Valve field failures have directed oil majors to look for viable and consistent quality products from valve vendors. This was then focused mainly on engineering control, and additional non-destructive examination (NDE). Oil majors have regulated this by means of valve technical specifications, and the same was ensured by periodical audits by these companies to the valve vendors, but still, this was not enough, and many of the end users have gone with type testing or valve validations.
By S. Mohamed Ali Jinnah, Engineer – Specialist – Flowserve India Controls Pvt Ltd.
Various specifications covering these aspects have been released. The notable customer-based validations are Shell Type testing (MESC SPE 77/300 – procedure and technical specification for design validation testing of industrial valves, SNAM rete gas based on EN I4141 Annexure A, etc.,).
Growing concern about the performance of the valves necessitates the need for an international standard to govern and control the requirements. Understanding the needs of the international standards have resulted in various validation testing standards and regulations.
- API6A has Annexure F for valve validation for the upstream segment,
- API6D in its 25th edition has included Annexure F for validations,
- API6D in its earlier editions and in the current edition covered quality specifications level (QSL) 2,3 and 4,
- ISO 23632 Industrial Valves-Design validation testing of valves, and,
- EN 14141 Annexure A Type test on general design and production of pipeline valves.
Applicability
- API6A Annexure F – ball, plug, and gate valves
- API6D Annexure F/QSL – axial, ball, check, gate, and plug
- ISO 23632 – butterfly and ball valves
- EN 14141 – plug, ball, gate, and check valves
Validation
The prime focus of validating a valve is evaluating its seating performance, the validation of valve torque, and maximum allowable stem torque (MAST), which is also important to assess valve performance. Validating stem strength is also part of some of the previously mentioned standards.
ISO 23632 instructs to validate the stem by applying MAST on the valve and EN14141 has got Annexure B wherein the stem should withstand two times of valve breakaway torque as declared by the manufacturer.
Additionally, the standards expect that the valve subjected to validation shall be representative of the products that are being shipped to the customer, and not specifically prepared for the validation tests. ISO 23632 demands a declaration of conformity in this regard; the manufacturer shall declare that the valve components and the assembled valve have undergone all the manufacturing, assembly, and hydrostatic shell test steps of the typical quality control plan provided by the manufacturer.
Basis of Design Validations
Pressure, temperature, and number of cycles are the critical factors determining the performance of the valve and the durability of its seat performance. API 6D QSL in addition to the mechanical cycles, calls for the extended duration of the seat and shell tests depending upon QSL levels. It also increases the stringency levels of NDE, including visual testing (VT), magnetic-particle testing (MT), penetrant testing (PT), and radiographic testing (RT).
EN 14141 Annexure A has gone a step ahead and even defined the material of the tested valves, and defines qualifications accordingly.
All of these standards recommend testing in both water and gas. The gas can be air, nitrogen, methane, or other gases or mixtures of gases. Though dry air is allowed as test media, ISO 23632 gives a warning on self-ignition risk when testing the valve above 5Mpa (50 bar). API 6D QSL level gas testing media shall be nitrogen only.
Scaling
ISO 23632 and API 6D Annexure F define almost equivalent scaling factors. On pressure classes in general, a test of any pressure class shall qualify any valves having an equal or lower pressure class, but 6D Annex F and ISO 23632 differentiate class 600 and below, and 900 and above.
- API6D Annexure F
- 1/2” – 2” – Any one size
- 3” to 24” – Three sizes smaller and two sizes higher
- 24” Above – One size smaller and two sizes larger
- ASME Class 600 validates Class 600, Class 300, and Class 150.
- ASME Class 900 validates only Class 900.
- ASME Class 1500 validates Class 1500 and Class 900.
- ASME Class 2500 validates only Class 2500
- ISO 23632
- 1/2” – 2” – Any one size
- 2” to 24” – Three sizes smaller and two sizes higher
- 30” – 80” – One size smaller and two sizes larger
Class 600 and below is one range and class 900 and above is another range – in these ranges, any tested valve qualifies itself and its lower pressure class.
- EN 14141
Testing any one valve in the following size range will qualify the respective size range:
- ≤ 6”
- 8” to 18”
- 20 to 28”
- Greater than 30”
A test of any pressure class shall qualify any valves having an equal or lower pressure class. This covers all steel shell materials having mechanical properties equal to or superior to those of the test valve. The valve manufacturer shall request the independent accepted body, usually once annually, to check the continuous manufacture and testing of valves, for which a type mark was granted.
Mechanical Cycle
On mechanical cycles, API 6D and 6A Annex F requires 160 cycles, ISO 23632 requires 205 cycles, while 160 cycles in 6D and 6A are hydrostatic, and in addition, 20 cycles at minimum and maximum temperatures in the gas are to be performed. After each 20-gas cycles there shall be gas seat/shell and low-pressure gas test to be performed. In addition to all these API6D ANNEX F calls for gas body shell gas pressure and temperature cycling and subsequent seat/shell test. All 205 cycles in ISO 23632 shall be conducted in gas.
Temperature cycles are built into the mechanical cycles very well picturized in the validation standards, wherein the valve shall be brought to minimum and maximum temperatures as defined by the manufacturer depending upon the respective product family.
API6D QSL is covered in Annex I and there are levels 2, 3, and 4, wherein level 4 is the most stringent and calls for three cycles each in hydrostatic seat, shell, and high-pressure gas seat and shell. The low-pressure seat also shall be conducted for three cycles.
These standards call for a torque/functional test at the end of each cycle to validate the valve torque.
Acceptance Criteria
All of the valves during validation should satisfactorily exhibit performance. The leakage rate during seat tests should be according to the standards of the respective products and during torque testing, the recorded torque should fall within the limit as declared by the manufacturer.
At the end of validation cycles, the tested prototype shall be disassembled and inspected, and all critical parts shall be photographed. The examination should be performed to ensure that neither the product nor the component design contains defects to the extent that any performance requirement cannot be met. The results of the examination shall be documented.
Conclusion
All of the previously-stated validation testing standards are voluntary except for EN14141, The Annexure F in API 6D, and API 6A, QSL 2, 3, and 4 with respect to customer requests, and ISO 23632. As of right now, some customers demand validation certifications on a project-to- project basis. In its latest specification 04-SAMSS-035, Aramco for example, mandates all products to be type tested in line with API6D ANNEXURE for ball/gate valves, ISO 23632 for plug valves, and API600 for gate valves. Due to variations in mechanical cycling, media, and scaling, these validations cannot be combined and if requested by the purchaser, these tests need to be done separately.
The figures used in this article are from the respective standards and subject to change during the revision of standards, therefore the author requests the readers to verify the standard for all values at the time of reading.
