Fugitive emissions testing, Standard and valve stem packing

Fugitive emissions can’t be tolerated by regulating agencies now a day. In general, it is observed that fugitive emissions are found in valves, especially in their stem portion. In the case of the Quater turn valve, Stem movement is much less so it provides respectively much protection against fugitive emission.

Many other types of piping components especially with flanged type connections are also contributing to fugitive emission. Because of this unwanted emission, our environment is getting affected. So, reducing fugitive emissions becomes a leading concern of all process industries.

According to an analysis on Valve are responsible for more than 65% of total fugitive emission. As a result, valves have been considered as the area of focus in ways to reduce this leakage. In this article “FUGITIVE EMISSION” you will get familiar with the following things:

1. What is fugitive emission and ways to reduce it.
2. Harmful effects fo fugutive emission and thermal leak risks.
3. Different testing standards for this emission/leakage and Comparison among different standards.
4. Ways to control fugitive emission.
5. Testing measurement methods

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What is Fugitive emission?

Fugitive emissions are a kind of leakage and other irregulars release of gas or vapor from a pressurized containment for eg: Storage tanks, vessels, pipes, wells, valves, etc. These emissions are actually accidental emissions because of pressure either because of faulty components or overlook mishappening.

This is not only limited to industrial settings but it also happens because emissions take place in factories, power plants, waste-water treatment, and such like plants. This emission is very harmful in a way to lose for the respective industry as well as environment.

Fugitive emission terms for all standards

Some common terms that everyone should need to know to understand fugitive emission are:

1. Parts per million volume (ppmv): Gases found in the atmosphere is a kind of mixture. To measure amount of some specific gas in a mixture ppmv is used. For e.g. 1 ppmv= 1 ml/1000ml.
2. Mechanical Cycle: This is defined differently for Isolation and control valves. In case of Isolation valve, A full stroke from fully open to fully close to fully open is called a mechnical stroke. While In case of control valves, 50% of valve travel and 10% of full stroke is know as mechanical cycle.
3. Thermal Cycle: Application of temperature on valve from room temperature to Test temperature to room temperature is considered as thermal cycle.
4. Type Testing: This is a new protocal developed from the fugitive emission standards. In this valves are tested at operating pressure and temperature specified by the manufacturer.
5. Test Gas: This is the gas with which fugitive emission testing is conducted to measure leakage rate. Helium and Methan are mostly used test gas.
6. Part per million (ppm): This is the method of measurement of concentration of very dilute concentration of substance in the mixture. 1 ppm= 1 part of the specific substance in 1M part of mixture.
7. Ambient temperature: This is the temperature of air around any object or environment where the object is store.
8. Room temperature: This is defined as the temperature in which most of the people feel comfertable in a room. In general room temperature is near to 20°C-22°C.

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Sealing types of Valve steam

In the process of reducing or stopping FE from valves, the type of valve stem sealing material is much important. each type of sealing material has its own advantages and capacity of holding pressure. There are normally 05 types of sealing we use for valves:

1. Die-formed flexible graphite : Die-formed Flexible graphite with minimum 95% carbon content is die-formed in the rings with braided carbon or graphite yarn end ring. These die-formed rings are used for higher service temperature from 850ºF to 1200ºF and pressure upto 4000 psi. It is capable for fugitive emission near about 500 ppm.
   Characterstics:
   1. This sealing type provides enough emission performance for a long time.
   2. These may not attain the low leak rates demanded by the most stringent air-quality-management districts, consent decrees and standard performance specified by plant end users
   3. Rings are made for a specific valve stem and box sizes, and may require adjustment to obtain and maintain low emission results.
   4. Multiple-step installation required
2. Braided flexible graphite: Unlike Die-formed flexiblw graphite, braided flexible graphite sealing has more than 95% carbon content. This is also wire reinforced with flexible graphite yard. Temperature capacity of this sealing is also ranging from 850ºF to 1200ºF with pressure handling capacity of 4500 psi. This sealing type of valve is capable of less than 500 ppm fugitive emission.
   Characterstics:
   1. With wire reinforcement and high carbon purity this give superior emission performance.
   2. One size of sealing component can be used to pack different size of valves with some adjustment.
   3. Multiple step installation and easy field replacement.
   4. Low emission result.
3. Engineered sets: This type of sealing is engineered with a combination of die-formed and braided graphite rings. It can handle same temperature range but its pressure handling capacity is much higher i.e. 10000 psi. In sealing fugutive emission is very low ranging from 100-500 ppm.
   Characterstics:
   1. Most effective emission performance
   2. Each sealing set is specially made for specific type of valve steam and box size.
   3. Its easy to install in comparison to other set.
4. Bellow sealed valves: In above 03 types of sealing set carbon ring were used with 95% or more purity. while in Bellow sealed valves, a mettalic bellow is used instead in valve design. And some packing type seals are also used as secondary seals. Temperature and Pressure holding capacity depends on metallurgy ,design, and construction to match with requirements as per flange pressure class of valve.
   Characterstics:
   1. Almost zero emission
   2. Comparatively costly than that of a standard packed valve.
   3. In case of any sealing failure, there is no possiblity for adjustment.
   4. In few cases, an elongate bonnet is require to accomodate the bellow. Here space becomes an issue.
5. Live loading valves: In this type of sealing disc spring washers are compressed on the gland follower under the gland stud nuts. Here temperature and pressure capablity depends on the type of seal used in the valve. A live loding arrangement doesn’t enhances emission holding capablities with increase in pressure rating.
   Characterstics:
   1. This can be used with any type of packing in the valve.
   2. effectively increases the energy in the stud bolts of gland.
   3. Live loading will cause added expenses.
   4. Tis provides some performance enhancement for valve in numerous thermal cycles.
   5. Preffered solution for valve that are difficult to monitor and access.

Why Reducing Emission is Required?

Because of the harmful effects of fugitive emission controlling fugitive emission in valves is a must-have agenda for oil and gas industries to save cost and environment. There are many reasons which fugitive emissions got special attention to minimize and eliminate them. Some of the reasons are as described:

1. Fugitive emission contribute to air pollution as harmful gases escapes via this.
2. This create other risk and hazards like:
   • Emission of volatile organic compounds from oil refineries and chemical plants create risk for local communities and workers.
   • In cases, where large amount of flammable liquids and gases are contained under pressure, it create a chance of explosion.
3. Valve is major component of piping system which is used for every industry can create toxic working environment if fugitive emission is not controlled.
4. Long duration emission starts leakage along the shaft or stem. This stops the gas or liquid being controlled from leaking.
5. This increases economic costs related to lost and damaged materials.

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Thermal leak Risk mitigation

The thermal leak happens after the thermal cycle in a chemical process. This can create a leak path at the body in the critical service condition with extreme pressure and temperature fluctuation.

Not all valve manufacturers present their thermal cycle test data. So, it is important to ask whether the vendor has tested and gathered this data or not. To eliminate the suspective leak risk with the thermal cycle following things should be considered during valve selection:

1. PTFE type body gasket is good choice for stable temperature but a thermal cycle will cause the PTFE to cold flow and reduce compression and results as leak.
2. Graphite type body gasket works well at most temperatures. This can handle many thermal cycle but usually required to achieve fire safe rating on valve.
3. Dual material body gasket combines the benefit of PTFE and gaphite both to provide an inert seal that can protect against fugitive emissions and also ensure fire-safe operation.
4. Spiral wound body gasket is industry proven design provide structural support and live loading via metal spring v-shaped rings. Most of the spiral wound gasket use either PTFE or graphite seal material.
   A dual material gasket in the spiral wound configuration is chemically resistive and and able to recover itself after thermal cycle and operate uncompromised in all temperature range.
5. Self-relieving seats are designed to relieve excess pressure builtup in tha ball valve because trapped fluid in its cavity, Under specific condition that trapped fluid in fully closed valve expand and result in high pressure and create potential leakage.

Fugitive Emission Testing Standards

Fugitive emission testing is done as per mainly 02 standards:

1. International Standard organization: In this type of testing standard, testing procedure is described in 02 parts.
   1. ISO 15848-PART 1: In this part of ISO classification system, qualification procedure is for fusitive emission test of valves for both rotary and rising steam.
   2. ISO 15848-PART 2: In this part testing procedure defines acceptance criteria for fugitive emission.
2. American Petrolium standard: There are 03 API valve and stem sealing emission standard that followed today.
   1. API 622: This section of API defines process valve packing for fugitive emission.
   2. API 624: All rising stem valve fitted with graphite packing are tested according to this procedure.
   3. API 641: Any type of quater turn valve is tested according to this API standard for fugitive emission test.

All of these procedures are described in a detailed way in the following sections.

ISO 15848 Part-1 Testing Procedure

In this testing procedure, helium or methane is used as a standard testing fluid. After fugitive emission test valve is accepted only when the valve qualifies for tightness, endurance, and temperature class.

Valve Testing Methods

Here, testing is done with a completely assembled valve using different testing techniques like:

1. Vacuum testing
2. Flushing Testing
3. Bagging testing and
4. Sniffing method

Vacuum Testing Method for Fugitive Emission

This method is used to measure the total leak rate from the stem seal. Helium is used as a testing medium and checked with a spectrometer. Stem packing is enclosed in a tight vacuum chamber, which is later evacuated then connected to a helium mass spectrometer.

Flushing Method for fugitive emission

It is used to measure the total leak rate from stem sealing when pressurized with helium or methane. The leak stem packing is closed in a flush chamber. Here, specified gas passes through that chamber then it is mixed with leak fluid from the valve stem.

This mixture is then passed through an exhaust piping network connected with a quick Exhaust valve for eg. Humphrey sqe2 valve. The concentration of leaked fluid in the mixture is then calculated in Part per million volume (ppmv).

Bagging Method for Fugitive Emission

Here helium is used as a testing medium. The leak source is then enclosed in a bag and air is passed through that bag. Then it is mixed with leaking fluid from stem packing. The amount of leak fluid is determined with the help of a spectrometer through a constant flow rate detector probe, also known as a sniffer.

Sniffing Method for Fugitive Emission

In this method of testing helium leak detector is used. Here detector is fitted with a sniffer probe to measure helium concentration in a mixture of helium and leakage fluid.

In case, when methane is used as a testing fluid, a volatile organic compound detector is used.

Test Pressure and Temperature

Generally testing pressure for ISO 15848 Part-1 is rated pressure corresponding to test temperature. For eg. At room temperature Valve is tested on 20 bars for 150# class.

Valves are normally rated at room temperature, but in some special cases, the valve needs to be tested at a specified temperature. When valve testing is done other than room temperature below table is used to choose the qualified testing:

From the above table, some important things to note are:

1. If the testing temperature is other than the rated temperature then a lower temperature will preferred for testing. For eg. if the testing temperature is 410°C then rated class selected will be 400°C. And if test temp. is 210°C then the selected class will be 200°C.
2. If some valve has a broad operating range like -196°C to 200°C, this will be tested in the 02 stage. i.e. First on -46°C and second on 200°C.

ISO 15848 Part-2 Testing Procedure

This standard is mainly applicable for the production acceptance of valves. For approving this standard valve should be type tested with ISO 15848 Part-1 testing procedure.

Valve Testing method

In ISO 15848 Part-2 standard helium is prioritized used with a minimum purity of 97% by volume for testing. Testing is done on completely assembled valves using the sniffing method.

And leak measurement is determined in accordance with ISO 15848 part-1 which is further expressed in ppmv. ( 1 ppmv = 1 ml/m³)

Steps for measuring valve fugitive emission

1. First, open half valve and pressurize it upto 06 bar and measure seal leakage using sniffing method.
2. Than, fully open the valve and pressurize it upto 06 bar and measure steam seal leakage using same, sniffing method.
3. Half open the test valve after mechnical cycle and measure stem seal using the sniffing method.
4. If sniffing probe reading exceeds the rated ppmv value as in below table than valve is certified as “NOT OK” from fugitive emission testing.

Test Pressure and Temperature

In the time part of ISO, testing pressure is the same for all valves i.e. 06 bar and testing temperature varies as defined in part-1 ISO standard.

API 624 fugitive emission testing standard

This standard of testing is applicable for valves with rising stems equipped with flexible graphite packing.

Scope of API 624 Testing: This API standard provide information about requirements and acceptance criteria for fugitive emission testing. Valve with rising or rising-rotating stem equipped with suitable packing for service temperature -29°C to 538 °C.

This applies to those valves that are already type-tested with API 622 standard. An API 598 certificate is also provided with the valve.

Testing and Leakage measurement: In this standard minimum of 97% pure methane is used as the test medium. Testing of the completely assembled valve is done using the sniffing method.

Leaking fluid concentration is measured with the sniffing method and expressed in parts per million volume.

Testing pressure and Temperature: The test pressure in this standard is as per B16.34 at 260°C or 42 bar whichever is low. And test temperature should be 260°C.

Acceptance Criteria for API 624: Fugitive emission testing is marked as passed when the measured leakage does not exceed 100 ppmv.

API 641 fugitive emission testing Standard

This standard is applicable for type testing of quarter-turn valves only.

Scope of API 641 testing: This standard gives the requirement and acceptance criteria for fugitive emission testing types and protocols. A quarter-turn valve equipped with preferred packing material is first tested according to API 622 standard.

Valve with a pressure rating at an ambient temperature less than 6.89 bar is out of the scope of this standard. Also, valves with the size of 24″ above and pressure rating above 1500# are out of scope from this standard.

Testing and Leakage measurement: Same as API 622, leakage to be checked with sniffing detection prob and expressed in ppmv.

Testing Pressure and Leakage measurement: Testing pressure and temperature under API 641 scope is as per the below table:

Note: Any conditions apart from the above mentioned in the table are out of the scope of API 641 standard criteria.

Acceptance Criteria for API 641: The fugitive emission test report in the test is certified as “pass” when the measured leakage does not exceed 100 ppmv.

API 622 fugitive emission standard

This standard is specifically developed to resolve the fugitive emission of valves in the petroleum and chemical industries.

Scope of API 622 testing: In the first edition of API 622 fugitive emission of valve packing test is in scope but it also allows valve company or packing manufacturer to test valve. In 2011, this standard is revised and In the new edition, a test facility is required to use the test devices.

Testing and Leakage measurement: The test requires five thermal cycles from ambient temperature to 260°C with 1510 mechanical cycles using a methane gas test medium. Each set is divided into 300 cycles per day and the final 10 cycles are used to measure the fugitive leakage.

Testing Pressure and Leakage measurement: Test is conducted as per the requirement with pressure in the range of 0 to 600 psi. ANd temperature at which a fugitive emission test is done is either ambient temperature or 260°C as applicable.

Acceptance Criteria for API 622: The leakage limit will fall in acceptance limit if fugitive emission leak is below 500 ppm with only one adjustment.

Overall comparison among fugitive emission testing standards

Performance class factor for Fugitive emission

Performance class for any valve is a combination of 03 standard criteria:

Tightness Class: This measured with leak test using helium or methane gas as test fluid and give a standard for maximum allowable leakage at each rating of valve.

The above table is for the Tightness class and valve stem leakage rate with Helium and Methan as the test gas. Leakage from body seal with both test gases should be less than 50 ppmv

Endurance Class: The number of cycles that a valve has to complete with successful operation without any leakage with minimum tightness class is considered as endurance class.

Temperature Class: This can be defined as the maximum operating temperature of the valve a which valve operates with no leakage. Criteria for this class are as per Table-03 and 04.

Conclusion

All of the standards described in the above section like ISO 15848 (Part-1 & 2), API 622, 624, 641 are used to evaluate the leak handling capability of the valve and its stem packing material.

Each standard has a special approach towards detecting fugitive emissions according to the requirements of the end-user but these different standards create confusion to choose the best possible approach to test valves for fugitive emission.

Today each industry wants to harmonize and hope for a single and best standard to evolve as a universal standard for testing fugitive emissions.