Fugitive emissions efficiency
Emission reduction has been a global megatrend for quite a long time. Emissions are being monitored for just about any product available on the market today, whereas regulations of emission measurements continue to develop under the pressure from society.
- Global valve suppliers are facing the challenge to design their valves to be compliant with global emission standards.
- Designing the valves certified according to ISO 15848, the most comprehensive fugitive emission standard today, ensures that they are also compliant with many other emission standards and fulfill the most stringent global requirements.
- ISO 15848 certification verifies a valve's high fugitive emission performance, consequently improving HSE issues and saving a considerable amount of capital.
Originating from the process industry, emission reduction has been a global megatrend for quite a long time. Emissions are being monitored for just about any product available in the market today. Whether one is buying an everyday consumer product, driving one's car or even conducting an internet search, one can find studies about the environmental effects this has and much, much more. This trend underlines why the regulation of emission measurements has been forced to develop, and continues to develop, under the pressure from society. In fact in many areas globally, the measurement of emissions has been increasing at the same pace as the growing awareness of environmental impact. When it comes to valves, this emission reduction trend is one of the biggest reasons why today many different standards exist to evaluate the emission level. Still, the basis of the standards lies in the valves themselves. Studies1 show that on average the most problematic valves are old valves, designed without the modern knowledge of emissions reduction possibilities. Approximately 2% of the most problematic valves account for 98% of related emissions. Therefore, it makes sense to invest some thought into the emission issue: what is the easiest way to prevent these unnecessary emissions and, at the same time, the costs that lie behind fugitive emissions?
Understanding the differences in emission standards
As the major standards have been developed from somewhat different perspectives, considerable expertise is required to understand the differences in the fugitive emission (FE) standards. Moreover, applying this knowledge to actual valve designs is even more demanding, as the actual process requirements for which the valves originally are intended should be combined with good design practices and likewise should conform to these regulatory standards.
Among the most commonly used global fugitive emission standards are the following:
• EPA 40 Parts 60/63 (EPA Method 21).
• TA-Luft (VDI 2440).
• ISO 15848.
• End user specification from Shell Oil Company (MESC SPE 77/312).
Foremost, it is good to notice that direct comparison of these standards is challenging, as they all rely on their own testing procedures. The most important variables are the test fluids, detection methods for leakage, and leakage limits.
The test fluid basically may be either helium or methane. Helium is a very permeable and safe gas to use, whereas methane has neither of these qualities. Due to these differences and precautions when using methane, leak test results are not strictly comparable.
When it comes to leakage detection, two basic types of methods for detecting emissions are used. Local method, also called sniffing, measures a concentration and is typically associated with methane based tests. As sniffing can only be used as an estimate of the actual leakage, a correlation factor is applied to the measured leakage to estimate actual leakage from a valve stem. Global methods, the second possibility for leakage detection, include basically two options: testing in vacuum and bagging (most likely being included in the next edition of ISO 15848). These methods capture and accurately measure the actual leakage from the atmosphere surrounding the valve. To summarise the differences, local and global leakage detection tests are not directly comparable.
Further differences exist in the procedures among the standards. These include different testing temperatures, temperature cycling and the number of operational cycles before the test data is collected. Moreover, some standards allow for more interpretation of the test results, and some less. To top it all, the emission limits set out by the different standards also vary and use differing units. Knowing this, it can be tricky to find the best standard for fugitive emissions. To facilitate this decision, let us take a more detailed look at understanding the intentions behind the standards and the actual benefits of using a particular standard to evaluate FE performance.
ISO emission standard stands for its versatility
EPA 40 Parts 60/63 or EPA Method 21, used mainly in North America, sets only 100/500 ppm leakage criteria, which must be met during the actual operating conditions, throughout the useful valve service life. The flow media used in leakage detection is a hydrocarbon directly from the pipeline, with the detection method being sniffing. This is a very practical and intelligent approach, since the individual process plants are thus tied directly to the legislation. Another advantage is that the FE levels are measured throughout the life cycle of the valves. The downside, however, lies in the flow medium. As different hydrocarbons have different leakage behaviours, depending on if the form of the flow medium is gas or liquid, the results differentiate a lot. In practice, it is thereby difficult to compare the fugitive emission performance of different valves. As a final comment, these leakage limits are not very strict and are easier to fulfill. TA-Luft, applied more globally outside North America and originating from Germany, is a standard that requires the valve designs to comply with certain requirements. Fundamentally, TA-Luft sets the maximum leakage rate to be equal to that of a rising stem bellows sealed valve. The flow media for leakage detection is helium, and only global leakage detection methods are allowed. Two different FE leakage limits are set based on the temperature of the application. As seen in Figure 1, the leakage limit under 250 °C can be regarded asstrict, whereas the limit above 250 °C is less demanding.
Figure 1. Comparison of emission standards, ø 25 mm stem.
At the core of TA-Luft, it is revealing that no exact number of operating cycles is defined for the test. An additional drawback is the fact there is no predefined temperature cycle procedure. This is definitely a disadvantage when evaluating the FE performance under actual service conditions.
ISO 15848, the newest standard and one aimed at meeting the global need of a single emission standard, does not regulate the valve design, but rather defines the type approval tests with which valve designs must comply. Consisting of separate parts for prototype and production testing, the flow media for leakage detection is typically helium, although methane can also be used. The current edition of ISO 15848 Part 1, has a three stage categorisation for leakage, which ranges from an extremely stringent, Class A, to a non-strict, Class C. According to the ISO standard, the global leakage detection method in vacuum is the only method allowed. Testing temperature classes range from -196 °C up to 400 °C.
Today, ISO Class A is the most demanding leakage criteria in the field of all FE standards. As with TA-Luft, this level, too, emulates the rising stem bellows seal packing. Quite interestingly, quarter turn valves can also reach ISO Class A with graphite packing. With shut off ISO testing at low temperature, Neles® D1F ball valves have this rare certification with graphite packing. Definitely noteworthy, this means the emission performance of current graphite packing actually can even exceed the polytetrafluorethylene (PTFE) based, chevron V shaped packing that traditionally has been the fugitive emission solver.
ISO Class B leakage can be regarded as a standard quarter turn valve performance level. This level can be reached with graphite packing also in long duration control valve tests, including high temperature (400 °C) testing.
ISO Class C leakage is the standard emission level for rising stem valves.
The ISO emission standard also has the static valve body joints in its scope. There, the sniffing method is applied and includes three endurance classes for both shutoff and control valve types. Temperature classification is the same for stem packing, ranging from -196 °C up to 400 °C. Additionally, it includes a compulsory temperature cycle procedure.
Accelerating the acceptance of ISO 15848 certification
The biggest challenge for ISO certification adoption so far has been in the wide variety of standards in testing methods and acceptance limits. The ISO standard chair committee is currently revising ISO 15848 to include new features, enabling simultaneous acceptance of other local standards. The new edition release most likely in 2015 should include the EPA qualification, which could boost the acceptance of ISO 15848, also in North America, and make the standard a globally accepted one. Being already the most comprehensive tool for measuring and realising FE performance objectives, this will be an even stronger incentive for global valve suppliers like Metso to actively pursue a wide range of theoretical and technical knowledge along with the testing and reporting capabilities needed to fulfill current ISO 15848 standards classifications.
Metso conclusions over the ISO 15848 standard
Metso has made the following conclusions regarding the current ISO emission standard:
• The mandatory heat cycling program effectively simulates the heat cycling of a real process. It is well known that a weak valve design may lose its emission performance after a single heat cycle, setting high demands on the valve design. Thus, the severe heat cycling sequence included in ISO 15848, combined with uncompromised emission limit criteria (at Metso, Class A or B) and secured with an accurate measuring method, is a strong indication of the low FE levels a valve may experience during its service life. This extreme is missing, for example, in TA-Luft, as there is no defined number of operational cycles the valve must complete. With regard to control valves, for example, there is a definitive difference if the valve is being cycled to 50 000 cycles instead of 500. In TA-Luft, both of these are acceptable to get certification.
• Third party certifications. Highly regarded third party certifications are frequently obtained to verify testing conditions and the actual FE performance of valves at Metso. In this way, end users can be sure that they receive high fugitive emission performance with their valves.
• Expanding the number of valve designs tested. In 2012, a corporate initiative was put in place at Metso to increase the number of valves certified according to the ISO 15848 standard. Continuing in 2013, numerous Neles® metal seated and Jamesbury® soft seated valve products were certified to meet the requirements of the ISO emission standard, providing simultaneous compliance with many other emission standards mandated in specific regions, including TA-Luft and EPA Parts 60/63.
Customer benefits of fugitive emission reduction
Besides the obvious compliance with the ever tightening environmental regulations, also other benefits promote using valves with a low fugitive emission design. First, emission certified valves improve safety and are therefore an health, safety and environment (HSE) issue as well. In the worst case, for example, a hydrogen leak from valve packing can auto ignite and cause barely detectable flames, inflicting an enormous health, safety and environmental issue at a process plant. Again, employing valves with properly designed emission control can minimise these issues. In addition, a healthier environment protects people and potentially also makes them more productive. For example, reducing H2S content in the air allows staff to work for longer periods in areas with high H2S content in the atmosphere.
"By employing valves that are compliant with ISO 15848 valve users can introduce significant operating cost savings while benefiting from the improvement of health and safety issues as well as reducing the environmental impact."
Second, a low fugitive emission design minimises the costs occurring when a product is lost via leaking valves. Even if the cost for the leaked medium could be measured in hundreds of euros per year, the lost flow medium results in secondary costs, such as smaller end product yields. The loss of flow medium is also lost energy, since the pumps or compressors need to operate more to compensate for the leakage. Consequently, minimising fugitive emissions using appropriate valve designs and proper maintenance can save a considerable amount of capital.
To conclude, by employing valves compliant with ISO 15848, the most comprehensive fugitive emission standard today, valve users can introduce significant operating cost savings while benefiting at the same time by improving health and safety, and reducing environmental impact.
Published in April 2014 Hydrocarbon Engineering
Text by Mikko Vuolanto