Consider ENP Trim Ball Valve where 316 trim is specified
May 10th, 2017 by Amanda Schumann
In floating ball valves 316 (CF8M ball) trim is the standard because under 150mm there is only a minimal cost advantage using ENP trim in 150/300 class as the ball wall thickness is not large and the balls are mass produced keeping the costs down. However, in the trunnion mounted arena 316 trim is utilised for customer’s believing this will have a higher corrosion resistance. The first problem with this is it’s a lot more expensive, especially in larger, heavier trunnion mounted ball valves. Also note: – CF8M ball is often used in lieu of 316 but it’s strength and smoothness is inferior to 316. (Even in cast trunnions are more reputable brands like APV use 316 balls with higher tensile stems for larger sizes or higher pressure classes). In higher classes and larger sizes, 316 trim materials are not strong enough and have to be substituted with F51 etc., again increasing the cost by another 30 percent. Also, from a corrosion point of view if you had a service that would corrode an ENP ball it would also corrode the seat pockets and body component of the stem area of carbon steel body valves. Of course some customers do specify seat pocket and stem overlays such as incoloy. The history of 316 trim dates back to when the standard trim was chrome plated which was inferior to today’s modern ENP plating. Also when left in a corrosive salt water environment, chrome plating that is peeled off could then allow rusting underneath the plating meaning the ball had to be replaced rather than being re-machined, such as with a 316 ball. Consequently valves for offshore environments started specifying 316 trim for this reason. Today, few valves are reconditioned under 250mm in size and with the advent of ENP plating this has become the new standard throughout most of Australia even for NACE service. Stockists like Global Supply Line order ENP trim in much larger quantities and the price is far more competitive. Also, reputable manufacturers like APV, Sfeova, JC & Starline pair A105+ENP ball with 4140+ENP of 410 SS stem and seats, especially in larger sizes & higher classes. If you order a 316 trim valve this also means the stem, trunnion and seat holders need to be 316. Once again in higher pressure classes 316 isn’t strong enough and then has to be substituted for even more expensive 17-4PH or F51/S31803 or XM-19 Nitronic 50, etc., increasing the costs further again.
GSL stock trunnion ball valves with A105 or LF2 or F52 + ENP .003” (76 micron) ball/trunnion. APV, Sferova, JC & Starline use high phosphorous EN plating that has a Vickers hardness of 500 minimum as standard (further hardening can be performed up to 900vh). 316 Stainless steel is very soft and has no minimum ASTM hardness level. An .003” EN plated ball resists scratches much easier than 316SS. Also it is smoother and provides a better seal in higher classes. A105/ LF2/ F52 + ENP is also stronger for higher pressure service compared to 316. Where 316 trim is specified in higher classes, it needs to be substituted with F51 further ramping up the price. EN plating is allowed by NACE MR-0175/ ISO 15156 and is suitable for higher temperatures than 316. 316 when used in NACE service is limited to a maximum of 60ºC due to dangerous interactions with chlorides above 60ºC causing sulphide stress embrittlement. Electroless Nickel plating has a surface coating of up to 85% nickel. 316 Stainless steel also relies on nickel for corrosion resistance but only contains 10 – 14% nickel. The corrosion performance of EN plating is proportional to the plating thickness. 3 mil plating provides an excellent barrier protecting the base material (A105N, A350 LF2 or F52). In acidic PH conditions as low as 4.1 mil (.001”) ENP should be limited to valves in sweet service and neutral pH process. GSL only stock .003” (76 micron).
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