Liquid piping systems are prone to collecting air from incoming fluids, pumps and connections. This air can cause inefficiencies and serious operating problems. Last year, VALVE Magazine published “Why Air Valves are Needed in Water Applications,” which outlined the sources and effects of air in pipelines. This article explains how the various types of air valves operate in liquid piping systems.
Air release valves, which expel trapped air in a pipeline, are familiar to most people, but many other types of air valves are out there that have special functions, unique construction for wastewater and sizes up to NPS 20. Safeguarding piping systems from air-related problems requires knowing how to select and install the right air valves.
TYPES OF AIR VALVES
Three basic types of air valves are defined in American Water Works Association (AWWA) Standard C512 for use in water and wastewater, including:
AWWA air valves are constructed of iron or stainless-steel bodies with corrosion-resistant trim for water and wastewater service. An important point here is that these air valves have a different function than pressure and vacuum relief valves, which are installed on the top of gas or steam pressure vessels and liquid storage tanks to provide overpressure protection. Relief valves have set points designed to provide overpressure protection and are beyond the scope of this discussion. The air valves presented in this article automatically control the flow of air or gases in and out of liquid piping systems at all operating conditions.
Air Release Valves
Air release valves are probably the most widely used type of air valve and are characterized by small orifices, weighted floats and leverage mechanisms. The combination of these three features allow air release valves to expel air or gas at full operating pressure. Since air release valves have orifices that range in diameter from 1/16 of an inch to 1 inch, they have a limited capacity for admitting and exhausting air. In other words, a typical piping system will not be filled or drained using just an air release valve; such an action would take weeks. Air release valves automatically vent small pockets of accumulated air or gases as those pockets accumulate in a liquid piping system. For example, an air release valve mounted on the top of the pipe could automatically release trapped air that accumulates in the top of the piping system (Figure 1).
When installed, air release valves are “normally open” and expel air (Figure 2). It is only when liquid enters the valve that the float rises because of its buoyancy and seals off the valve’s orifice. Conversely, as air accumulates in the valve body, the float will drop because of its weight and reopen the valve. To reopen an air release valve under operating pressure requires a mechanical linkage for magnifying the weight of the float and breaking the pressurized seal on the orifice. Even a quarter-inch orifice will require 3.9 pounds of force to break the seal when operating at 80 psig, and a typical 3-inch diameter air release valve float only displaces about half a pound of water. Because of this, mechanical linkage is needed to multiply the weight of the float, and the orifice diameter on air release valves is limited in size to reduce the breaking force needed.
An air/vacuum valve has a full-size orifice ranging from half an inch to 20 inches. Because of this, the valves can exhaust large volumes of air. The valves also will admit large volumes of air to prevent a vacuum condition from occurring in the pipeline and to allow for draining. Air/vacuum valves are normally open (Figure 3) 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 is lost because of draining, line break or column separation, the float drops and allows air to re-enter 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 mechanical linkage and because of the large diameter orifice, have no ability to open while the system is pressurized. Therefore, an air release valve is needed to relieve air and gas during system operation.
A common application for air/vacuum valves is for the discharge of vertical turbine pumps. This is because after shutdown, the piping between the pump and the check valve fill with air. When the vertical turbine pump is started, it rapidly lifts the column of water, and the trapped air must be expelled before the water opens the check valve. An air/vacuum valve is the right fit for this application because it can rapidly expel large volumes of air and close when fluid fills the air valve.
An air/vacuum valve can be piped to the top of the pump column so that when the pump is started, the air trapped in the pump column is expelled through the air/vacuum valve (Figure 4). 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 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.
The slow-closing device has a disc that closes automatically when high air exhaust rates occur (Figure 5). The disc contains reduced ports that are typically 5-10% of the full orifice size. This is important when the valve is subjected to column separation or vacuum conditions in a pipeline. If the air valve location is subjected to a sudden vacuum pressure after a power outage and pump stoppage, the air valve will admit a large volume of air into the pipeline to prevent a vacuum. When the pipeline pressure returns, or a pump is started, the slow-closing device controls the exhaust rate of the air so that the water column slowly enters the air valve to prevent air valve damage and water hammer in the pipeline.
Combination Air Valves
The third type of air valve is the combination air valve, which contains the functions of both the air/vacuum and air release valves. A combination air valve can be furnished either as a single-body design, where a singlebody contains both air release and air/vacuum components, or as a dual-body design (Figure 6), where an air release valve is piped to the side of an air/vacuum valve. The two configurations perform the same functions. However, the single-body design can be more economical while the dual-body design can provide design flexibility when sizing the orifices.
Some piping designers use only combination air valves on a pipeline because all air valve functions are included; a mistake in field installation will not leave the pipeline unprotected. Other applications for combination air valves include pump discharge headers and use upstream for flow measurement devices (Figure 7). The combination air valve automatically releases air to improve the accuracy of the flow measurement device.
LOCATIONS ALONG A PIPELINE
Air valves are generally installed on liquid piping systems to exhaust air or gases and admit air to prevent vacuum conditions and air-related surges. The AWWA Air Valve Manual recommends air valves at various locations including high points, long runs, adjacent to mainline valves, downstream of pumps and where there are changes in pipe slope. Valve manufacturers provide online computer software to assist in locating and sizing air valves.
Installation methods for air valves are important to ensure their proper function (AWWA, 2016). The best results are achieved when the air valve is mounted directly on top of the pipe. Unfortunately, some pipelines are located under roadways, which requires the air valve be mounted in a separate vault. In these cases, it is important that the connecting pipe be sized for the flow conditions and slope upward to the air valve. Furthermore, extended air valve piping can have a multiplying effect on surges, so a transient analysis may be needed to evaluate this piping.
For maintenance purposes, all installations should include a shutoff valve under the air valve. Also, to help in collecting the air that travels along a pipeline, a riser pipe larger than the air valve inlet is recommended (Figure 8). A drain valve can be used to annually check the function of the air valve. If the drain valve expels air, the air release portion of the air valve may require maintenance or repair. Otherwise, the air valve should be observed during a pump 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 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, pressurized air can be trapped in the air valve. This is why care is needed when venting the air through a drain valve or pipe plug before removing the air valve cover.
Wastewater air valves can be subject to clogging from the collection of greases, grit and solids in the valve. For best performance, these valves should be equipped with elongated bodies, sloped bottoms, a minimum of a 2-inch inlet and smooth coatings. Depending on the service, a backwash kit can be added to the valve to facilitate maintenance. Also, a backwash operation (Figure 9) can be performed by 1) closing the isolation valve, 2) connecting the blowoff valve to a drain opening and 3) supplying water for several minutes using the water hose to flush out the valve. Some air valves may have a 2-inch cleanout for large debris or multiple ports on the top of the valve to flush.
When air is allowed to accumulate in pressurized pipelines, efficiency is sacrificed, and serious system damage can occur. By having an understanding of the various types of air valves, system designers can better select and install air valves to protect liquid piping systems.
1. American Water Works Association, VALVE Magazine, “Why Air Valves are Needed in Water Applications,” Spring, 2017, pp. 32-34
2. American Water Works Association, AWWA C512-2015, Air Release, Air/Vacuum, and Combination Air Valves for Water and Wastewater Service
3. American Water Works Association, AWWA M51, 2nd ed., Air Valves: Air Release, Air/Vacuum, and Combination, 2016