In order to maintain system design efficiencies and provide protection from transients, water pipelines and distribution systems require the installation of air valves at high points and regular intervals to exhaust and admit air during system operations including filling, draining, and critical conditions such as surges or line breaks.
A significant risk to public health exists when a drinking water distribution system is exposed to the negative pressure with possible entrance of contaminated water through the air valve.
CSA SUB series, available for the water, sewage and industry line of CSA air valves, are equipped with a threaded outlet connection to avoid the entrance of contaminants, in case of underground installations, reducing at the same time noise and spray effects coming from a possible rapid filling.
It is important for engineers to be fully aware of the need of protection of water and wastewater pipeline from negative pressure occurring in case of pipe drainage or burst, in presence of undersized or poorly located air valves, and in case of transients associated to flow rates changes, like valves operation or pump trip.
Failing in doing that may lead to the entrance of contaminated water into the system, pipe collapse, cavitation and column separation because the negative pressure can be just as damaging to the network as high pressure.
Alleviation of sub-atmospheric conditions commonly requires the introduction of properly located air valves at the high points, changes in slopes, long segments and whenever the risk of column separation may appear.
The air flow capacity through air valves depends on the effective section of the seat, following physical laws and mathematical formulas. Engineers should therefore always base their analysis on the air flow charts provided by the manufacturer and not only on the connection which may lead to inaccuracies and great danger.
The following pictures shows, purely for illustrative purposes, the use of two sewage air valves both with flanged DN 150 mm. On the image on the right CSA full bore model with an intake capacity of approx. 5000 m3/h with a 0,5 Dp, suitable for the application of pipelines up to 700/900 mm , while on the left a model with reduced passage on top, features a capacity less than 300 m3/h, strongly advised for pipeline not larger than 150/200 mm. If used for larger pipes the latter will inevitably lead to negative pressure.
The video below shows the power of vacuum as a reminder of the great danger coming from a poor judgment and air valves assessment.
CSA AVS sizing program has been designed to help users in the choice and location of air valves, in case of pipe drainage and burst, then seamless integrated with the CSA surge water hammer analysis program, able to carry our dynamic simulations and transient analysis.
The innovative anti-surge tank A.V.A.S.T. has been designed to contain the devastating effects of water hammer, more precisely the transients coming from the sudden pump failure both for water and sewer systems. The device, fully automatic, proved to be an innovative and reliable solution thanks to the absence of air compressors, electricity, panels, bladders, pre-charges. A.V.A.S.T. is the ideal solution to avoid damages sometimes fatal for our systems as a consequence of uncontrolled overpressures and negative pressure waves.
It is an object of this engineering evaluation is to determine whether any adverse hydraulic conditions or unacceptable surge will occur during the operation of hydraulic systems. The following video presents the simulation of the transient event generated in a pipeline whose profile is represented by the two red lines, with the pumps on the left side at the bottom and the downstream boundary condition on the right. The software shows the evolution of the HGL during the event and the pressure variations on critical point of the system, associated to the coloured vertical lines. In case of unexpected power failure at the pumps, without any surge protection devices nor air valves, it is evident the negative pressure followed by a pressure surge in the second phase of the transient.
The picture below shows the minimum pressure envelope, depicted in purple and indicating severe negative pressure values, and the maximum pressure envelope in green reached in the course of the simulation.
Thanks to the use of CSA AVAST (pat. pending) the system is protected against unwanted and excessive pressure variations. The following video shows the same system with the introduction of CSA anti surge tank AVAST at the pumping stations, in addition to CSA non slam air valves along the profile, The purpose of AVAST is reduce the drop of the HGL in order to avoid negative pressure consequent to the pumps failure, to contain the pressure surge on the second phase of the transient by means of a patented air outflow control system.
The picture below shows the minimum pressure envelope, depicted in purple, and the maximum pressure envelope in green reached in the course of the simulation. It is evident, compared to the same case without protection, the effectiveness of the solution chosen to prevent negative pressure and unwanted pressure spikes from occurring.
CSA RFP technology
The CSA RFP technology can be considered like some sort of air bag for pipelines and is based on the purpose of ensuring unrestricted flow of air in during vacuum, to provide the same degree of protection of a standard air valve during critical operations like pipe draining, burst of pump failure with the forth functions or RFP mechanism being triggered only in case of excessive air out flow, to prevent the valve from being slammed shut from water during the closing phase. RFP can be used everywhere except for those sections of the pipe where column separation may occur and exposed to severe negative pressure conditions.
Below particular of pressure measurements taken on a pipeline subject to rapid filling with frequent failures due to the water hammer generated by conventional combination air valves, depicted in dark blue, and with CSA 3F RFP, light blue, necessary to solve the problem.
See the animation on the operation of RFP vents:
CSA AS technology
The CSA AS technology, first introduced on the market by CSA, is obtained by an automatism composed of a metallic disk, spring and shaft, and located on top of CSA air valves therefore never in contact with the liquid thus maintenance free. As soon as negative pressure conditions occur the disk is pulled down and air will enter through the main orifice unrestricted. Upon termination of the negative pressure phase the spring will pull up the flat, allowing air outflow through adjustable orifices and creating the anti surge protection with the effect of reducing the water column approach and acceleration towards the air valve. The anti slam or anti-shock technology is definitely the safer choice under many circumstances.
Below particular of pressure measurements taken on a pipeline subject to frequent pipe failures due to the water hammer generated by pump cycles, depicted in dark blue, and with CSA 3F AS in light blue, necessary to solve the problem.
See the animation on the operation of AS vents:
Cavitation has been a familiar phenomenon for a long time for valves and fluid control applications involving the areas of acoustics, hydrodynamics, thermodynamics, physics and chemistry. The consequences for a control valve are often destructive with loud noise, vibrations, vapor, erosion of the valve component.
CSA thanks to the use of advanced CFD, state of the art testing facilities and years of experience has developed a series of cages to prevent cavitation and ensure the maximum stability of the valve under low flow events.
Pictured above are the XLC 300 and 400 series CSA valves.
The following versions of the moving block are available and are interchangeable on the same valve:
Standard version of the gasket holder and seal seat
LF version for stability with low flow rates
The LF version, with a gasket holder (1b) that allows progressive opening, ensures stability even at extremely low flow rates.
AC version for low flow rate and cavitation resistance
The AC system includes a device (1c) with progressive opening to ensure stability at low flow rates and resistance to the cavitation phenomenon.
CP version for maximum cavitation resistance
The CP system provides dual stage (1d, 2d) energy dissipation for maximum resistance against cavitation.
Detailed animations are available to understand the operation of CSA control valves, which are equipped with the unique GR.I.F.O. control device, made entirely of stainless steel, with filter included, which allows independent adjustment of closing and opening speeds and valve reaction time.