API standards on fugitive emissions

Published: 08 February 2016 Written by Rich Davis
   

 

The API Standard Committee on Piping and Valves has published some standards on fugitive emissions. While other standards do address the issue of fugitive emissions, the API standards have also attempted to address leak rates and other aspects of valve performance.

API 622

The standards committee started with API 622 Type Testing of Process Valve Packing for Fugitive Emissions. This standard establishes requirements and parameters for the following tests:

Fugitive Emissions
Corrosion
Packing Material Composition and Properties
Oxidation Evaluations

The fugitive emissions testing includes 1510 mechanical cycles with five thermal cycles—ambient to 500°F. The test methods apply to packing for use in on-off valve rising stem and rotating stem motions.

The latest standard was published in 2011 and currently is undergoing revision. A test chamber for 1/8-inch packing is being added as a number of tests on API 624 have indicated issues with the smaller packing cross-section.

API is also changing the leak test monitoring equipment by opening up the types of equipment allowed to detect fugitive emissions. An important note on this section: The review of the packing manufacturers testing should include an overview of all sections of the testing. 

The expectation from the valve buyer some standards on fugitive emission still be an extended warranty and this means that the potential for oxidation and volume loss due to the heat loss of lower temperature materials may cause premature failure, so it is essential to look beyond the fugitive emissions numbers and review all of the results.

The committee hopes the new standard will permit up to 100 ppmv methane leak rate with no allowed adjustments, along with the above-referenced changes.

API 624

Following the development of the API 622, the API 624 Type Testing of Rising Stem Valves Equipped with Flexible Graphite Packing for Fugitive Emissions standard was developed and published in 2012.

The API 624 standard specifies the requirements and acceptance criteria (100 ppmv) for fugitive emission type testing of rising and rising-rotating stem valves equipped with packing previously tested in accordance with API Standard 622. The fugitive emissions testing includes 310 mechanical cycles with three thermal cycles—ambient to 500°F.
An optional low temperature test at -2°F (-29°C) may be performed if requested by the purchaser. The elevated test temperature is to be 500°F ± 5% (260°C ± 2%). 

The test pressure must be the lower of 600 psig or the maximum allowable pressure at 500°F per ASME B16.34 for the applicable material group and shall be held constant throughout the test. The packing must also be suitable for use at service temperatures 20°F to 1000°F (–29°C to 538°C). Valves larger than NPS 24 or greater than class 1500 are not covered in the scope of the standard.

Both of these current standards identify requirements of requalification, i.e., if the manufacturing location changes or if the valve design changes.

API is currently working on developing a standard to cover quarter-turn type valves. It will be titled API 641 Quarter Turn Valve FE Test. 

The standard specifies the requirements for type testing quarter-turn valves for fugitive emissions and applies to all stem seal materials. Conversations have been held about the types of mechanical and thermal cycles, and the committee has settled on 100 ppmv as the maximum allowable leakage. 
The wide range of quarter-turn valves complicates the development of the standard. Also, the inclusion of all types of stem seals further complicates the test temperature and pressure requirements.

Finally, the Upstream & Midstream API 6D Valves group has decided to develop a separate standard covering their valves and are currently working with the Association of Wellhead Equipment Manufacturers to make that happen. The group is aiming for completion by the end of 2016.

Emergency SHUTDOWN Applications!

 
Many critical applications must be prepared in the event of a catastrophe, such as an explosion, fire, flooding, earthquake, ect. In the past, process engineers often specified remotely operated on-off valves for standby applications. However, today’s emergency shutdown (ESD) standards have been increased so that valves must now be applied using a strict code of testing, identification, auditing, and performance under adverse conditions associated with catastrophic event. 

Under common ESD codes, before an ESD valve is applied in a critical application, it must be designed and tested so that the valve goes to the required fail safe position ( fail open, closed or fail in place) and then holds that position for a particular length of time under given parameters of a possible catastrophic event.

Of the three fail safe positions, fail closed is the easiest to achieve since the actuator spring and fluid pressure are both used to move or assist the valve to the required closed safe function. 
However, although rare in emergency shutdown applications, the fail in place position often requires an independent power or air supply source to hold the required position. With the use of digital positioners such as DVC 6200 and controllers, throttling control valves used in ESD applications are expected to perform at a higher standard as a critical part of the safety instrumented function (SIF), which calls for a safety loop between distributive control system (DCS) and the ESD final control element.

When a catastrophic event occurs, sensors send a signal to the (SIF) which calls for a safety loop between the DCS and the ESD final control element. When a catastrophic event occurs, sensors send a signal to the SIF, which disconnects the power to the ESD control valve, moving the valve to the correct failure position. 

For self-sustaining control valves employing smart technology with an internal safety integrity level (SIL) function, the digital positioners and controller are linked to self-contained sensors, which after sensing the event can employ the required failure mode. For this reason, the digital positioner or controller must be designed with the necessary Intrinsic temperature, and exterior conditional requirements of the ESD valve. 

The likelihood of catastrophic event is rare, it’s critical that the ESD valve must be tested and designed for a particular mean time between failure MTBF parameter which is an important part of SIL requirements. The problem with most valves is when they stay in the one position for a very long period of time, they are progressively more prone to stick or require a greater force to move to the fail safe position upon failure. Knowing the MTBF for a particular valves allows maintenance or a failure test to be performed at certain time intervals to ensure proper performance. 


ESD valve standards 

The most common standards applied to ESD are IEC 61508, 61511. IEC (International Elecrotechnical Commision) is commonly recognised worldwide as the safety standard for ESD valves, and is a requirement for many global insurance companies. This standard provides stringent guidelines for testing and auditing of the functional safety requirements for safety integrity loop, which includes all aspects of the process loop, sensors, positioners or controller, control valve or any exterior fail safe equipment. IEC demands a zero tolerance level of hardware failure (valve, actuator yoke or cylinder, spring ect.) and a minimal tolerance failure rate, diagnostics associated with ESD valve and SIL software requirements. 


Partial Stroking Testing
 

In the past the only way when the testing of the ESD valve was required, was to carry this out during a system shutdown for routine maintenance. This can be an expensive and time consuming process. 
However now with the development of smart positioners on ESD valves. Not only does this save time and expense but now gives diagnostic information of the systems performance and strokes the valve partially 10 to 30 percent of stroke, to ensure their is no stiction on the valve stem or shaft. Each test records the ESD valve signature so you can see from one test to the next if any changes occur and look for obvious signs of high friction levels, this would suggest a possible stuck stem or shaft or stiction taking place. Self diagnostic tests can also be conducted to detect leaks to the actuator, or determine spring rate or bench set. 
    

 

Fisher FIELDVUE™ DVC6200 Digital Valve Controller
  
Product Description
FIELDVUE™ DVC6200 digital valve controllers are HART® communicating, microprocessor-based current-to-pneumatic instruments with linkage-less, non-contact travel feedback. Important functions performed: 1 Traditional valve positioning 2 Automatic calibration and configuration 3 Provide instrument and valve diagnostic information.

Key Features

Linkage-Less Non-Contact Position 

Feedback—There are no wearing parts so cycle life is maximised

Encapsulated Electronics–Resist the effects of vibration, temperature, and corrosive atmospheres