Depending on the service conditions, disks may be of all metal construction or may be in the form of a disk holder fitted with a rubber or composition disk ring. Like the globe valve the flow path through a lift check valve follows a twisting course. Consequently the pressure drop across the valve is greater than in the swing type of check valve where the flow is less restricted.
Lift Check with Dashpot:
In this design a dashpot is arranged above the disk so as to provide a cushion effect on services where pulsations in the line may cause an ordinary check valve to hammer.
Similar to the normal lift type check valve but utilizing a ball as the disk. The ball is guided in the body cap but free to rise and rotate.
Screw-down Stop and Check:
Also known as a screw-down non-return (SDNR) valve (Fig. 10). This is a globe type of valve except that the disk is not attached to the screwed stem. The stem is used to regulate the lift of the disk when it is acting as a check valve, or, to hold the disk closed in the same manner as an ordinary globe valve.
Cone and Diaphragm Check:
This is a modern development and a new concept in the field of check valves. The essential elements are a stainless steel perforated cone and a flexible diaphragm of natural or synthetic rubber. Service fluid passes through holes in the cone and deflects the rubber diaphragm inwards. As the flow increases the diaphragm is progressively deflected inwards until the valve is fully open. When flow is reduced the diaphragm tends to open outwards to its original shape and when flow ceases the diaphragm snaps shut, closing the valve. There are no metallic moving parts in the valve, which is extremely lightweight and can be installed in horizontal or vertical pipes even where the flow is vertically downwards. Operating temperatures and pressures are limited to the capabilities of the diaphragm material.
Sizing of Check Valves:
Care should be taken to avoid oversizing of conventional swing and lift check valves. To obtain the minimum velocity required to lift the disk to the full open and stable position it may be necessary on some applications to fit valves smaller in size than the pipe in which they are installed.