API 622 – Helping you to choose the best fugitive emissions packing

Valve fugitive emissions standards and legislation are not new ideas or concepts. Beginning in the USA in the 1960s, formalisation of criteria and test methods for the monitoring and reduction of fugitive emissions are now well known throughout the valve industry. The European territory and specific governments - in addition to end users such as SHELL, Chevron, and TOTAL - have all developed separate specifications and test methods for evaluating fugitive emission performance.

All these specifications however, have one thing in common; they evaluate the complete valve assembly. For this reason compression packing manufacturers work with individual valve companies to maximise the performance of their valve with a chosen sealing product. However, no two valves or housing assemblies are the same and extrusion clearance; stem surface finish; seal section and valve materials all influence the final valve assembly performance.

As a valve company supplying fugitive emissions valves, or one looking to enter this market, how do you know which packing to select in order to achieve the best fugitive emissions performance? You could rely on aspects such as brand, purchasing inertia or historical performance to guide your choice of packing, but there is a more scientific and impartial method of evaluating packing performance.

The API 622 fugitive emissions specification, originating in the USA, differs in one fundamental way from all other fugitive emissions specifications. This specification tests the packing in a standardised housing, not a valve. This allows end users to evaluate and directly compare the performance of compression packing products without the consideration of valve type and form.

Introduced in 2006, API 622 is now in its 2nd edition and arguably represents the most arduous fugitive emissions test conditions for today’s high performance valve stem packing. Five cycles from ambient to 550°F over 1510 mechanical actuations offers a difficult test for any packing and has been designed to replicate a 5 year service period for a globe or gate valve. The test is carried out on standard apparatus as detailed within the specification. The apparatus is horizontally mounted with methane leakage levels detected using EPA Method 21 bagging and sniffing method.

Unlike other specifications, there are no pass or fail limits for leakage within API 622. However, over the course of time, industry accepted limits have been established. Originally in 2006 leakage levels of 100PPMv were deemed to indicate high performance, but with the development of impregnation technology and braiding structure refinements, the industry trend is now for this to be reduced to 50PPMv.

In addition, a regularly overlooked element of API 622 is the corrosion test. A poor result in this element of the test can result in reduced service life, increased friction and high maintenance costs.

So, with this new found insight, you request from your packing suppliers a copy of their API 622 certificate, but how do you compare the results and which elements are truly important and what affect can these have on the performance of your valve.

• Average and maximum Leakage
Measurements are taken in both the static and dynamic phases of the mechanical movements, vastly different results can be generated from either phase so care should be taken that figures published take into account both phases of measurement.

Average leakage is of course a key criterion but as relaxation can occur after a thermal cycle, notice should be taken of the stage at which the maximum leakage occurs as well as the size of this leakage. Consider - does the packing recover or is the maximum level rising throughout the test?

• Number of stem adjustments
Next to leakage this will have the largest direct impact on a valve company or end user. Stem adjustments are used to bring leakage under control during the test. Effectively, adjustments represent inadequacies in the fitting procedure and/or packing construction. The consequences are higher maintenance costs, possible down time and, if unchecked, valves performing well outside accepted fugitive emissions performance levels.

• Load applied to the packing
As the test apparatus is standard, this figure can be compared directly from test to test. A lower bolt load figure may have consequences for valve design through smaller bolts and reduced metal work, but also as a general rule the higher the load applied, the higher the dynamic friction between the packing and valve stem. Higher friction can adversely influence actuator size which in the case of control valves will affect response time.

API 622 can be used by valve manufactures and end users to assess the performance of a fugitive emissions packing before embarking on costly and possibly fruitless valve specific trials. For a valve OEM, achieving any of the fugitive emissions specifications such as ISO 15848; TA Luft VDI 2440; SHELL SPE 77-312 can be time consuming and costly. Using API 622 and the criteria outlined in this article will provide an advantage in selecting the right packing and, along with other criteria, can be used to select the best sealing partner with which to carry out this work.

James Walker
Tel: 01900 823555
www.jameswalker.biz

Published: 17th October 2013