Leakage Standards
To properly understand allowable valve leakage rates, you must understand the varying standards used to describe leakage in isolation valves. Each of these standards uses an array of pressure tests that all have different allowances for pressure, media, and test duration. This article will review the most cited standards in North America – MSS Standard SP-61, API Standard 598, and ISO 5208. It will also highlight the shortfalls in using these standards for severe service applications where the pressures, media, and cycle times would need a have their own, much higher standard to ensure safety and efficiency of operations.
Over the years, the industry has demanded better tools to help improve and control their investments and the valve industry has seen engineering and architectural firms use increasingly detailed specifications that attempt to remove ambiguity. The valve industry in turn has responded with better compliance through design, materials, and quality control. Many Severe Service Valve manufacturers have created their own specifications for testing that hold their valves to a higher standard though most of these are not recognized on a global scale.
One must be careful, however, to substantiate and quantify claims and expressions that appear to be clear. Class VI, Tight Shut-Off, Bubble-Tight, and Zero Leakage all evoke positive isolation abilities in performance, however, without more details, many valve users may be disappointed if they rely on valves sold with these buzzwords. These “designations” don’t have defined objective metrics to delineate them and are often used as marketing terms.
Valve leakage may be viewed from four discrete sources: 1) Seat Seal, 2) Stem Seal, 3) Body Seal, and 4) Pipe Connection, in order of potential of leakage or fugitive emissions. Each of these areas will have different criteria for their design, execution, and testing.
Test standards have been established to provide uniformity, however there is no uniformity between the different standards and therefore decisions must be made by the manufacturers as to which one(s) are adopted for use. The end-user or specifying engineer must also decide which tests he wants as the guardian of his concerns.
Once the standard or standards are chosen, the minimum test requirements must be considered for the intended application. In addition, the verification of the standard by the manufacturer of each new valve and the continuing conformance while in service must be addressed.
