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Modeling and Field Verification Study of Air Slam Conditions on kalanit Pipeline System

 

Air valves are integral part of long water transmission mains and are essential for removal of bulk and residual air during initial and subsequent filling operations. Air valves are also becoming increasingly popular as part of transmission main surge protection system. Whether air valves are considered as part of surge protection system or not during design stage of a transmission main, they may get activated during certain transient events allowing air into the transmission main. While allowing air into the pipeline system may help minimize extreme negative pressure conditions, uncontrolled release of air can lead to air slam conditions and the associated sharp increases in surge pressures. This paper presents a case study on a water transmission main that has experienced several damages along the pipeline system due to uncontrolled release of air during pump trip conditions. Preliminary modeling studies incorporating all existing air valves (kinetic air/vaccum valves) on the transmission main model during a pump trip event revealed the potential for dangerously high surge pressures at several different locations along the transmission main. The models also helped identify critical air valves and the potential for substantial reduction in surge pressures when valves at those locations were replaced by either non-slam air valves with low switching pressures or a new breed of dynamic air valves that eliminate the uncertainty on switching of large orifice to smaller orifice associated with conventional non-slam air valves. Subsequently, extensive field measurements were conducted with three different types of air valves; ordinary kinetic air valves, conventional non-slam air valves and the dynamic air valves at critical locations on the transmission main. High speed (1000Hz) pressure logging devices were able to accurately capture the air slam pressures that resulted from ordinary kinetic air valves on the transmission main. Controlled release of air through dynamic air valves completely eliminated air slam conditions and the associated field measurements were similar to those predicted by mathematical models.

 

 

Figure 1. Overview of Kalanit Pipeline System

 

System Description.Kalanit Pipeline System (KPS) is a groundwater collection system, pumping water from three different deep well pump stations to a common reservoir that serves the drinking water needs of large settlements near Sea of Galilea, Israel (Figure 1). There are direct tappings on KPS for supply of drinking water to two different settlements along the way. KPS is owned and operated by the Israeli National Water Company “Mekorot” and comprises 12 km of ductile iron and covered steel pipeline ranging from 250mm to 800mm diameters. Ground levels vary from 45m below sea level (-45m) near pump station to 66m above sea level (+66m) at delivery end. Figure 2 shows general arrangement of pumps and associated valves at typical deep well pump stations on KPS. There was no explicit surge protection on KPS except for the ordinary kinetic air valves that are essential for normal operation of pipeline. KPS has experienced repeated damages at several locations along the pipeline and over a period of time and most of the incidents were associated with the pump trip condition at pump station 1 (PS-1). In particular, repeated damages occurred at the air valve marked as critical air valve on Figure 1.

 

Figure 2. General arrangement at pump station PS-1

 

Attempts were made to provide additional surge protection by way of a connection to a nearby existing elevated storage reservoir (mimicking a one-way surge tank connection). The storage tank is located at an elevation of +43m about 2km from pump station PS-1 and received water from other pumps located within PS-1 area through a 500mm pipeline. Temporary arrangements were made to connect this storage reservoir to the delivery side of PS-1 pumps through a check valve so the potential for extreme negative pressure conditions at pump discharge could be reduced following the pump trip event. Though this arrangement has helped reduce the severity of pressure surges at critical locations, damages to the pipeline system continued. Since all the air valves on KPS were ordinary kinetic air valves with 100mm nominal inflow and outflow orifices (along with an automatic air release valve for removing residual air), it was suspected that air slam could be the reason for repeated damages to the pipeline systems and a comprehensive modeling study was undertaken.

 

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