B – Endurance Class It is based on Number of Mechanical Cycles (Close-Open-Close). The Three types of endurance class for Isolation Valves are as follows: • C01: Minimum required 500 Mechanical cycles with two thermal cycles. • C02: Addition to CO1 of 1000 Mechanical cycles with one
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E M I S S I O N C O N T RO L project, Fugitive emission testing norms are more stringent than ISO 15848 specifications, like achieving tightness class A at high temp (e.g. 400 deg C) using flexible graphite based packings which is a real challenge. The performance criteria selected of the metal seated trunnion mounted ball valve for the prototype testing as per ISO 15848-1 were Tightness Class = AH, where H indicates test fluid as Helium, Endurance Class = CO2, and Temperature Class = t (-) 29 deg. C to +400 deg. C The valve that was selected for test was Virgo metal seated trunnion mounted ball valve, with 12” full bore and class # 300 including a flexible graphite packing ring set at stem seal. The testing conditions were: • Test Pressure : Rated Pressure as per ASME B16.34-2009 • Test Medium : Helium (97% minimum purity) • Operating Equipment: Actuator. • Stroke: Quarter Turn. • Test Temperature: Ambient to +400°C • Test Method: Vacuum method • Test Equipment: Helium Leak Detector • Equipment Make: Varian Make or equivalent • Sensitivity of leak detector: 2.0 x 10 -12 atm. cc/sec for Helium • Standard leak used: 2.0 x 10-08 atm. cc/sec Acceptance Criteria for tightness Class A were for the stem seal 1.76 x 10-8 atm.cc/sec per mm of Stem diameter and with Stem Diameter 73 mm. Allowable leakage through stem is: 13.0 x 10-7 atm.cc/sec. Given the stringent performance criteria to be met, following parameters were considered while designing the valve prototype:
Packing Set Design
Design features at Stem sealing area
Emissions can happen through all static seals & dynamic seals of the valve. But, majority of valves leak through stem i.e. dynamic seal since it has got a rotatory or linear motion to operate valve under pressure & temperature. Hence it is essential to have a very robust stem packing design. • Graphite being flexible and soft, various shapes and hardness of rings can be achieved. • V-shaped ring gives more sealing effect and adjustments than standard rectangular section.
• • • • •
Two bearings, ensures alignment of stem with the rotation axis Appropriate Clearances in Stuffing Box Appropriate Surface Finish Bolting for 5500psi (38MPa) Compressive Packing Stress Live loading to maintain packing load at high temperature and through mechanical cycles.
FET observations, results & conclusion Conducted FET using Vacuum technique, i.e. a vacuum chamber is constructed above stem sealing area & connected to HMS (Helium Mass Spectrometer). Observed maximum leakage of 8.1 x 10-7 atm.cc/sec. as against allowable value of 13.0 x 10-7 atm.cc/sec. Hence the Virgo metal seated trunnion mounted ball valve with flexible graphite packing is qualified for: • Tightness Class: AH • Endurance Class: CO2 (1500 number of Mechanical Cycles ) • Temperature Class: t(-)29 deg.C to +400 deg.C
Graphite material properties • • • • • • • •
Suitable for temperature from (-) 250 deg. C to @ +550 deg. C. Impermeable to gas & liquid No ageing Better long term stability of compressibility and recovery Lower coefficient of friction Sustains high thermal shocks Absorbs high residual stresses Good duration of life
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About the author Mr. Narayan Patil is currently heading the Design function for Virgo Valves & Controls Ltd. With a Bachelor’s degree in Mechanical Engineering, he has over 25 years of experience in the engineering industry, both in manufacturing and design. With thorough knowledge of national / international standards and codes he has developed a wide variety of products for the Oil & Gas Industry which include Ball valves, Check valves, Gate Valves, Triple Offset valves and Wellhead products.
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