Engineering of Water Systems

Part 17(d)—Mechanical Design, Valves, Part 4

By Ed Butts, PE, CPI

In the past three installments of The Water Works, we have discussed the mechanical equipment used in larger water systems with an overview on piping types, design, and pipe protection methods as well as isolation and throttling valves. This month, we will discuss check valves and air/vacuum release valves. Our series will conclude with a review of control valves in July.

Check Valves

Check valves are a one-way type of mechanical valve used on most pumping applications. Their use is to limit the flow of fluid in a system to a single direction only and are fundamentally simple valves in operation.

They are classified as one-way directional valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to exit. Fluid flow in the desired direction opens the valve, while a zero-pressure differential and/or reversal of flow forces the valve closed.

They use inlet or upstream pressure to open the valve, then automatically close when the flow either stops forward travel, pressure equalizes on both sides of the valve, or the downstream net force, by virtue of a spring-assisted closure of 1 to 5 pounds higher than the upstream force, prevents a reversal of flow from occurring through the valve.

Check valves do not normally require power to function nor need much routine attention or maintenance. They are a required valve on most water and wastewater systems as well as most industrial and irrigation service applications.

Without check valves, fluid pumped to a higher elevation would return to the suction or water source as soon as the pump was deactivated. In some cases, this flow reversal can result in backspin of the pump and driver and cause damage to both should the unit restart while backspinning.

Issues with check valves and potential problems start with normal wear of the valve disc and seat, up to abnormal wear and failure caused by excessive velocity, sand, slamming, or extreme cycling.

The most common anomalies with a check valve are slamming of the valve when starting or stopping the pump, incomplete or partial opening when the pump is on, and incomplete or partial closure when the pump is off.

A typical check valve primarily relies on gravity, spring-assisted force, and reverse flow to close completely.

An opportunity for a check valve to slam frequently occurs during a pump shutdown. When a pump stops, gravity and reverse flow can slam the check valve shut. Because fluid is non-compressible, it creates a pressure or shock wave, more commonly known as water hammer.

The fluid continues to flow back and forth until friction losses dissipate the pressure wave and cause the pressure to settle down to the static level. Slamming check valves can cause serious pressure surges in a system, from minor inconveniences that rattle pipes to serious conditions that can damage equipment and burst pipes.

Many people assume that’s just what check valves do, since “No matter what we do, they’re going to slam.” This is not usually the case. The real cause of check valve slamming problems occurs from poor sizing and selection, not from the type of valve.

Check Valve Types

Different types and styles of check valves display various values of reverse acceleration and non-slam performance. The selection of a system line check valve should not be made by simply choosing a non-slam type of valve as the default check valve, but by tailoring the check valve to the water system dynamics.

For example, a deep-well pumping installation using a vertical turbine pump (VTP) without a riser check or suctionfoot valve will display a much higher tendency toward reverse f low and acceleration as soon as the pump is disabled. This will cause water within the pump column to evacuate and empty the column through the pump bowls rather than the same well with a submersible well pump equipped with a riser check valve that closes immediately upon pump shutdown and retains the water within the riser column.

This type of system disparity demonstrates that while a conventional swing check valve may operate satisfactorily in the submersible pump installation, the likelihood of reverse flow in a VTP installation makes the potential for water hammer much greater.

The most common types of inline check valves used in well or booster pump service are illustrated in Figure 1. The degree of reverse velocity and graphical impact to the water system from the dynamic characteristics of the various styles of check valves are shown in Figure 2 (Ballun 1997).

Even with the proper type of check valve, many instances of water hammer can occur from sudden pump failure during an instantaneous power outage or normal pump operation without a flow transition method where the entire flow velocity from a single pump or facility changes over a matter of just a few seconds.

The most severe example of water hammer during a power failure is often the result of a stalled check valve function or the generation of flow reversal.

In the case of flow reversal during normal operation, the fluid in the pipeline has developed forward momentum as the result of the energy imparted by the pump to the fluid (Figure 3a). When a sudden power failure occurs, the pumped fluid retains the same tendency to continue unabated forward travel due to the momentum imparted by its kinetic energy.

This momentum can result in a situation where discontinuance of the energy provided by the pump can result in the interior of the pipeline developing a negative or reduced pressure region or zone immediately adjacent to and downstream of the pump, due to the physical separation of the fluid molecules or a shearing action of the fluid inside the pipeline (Figure 3b).

The fluid eventually can no longer sustain its forward momentum due to the combined effect from the resistance imparted due to gravity forces along with the loss of energy developed by the pump. The fluid will rapidly reverse in direction back toward the pump to relieve the lowered pressure zone developed by the separation of the fluid (Figure 3c).

Often, due to the higher pressure that exists on the delivery side of the pump combined with the vacuum effect of this lowered pressure, the fluid can develop an extremely high reverse velocity component needed to relieve this void in less than a second. When the reversed flow encounters a boundary, such as a check valve disc, the force created by this sudden stoppage of flow results in the familiar slamming noise that can result in destructive forces up to rupturing of pipes or fittings (Figure 3d).

This instantaneous change in energy—from a state of kinetic to pressure energy—results in one cause of the familiar phenomenon known as water hammer. This sudden and high-pressure spike continues back and forth within the pipe as the pressure wave oscillates inside the pipe until the pipe’s interior wall elastic properties begin to diminish and frictional forces eventually dissipate the remaining energy (Figure 3e).

The best way to prevent this type of problem is through adequate prevention by using non-slam or silent check valves. This type of check valve, popular with well pump service, has a relatively short travel distance that does not depend on gravity to close. This, along with aid from the stored force from a compression spring, will cause the valve to fully close upon a small pressure differential occurring on each side of the valve before flow reversal can start with a differential of less than 0.5 psi (1 foot of head).

In other types of systems, it is sometimes desirable to allow some reverse flow back through the valve before fully closing. In these cases, the valve is often equipped and cushioned with a dashpot, an air- or oil-cushioned device that provides an initial rapid closure to around 75%-90% of full closure followed by a slow and orderly closure of the remaining travel of the check flapper. This also prevents a slamming action of the valve’s internal components by providing a controlled closing speed.

When a swing or tilting disc check valve is used, external access to the check valve disc is often required so the valve can be equipped with a weighted external lever or spring to aid in rapid closure of the valve.

Either of these methods can greatly diminish the degree of surge pressure developed by preventing any acceleration of the pumped fluid in the reverse direction.

Yet another type of check valve, one used in certain booster pump or gravity service, works in an opposite manner. The valve remains fully open following pump deactivation. This allows flow to continue its downstream movement along with the addition of fluid from the head provided from the source through the inoperable pump to prevent column separation and then through the use of external controls, which slowly closes the main valve so that the time of closure is extended.

This type of application can be used with many booster pump systems, as a bypass feature around the pump to provide a low head loss route of water or on the pump’s discharge.

Sizing and Selection

The most appropriate size and type of check valve must be picked in order to render the best performance and service life. An undersized valve will cause higher pressure losses and create excessive noise and vibration. An oversized valve can lead to premature wear and failure of the valve’s internal components.

Choosing the best valve will ensure proper flow, optimize overall efficiency, and enhance the integrity and longevity of any fluid handling system.

Check valves are often selected based solely on the pipe size and the largest rate of flow possible.

Although you may want to send as much fluid through the valve as possible, this is not always the best or most desirable. In fact, the flow conditions should determine the internal performance of a check valve because the disc is situated directly in the flowpath.

If there is not enough flow to keep the valve disc fully open, the disc will flutter and move up and down or around in the flow stream, resulting in premature wear of the internal components, potential for rapid failure, and a higher pressure drop. For a generic check valve selection, consideration of the following elements is needed:

  • Pipeline sizing (use of a line size, full or reduced port?)
  • Application data (fluid characteristics including viscosity, presence of abrasives, and temperature)
  • High, design (optimum), and low flow rates with associated pressure drops (in feet of head or psi)
  • Cavitation potential and resistance at high velocities or differential pressures
  • Seat (trim) material type (rubber, bronze, stainless steel, steel alloy, etc.)
  • Permitted installation orientation (horizontal or vertical)
  • End connections (flanged, threaded, grooved, wafer, plain end, welded end, etc.)
  • Pressure rating (125 or 250 pounds is most common)
  • Material chemical compatibility with the fluid (cast/ductile iron, steel, PVC, etc.)
  • Water hammer potential and prevention (short disc travel, spring assisted closure, cushioned, etc.)
  • Sealing/leakage tolerance allowance.

In order to properly size a check valve, you must either consider the valve’s K value, relative to the velocity through the valve in the Darcy equation or the coefficient of flow (Cv) value (defined as the flow in U.S. gallons of water per minute at 60°F that will travel through the valve with a resulting pressure drop of 1 psi) to be able to customize a check valve to the application.

This involves changing the distance the disc travels from the closed to full open position. These values are shown for typical check valves in Table 1. When the valve’s disc is stable and fully open or closed against the seat, no fluttering, chattering, or excessive vibration will generally occur.

Once again, simple problems with a check valve, if ignored, tend to lead to more serious problems with the system if not soon corrected. The two most obvious of these problems are slamming and incomplete opening or closure.

Diagnosis of a slamming check valve, generally caused by water hammer from too rapid a change in pipeline velocity, is usually done by no more than listening to the valve and observing downstream pressure fluctuations during pump startup or shutdown. Incomplete closure or opening is discovered by observing the valve and system during pump cycling.

In either case, the cause is often traced to excessive wear of components within the valve itself. Slamming of a check valve is usually the result of liquid partially or fully reversing direction through the valve before it closes. When the valve does finally close, velocity of this reversed flow may be high enough to result in the familiar and loud slam when the valve disc ultimately engages the seat.

This event can lead to severe, if not disastrous, results to the pump or system depending on the location of the valve and magnitude of pressure rise. In some instances, the pres-sure rise from this sudden closure can cause rupture of the piping or pump case.

In other cases, the reversal of flow through the pump may be adequate to cause a low-speed reversed rotation of the pump and motor. If the unit was to restart during this reversed condition, the resulting strain on the pump and motor would most likely result in total failure of the unit.

The solution to this problem is typically simple—stop the fluid from flowing backwards before it starts. This is often done by using a spring-loaded valve with a short disc travel. The spring is designed to fully close the valve at zero velocity, preventing flow reversal, before it can start.

This type of valve is referred to as a non-slam or silent check valve and is commonly used on water applications.

Most swing types of check valves (Figure 1B) are especially prone to this problem. However, it can often be corrected by the addition of an external spring or counterweight on the external valve lever.

Incomplete opening or closure of a check valve almost always requires removal, examination, and repair of the valve internal components. Incomplete opening of a check valve can cause reduced flow into the system.

Check valves have a cracking design pressure which is the minimum pressure upstream required to open or operate. Customers can carefully match application requirements through many different types of valves. Verification of this is performed by installing pressure gauges immediately on both sides of the valve. In most instances, the pressure difference between the two sides of a check valve should be less than
2 psi, and less than 5 psi in virtually every case and flow rate.

The upstream pressure can then be compared with the pump curve data to determine the difference from the actual pump head. A significant differential in pressure is cause for investigation and confirmation before the pump itself should be suspected.

While an incomplete opening of the valve is generally not a cause for immediate alarm, the opposite is true for a valve with incomplete closure. This situation can lead to total failure of the valve in a short time, causing complete failure of the pumping plant due to full flow reversal problems as described previously.

Although silent-type check valves (Figure 1G) offer significant advantages in most water pumping applications by preventing (or at least lessening) slamming or water hammer, the valve components are internal and unfortunately not easily examined for wear.

The short travel distance of the disc, while an advantage against water hammer, can result in wear along the stem guidesurfaces, which are  not visible to external examination. This wear, if allowed to become excessive, can result in misalignment of the disc, causing flutter and/or incomplete closure.

Periodic examination of the valve internal components, which requires removal from service, is recommended on critical installations with high pressure or flows to avoid this occurrence.

Submersible Well Pump Check Valves

Check valves used in wells as riser check valves require different selection and design considerations than above-ground valves. To provide adequate strength and pressure capabilities, many riser check valves use standard poppet arrangements and are constructed from ductile iron bodies and steel internal components as shown in Figure 4a.

Another unique factor involves the physical size of the valve, particularly for submersible pump installations. Check valves used in fixed well diameters must be able to physically fit into the well, while accommodating the drop cable and any other downhole pipe or devices. This can be challenging when needing to deliver high flow rates from smaller wells is necessary.

For example, consider 1000 GPM from a 12-inch well where an 8-inch check valve made from carbon steel and flapper internals is used (see Figure 4b).

The second pertinent factor is strength. Check valves used in water well applications must often contain internal pressures up to 1000 psi as well as suspend a significant amount of weight of pump, motor, drop pipe, water, and cable set below the valve. This means the valve must possess adequate strength in both body construction and thread strength. This typically limits the selection to ductile iron or steel construction, although thermoplastics are capable of supporting a limited amount of weight.

Next is the type of threads. Check valves used for deepwell submersible pump service must be capable of supporting the weight of the pump, motor, drop pipe and cable, and water below the valve while resisting the torque during motor startup and acceleration without unscrewing or loosening.

On across-the-line starting with large horsepower units, this torque can generate several hundreds of foot-pounds of torque.

Typically, assuming adequate thread depth and engagement, tapered threads using ¾-inch per foot of taper with eight threads per inch on nominal sizes of 2.5 inches and larger are used to provide both structural axial strength and resistance against possible unscrewing.

In certain cases with pipe sizes greater than 12 inches, straight threads are used with pinning or tack welding of the joints to provide equivalent resistance.

Lastly are the various conditions related to variable flow. When a submersible pumping system is designed and sized for a specific flow rate but will operate on variable flows and is using a variable frequency drive, control valve, or other means to provide variable flow conditions, the valve internals— specifically the disc—may experience inline flutter due to incomplete opening.

Depending on the valve, this can result in vibration, wear of internal guides on the disc and body, and premature failure. In these instances, particularly if the expected low flow velocity condition is less than 50% of the design flow, the use of a specialized disc stabilizer or buffer or V-port valve/seat arrangement may be necessary.

Figure 4c is an example of a riser check valve designed for variable frequency drive deep-well submersible pump service along with a breakoff plug to enable draining of the drop pipe before pulling the unit. This can substantially reduce the weight of the installation and lessen the impact and potential water damage from pipes full of water.

Air and Vacuum Release Valves

Pressurized, two-phase (air and water) flow in a hydraulic system can be a complicated and difficult to manage condition. When combined into a fluid stream, the inherent lack of compressibility of water combined with the compressible qualities of entrained air create a solution with unpredictable states that can impact a hydraulic pumping or conveyance system.

This requires the separation and removal of air from the system whenever possible. There are three primary sources of air that occur within a pipeline or pumping system.

The first source occurs at startup. The pipeline contains air which must be transported and exhausted during filling. As the pipeline is filled, much of the air will be pushed downstream and released through hydrants, blowoffs, and other mechanical apparatus.

A considerable volume of this air, however, will become trapped at high or obstructed points in the system. Trapped air can have serious effects on system operation and efficiency. As air pockets collect at high points, a restriction of the flow occurs which produces unnecessary head loss and energy consumption.

As shown in Figure 5, trapped air forms a long pocket along the pipe descent with a constant depth (d). Since the air is at the same pressure along the air pocket, it can be shown that the head loss is equal to the vertical height of the pocket or dimension (H). A pipeline with many air pockets can impose enough restriction to stop all flow.

Also, sudden changes in velocity can occur from the movement of air pockets. When passing through a restriction in the line such as a control valve, a dislodged pocket of air can cause surges or water hammer. Water hammer can damage equipment or loosen fittings and cause leakage.

Finally, corrosion in the pipe material is accelerated when exposed to the air pocket, which can result in premature failure of the pipeline.

The second source deals with naturally dissolved air entrapped in the fluid. Water generally contains about 2% air by volume based on the normal solubility of air in water. This dissolved air will usually separate from water with a rise in temperature or a drop in fluid pressure which will occur at high points due to the increase in elevation.

The third source of air occurs during operation. Air can enter the system through equipment such as pumps, fittings, and valves, especially when vacuum conditions occur.

Trapped air in a pipeline can have serious effects on system operation and efficiency. As air pockets collect at high points, a restriction of flow occurs that produces unnecessary and higher head loss. A pipeline with numerous air pockets can impose enough restriction to conceivably stop all flow. Rapid changes in velocity can also occur from the sudden movement of air pockets. When passing through a restriction in
the line such as a control valve, a dislodged pocket of air can cause surges or water hammer, which can damage equipment, loosen fittings, and cause leakage.

Finally, corrosion in the pipe material is accelerated when exposed to an air pocket, which can result in premature failure of the pipeline.

Four basic types of air valves are outlined here with three defined in the American Water Works Association (AWWA) Standard C512 for use in water and wastewater. The four primary types of air valves include:

  • Air/vacuum valve (AWWA Std. C512)
  • Automatic air release valve (AWWA Std. C512)
  • Combination valve (AWWA Std. C512)
  • Kinetic air release valves (mainly used in irrigation and low head applications).

AWWA air valves complying with C512 are constructed of iron or stainless steel bodies with corrosion-resistant trim for water and wastewater service. Irrigation-grade air valves are often constructed from aluminum, PVC, or nylon bodies.

An important point is that air valves have a different name and function than pressure and surge relief valves, which are installed on water systems and liquid storage tanks to provide overpressure protection. Relief valves incorporate pilot devices with set points designed to provide overpressure and/ or surge protection. (This will be discussed in greater detail in the next edition of The Water Works.)

The air valves being talked about here automatically control the flow of air in and out of liquid piping and pumping systems at all operating conditions and are generally defined according to their orifice size: small and large.

Air/vacuum release valves (Figure 6a for 2-inch sizes and smaller and Figure 6b for 3 inches and larger) are also used for quickly venting large volumes of air during pipeline filling or startup, so they are also equipped with a larger orifice. However, in this case, they also permit the admittance of air back into the pipeline during emptying of the pipe. Thus, this valve type is often regarded as a two-way valve. This is important because some pipe materials—particularly large diameter, thin walled steel, or PVC—can easily collapse under negative pressure.

An air/vacuum valve has a full-size orifice ranging from ½-inch up to 20 inches. Because of this, the valves can exhaust large volumes of air. The valves will also admit large volumes of air to prevent a vacuum from occurring in the pipeline and to allow for pipe bursting or draining.

Air/vacuum valves are normally open in design and a float in the valve rises with the water level to seal the large orifice after the air has been exhausted. Conversely, when system pressure falls below atmospheric values from draining, a line break, or column separation, the float drops and allows atmospheric air to reenter the pipeline.

It is important to note that under normal operation, the float is held closed by the line pressure and will not relieve accumulated air.

These valves do not have a mechanical linkage, and due to the large diameter orifice, have no ability to open while the system is pressurized. Therefore, an automatic air release valve is needed to relieve any trapped air during system operation.

Another common application for air/vacuum valves is on the discharge of vertical turbine or submersible pumps without well check valves. This is to allow the piping between the pump and the check valve to fill with air and relieve any vacuum upon pump shutdown.

When the pump is started, it rapidly lifts the column of water and fills the pump riser pipe. The air/vacuum valve releases this trapped air before the water reaches and opens the check valve.

An air/vacuum valve is appropriate for this application because it can rapidly expel large volumes of air and close when fluid fills the valve. An air/vacuum valve is placed at the top of the pump column or riser pipe so that when the pump is started, the air trapped in the pipe is expelled through the air/ vacuum valve.

The valve in this case is equipped with a throttling device, which is an adjustable device mounted on the outlet of an air valve to control the exhaust flow rate. Since the pump can reach full speed in a few seconds, a throttling device is used to slow down the exhaust of air, preventing the water from rising too fast, slamming into the downstream check valve, and causing water hammer in the pump column.

Another optional device for an air/vacuum valve is a slow-closing device. This device is commonly used for pipeline applications where column separation may occur. One purpose of this device is to close when high exhaust rates might occur. It’s also used to regulate the exhaust rate of the air valve so that the water column does not slam into the air valve and cause water hammer or damage to the air valve.

The slow-closing device can be mounted on the inlet of clean water valves and on the outlet of wastewater air valves when column separation or vacuum conditions might occur.

Air that is trapped or accumulates in a pipeline will naturally rise and collect at high points within the system. This trapped air can cause pump failures, corrosion, flow issues, and water hammer and pressure surges. Unnecessary air in the pipeline also makes the pump work against higher head, resulting in additional energy consumption.

An automatic air release valve continually releases excess air out of the system, resulting in smooth and efficient flow and pumping operations. This type of air release valve operates either using a simple lever action (Figure 6c) or a compound lever action for higher pressures (Figure 6d).

In both cases, the orifice size is small compared to the float diameter. This differential in size, combined with the leverage afforded by the lever, allows the seating of the needle valve against the seat at high pressures.

Once an automatic air release valve is installed, it constantly operates automatically to release pocketed air. Air release valves are installed at the highest points in a pressurized pipeline where air naturally collects. Air bubbles slowly migrate to and enter the valve and displace the  liquid inside,lowering the liquid level within the valve body.

When the level drops to where it no longer buoys the internal float, the float drops. This motion pulls the seat away from the orifice using a lever advantage, triggering the valve to open and venting the accumulated air into the atmosphere. As the air is vented, liquid reenters the valve, once again buoying the float, lifting it until the seat presses against the orifice, closing and sealing the valve. This cycle automatically repeats as often as necessary to maintain an air-free system.

The third type of air valve is the combination air release valve (Figure 6e), which contains the combined functions of an air/vacuum and automatic air release valve. A combination air valve can be furnished either as a single-body design where a single body contains both automatic air release and air/vacuum components, or as a dual-body or valve design where an automatic air release valve is piped in parallel with an air/vacuum valve.

The two different configurations perform the same functions in tandem. However, the single-body design can be more economical while the dual-body design can provide greater design flexibility when sizing the orifices.

Some piping designers use only combination air valves on a pipeline because all air valve functions are included into a single valve. Other applications for combination air valves include pump discharge headers and for use upstream of flow measurement devices as a combination air valve will automatically release air to improve the accuracy of the flow measurement device.

Installation methods for all air valve styles are important to ensure their proper function. The best results are achieved when the air valve is mounted directly to the top of the pipe.

Unfortunately, some pipelines are located under roadways, which requires the air valve to be mounted in a separate vault and offset from the pipeline. In these cases, it is important that the connecting pipe is sized for the flow conditions and sloped upwards to the air valve.

Furthermore, extended (offset) air valve piping can have a delayed air release function, so a transient analysis may be needed to evaluate this piping configuration.

For maintenance purposes, all installations should include a shutoff valve placed under the air valve at the pipeline connection. Also, to facilitate the collection of any air that may travel along a pipeline, a riser pipe at least one to two nominal sizes larger than the air valve inlet size is recommended. This will help to assist and guide the air pocket into the valve as well as compensate for the offset connection.

A drain valve, set at the same elevation as the air valve, should also be used to depressurize the assembly and semiannually check the function of the air valve. If the drain valve expels air, the automatic air release portion of the air valve may require maintenance or repair. Otherwise, the air valve should be observed during normal operation to verify it is exhausting air and closing without excessive leakage.

Caution is needed when inspecting or performing maintenance on an air valve. This is because when the system is functioning, an air valve can release large quantities of air under pressure or admit large quantities of air under vacuum conditions.

Both can potentially cause bodily harm. Any maintenance on an air valve requires closing the shutoff valve under the air valve. But even with the shutoff valve closed, a pocket of pressurized air can be trapped in the air valve. This is the reason care is needed when venting the air through a drain valve or pipe plug before removing the air valve or its cover.

Air valves are routinely installed on a pipeline to exhaust and admit air to prevent vacuum conditions and air-related surges. The AWWA Steel Pipe Manual recommends air valves at the following points along a pipeline:

  1. High points: Use a combination air valve.
  2. Long horizontal runs: Use an automatic air release or combination valve at 1250- to 2500-foot intervals.
  3. Long descents: Use a combination air valve at 1250- to 2500-foot intervals.
  4. Long ascents: Use an air/vacuum valve at 1250- to 2500-foot intervals.
  5. Decrease in an upslope: Use an air/vacuum valve.
  6. Increase in a downslope: Use a combination air valve.

For long horizontal runs, automatic air release and combination air valves should also be used alternately along the pipeline. It should be noted that combination valves can be used at any location instead of automatic air release or air/vacuum valves to provide added air release capacity on the pipeline. When thin-walled steel, plastic, or any potentially collapsible pipe is used, it is important to determine if there is a risk of pipeline collapse due to the formation of a negative pressure (vacuum) within the pipe.

These general locations of air release valves are shown for a typical pipeline in Figure 7. In most cases, the size of the air release valve is a judgment decision based largely on experience. The 1.2% to 6% of air content can be variable and depends on the potential for entrained air emanating from the water source.

The air release valve inlet connection should be as large as possible to maximize the exchange of air and water within the valve. Smaller valves may be grouped together in parallel valve arrangements to provide an equivalent size to accommodate an excessive air discharge requirement, particularly at higher pressures.

For irrigation systems, kinetic air release/vacuum relief valves (Figure 8) help protect the pipeline system and maintain its efficiency. As inexpensive additions, these valves often use floating balls to facilitate venting and are used for quickly venting large volumes of air during filling or startup as they are equipped with a larger orifice.

Kinetic air valves are essentially bidirectional valves and designed for high-capacity air venting of pipelines during the initial filling and relieving vacuum on emptying. Since they are intended for high-velocity venting, this valve style is more exposed and prone to sudden closure from the force of the rushing air. Therefore, they are often equipped with throttling or slow-closing devices to control the filling flow rate with no more than 1 to 2 psi of backpressure on sources that cannot be easily controlled for the fill rate.

They do not function as automatic air release valves and remain closed once pressurized. This style of air valve is used on many irrigation systems and applications on level ground where vacuum release and air admittance are not critical design factors.

These valves are often sized using a fill rate analysis that is based on the maximum filling rate. It must be noted that the application of this style is based on a zero backpressure and reflects the maximum fill rate. Thus, any backpressure applied to the valve will lower this rate. A properly conducted fill-rate analysis evaluates the velocity of filling the line with the inside (ID) area of the pipe.

The valve is thereafter sized based on a typical or maximum filling velocity with this pipe area by applying these two values to generate a volume in cubic feet to determine an air-escape velocity.

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This concludes this edition of The Water Works. We will return in July with a discussion on control valves, their application, and design parameters.

Until then, keep them pumping!

Reference

Ballun, John. 1997. A methodology for predicting check valve slam. AWWA Journal March.


Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.