Secrets to choosing check valve

Check valves, also known as one-way directional valves, serve as automatic safety measures, allowing the unidirectional flow of gases and liquids. These valves are frequently employed to safeguard pumps in fluid systems and compressors in gas applications. They play a crucial role in preventing backflow, which could potentially disrupt the operation of pumps or compressors. Here are the essential in for selecting the check valves. 

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Check valves – proper selection is the secret to performance

What would you say to a person who is trying get the hens back into their coop with a stick? He should be using a food or treats instead! The stick is not the problem. In fact, it may appear to do the job…but not exactly in the smartest way. And that’s the problem. The use of stick is misinterpreted. Wondering why bring this up here? Well, that’s somewhat true when it comes to check valves too. check valves should be selected based on the parameters available in the system. 

Check valves selection concerns

Check valves usually generate a lot of concern from plant maintenance and users. The first complaint is often that ‘they don’t work’. It is not uncommon for the technicians to open and modify internal components of the check valve for their needs. Overall, the popularity of check valves is often challenged on multiple levels. The doubts and skepticism is a misinterpretation in many ways. When selected with due process Check valves are very effective.

So, here are some secrets to select the proper check valves:

1. Proper check valve sizing is half the job done right:

   
   An ideal check valve should be sized for the specific application, not just for the line size. This is probably the most common mistake in selection of check valves. The selection should be such that the valve disc should be stable in the open position during flow or fully closed during no flow to prevent chattering and premature failure.
   
   

2. Flow sensitivity of check valve – the make or break factor:

   
   Check valves are flow-sensitive, and the valve must be fully open to prevent premature wear and failure. Insufficient flow and pressure can cause the valve to partially open, leading to increased pressure drop and decreased effectiveness. If not considered in initial stages, over the long term, the check valve is bound for premature failure, complete damage or erroneous functioning.
   
   

3. Check valve durability of components:

   
   A check valve should be durable to prevent component failure and to ensure long-lasting service. A component failure can result in the valve not performing its function, which in the case of a check valve is to prevent reverse flow. Often, the scrutiny of components is not done to the required details – the use of materials, compliance to standards, allowances required for use over the long term, compatibility with the material and environment conditions must be carefully considered before selecting the check valve.
   
   

4. Pressure drop – the check valve critical factor you cannot drop:

   
   Pressure drop is an essential consideration when selecting a check valve. If the flow is not sufficient to achieve full opening, the pressure drop will be higher than calculated, leading to chattering of the disc and eventual failure. This is also the most common cause of concern for plant personnel. The chattering sound is invariably misinterpreted as the problem in quality of the valve. But, in fact, it is the first sign of wrong selection.

While there are several other factors that influence the functioning and performance of check valves, the ones mentioned above are the first to look at among the others.

Swing check valves

Swing check valves are the most common check valves. They are inexpensive and as they are automatic they do not require any external power or control to operate – only the flow direction determines the valve operation. Swing check valves with closed bushings do not have an open/close indicator but often the valves are mounted with lever and weight or lever and spring which enable visual check. For more details, please visit the section about swing check valves.

Ball check valves

In general, ball check valves are simple and cost effective. Advantages are their compactness and the fact that they have no external parts which helps keeping the price low and the reliability high. A disadvantage may for some applications be that they do not have an open/close indicator. For more details, please visit the section about ball check valves.

Recoil check valve

The recoil check valve is one of the oldest and recognized types of high performance check valves. They are used in pumping systems where high rapid flow reversal takes place. Typically, these active systems are pumping stations which adopt high branch velocities and, in addition, provide automatic stopping and starting, deliver into a vertically rising main and have multi‐pump sets delivering into a common main.

The recoil check valve is designed to incorporate every mechanical and hydraulic assistance for accelerating the closure of the door.

• Mechanical assistance is given by a combination of inertia reduction, concentration of mass and ideal suspension of the moving elements.
• Hydraulic assistance is given by the provision for forward and backward water streams within the body and around the valve door

For larger valve sizes, a multi‐door design is used. By sharing the volume of flow by two, three or even four doors, this reduces the required angular travel of each door, thus reducing the closure times of the doors.

The recoil check valves have metal to metal seats and an allowable leakage rate is to be considered.

Tilting disc check valves

The tilting disc check valve is similar in appearance to an eccentric butterfly valve. The valve body is double‐flanged and of a short length. The disc is held in place via a shaft which is positioned eccentrically from the body centreline in both the horizontal and vertical axes.

The double eccentricity of the shaft results in the lower section of the disc occupying a greater area in the flow path. Consequently, the disc begins to open at very low flow rates. The tilting disc check valve is therefore commonly used in pumping systems with low flow rates and also for pulsating flows.

During flow reversal, the disc closes due to gravity and the upper part of the disc above the shaft centreline pushes against the flow thus acting as a hydraulic brake which cushions the disc as it returns to the closed position. Tilting disc valves can be supplied with optional levers and weights to adjust the closing characteristics of the valve. As such, the tilting disc check valves are well suited to reduce the risk of water hammers.

Because of the internal shafts which are within the flow stream, the tilting disc check valve is used for water and treated effluent applications. For more details, please see our tilting disc check valve product information.

Check valves are critical in backflow prevention. They protect pumps and compressors in a process system and prevent wet wells from flooding. Proper check valve selection and installation will prevent premature wear of the valve and an unscheduled shutdown. The keys to successful check valve performance are valve selection, size, pipeline installation and cost of ownership. Time spent evaluating these key considerations will maximize check valve operational performance.

If you are looking for more details, kindly visit double door check valve.

Evaluation Before Valve Selection

The design of the pumping system should be evaluated before selecting a check valve. First, consider whether the pump system design has potential surge issues. If surge issues are a concern, then a surge investigation should be performed by a valve manufacturer. The surge investigation may determine whether a pump control valve is better suited for the application than a check valve.

Key Check Valve Considerations

Valve Style: Selecting the appropriate style check valve starts with understanding the process media. The type of media – whether it is clean, abrasive, corrosive or slurry –  determines what style of check valve should be selected. For clean service, check valve styles such as slanting disc, double door and silent are most common for low head loss, large pipe or quick closure requirements. Although rubber flapper and swing check valves can be used in clean service, they are typically used in wastewater service. These valves are more suitable for abrasive/corrosive/dirty applications and rapid flow reversal. Swing check valves are commonly used for pump discharge applications due to control options that accommodate varying forward and reverse flow conditions.

Valve Size: Correct valve size is important to valve performance and may or may not be the same as the pipeline size. For best valve performance, sizing calculations for minimum, normal and maximum flow conditions are necessary to optimize valve life and minimize valve maintenance. The American Water Works Association standard (AWWA C508) for swing check valves states: “Valves may be subjected to excessive wear if there is insufficient flow to open the valve.”

Rubber flapper valves are typically sized to be fully closed or fully open with sufficient flow. Conversely, full waterway swing check valves are not typically sized for full open but are sized for the lowest acceptable head loss per the design. If the valve is sized too large and normally operates nearly closed, the disc connection can be worn. Premature wear can occur on the disc connection through vibration, oscillation and force until the metal connection is ground away and no longer allows the disc to seat properly in the valve body.

Case Study 1 

In one installation, a swing check valve was oversized because they were planning for future growth. The low flow rate of the valve resulted in the valve only being open 17 degrees. The near-vertical disc was subjected to high-velocity water flow near the top of the disc and low-velocity swirling water at the bottom of the disc. The unbalanced dynamic forces on the disc caused the disc to wobble and wear the connecting parts. The pin and connections were worn down and the center hole in the disc was worn out, not allowing the disc to close properly. In time, the disc became loose and failed to close.

Valve Installation: Valve installation in the pipeline is a critical consideration to the success of the check valve. The recommendation by Manufacturers Standardization Society of the Valve and Fittings Industry (MSS SP- 92) is to install a check valve at a minimum of 10 pipe diameters of straight pipe on the downstream side from tees, fittings, increasers or pumps and five pipe diameters from elbows to ensure laminar flow with minimum turbulence to minimize disc movement and premature wear. However, many facilities with smaller footprints have achieved acceptable performance in systems with the check valve installed five pipe diameter lengths of straight pipe from the downstream side of tees, fittings, increasers or pumps and three pipe diameters lengths from elbows, as shown in the illustration below. Consult the valve manufacturer for design pipe distances less than the recommendation by MSS.

Design Goal: The goal of design planning is to minimize the facility footprint without compromising equipment performance. Equipment will last longer under laminar flow and with no sudden changes in flow velocity. Sudden changes in flow velocity occur during a sudden pump shutdown when the flow reverses and is stopped by the closing of the check valve. This sudden stop in fluid flow or flow velocity change instantly reduces the pressure below the vapor pressure and may cause momentary column separation. The vacuum created by the column separation draws the two columns together violently. When the columns rejoin it creates a high-pressure shockwave, also known as water hammer. Water hammer creates a loud banging sound which is disruptive and the accompanying pressure surge could be destructive if left untreated.

AWWA C508 states: “Conditions of water hammer, hydraulic pulsation, and excessive operating noise are results of system design rather than valve design and are beyond the scope of this standard and require special design and construction considerations.”

Flow Characteristics: Shown below are Computational Fluid Dynamics (CFD) analysis of a check valve near an increaser. Eddies formed at the bottom of the increaser and created turbulent flow through the check valve. Moving the check valve away from the increaser removed the eddies and created a more laminar flow through the check valve, as shown in the second CFD image.

Increaser: Another way to ensure more laminar flow is to limit the size of the increaser between the pump and check valve so it is not greater than two pipe sizes. For example, if the pump discharge is 12 in. it would be acceptable to use an increaser to a 14- or 16-in. check valve size, but any larger would be too extreme.

Case Study 2:

In another installation, because of the facility’s extremely compact footprint, the check valve was installed immediately after the pump and increaser with little pipe length distance between the check valve and pump. The high-velocity pump discharge focused a jet of water directly at the check valve disc and pin connections, prematurely wearing out the valve components and causing improper seating of the disc.

Cost of Ownership: When selecting the proper check valve, it is important to consider the cost of ownership. The cost of ownership includes initial cost, operating cost and maintenance cost. An inexpensive check valve may end up costing less at startup but may cost more in the long run with unscheduled maintenance and downtime than a valve that is better suited for the application. The more suitable valve will perform better, require less maintenance, and provide system longevity.

Field Experience

Issues: Failing check valves are often caused by installations too close to pumps and increasers. In addition, there is a tendency in design planning to oversize discharge pipes for future expansion which then require extreme reducers to fit the pump size. Both these issues create turbulent flow that can wear the disc and pin connections and cause premature check valve failure.

Solutions: Ideally, the pipe distance between the pump and check valve should be calculated during the design phase of the project. For existing facilities, one solution is to move the check valve further away from the pump to achieve a more laminar flow, preferably five pipe diameters. If there are space limitations and moving the valve is not an option, then consider another style of check valve. Another solution is verifying the check valve is properly sized for the process conditions and, if possible, avoid utilizing extreme increasers.

Keys to Successful Check Valve Performance: The role of the check valve is to prevent backflow from damaging pumps in the process system. Check valves play a vital role in the successful operation of a facility. When designing a pumping system, it is beneficial to request a surge investigation from the valve manufacturer to determine whether the process conditions require a check valve or a pump control valve. If a check valve is required, then valve selection, size, installation and cost of ownership should be considered before specifying the valve. These key considerations will extend the life of the check valve, ensure proper protection of process equipment, and prevent an unplanned shutdown.

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