Learn the distinctions between ISO 15848-1 and API 641
by Phil Mahoney
February 14, 2018

ISO does a similar process of ambient and elevated temperature cycling, but it details two specific endurance classes, each with three stages. For valves intended for on/off service, the class ‘CO’ is used. CO1 is two full thermal cycles, each with 50 stem cycles (fully open to fully closed) at ambient and 50 at elevated temperature; 10 ambient cycles are done at the end of the two full thermal cycles/stages. CO2 is a continuation of CO1 with 790 stem cycles at ambient and 500 stem cycles at elevated temperature (for a total of 1,500 stem cycles). CO3 extends from CO2 with 500 stem strokes at ambient and then 500 at elevated temperature for a total of 2,500 cycles.

For control valves, the three levels are CC1, CC2 and CC3. CC1 is 10,000 cycles at ambient followed by an equal amount at elevated temperature, CC2 continues with 20,000 more cycles at ambient and 20,000 at elevated temperature and CC3 repeats CC2 once more. In CC testing, all stem cycles are plus-10 percent of their stroke or angle from the midpoint of the valve stroke or angle. Speed of the stem is also dictated to be 1 to 5 degrees per second. One packing gland adjustment is allowed in each endurance class level (i.e. one in CO or CC1, an additional adjustment in CO2 or CC2, etc.). The ISO standard contains multiple leakage classes as well. If we consider only methane for the purposes of comparison to API 641, ISO specifies leakage classes- AM (≤50ppm), BM (≤100ppm) and CM (≤500ppm).

Valve measurement location diagramImage 1. Valve measurement location diagram (Image courtesy of the author)

Measurement Point

While on the subject of thermal cycles, another key consideration between the two standards is the measurement point on the valve from which the temperature for the test is monitored. (See Image 1 and note: valve shown is not a rotary valve and is used for illustration purposes only as found in ISO 15848-1, 2nd edition).

API 641 specifies the temperature is measured in two places: within ½ inch (12.7 mm) of the stem seal (3) and externally on the valve body adjacent to the flow path (2). The test temperature must be controlled within plus-5 percent at both points. API 641 specifies the temperature is measured in two places: within ½” (12.7mm) of the stem seal (3) and externally on the valve body adjacent to the flow path (2). Both measurements have to be controlled within plus-5 percent of the specified test temperature. ISO specifies three measurement points: the flow path inside the test valve (1), an external point adjacent to the flow path (2) and an external point adjacent to the stem/shaft seal (3). The flow path temperature defines the test temperature (and is controlled within plus-5 percent of the target temperature); the other two locations (2 and 3) are taken for informational purposes.

This is a significant point as valve testing has shown that the fluid process temperature can be significantly higher than the temperature at the stuffing box where the seal is located. This, of course, will vary depending on the valve design. Where API 641 specifies that the stuffing box temperature defines the test, the actual temperature of the fluid (i.e. test gas) can be significantly higher than the test temperature. Conversely, with ISO using the test fluid as the controlling factor, the stuffing box temperature can be much lower than the test temperature. To be able to practically compare ISO 15848-1 and API 641 test data, you would have to compare the API 641 test temperature with T3 temperature measurement in the ISO test.

So, looking closely at the two standards, there are significant differences between the two. Number of temperature and stem cycles differs, one allows adjustments and one does not, and it is difficult to compare data given that the test temperature is defined differently in each standard. Both procedures, however, are considered examples of good engineering practice.

Which Standard Should I Use?

The answer will depend on the valve design standard that end-users are requesting. API 641 first edition was released in October 2016, whereas ISO 15848-1 is in its second edition, released in 2015. Either could be deemed acceptable assuming methane is used as the test gas. However, one has to consider what valve standards currently call out for required type tests, as well as what will be added to valve standards in the future. API has done this for multi-turn valves by adding a requirement to various standards such as API 600, 602, etc. specifying that the valves shall be API 624 compliant (which requires API 622 compliant packing) in order to be tagged as API 600 or API 602 valves.

The current editions of API quarter turn valve standards (API 608, API 609) do not have API 641 certification as a requirement. However, the next edition of API 608 will have this requirement added and API 609 and API 599 are expected to require API 641 testing as well. ISO is not referenced in any of the API standards, nor is there any expectation that it will be in the future. However, given the global environmental concerns, there is a good chance that ISO 15848-1 could be added as a requirement to international valve standards as they are re-published.

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