Sunday, 16 February 2014

Comparison of Surge Protection Systems - HCV vs ZVV vs Surge vessel vs One way Surge tank

Comparison of Surge Protection Systems-

HCV vs ZVV vs Surge vessel vs One way Surge tank


Hydraulic Check valve [HCV] is an hydraulic valve, that closes in controlled pace when outlet pressure exceeds the inlet pressure (returning flow conditions). 

Installation of Surge Anticipating Valve

Zero-velocity check valve [ZVV] is a mechanical valve that closes very fast when outlet pressure exceeds the inlet pressure (returning flow conditions).

 Installation of Zero Velocity Valve

Surge vessel (Believe in this case is a hydro-pneumatic tank) is a closed tank, that contains a specific volume of water and the rest compressed air. It is located adjacent the surge- generating device (pump, valve), connected to the main by suitable pipe, fills the main by water on down-surge, and dissipates the kinetic energy of returning flow by re-compressing the air.

Installation of Surge Vessel

One- way surge tank is an open tank, which fills the main by water when the pressure in its location falls below atmospheric value. It refilled gradually by water from the main when the pressure rises, controlled by a float valve.

Installation of One Way Surge Tank

Relevant comparisons: HCV and ZVV, Surge vessel and one-way tank :

HCV vs ZVV: Water hammer is result of velocity change: kinetic energy of the water mass converted to pressure when the water decelerates or accelerates. The time of change is a very important factor: increasing the pace of the change reduces the magnitude of pressure spike.
  • HCV closes slowly, hence reduces the water hammer potential.
  • ZVV closes very fast, before the returning flow accelerates, so the velocity change is minor.  
 Hence the solution provided by the two is entirely different and the efficiency depends on the conditions of the specific case.
  •  HCV allows returning flow for considerable period- that causes backward-rotation of the pump. Not possible when the suction is negative (above-water pump) as it applies the pressure on the foot valve.
  • ZVV may generate high water hammer if located incorrectly in the middle of the pipe, is an inline device that cannot be inspected or tested, may generate cavitation conditions and pipe-collapse if not accompanied by suitable air valves.
·         Both create certain head loss and energy waste.

Closed hydro-pneumatic tank is a very good solution for transient conditions, as it is practically an energy storage that is activated automatically when the pump energy is gone, and dissipates kinetic energy of decelerated water mass.

Open tank can be used only to prevent column separation by inserting water to the pipe in its location. It can replace air valve- but it can not prevent pressure spike caused by returning flow. Normally used in the peaks along the main.

Both are relatively-expensive solution; require professional maintenance, freshening of the water volume, special control valves. Closed tank requires also compressor[s] and sophisticated water-level control. The sizing of both types is a critical factor, as too-small tanks will not provide the expected results. Both cannot be inspected without simulation of the water-hammer, meaning stop of the pumping.

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