Valve Seat Leakages - FCI 70-2 Class 1

[svi]

The FCI 70-2 (Fluid controls institute Inc. intially ASME B16.104) Leakage classes are specified for an ESD ball valve and a Pipeline MOV Gate valve having composite seats. The composite seats are a metal + soft sealing combination. Class VI and V are specified for the ESD and MOV valves respectively. These leakage classes actually apply to control valves. Class VI is further valve nominal size depended and is stated upto 12". None of the classes are TSO.

The design standard API 6D / ISO 14313 also has leakage acceptance criteria referred to ISO 5208. "Leakage for soft-seated valves and lubricated plug valves shall not exceed ISO 5208 Rate A (no visible leakage)."

What may be the reason of specifing the test leakages by the FCI Control valve classes? API 598 resilient seated valves by default require 0 leakage rates. If testing is required to API 598 or API 6D, is it again required to state a test leakage rate?

 

[JLSeagull]

FCI applies to control valves. Control valves leak and FCI classifies the rate of the leakage.

Many instrumentation and control systems engineers who specify control valves also specify shutdown valves without ever reading piping standards. API 598 and 6D are more applicable to isolation valves.

It could be difficult to train some of those control systems engineers who may think that they are superior to piping engineers.
;-)

Piping engineers often specify leakage testing per API 598. Some may not realize the damage done by dirty hydrostatic fluid during testing thus don't know that those zero leakage valves leak lots when placed in service.

Life

42

 

[NGiLuzzu]

svi,
in general and physical terms, Class VI appears to be a more realistic criterion, as it also takes test pressure into account.

But, in practice, the difference between that and "no visible leakage" (i.e. less than one air bubble, which is usally assumed to be 0.15 ml) for the minimum test duration (15 to 300 s, depending on the valve size) becomes very thin, especially for small sizes and low pressures: just try to calculate and compare the maximum allowable leak rates for some specific cases...

About this issue, please take also a look at thread408-150132 within this Forum.

Hope this helps, 'NGL

 

[svi]

anegri,

http://www.galloup.com/downloads/control_valve_seat_leakage_classifications.pdf

The Class VI test leakage rates are upto NPS 12 size. The valve size that I have is 16". The through conduit gate valve features additional soft sealing at the slab seating.

API 598 has the same acceptance leakage rates for both the low pressure and high pressure closure tests.

How is that you say that Class VI takes the pressure into account. Does not each of the testing standards state the test pressures?

 

[NGiLuzzu]

svi,
first of all, the table of your link refers to the 1991 edition of the standard in object, while the latest one is dated 2006 (see, for example:

http://www.techstreet.com/standards/FCI/70_2_2006?product_id=1261166).

Unluckily, I don't hold the 2006 edition, either, but I have the 2003 one; moreover, I also have 2006 of IEC 60534-4, which is the European equivalent of FCI-70-2 standard.

About valve size, the second note to Table 2 of FCI-70-2 suggests "interpulation assuming that the leakage rate varies as the square of the seat diameter" for intermediate diameters; of course, one can also think about extrapulation for diameters over 12"...

About test pressure, Class VI definition from the above mentioned IEC 60534-4 standard (see Table 3 on page 25) states clearly the direct dependance of leak rate on differential pressure.


Hope this helps, 'NGL