Diaphragm valve

1. A diaphragm valve comprising:

a valve body provided with a fluid channel;

a valve seat provided on a peripheral edge of the fluid channel;

a diaphragm having a spherical surface and configured to be pressed against or separated from the valve seat to close or open the fluid channel;

a diaphragm presser having a lower end that has a spherical surface and serves as a pressing surface and configured to press the diaphragm; and

an actuator configured to move the diaphragm vertically;

wherein SRb/SRa =0.4 to 0.6 is satisfied, where SRa represents a radius of curvature of the diaphragm and SRb represents a radius of curvature of the pressing surface of the diaphragm presser.

2. The diaphragm valve according to claim 1 , wherein the valve body has a depression with a bottom surface having: a circular flat portion, an annular depressed portion at the periphery of the circular flat portion and an outer periphery diaphragm support portion between the circular flat portion and the annular depressed portion.

3. The diaphragm valve according to claim 2 , wherein a surface of an outer peripheral edge portion of the diaphragm contacts the diaphragm support portion.

4. The diaphragm valve according to claim 2 , further comprising:

a bonnet extending upwards and having a lower end portion that is screwed into an upper portion of the depression of the valve body; and

a press adapter that is placed between a lower end surface of a bonnet and the bottom surface of the depression of the valve body and that holds an outer peripheral edge portion of the diaphragm between the press adapter and the bottom surface of the depression of the valve body, wherein an entire lower surface of the press adapter is tapered.

5. The diaphragm valve according to claim 1 , wherein the diaphragm is made of a nickel alloy thin sheet, a stainless steel thin sheet, or a layered product formed of a stainless steel thin sheet and a nickel-cobalt alloy thin sheet.