Points to Consider When Sizing and Positioning Air Valves – a 27 Year Review
27 years ago, the author wrote a booklet titled “Points to Consider When Sizing and Positioning Air Valves”, this was followed up with another book “Air Valve Technology Reviewed” in 1996 – both contained Air Valve Sizing and Positioning guides.
These two publications added to the many air valve design guides that have been developed before then and which, in the mid 1990’s, was incorporated together with other “rules of thumb” on air valve sizing into software programmes to assist the designer in simplifying the sizing and positioning of air valves.
The Problem with the use of Air Valve Sizing and Positioning Air Valve Software
By the early 2000’s, the popularity of the sizing software created the trend for many designers to accept the results without interrogation of the correct positioning and/or sizing of the Air Valves. This has resulted often in:
- Too many air valves being sized for a pipeline – more is not necessarily better when it comes to any pipeline component, specifically with air valves
- Air Valves being oversized – most the air valve sizing programme allows for standardizing the Air Valve size and/or pressure rating. This is normally on the largest valve size selected.
The problem of too many, or too large an air valve size will result in some air valve designs not having sufficient air velocity to “switch” to the anti-shock mode and therefore behave like a Kinetic Air Valve creating Air Valve Slam upon closure. Too many air valves in addition, add to the initial capital cost as well as potentially, the future maintenance cost of the pipeline.
An important point to consider is that virtually all air valve sizing programme will size the Air Valve for only one of two conditions i.e., for burst and/or scour conditions and select the large valve if both conditions are analysed. Some software programmes have a built in “nudge” in the form of a minimum flow to put an air valve on the change of a slope no matter how slight the change in slope may be. Lastly most software programmes will insert an air valve every 500 to 600m as a default. This is done regardless of the length and/or diameter of the pipeline and is also positioned from the start of the pipeline. This indiscriminate positioning of the air valves may result in two to three air valves being positioned sometimes within metres of each other.
In designing the pipeline, the designer should utilise his/her own discretion in checking the number and size of the air valves and eliminate those deemed no to be needed. The way in doing this is to visualize the pipeline under various operating conditions starting with pump trip conditions and the point from which the negative wave will be developed, Consideration should be taken for filling conditions – how will the pipeline be filled, size of the pipeline – i.e., is the spacing on a 1000mm diameter pipeline the same as that of a 200mm pipeline? Further, is the spacing required for pressurized air release or for adequate vacuum protection or, for surge protection etc.
The need to Link the Number and Sizes of the Selected Air Valve to a Surge Analysis
The assurance often given by the Air Valve manufacturer to the designer, is that if the Air Valve is sized adequately for Vacuum function, then the valve will be correctly sized to prevent Surge and Waterhammer. There are at least two points to consider with this statement namely:
- This statement is highly dependant on the design of the air valve – if the valve is of an Anti-Shock design, then, the valve may be too large to “switch” to the Anti-Shock mode as the air discharge velocity may not be sufficient across the large orifice – this valve will induce Waterhammer on closure. The statement will however be true for the AirFlo Variable Orifice Air Valve where the operation is such that the valve work in sync with the flow conditions in the pipeline and will always provide protection for the pipeline regardless of pipeline flow conditions.
- If the valve is oversized and/or there are too many Air Valves on the pipeline then, the Anti-Shock orifice may be too large to provide surge damping thereby still creating Waterhammer on closure.
This point can be adequately demonstrated in some Surge analysis software programmes. Designers have been aware of this problem with Anti-Shock air valves and have been guided by some so called experts to utilise the diameter of the small orifice.
This has led to the term Three (3) stage Air Valves or Triple Orifice Air Valves to signify that the valve has three discharge ports under Surge conditions. The reality is that the small orifice float does not get lifted once the Anti-Shock float has sealed off the large orifice, distorting the Surge results by a factor of 1000.
The Need for Manufacturers to Substantiate the Published Performance Figures of Their Air Valves
There is a peculiar and disturbing phenomenon in the market where most manufacturers have either copied the performance curves of the more popular air valve brands. With many of the most popular brands themselves having only done partial performance testing. Or a growing trend where many manufacturers provide performance curves substantially higher than valves designs of which they are virtual clones of.
The reason why most manufacturers either copy published data or utilise theoretical data or, even embellish the data has to do with many factors including the cost of air valve testing, wishing to stand out in a congested market or, basing a complex device such as an air valve on the performance of an orifice.
This phenomenon has a severe impact on the designer accurately sizing air valves. If the manufacturer’s performance data is embellished to show higher than actual performance figures, then the end result is the Air Valves may be undersized. This may result in the failure of pipeline materials such as HDPE, GRP or PVC that cannot withstand low negative differential pressures. Often when welds on large diameter HDPE pipes fail, this may be an indication of undersized or inadequately sized air valves.
All AirFlo Variable Orifice Air Valves have undergone independent 3rd party performance testing ensuring that our published data can be confidently referenced.
The Need to Revise the Sizing and Positioning of Air Valves in the Pumphouse
Most Air Valve sizing software programmes automatically place the air valve at the start of a pumping main. This is done without determining the type of Check Valve utilised on the pipeline or, without considering the pipeline profile.
The logic behind sizing this Air Valve is based on one of the rules of thumb for Air Valve sizing namely that the Air Valve subsequent to the pump should be sized assuming full pump flow. It is assumed that the column will separate upon pump trip drawing in a large volume of air. When the column returns, the pipeline will act like an accumulator and the air valve (if one with an anti-shock mechanism) will discharge air through the anti-shock orifice slowing down the advancing water column and therefore reducing the surge pressure at the pump.
The consideration for the designer is what type of Check Valve is being utilised – If the type of Check Valve being utilised is a swing type such as Swing, Slanted or Tilting Disc then, the check valve will only close upon flow reversal and upon closure, may create Waterhammer commonly known as Check Valve slam. In addition, the Check Valve will allow damaging flow reversal through the pump. It is therefore the shortcoming of the Check Valve design that would necessitate the need for an Air Valve at this location.
By installing a Nozzle Check such as our CheckFlo design subsequent to the pump, the valve will respond in a low milli second time span prior to the column separating thus obviating the need for an Air Valve at this location. In addition, this will not only prevent damaging Waterhammer but will save on the cost of the Air Valve. In addition, a Nozzle Check Valve has a low headloss and is maintenance free thereby saving in both energy and future maintenance.
The insistence of locating an air valve at this point will result in water spillage upon the air valve closing. Thus, the Air Valve will often require a modification on the outlet to allow the water to drain into a sump. Often, the air valve located at this point is further modified by some manufacturers with a Bias Mechanism that will allow vacuum breaking but will control the air discharge through a static discharge port. Due to the often, low milli second times span in which flow reversal can occur, this valve will still create some spillage on closure.
Investing in the use of a Nozzle Check Valve will not only enhance the surge protection of the system but may result in savings in initial capital costs and in the life cycle costing of the pipeline.
AirFlo Variable Orifice Air Valves and CheckFlo Nozzle Check Valves work in harmony in effectively and comprehensively protecting a pipeline from Surge and Waterhammer.
A well-designed pump suction arrangement will eliminate the need for an air valve on the suction side of the pipeline. There may depending on the size of the pumps, a need for an Air Release (Singe Orifice) air valve on the top of the pump volute.
The AirFlo Approach to Sizing and Positioning
We have, with the AirFlo Variable Orifice Air Valve design, taken special care to distinguish between the sizing regime for different type of pipeline materials based on the limitations for each material type including the installation method and the jointing. We also consider, as part of the Air Valve sizing and positioning, the use of other components on the pipeline such as Check Valves, Surge Anticipating Valves, the use of bypasses, the type of endline control valve utilised etc. This is all with the aim of ensuring that there is an optimum number of Air Valves used to protect the pipeline and that the Air Valves are working in harmony with other components on the pipeline through every possible operating cycle.
We have developed our own Sizing and Positioning software but emphasise that this should be a guide and that the designer should utilise his/her discretion when selecting Air Valves. To this end, we share our knowledge and insights gained over many years, free of charge. Please contact us for assistance with your Air Valve Sizing and Positioning or attend one of course on Air Valve Sizing and Positioning, Surge and Waterhammer or Pipeline Material selection. These are free of charge an attract one (1) Continuous Professional Development Point per course.
Allistair Balutto (c)
