Definition of check valve
Check valves are automatic valves that open with forward flow and close against reverse flow.
This mode of flow regulation is required to prevent return flow, to maintain prime after the pump has stopped, to enable reciprocating pumps and compressors to function, and to prevent rotary pumaps and compressors from driving standby units in reverse.
Check valves may also be required in lines feeding a secondary system in which the pressure can rise above that of the primary system.
Grouping of Check Valves
Check valves may be grouped according to how the closure member moves onto the seat.
Four groups of check valves are then distinguished:
1. Lift check valves.
The closure member travels in the direction normal to the plane of the seat, as in the valves shown in Figure 4-1 through Figure 4-7.
Figure 4-1. Lift Check Valve with Piston-Type Disc, Standard Pattern. (Courtesy of Eaward Valves Inc.)
2. Swing check valves.
The closure member swings about a hinge, which is mounted outside the seat, as in the valves shown in Figure 4-8 through Figure 4-10.
3. Tilting-disc check valves.
The closure member tilts about a hinge, which is mounted near, but above, the center of the seat, as in the valve shown in Figure 4-11.
4. Diaphragm check valves.
The closure member consists of a diaphragm, which deflects from or against the seat, as in the valves shown in Figure 4-12 through Figure 4-14.
Rapid fluctuating movements of the closure member must be avoided to prevent excessive wear of the moving valve parts, which could result in early failure of the valve. Such movements can be avoided by sizing the valve for a flow velocity that forces the closure member firmly against a stop. If flow pulsates, check valves should be mounted as far away as practical from the source of flow pulsations. Rapid fluctuations of the closure member may also be caused by violent flow disturbances.
When this situation exists, the valve should be located at a point where flow disturbances are at a minimum.
The first step in the selection of check valves, therefore, is to recognize the conditions under which the valve operates.
Assessment of Check Valves for Fast
In most practical applications, check valves can be assessed only qualitatively for fast closing speed. The following criteria may serve as a guide:
- Travel of the closure member from the fully open to the closed position should be as short as possible. Thus, from the point of speed of closing a smaller valve is potentially faster closing, than a larger valve of otherwise the same design.
- The inertia of the closure member should be as low as possible, but the closing force should be appropriately high to ensure maximum response to declining forward flow. From the point of low inertia, the closure member should be of light construction. To combine light-weight construction with a high closing force, the closing force from the weight of the closure member may have to be augmented by a spring force.
- Restrictions around the moving closure member that retard the free closing movement of the closure member should be avoided. Application of Mathematics to the Operation of Check Valves
The application of mathematics to the operation of check valves is of relatively recent origin. Pool, Porwit, and Carlton40
describe a calculation method for check valves with a hinged disc that involves setting up the equation of motion for the disc and applying to that the deceleration characteristic of the flowing fluid within the system. Before the equation of motion for the disc can be written, certain physical constants of the valve must be known. The calculation determines the reverse flow velocity at the instant of sudden shut-off.
It is important for the valve user to know that valve manufacturers can use mathematics in designing check valves for given critical applications and predicting surge pressure.