The primary purpose of check valves is to permit flow in one direction while prohibiting or stopping the flow in the opposite direction. The key point is that the normal resting state of a check valve is the closed position.
WHY IS A NON-SLAM CHECK VALVE USED?
When flow is reversed in a pipe, this can cause hydraulic shock waves to pass through (also known as water hammer). This can cause severe damage to equipment in the pipeline if measurements are not taken to prevent this. The introduction of the correct check valve, in a fully closed position during reverse flow can prevent this from occurring and causing severe issues at the site.
Valves that prevent water hammer and close rapidly without slamming are called non-slam check valves.
HOW ARE AXIAL FLOW CHECK VALVES USED?
Axial check valves are specifically designed for fast-reversing systems where backflow is a constant concern. It is categorized as a non-slam check valve as the valve closes without slamming meaning no excess pressure spikes are created. Most axial flow check valves are spring-loaded, with a single low-mass disc, which facilitates a fast, dynamic response to reduction in flow. Due to having a single disc and spring when the disc is opening, the force is balanced resulting in an even flow.
METHOD OF OPERATION
When pressure differential between the upstream and downstream side of the valve exceeds cracking pressure of the spring, the obturator will move and allow flow. The flow area of the valve decreases steadily down to the seat diameter; this is the venturi design.
As a result of the venturi design, the reduced flow area increases the kinetic pressure and reduces the static pressure allowing the valve to fully open quickly. The spring is selected so that the disc is fully opened and stable against its backstop under normal flow conditions.
When a noticeable reduction in flow occurs, the spring reacts early against the force of the reducing flow.
PRODUCT DYNAMIC PERFORMANCE COMPARISON
Simple conventional swing check valves tend to have a very poor response. Under dynamic conditions, relatively long response times are experienced. This yields high reverse velocities resulting in slam and unwanted pressure surges.
Dual plate spring-assisted valves versus conventional swing checks are designed with small plates, which have less inertia than a single thicker disc. Pre-loading with a spring provides a closing force at all angles, thus helping to reduce response time.
As mentioned, axial flow check valves have an excellent response and are recommended where a high-level performance is essential. As flow decelerates in the pipeline, the forces acting on the disc are reduced, and the spring is able to overcome these smaller forces causing the valve to close. Short displacement of the disc combined with axial spring assistance greatly reduces the response time giving a fast slam-free response with minimal reverse.
Axial flow valves:
Cover any service conditions (lowest to highest deceleration systems)
Have the fastest response time and are non-slam and mitigate against pressure surge or the impact of water hammer
STANDARD FEATURES
The base valve design is available in various body configurations to suit the customers’ needs for industry standards, specification, weight and connection requirements. The majority of axial flow check valves will be sold as flanged.
STANDARDS
The list below summarizes the most important standards for axial flow check valves:
TYPICAL APPLICATIONS
Although axial flow check valves can be utilized in a multitude of applications, a growing number of niche sectors the valves have primarily been utilized in the following:
Oil and gas production, including midstream:
Regasification
Liquefaction
Centrifugal compressor discharge
Fire water lines
Oil/steam separation
Steam and CO₂ injection
Oil and gas gathering systems
Chemical and pharma:
Chlorine
Phosgene
Aromatics
Polymers
Acids
Air separation
Caustics
Petroleum refining and petrochemicals:
Hydrogen
Cracking
Steam
Crude oil
Ethylene
Propylene
Steam
Power:
Steam
Condensate
Boiler feed pumps
Cooling towers
Service water recirculators
River water intake
Renewables
The most important thing to remember is that axial flow check valves are specifically designed for fast-reversing systems where backflow is a constant concern.
Post time: Apr-18-2022