One size-fits-all is not a good way of thinking when it comes to selecting air and vacuum valves and air release valves. In fact, choosing the wrong size valve can negate the purpose of a release valve completely.
Air valves are hydro-mechanical valves designed to automatically release or admit air during the filling or draining of a piping system. Air comes out of solution in a pipeline because of low-pressure zones created by partially open valves, variations in flow velocity and changes in pipeline system elevation. An air pocket in system high points may reduce the flow of water in a pipeline by reducing the flow area and in severe cases, completely air bind the pipeline and stop the flow of water. Air pockets in pipelines are difficult to detect and will reduce the pipeline system’s overall efficiency due to additional head loss and increased power consumption required to pump the water.
Proper sizing of air and vacuum valves is an important factor to consider when installing a new system or repairing an older one. Doing a little background research on your operation and following a few simple equations will alleviate future headaches and potential catastrophe from wrongly size or type air/vacuum valves.
There are three main types of air and vacuum valves — air/vacuum valves, air release valves and combination air valves — and each one has its own unique requirement to consider when sizing.
The American Water Works Association (AWWA) cover these in much more detail in their Standard C512-15: “Air-Release, Air/Vacuum, and Combination Air Valves for Water and Wastewater Service.”
Air/vacuum valves, also known as large orifice valves, are automatic float operated valves with two overall functions. The float operated valve controls the exhaust of the air during the filling of a pipe system and will simply close when all air is exhausted. The valve will also fully open during draining or if a negative pressure occurs. When sizing this valve, consider both the filling and draining of the piping system separately, as explained in more detail below.
An important valve sizing consideration is that this type of valve will not release accumulated air from a piping system while the system is in operation and under pressure. The air/vacuum valve is typically equipped with a simple double guided, float-operated valve seat that is normally open during vacuum piping condition and fully closed during pressurized system operating conditions.
The first sizing step is to determine your maximum flow rate in gallons per minute (gpm) in the pipeline. This may be a known constant for your specific system, or if gravity-based, you may need to calculate it using the diameter and slope of the pipeline. Once that is understood it can be used to determine the rate at which air will exhaust in cubic feet per minute (cfm) when filling the piping system. Using the flow rate in gallons per minute, divide that value by 7.48 gallons per cubic foot to give you your cfm of exhausted air.
The example below uses a 12-inch pipeline with a designed flow rate of 3,000 gpm, which is about 8.5 feet per second flow velocity and equates to 401 cfm. Use the calculated cfm of air discharge and a pressure differential no greater than 2 psi to determine the appropriate valve size represented in the chart below.
Using the table, 401 cfm falls between 387 and 445, in the 2-inch orifice size column, indicating a 2-inch valve in necessary for proper exhaust.
When using an air/vacuum valve to admit air into the pipeline to drain for repair or maintenance, it may be necessary to use a different size than the valve previously selected for filling the piping system.
When calculating and sizing this vacuum valve function, take into consideration the pipeline slope (S) and a percentage of vertical height (h) of the slope in relationship to horizontal distance (d) to determine the gravity flow. When calculating the percent of vertical slope or grade, use the steepest grade. In this example, the steepest grade is between stations 1,000 and 1,500. Between these stations with a horizontal distance of 500 feet there is a vertical height difference of 40 feet or 8%.
Based on this piping slope information, use a different equation to calculate the air volume (Q) in cfm needed to have the water properly drain back under these gravity and vacuum conditions. Use a Chezy flow coefficient (C), which in this case is 110. That value indicates the roughness of the pipe. For concrete it would be 120, steel is 130 and PVC pipe is 190. For this example, with a Chezy flow coefficient of 110, the calculated slope (S) of 8% or 0.08 and the pipe inside diameter (D) of 12 inches; the calculation results in 730 cfm as shown.
The air valve orifice size is typically based on a maximum pressure differential of 5 psi. Since the piping inlet pressure is atmospheric pressure (14.7 psia), then any negative low pipeline pressure may produce sonic flow. Sonic flow will occur when the outlet-to-inlet pressure ratio falls too low which may damage the system and potentially collapse larger diameter piping.
The valve needed to allow water to drain out freely at a 730 cfm flow rate (between 688 and 824 as indicated), within the 5 psi pressure differential is a 3-inch size according to the “Air-Inflow Valve Orifice Diameter” table below.
After determining valve size using both methods, select the larger of the two air valves calculated, or the 3-inch valve orifice in this case, rather than the smaller 2-inch size valve calculated above needing to exhaust 401 cfm of air from the pipeline.
It is also important to note that when filling and draining a piping system, it should be done at a control flow velocity of about 1 to 2 feet per second to minimize pressure transients. If there is a risk of larger pipes collapsing from vacuum formation, you will need to determine the maximum tolerable pressure differential using the formula below, which includes a safety factor of 4.
P = 16,250,000 x (T/D)3
P = Pipe Collapse pressure (psi), T = Pipe wall thickness (in.), D = Pipe diameter (in.)
After calculating P, if it is lower than 5 psi, use the cfm value determined with the above equation. If P is greater than 5 psi, pipe collapse may be a concern on large-diameter pipe. It is recommended that a different type pipe or wall thickness is used, and the pipe manufacturer be consulted to provide maximum external collapse pressures.
The above pipe collapse formula is applicable to a pipe submerged or an aboveground environment. Pipes used in buried service with firm soil compaction are not as prone to vacuum collapse.
In the example above, with a 12-inch pipe, there is no concern with the collapsing pressure calculated over 715 psi. However, if a 24-inch pipe was used with a wall thickness of only 1/8 inch, the collapsing pressure would only be 2.3 psi. This could be a serious problem when 5 psi is the minimum pressure differential needed to protect the pipe under vacuum conditions.
The air release valve is the workhorse of air and vacuum valves automatically letting air out from the system high points all the time while under pressure during system operation. Clean water can typically contain 2% air, while wastewater contains about 6%. Air is compressible in the piping system while water is not, and as a result of the changes in pressures in the system, it will come out of solution. The air will accumulate in the high point of the piping system, causing higher flow restriction, which leads to higher operational costs. Also, if air is not properly vented out from the piping system, you may have water hammer and higher corrosion and maintenance costs.
Air release valves are also referred to as small orifice valves due to their smaller orifice size in comparison to air/vacuum valves. They are typically connected to a compounded hinged float mechanism versus a simple float seat operation in the air/vacuum valves. This is how air release valves get their hydro mechanical advantage to open/close and vent air under pressurized system operation.
Sizing an air pressure relief valve can be a challenging task without a little background knowledge and experience. The above example of a 12-inch pipe with a flow rate of 3,000 gpm, falls in the table between the range of 2,001-5,000 gpm, indicating a 2% air release factor. Use that and the formula below to calculate air discharge rate in cfm.
EXAMPLE 1
Based on this calculation, there will be 8.0 cfm of air discharge in the 12-inch pipeline at 3,000 gpm.
Use the table labeled Air Discharge Capacity (next page) that shows orifice size based on working pressure to determine that a valve with a 3/32-inch orifice diameter is needed if working pressure is 100 psi. The 3/32-inch orifice has a 9.5 cfm capacity (see circle in table), well within the requirements.
The air volume vented through the orifice of the air-release valve at the pipeline is directly related to the working pressure at that valve location. As you can see from the Air Discharge Capacity table, the higher the pressure the more air volume capacity or cfm flow.
Once the required orifice size is determined, in the example above, you need to select the appropriate release valve connection size. At the 100 psi system pressure in the example, a 1-inch valve with a 5/16 inch orifice will work fine for the application. It is still important to go through the above sizing steps to make sure your piping system is properly protected.
It’s not over yet. A 3/32-inch hole on the side of a 12-inch pipe would not do much in relieving air. A valve body that has an internal body is needed so that it can accumulate enough air to activate the cam and lever float mechanism to open/close the valve. Here comes the part you cannot calculate: selecting the air release valve body size and connection.
Combination air/vacuum valves are simply an economical and practical combination of the air/vacuum valves and the air release valve in one body configuration. Typically, these air combination valves are available in smaller sizes, from 1/2 to 3 inch. In larger sizes, the larger air/vacuum valve may have the smaller air release valve attached on the side of its valve body. The sizing of these combination valves follows generally the same steps as explained above.
Specifying and selecting air and vacuum valves are important, as is the proper installation. The air valve, regardless of type, should be installed as close to the pipe as possible with an isolation valve. Isolation valves need to be full-ported and connected to the top of the pipeline to facilitate maintenance.
A good preventive maintenance schedule is also very important for all air and vacuum valves, as they are often located throughout the piping system in hard-to-access places. This is often overlooked and misunderstood after the valves are in service. You will find that when your air and vacuum valves are properly sized and maintained they will make your piping system more efficient with less maintenance.
When it comes time to install or replace air pressure relief valves and air and vacuum valves, make sure you consider these factors, and when in doubt, contact a valve professional. Experience and familiarity go a long way in getting the proper size valve for your specific application.
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