How to choose the right check valve? Gain insight!

Some of the things you need to consider are fluid compatibility, flow characteristics, headloss, non-slam characteristics and total cost of ownership. For optimal performance, it is of course important to choose your valve respecting the characteristics of each specific installation.

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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.

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.

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Some of the selection criteria to consider when choosing your check valves

Fluid

All check valves are designed to handle water and treated wastewater but handling of raw waste water/sewage may cause some issues. When selecting a valve for these fluids, you should probably consider how the presence of solids may potentially affect operation of the valve.

Flow characteristics

If a check valve closes very fast, it may prevent slamming. However, the rapid closure will not protect against the surges caused by pumps being started and shut down. If the valve opens (and closes) quickly, the flow will be changed suddenly and surges will more likely occur.

Total cost of ownership

The costs for your check valve may consist of more than just the purchase price. For some installations, the most important costs may be purchasing and installation but in other cases maintenance or energy costs may be equally or even more important. When considering costs as a selection criteria for your check valve, the total costs over the life of the valve should therefore be considered. In general, the more simple the valve construction is, the lower are the maintenance requirements. Both swing check valves and ball check valves are very cost-effective.

Headloss

Headloss is a function of fluid velocity and the valve headloss is affected by the flow conditions of the system and the internal surface of the valve. The geometry of the valve body and the closure design determines the flow area through the valve and thereby also affects the headloss.

The headloss to be considered is a combination of static head (caused by elevation difference) and friction head (caused by the internal of the pipes and valves). There are a number of formulas for headloss and rating of valves based on this. The most common is probably the flow coefficient of the amount of water passing through the valve at a certain pressure drop during a certain amount of time. However, for comparisons the resistance coefficient Kv is considered the best choice.

Both ball check valves and swing check valves have low headloss.

Non-slam characteristics

Check valve slam results in system pressure surge. First step of the process is when the pumps stop and the flow is reversed. That may cause some reverse flow through the valve before it gets to a fully closed position. Then the reverse flow is shut off and the change in flow velocity turns the kinetic energy of the fluid into pressure.

A slam sounds like if the disc or the ball from the check valve is hitting the seat and can make quite some noise. However, the sound is not caused by the physical closing but by a sound wave arising from a pressure spike stretching the pipe wall. To completely avoid slamming, the check valve should close before any reverse velocity can occur. Unfortunately, that does not happen. The geometry of the valve decides how much reverse flow will occur so the faster the valve closes, the less slamming.

As swing check valves have the disc in the flow stream helping with rapid closure, they have better non-slam characteristics but today most pumps are frequency converted enabling them to adjust the start-up and closure time in order to avoid water hammer.

Check valves can be installed both horizontally and vertically. Slamming may be a bigger issue for vertical installations since a vertical flow reverses more rapidly but a vertical installation allows gravity to assist in closing the valve rapidly.

There are many questions you should ask to determine which check valve is best for your application

Check valves are often one of the most overlooked and misunderstood components of a water flow system, but using the proper size and style is essential for efficient operation.

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Simply stated, check valves are designed to allow flow only in one direction and prevent back flow or reverse flow. Improper sizing or selecting the wrong valve can pose serious hydrodynamic issues on the pumping system performance and possible failure.

To choose the right check valve, it&#;s important to analyze the system hydrodynamics from the beginning. Jim Tucci, National Sales Manager for Flomatic Valves, says, &#;It&#;s all too often that people order the wrong valve because they aren&#;t assessing the system as a whole when choosing the valve size or type&#;. It is crucial to use a &#;system approach&#; when selecting size and type valve because the hydraulic conditions of each and every system are different.

When selecting a check valve, the following series of questions can help establish the application needs and find the proper size and type of check valve needed.

• What is the flow rate or anticipated flow rates of the system (GPM)?
• What are the pressures, under both static and pumping conditions (PSI or FEET)?
• What is the pipeline diameter size?
• What is the media (Potable water, wastewater, slurries etc.)?
• What potential chemicals will be used in the system?
• What size solids will the system deal with, if any?
• Is it a multiple pump application?
• Is it a header system?
• Where and how far is the discharge pipeline going (Feet)?
• Open or closed discharge line?

How water will flow through the system directly affects check valve size and type. &#;A lot of times people don&#;t realize it&#;s so important to know the answer to all these questions,&#; Tucci says. &#;But it is very important for a trouble free system operation. If all that isn&#;t taken into consideration, you can have potential problems.&#;

Specific information on what media is used, the type of water and size of solids that will be passing through will dictate the size and style needed to handle the debris. &#;If you&#;re running grey water or regular sewage, you want to specify a full port check valve to allow solids and stringy material to pass easily and avoid the hinge arm of the swing check valve to get stuck.&#;

Internal components, like the seat trim, may also vary depending on the circumstances unique to each system and application. &#;As far as materials, that is dependent on a lot of things, like if there are any additives that may be in the flow stream or where it is in the treatment process,&#; Tucci says. &#;Another important factor to know is where the valve is going to be located. Is it outdoors in a coastal environment? If you have really salty air, you want to make sure you have stainless steel components.&#;

Where and approximately how far the pipeline is going is information needed to calculate head pressure and surge potentials. It&#;s important to know whether the water or wastewater flow will be open discharge in a gravity system or a pressurized closed system. Each variable impacts the calculations and may require different valve type with an external assisted closing system.

Knowing your flow rate and pipe size are particularly important. &#;I often ask the customer what their flow velocity is, and they can&#;t answer that question. If they provide us flow rate and pipe size, we can calculate it.&#; Tucci explains that it&#;s all based on flow velocity. &#;Certain check valves have certain velocity ranges and certain acceptable head parameters. If they aren&#;t sized properly, you&#;re going to have maintenance issues with the system.

&#;With increased flow velocity, you have increased surge potential if a pump were to fail. You will have the possibility for greater slamming potential or waterhammer if something goes wrong.&#; Surges will travel down the pipeline providing another example of why it is so important to think of the system as a whole, and how a wrong sized valve can impact more than just the area or pipe where it is located.

Too-low velocity can result in chattering of the internals, depending on the style of the valve, causing the spring or closing mechanism inside the valve to wear away at a faster than normal rate. The valve may barely open under low-flow conditions which will eat away the seating components, also leading to shorter lifespan.

&#;Check valves want that sweet spot,&#; Tucci says. &#;If you&#;re too low or too high, you have the potential to wear that valve out.

Swing style check valves like Flomatic&#;s model 745 AIS are common across the spectrum of water systems and ideal for systems where flow is constant to minimize pressure drop.

&#;It&#;s a great overall check valve for wastewater applications or clean water applications. It&#;s a full port check valve and it has a broad range of acceptable velocities.&#;

Check valves that are factory compatible with optional features allow operators more control and monitoring over the valve&#;s functions. Add-ons like a position indicator provide a visual indication of the internal disc&#;s location during operation and shutdown. Another option to consider is a backflow device. These devices can be installed in check valves and manually operated when backflow, pump priming, line draining or system tests are required. Limit switches are also extremely useful for remote indication of valve position and a positive sign of flow.

Check Valves are Designed to Protect The System

&#;Check valves are designed to protect your system and pump. You want check valves closing smoothly, you want the system be happy and to protect the pump,&#; Tucci says. &#;When you get outside the recommended operating conditions the system becomes unhappy and things are going to prematurely cause grief. It might be as minor as having to replace the seal, but it could also be as major as the side of a valve blowing out and dumping 30,000 gallons of waste on the ground.&#;

Taking the time to establish the needs using the &#;system approach&#; of your application will help ensure you always select the right check valve for the job.

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