The purpose of a surge control tank is to provide a pre-determined amount of gas (often air) and liquid (often 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 occur 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 occur which can lead to pipes bursting or pipes collapsing.
To ensure that the air/water volumes are correct, surge tanks are often provided with a control system which will maintain the amounts 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 PV Control.
On this page the most common types of surge tank are discussed;
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 method 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 vessel 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 do not generally reach extremes, such as where the liquid freezes, or vaporises.
A bladder tank provide low cost of and is ideally suited to remote locations due to low level of complexity of the equipment and the low level of skills required to perform maintenance.
A compressor type pressure surge tank is the generic, or common name, for a pressure surge tank that is linked to an air compressor and level control system that adjusts the liquid level in the vessel to some pre-determined requirement.
The level control system may be something as simple as a small control panel mounted near to the equipment on the floor, or a nearby wall, perhaps using relay logic, or something more complex that is remote from the equipment and uses a programmable logic controller (PLC) within a main panel in the local control room.
In whatever manner the system is controlled the purpose of the control system is always to ensure that the liquid level inside the surge vessel is at the correct position when a pressure surge event occurs.
To do this the control system will control the addition of extra air, or the release of excess air from the vessel in order to change the level up, or down, as may be necessary.
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.
The liquid level in this design of system will be controlled to a set level for the tank. Whilst it is possible to manually change the position of this level, it is not normally intended for this type of vessel to provide variable level control. 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 which are connected to a control panel.
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 constant.
Out gassing from liquid, or absorption of the gas by the water will therefore be automatically corrected by the system.
Because of the simplicity of the control arrangement, and the prevalence of single speed pumps, this type of tank and control system are the most commonly used.
In this arrangement there is no air compressor or internal bladder as the air charge is controlled by means of an air admittance valve on top of the vessel and an internal dip tube that penetrates the vessel to a set depth.
During normal operation the liquid inside the vessel will have a normal level that is above the bottom of the dip tube.
When a surge event occurs the liquid level in the tank will rise and fall within the length of the dip tube.
Should the air charge be absorbed by the liquid in the tank, or be lost through a leak, then on the next surge event the water level in the tank will fall below the bottom of the dip tube, thereby allowing the air admission valve to open and replenish the air charge.
In some arrangements the size of the vessel is reduced and the liquid level during a surge event is deliberately allowed to fall below the bottom of the dip tube, at which time the air admission valve will open and allow air into the system in order to sustain system pressure.
Whilst dip tube vessels can in some circumstances be smaller than other types of surge tank, their correct operation requires a greater certainty of the piping geometry than other types, they are often less tolerant of changes in the system, and require all the associated equipment, air valves, check valves, isolating valves etc to be working perfectly.
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 maintain the volume of the air according to the Pressure to volume, or PV, relationship.
This type of control system is ideal for systems where the operating conditions vary due to changes in the pumping conditions which would normally require a surge tank with multiple set levels, one for each pumping condition. In such conditions where the pumping pressure changes, usually due to a change in demand, but sometimes due to a change in the suction and/or discharge reservoir water level then a control system following a P-V relationship can permit the sizing of a surge tank that is suitable for all possible conditions.
This type of tank is widely used in Asia, Africa, Europe and the Middle East.
An expansion tank is used either to protect piping systems and vessels from over-pressure, due to expansion of the contained fluid, and/or under-pressure or vacuum due to contraction of the contained fluid, or as a reservoir that can provide instantaneous supply of the liquid on demand, to either sustain flow in the event of system failure, or to initiate flow where system start up takes too much time.
The most common cause for fluids in a piping system to expand, or contract, is due to a change in temperature of the fluid. So whilst is is commonly known that solids will expand or contract when their temperature changes, according to their coefficient of expansion, the same is also true for fluids.
If there is a miss-match between the expansion/contraction of the piping system and that of the fluid that it contains, then an expansion vessel will be needed to prevent the piping from being over-stressed as the fluid expands more than the piping, or be subjected to vacuum like conditions as the fluid shrinks more that the piping.
Expansion tanks are normally provided with an internal bladder, or membrane to separate the process liquid from the gas charge. Bladders, or membranes are used because they separate the liquid from the gas and prevent absorption of the gas by the liquid. They also prevent escape of the gas charge from the vessel when the system is depressurised.
The tank will allow the process liquid to flow into, and out of it's volume, thereby regulating the pressure in the system. In this respect they act like a surge vessel, however the detail design requirements are quite different.
Where expansion tanks are provided to deliver instantaneous flow to support a system then it is usually necessary to regulate the flowrate out of the vessel by way of an orifice plate, otherwise the pressure in the tank may fall too quickly.
The mass of the air or liquid in the tank may be controlled to give the correct level in the tank that correlates with a pre-determined mass characteristic. It is important to be aware that nearly all commercially available surge analysis software determines the requirements of a surge tank in terms of liquid and gas volume. Using a mass controlled tank will often mean that there is no direct correlation between the levels and volumes that the surge engineer decides are necessary for surge protection as the mass controlled surge tank does not provide a level, or volume, it provides a mass of gas instead.
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, can be used on systems where the operating conditions, system configuration and ambient conditions do not vary significantly.
