Posted on 3 November 2013

Corrosion resistance of valve interlocks

Valve safety products such as valve interlocking devices have to withstand harsh conditions for long periods of time. Therefore, stringent requirements are imposed on this kind of equipment. Which materials can be best used and which design principles have to be taken into account?

Durability and reliability

Valve interlocks are nearly always used in environments where safety is paramount. For this reason, companies in the petro-chemical industry impose stringent requirements on levels of durability and reliability for this kind of equipment. Naturally, resistance to corrosion is of great importance, taking into account the harsh conditions and environmental influences that these devices often have to withstand.

Corrosion resistance of Netherlocks valve interlocks

Field experience acknowledges the reliability of uncoated AISI316 stainless steel valve interlocks

AISI316 as a starting point

Choosing and using materials that are inherently resistant to corrosive environments is a good starting point as it comes to designing interlocking devices. All Netherlocks interlocks are fully AISI316 stainless steel, AISI316 is one of those few stainless steels that are considered to be resistant to all different corrosive environments, rather than a just a few in particular. As can be found (amongst others) in the Handbook of Corrosion Data 2nd edition (ASM International; table 3 – Relative corrosion resistance of AISI Stainless steels), AISI316 is corrosive resistant to Mild atmospheric and fresh water, Atmospheric industrial and Atmospheric marine, Salt Water, Mild chemical, Oxidizing chemical and Reducing chemical. In all, the material characteristics of AISI316 stainless steels are ideal for use in harsh and demanding environments.

Field experience

Field experience acknowledges the reliability of uncoated AISI316 stainless steel valve interlocks. Netherlocks installed 100.000 uncoated valve interlocks, of which on average 50% are for offshore applications. As a general requirement, the interlocks have to perform under the toughest conditions for extensive periods of time, without requiring replacement or even much maintenance. Over the last 20 years, a total of zero corrosion-related malfunctions or difficulties were experienced on any of the installations.

Coating offers no extra protection against corrosion

Because of typically valve-based ‘generalized specifications’, or signaling functionality, coating of the outside of the interlocking products is sometimes considered by contractors and/or end-users in the petrochemical industry, particularly in offshore conditions. In practice, layers of coating can only be added to the outer side of the valve interlock, the lock body. The moving parts of the interlocking mechanism (such as counters and lock fingers) all are inside the interlock, and therefore cannot be (organically) coated without interfering with functionality. As all interlocking devices are intrinsically ‘open’ in their design (despite dust covers and other related products), corrosive environmental influences are still able to freely enter the inside of the valve interlock. Therefore, coating parts of the interlock does not offer any extra protection.

Corrosion protection by design

Effectively, choosing superior corrosion resistant material such as Stainless steel 316 proves to be a good design philosophy. The right material choice and design of both internal and external parts of the interlock enable a high degree of protection against corrosion without extra coating. Adding partial or full layers of coating might sometimes offer visual signaling functionality, but it does not provide extra protection against corrosion.

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