How It Works: Check Valves

Types

Swing (tilting-disc) check valves consist of a clapper with a disc that is convex on the upstream inlet side and flat on the downstream outlet side. For API Spec 600 valves, this disc swings on a hinge that is mounted to the bottom of the valve bonnet. For API Spec 6D valves, the typical construction is a cast pocket in the valve body with a drop-in shaft or pin-and-bushing arrangement that the clapper will turn on. This type of construction requires some sort of restraint to keep the clapper in the pocket. The new design features improved modularity and facilitates easier assembly, enabling different configurations to be assembled with a base design.

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Nozzle (axial flow) check valves have a spring-loaded disc that translates horizontally. The spring allows for a nonslam design in that the effects of water hammer are eliminated. These are designed to be in the full open position, even at minimum flow rate, to ensure performance. Nozzle check valves can be installed in both buried and above-grade applications and are ideal for gas applications.

Ball check valves include a spherical ball clapper that is sometimes spring loaded to seal at pressures below the cracking pressure. Because of the spherical design, these valves can easily wear from prolonged use and require frequent maintenance; therefore, they should be installed in places that are easy for repair teams to access.

Diaphragm check valves consist of a rubber diaphragm clapper that flexes open when the pressure on the upstream side is greater than the pressure on the downstream side and closes when this pressure is equalized or lowered below a set pressure differential.

Stop check valves are usually constructed similar to a swing check valve but have an additional external control mechanism&#;such as an actuator, lever, or handwheel&#;that enables the valve to be deliberately closed regardless of flow pressure.

Duckbill valves enable flow to proceed through a soft tube that feeds into the downstream side of the valve wherein backpressure collapses the tube and cuts off the flow.

Features

The main advantage of a check valve is its simple design. Generally, check valves also are smaller and easier to install compared with other valve types, making maintenance easier and more efficient. Check valves can be designed with specific nonslam features to reduce noise and seal wear. ENTECH nozzle check valves are commonly selected for this feature in LNG pump stations and gas compressor stations. Additionally, because of their simple, streamlined design, TOM WHEATLEY check valves and ENTECH check valves have lower pressure drops (less than 1 psi) than comparable piston check valves.

Tilting Disc check valve

The tilting disc check valve, as check valve is opened when flow goes through one direction and closes at backflow

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Thanks to the fast closure of this versatile valve, pump protection is one of its main applications. It is used for gases and liquids, bus it is not recommended when there are solids in suspension. The head drop is relatively low.

Tilting disc valves are designed with three eccentricities:

&#; First eccentricity: it is the offset between the shaft and the seat plane. This eccentricity removes any interference of the seat with the shaft and therefore the seat has an integral and continuous construction both in the body and disc, and potential leakage can be avoided.

&#; Second eccentricity: it is the offset between the shaft and the disc diameter. This eccentricity works when reverse comes or normal flow pressure is so low than it can't make enough force to hold the closing torque caused by the weight of the disc.

&#; Third eccentricity: it is the angle of tilting of the seat plane with the normal plane of the pipe. This tilting reduce the stroke of the disc, therefore it could close faster at backflow.

Furthermore of geometry and weight of the disc, in same cases other elements can optimize the closing sequence. Springs can be used in order to increase the speed of the disc closure before reverse flow starts. In the other side, when reverse flow is a reality but the valve still not close at all a slow closure can avoid slam over the valve. There are several oleo-hydraulic damper systems.

In order to tighten the valve, springs and also counterweights assemble at the end of the shaft can be used.

Minimum work pressure provided by manufacturers should be take into account when selecting valves for low pressure applications, if not disc can be buffeted and in constant motion between the seat and partially open. This leads to seat damage and shaft wear. This effect is more important in swing check valves.

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