This might be relevant. Without retainer indicates perhaps a direct metal to metal sealing with direct contact, again the definitions for HP butterfly valves might be significant. The above said, I must admit that I have never seen this descriptin in connection with a check valve. On the other hand it is quite common that the description for a check valves leans towards somthing used in 'an almost similar' , but older/previous solution, without adeqatly giving the necessary fluid and operation parameters necessary to dimension and select the optimum type: eg.: correct dimension, type, placement etc. necessary to avoid pressure peaks and slamming or gulping operation, partially open valve, minimum pressure loss and soft opening and closing.

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