"Zero leakage" Valves – Let's have a look at the facts!

The goal of this article is to put into perspective some misunderstandings on the subject "zero leakage" for industrial valves.

The use of the term "zero leakage" for a valve seat is probably one of the most misunderstood subjects in the industrial valve market. 

A good example are manufacturers of triple eccentric butterfly valves and API6D metal seated ball valves. They differentiate their product by claiming this style of valve is "zero leakage" or "bubble tight" shut off.

Let's fact facts! From a technical & design point of view, there is no such thing as true "zero leakage". Almost all valves leak by some measure. Did you know that even glass leaks 1cc every three years per square cm?

The most common standard on the market to define the allowable leakage rates are API598/ISO5208/API 6D/MSS SP-61/FCI 70-2. Each of them might have different acceptance criteria. From a technical point of view, when we speak about leakage rates, there must always be a direct link to the applicable standard. Example: Allowable leakage rate Class IV according to FCI 70-2.

? Metalyte Pipeworks UK – Leaking Globe Valve

There is no definition of "zero leakage" concerning valves in API 6D or ASME B16.34 and this is where the confusion arises. Vendors often claim that the valves offered are tight shut off without commenting on the leakage rates or the type of test conducted (low pressure or high pressure), or the testing medium (air or gas).

Other standards like API598 might define "0" leakage for soft seated valves.

Knowing the most used valve types and testing standards

When establishing a technical specification for valves it should be mandatory to be very precise about the leakage acceptance criteria. The above table gives a good overview of the most common test standards. A useful checklist which should be on the desk of every valve specifying engineer.  

Understanding "zero leakage"

In general, specifications such as API598 (mirrored with ISO 5208) or API 6D (ISO 5208 leakage rates) that govern leakage for soft seated valves call for '0' bubbles of air or '0' drops of water under the specified test conditions over the minimum test time period. These valves are therefore sometimes referred to as 'zero leakage' valves. There really is no such thing as 'zero leakage', since microscopic amounts of material may indeed cross the seat or packing boundaries, especially if helium or hydrogen or other small molecule gases are used. Another common term for soft seated valves is 'bubble tight'.

API 598 acceptance criteria

The API standard 598: Valve Inspection and Testing covers the testing and inspection requirements for gate, globe, check, ball, plug and butterfly valves. It has acceptable leakage rates for liquid as well as gas testing. All valves built to the various API standards are required to meet API-598 leakage criteria prior to shipment from the manufacturer or supplier.

The API STD 598 requires the valve being tested at 110% of the maximum allowable pressure at 38 °C (100 °F). The ASME B16.34 requires the test be performed not less than 110% of the 38°C (100°F) pressure rating which almost is the same. 

API 598 states for shell and BACK SEAT tests, no visible leakage is permitted. If the fluid is a liquid, there shall be no visible evidence of drops or wetting of the external surfaces (no visible leakage through the body, body liner, if any, and body-to-bonnet joint and no structural damage).

If the test fluid is air or gas, no leakage shall be revealed by the established detection method. For both the low-pressure closure test and the high-pressure closure test, visual evidence of leakage through the disk, behind the seat rings, or past the shaft seals (of valves that have this feature) is not permitted (Plastic deformation of resilient seats and seals is not considered structural damage). The allowable rate for leakage of test fluid past the seats, for the duration of the tests, is listed in the following table:

General notes:

• 1 milliliter is considered equivalent to 16 drops.
• For check valves larger than NPS 24, the allowable leakage rate
• shall be per agreement between purchaser and manufacturer.

Notes:

1. There shall be no leakage for the minimum specified test duration. For liquid test, 0 drop means no visible leakage per minimum specified test duration. For the gas test, 0 bubble means less then 1 bubble per minimum specified test duration.
2. The maximum permissible leakage rate shall be 0.18 cubic inch (3 cubic centimeters) per minute per inch of nominal pipe size.
3. The maximum permissible leakage rates shall be 1.5 standard cubic feet (0.042 cubic meter) of gas per hour per inch of nominal pipe size.

? Pentair Valves & Control

FAT (Factory Acceptance Test) vs Field Use

When we speak about seat leakage acceptance criteria, in most cases, this means; What is required by the applicable standard during FAT, usually done at the valve manufacturer's location.

Achieving the acceptance criteria during this test is one thing, having the valve in use on-site is entirely different.

A study made some years ago by a Swiss EPC contractor involved the testing of five Globevalves, class 2500, 1" according to API 598. All valves fully complied with the acceptance criteria.

After several fast closing cycles (FC pneumatic actuator operated), all valves failed the second test. Some started to leak badly after only five operating cycles.

And here we speak about testing with clean media. Imagine the same thing for a metal seated ball valve, where you might have particles of sand in the natural gas..

Please see also my article: "How leaking globe valves massively drain profit of Power Stations!"

How leaking globe valves massively drain profit of Power Stations! | LinkedIn

Depending on valves' choice, the user must be aware that FAT testing is one thing, but using the valve on site is entirely different. Not only a valve seat but also valve packings and back seats are wearable parts. They won't last forever.

The good news are, that many valves manufacturers have innovated during the past 20 years to improve the durability of their valves in terms of seat leakage. 

Conclusion

Valve manufacturers often claim that the valves offered are tight shut off without commenting on the leakage rates or the type of test conducted (low pressure or high pressure) or the testing medium(air or gas).

Vendors try to introduce more confusion when they mention leakage class I to VI of ANSI/FCI-70-02: Control valve seat leakage for the manual valves. Although it is possible that the vendor can achieve a tightness class V or VI we still need to know the pressure at which the valve was tested and the testing medium, be it air and water or inert gas like helium.

The most important thing is to identify, which is the applicable standard for defining the acceptance criteria for a particular valve in terms of seat leakage.

Michel F. Bolle – 15.4.2021

Commercial Director LK/IPI Group Valve Division

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Article credits:

- https://globalsupplyline.com.au/

- https://www.piping-world.com/

- https://www.wermac.org/

- https://www.valve-world.net/

• Steve Parker & Andy Williams