Check valves are also described as reflux, non-return, back pressure, retaining and clack valves. In some areas of usage the term 'reflux' is reserved for the swing disk type of valve and the description 'check' is largely associated with the lifting disk type of valve. Current British Standards use the term 'check valves' and this includes both swing and lift types.
The purpose of a check valve is to permit flow in one direction and to prevent it in the reverse direction. Commonly such valves are automatic in operation, the pressure of the fluid flowing in one direction holding the valve disk open and reverse flow, together with the weight of the disk, acting to seat the disk and so cut off the flow.
Most check valves are based on either the swing concept or the lift concept. There are many variants and some .of the more commonly used of these are referred to in the following. A brief description is also given of a new concept in this field, the cone and diaphragm check valve.
Swing Check (conventional):
In this design a hinged, gatelike disk is free to swing against a body seating face which is tilted. To ensure that the disk takes up a satisfactory seating position at all times, the disk to hinge connection is designed so that the disk has sufficient freedom to swivel and make true and full contact with the, body seat. Disks may be all metal or may be fitted with leather, rubber, or composition facings, depending on the nature of the service. In the fully open position there is little obstruction to flow in a swing type check valve and consequently the pressure drop across the valve is relatively small.
Swing Check with Outside Lever and Weight:
In this arrangement, the disk hinge pin is extended through the body and fitted with an outside lever and weight, which can be set in various positions. In the position illustrated the weight will assist the disk to close quickly once forward flow ceases. Quick closing action is desirable in systems where sudden reversals of flow may occur and accumulate considerable momentum before an ordinary unweighted valve disk would close. Quick closing minimizes possible damaging shock and disk chatter. The lever and weight may be mounted so that the weight balances or partly balances the weight of the disk. The valve will then be more sensitive to low pressures and velocities.
Swing Check with Outside Lever and Quadrant:
The lever has a drilled hole and a locking pin fastens the lever to a quadrant, thereby holding the valve disk in the open position. Used mainly to assist in line pressure testing.
Swing Check with Fusible Link:
The disk is held in the open position by means of a fusible link attached to the lever. The fusible link has a predetermined melting point and the valve is used mainly as a fire control device.
Swing Check with Shock-free Operation:
This is a type of swing valve in which the design criteria are directed particularly towards the prevention of shock closure in conditions of very rapid reversal of flow, for example in the case of discharge by electrically driven centrifugal pumps. Special features are incorporated to achieve valve closure in the shortest possible time combined with low head loss.
Tilting Disk Check:
A sophisticated design of swing check valve in which a specially shaped disk is pivoted at a selected point, instead of being hinged as in the conventional type. of swing check valve. The seat faces on the disk and in the body are bevelled and in the closed position the valve resembles a simple 'lift' valve with conical seat. Characteristics are low pressure drop at low velocities and, depending on the design, a quick response to flow reversal.
The disk sits on a seat face provided on a horizontal bridge wall across the valve body, rather as in a globe valve except that in this case the disk is free to rise. Flow pressure lifts the disk from its seat and back flow, or no flow, causes the disk to drop back on to the seat and so shut off the flow. The disk is usually guided in both the body seat opening and the body cap but sometimes within the seat opening only. For some applications a spring may be fitted above the disc to ensure the valve is closed.