SURGE TANK OPTIONS
The purpose of a surge control tank is to provide a pre-determined amount of gas (often air) and liquid (often water) water at the moment a surge event occurs. The air and water volumes, normally calculated by a hydraulic study which examines the hydraulics for the whole pipe system, are chosen to mitigate the transient pressures during a surge event. When low pressures ocurr in the connected piping the air in the tank will expand pushing the water into the system to mitigate low pressures, when high pressures arise, the air in tank is compressed and acts like a shock absorber to reduce the magnitude of the high pressures.
If the air and water volumes in the tank are not correct at the time of the surge event then excessive high and low pressures will arise which can lead to pipe bursts or collapses.
To ensure that the air/water volumes are correct surge tanks are often provided with a control system which will maintain the ammounts of air and water at the required levels within the tank at all times when the system is operating. There are many different ways of providing such a control system with the most commonly used options being 5 Point Control and P-V Control,
The most commonly provided solutions are;
5 Point Control - A fixed level control system
The liquid level in this system will be controlled to a set level for the tank. The most common configuration of level instrumentation is that of a magnetic float liquid bypass level gauge with magnetically sensitive switches set at the control points.
This type of tank will provide the correct liquid / air volumes at the design point for the system and is ideal for pumped lines with fixed speed pumps and a consistent system configuration where pumping pressures and flow remain constan. Outgassing from or gas absorption by the water will automatically be corrected for by the system.
The liquid level and air volume in a P-V tank is continuously monitored and adjusted according to an algorithm in an electronic control system which will take action to either raise or lower the liquid level in the tank depending on the operating conditions. The tank requires a control system that can maintin the volume of the air according to the P-V relationship.
This type of control system is ideal for systems where the operating conditions vary and the air/liquid volume requirements strictly follow a P-V relationship and the tank must be sized to accomadate all possible regimes.
Controlled Level Tank
A controlled level tank is similar in operation to the P-V tank but instead of strictly controlling the air volume according to the P-V law, a hybrid algorithm is used in which the tank pressure is one of several inputs used to determine the liquid level that is provided by the system.
This type of control system is ideal for systems where the operating conditions are continuously changing and where the air/liquid volume requirements may not strictly follow a P-V relationship across the entire operating range.
An internal bladder is used within the tank to provide a barrier between the liquid and the gas charge. The purpose of using a barrier type maethod is to prevent absorption of the gas charge by the liquid, and thereby maintain a fixed mass of gas within the bladder under all operating conditions. A bladder tank is one of the simplest solutions available to provide a surge tank, the absence of a control system means that it is a low maintenance solution.
The use of a bladder can be both an advantage and a disadvantage, the bladder material may or may not be compatible with the liquid in the tank, it can be difficult to detect if the bladder has failed, in which case the tank may not be providing any surge protection at all, and with extreme ambient conditions the bladder may not provide the correct gas volume needed to provide full surge protection.
A bladder tank is ideal for systems where the operating conditions, the system configuration or ambient conditions are generally do not reach extremes.
Mass Controlled tank
The mass of the air or liquid in the tank may be controlled to give the correct proportions of live tank may be used where the gas charge can be adjusted to maintain constant mass. In this arrangement instrumentation may be used to detect conditions within the tank, which, when their signals are passed to a control system that can perform suitable calculations, enables the addition or release of the air charge to maintain the mass charge within the tank.
A mass controlled tank, where the mass of the air charge is controlled, is ideal for systems where the operating conditions, system configuration and ambient conditions do not vary significantly.
Whilst the above methods are the most commonly provided solutions surge protection measures that sometimes employed may include;
Dip tube tanks
One way surge tanks
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