A diaphragm control valve comprising: a bonnet for securing to a valve body, the bonnet including a mounting flange and a bonnet clamping surface; a valve body having a fluid inlet, a fluid outlet, and a flow passage, the flow passage including an inlet passage, an outlet passage, and a control passage located between the inlet passage and the outlet passage, the valve body further including a mounting portion having a body clamping surface located proximate the mounting flange of the bonnet, the body clamping surface facing the bonnet clamping surface; a diaphragm located within the control passage, at least a portion of the diaphragm being clamped between the bonnet clamping surface and the body clamping surface; a clamping washer being disposed between the diaphragm and the bonnet clamping surface, the clamping washer comprising a body; wherein the bonnet clamping surface transitions into a first annular retention surface formed by a skirt, and the body clamping surface transitions into a second annular retention surface that is formed by an upturned annular lip, and the skirt extends over at least a portion of the valve body, a gap being formed between the upturned annular lip and the skirt so that the upturned annular lip is spaced apart from the bonnet; characterized in that the clamping washer further comprises a ledge or projection, wherein the ledge or projection extends beyond the perimeter of the diaphragm and is disposed between the upturned annular lip and the bonnet clamping surface; and wherein a thickness of the body of the clamping washer plus a thickness of the diaphragm in an uncompressed state is greater than a thickness of the ledge or projection plus a height of the upturned annular lip. a bonnet for securing to a valve body, the bonnet including a mounting flange and a bonnet clamping surface; a valve body having a fluid inlet, a fluid outlet, and a flow passage, the flow passage including an inlet passage, an outlet passage, and a control passage located between the inlet passage and the outlet passage, the valve body further including a mounting portion having a body clamping surface located proximate the mounting flange of the bonnet, the body clamping surface facing the bonnet clamping surface; a diaphragm located within the control passage, at least a portion of the diaphragm being clamped between the bonnet clamping surface and the body clamping surface; a clamping washer being disposed between the diaphragm and the bonnet clamping surface, the clamping washer comprising a body; wherein the bonnet clamping surface transitions into a first annular retention surface formed by a skirt, and the body clamping surface transitions into a second annular retention surface that is formed by an upturned annular lip, and the skirt extends over at least a portion of the valve body, a gap being formed between the upturned annular lip and the skirt so that the upturned annular lip is spaced apart from the bonnet; characterized in that the clamping washer further comprises a ledge or projection, wherein the ledge or projection extends beyond the perimeter of the diaphragm and is disposed between the upturned annular lip and the bonnet clamping surface; and wherein a thickness of the body of the clamping washer plus a thickness of the diaphragm in an uncompressed state is greater than a thickness of the ledge or projection plus a height of the upturned annular lip.