Why is methane used rather than helium test gas?
Already this question is being asked and, I must admit, so it should.
As we all know helium is a more user friendly test gas when compared with methane. Valve manufacturers and test houses prefer helium as testing is cost effective and can be undertaken locally. There are of course far more helium test houses than methane which indicates the difficulty in setting up methane test houses. The diffi culty is compliance with health and safety requirements. Methane testing relative to helium is more costly. As a result of the release of API 624 existing test houses have been gearing up for the increase in number and size of test valves. New test houses are being discussed and appearing. ISO 15848 Parts 1 & 2 are primarily based upon helium though an appendix is being added to permit methane. All testing to this standard currently is with helium. Some major end users like Shell have adapted this standard in their MESC standards and it has therefore had a very strong following among valve manufacturers. German end users likewise use
helium with their own local standards.
Historically the majority of end users such as ExxonMobil, BP and ChevronTexaco tend to prefer methane as it’s more representative of VOC products at the operation plants. Over the years these end users have embarked on their own test protocols using methane which in themselves have been used to form the basis of API and other American standards. As mentioned in September’s article the major driving force for methane in the USA is the Environmental Protection Agency, EPA, as they appear to only accept valve leakage measurements in parts per million, ppm using methane. The mass flow measurements with helium are not legal tender or accepted. So, end users in the USA follow methane testing for compliance or face fi nes etc.
So 2 international standards are established and their use varies around the world one using ppm with the other mass flow.
Is there methane and helium correlation?
Many, many attempts and studies have tried to correlate methane and helium but this has proven to be unsuccessful. It would be great to have such a correlation then true harmony could be achieved in the market. However the difference in methane and helium molecule size, variance in Reynolds numbers, issues with repeatable results with varied pressure and temperature has prevented world experts in achieving a correlation that is universally accepted. Simple correlations at ambient temperature are claimed up to 500 ppm by some and are indeed useful as a guide but for the precision laboratories
and general valve community they are rejected.
It would be informative at a future conference if an expert could share reasons / test results why such a correlation can not be achieved.
The helium supporters explain that helium is more sensitive than methane and therefore gives better results. The methane supporters claim that the methane sensitivity gives adequate fugitive emission reductions on operation plants as demonstrated via various Leak Detection and Repairs (LDAR’s).
Why is the methane test pressure of 41.4 barg used for class 300 to 1500?
API 624 limits the methane test pressure to class 150 rating and 41.4 barg for class 300 to 1500. “Wow, surely this test pressure is too low for the high classes?”
- The decision to use the above was based upon proven testing experience with methane, to minimize health and safety risks at the test houses and to refl ect API 624 application on applicable plants.
- The majority of methane testing has been on 4” class 300 and ¾” class 800 valves and a very few 12” class 300 and to stretch the test envelope to the class pressure rating for high classes than 300 / 800 and to larger valves was considered unadvisable.
- Also the scope of API 624 should be noted as it is effectively written for refi neries and not upstream or landline applications.
- Interrogation of valve usage on refi neries indicates that the majority of valve classes are 150 & 300 with only a few classes 600 & 900 and very few classes 1500. Above class 1500 are normally on utility duty and are not relevant for methane testing.
- Current site measurements in the USA do not reveal that classes 600 to 1500 are major leakers.
So a “safe balanced view was taken for API 624”or should I say “common sense reigned”. Remember the valves have already been successfully tested hydrotested to API 598.
The 41.4 barg has again been challenged in the community as insuffi cient, meaningless etc. If some one is really concerned about fugitive emission performance the option to test with helium at the pressure rating to ISO 15848 still remains.
Why is the maximum allowable leakage 100 ppm methane?
Historically in the USA the fi rst LDAR initiatives targeted valves leaking greater than 10,000 ppm and then 500 ppm leakage. Recently 100 ppm is being becoming required and is anticipated that future requirements within the USA will refl ect this level. Sealing solutions have advanced such that they are cost effective and are available in the market to achieve this level. Test results in the last 5 years have clearly shown this with several valve manufacturers completely changing their production to engineered fugitive emission packings.
The challenge to the valve manufacturers is to clean up their production processes to achieve repeatable and acceptable stuffing box, stem tolerances and fi nishes. Perhaps I will address this in another article.
Why is there no retightening of the gland bolts allowed?
API 624 establishes a sealing system in a production valve to leak no more than 100 ppm without retightening which offers end users confi dence of success in the plant that leakage will not occur.
As stated before sealing solutions have advanced and can meet this requirement. Success is linked with the tolerances and finishes of the valve components and assembly.
I apologize if readers are familiar with the above but I hope that the explanations shared have assisted in understanding the background of API 624.
The differences between methane and helium testing requirements will no doubt still promote much discussion.
The challenge to end users is:
- They specify an international fugitive emission standard which is mandatory,
- It’s enforced globally and,
- Operation plants are checked to have actually reduced emission levels.
Many thanks for your time. I look forward to any comments or different views. They are always most welcome.
- Valve World 2014 in Dusseldorf has already included API 624 into their program with standards and test result discussions.
Discussions should be most interesting.
- The series on Understanding end user valve approvals will continue in upcoming magazine issues.