Surge tanks are commonly arranged with a compressor, or compressors, to provide additional air in to the tank if the liquid level rises above the normal condition. The change in level is detected by a level instrument, which may be a simple bypass level gauge containing a magnetic float which rises and falls with the liquid level activating magnetically sensitive switches attached to the level gauge or a more advanced level instrument, such as a radar or ultrasonic level detection instrument.
The surge tank will be fitted with level switches and instrumentation to enable level detection and control actions to be taken.
Typically control actions will be taken at liquid High Level, Low Level and Normal positions and there will be two alarm positions, one at High Level Alarm and one at Low Level Alarm.
The most common arrangement is known as Five Point control and is ideally suited to surge applications where the pumping condition does not vary significantly or where there is a fixed speed pump.
Typically a bypass level gauge will be fitted to the tank to which magnetically operated switches will be attached, these switches will be operated by a magnetic float within the level gauge.
In cold environments the level will require insulation to prevent freezing of the water inside.
Additionally the surge tank will be provided with a solenoid activated valve which is arranged to release air from the vessel should the level detection instrumentation show that the liquid level has fallen below the normal level.
The operation of the compressor and the solenoid air release valve are controlled by a suitable control panel which is usually mounted in a local plant room or within a local housing.
The control panel will respond to the inputs from the level measurement system and may be of single or multiple compartment construction dependent on local or national regulations.
In multiple compartment panels the control switchgear, power transformers, and relays are all separated.
The compressor system should be selected to give both a suitable time to top up the vessel as well as complete the initial filling. The compressors may be directly connected to the vessel so that the discharge from the compressor goes straight to the vessel. It is not usually necessary to install an air receiver between the compressor and the surge tank as the surge tank is itself an air receiver.
The compressor may be base mounted on the floor or it may be skid mounted and delivered direct to site and generally fall into two categories.
Oil free - where the compressor is free of liquid lubricants and relies upon solid low friction self lubricating bearings and moving parts. Such compressors are economically available up to a discharge pressure of 10 BarG. In order to meet air quality requirements for use with drinking water it is normally necessary to fit air filters on the discharge of the compressor. Depending on the exact requirements it may be necessary to fit multi-stage air filters for removal of oil droplets and odour. Compressors may be single or multi-stage and will normally be fitted with belt guards, anti-condensation heaters, inter-stage coolers as well as local emergency stops and solenoids. All compressors will be fitted with local safety relief valves.
Oil free air - Oil free air can be supplied from compressors which rely upon oil lubrication where the oil is of "food grade", and fine filter are used to eliminate any carry over from the compressor.
Such compressors are economically available up to a discharge pressure of 40 BarG and will normally require filtration to meet suitable air quality requirements for contact with drinking water.
Oil free - With oil free compressors the compressor is free of liquid lubricants and relies upon solid low friction self lubricating bearings to ensure smooth operation of all the moving parts. Such compressors are economically available up to a discharge pressure of 10 BarG and will normally not require filtration to meet suitable air quality requirements for contact with drinking water. However, care must be taken if the equipment is to operate in a dirty or dust laden environment where the absence of suitable inlet or discharge filtration may permit contaminants to enter the air supply. The vessel will be supplied with a safety relief valve and a drain valve.
