High purity, otherwise known as ultrapure, water has been created to stringent specifications to remove contaminants that can adversely impact affect people, products or equipment. These include organic or inorganic compounds, bacteria, toxins, particulates, and gases. Ultrapure water is used extensively across the semiconductor, electronic, medical, pharmaceutical, biotech and food industries. Examples of water purification processes include reverse osmosis and deionized water systems. Reverse osmosis removes molecules and ions from water through a filter membrane, whereas deionization uses specialty resins that transfer hydrogen and hydroxide ions for dissolved minerals, followed by a recombination approach that turns the ions back into the purified water. Both approaches create ultrapure water which is then contained in a storage vessel (usually termed a tank) for use in high purity applications. The storage vessels possess two basic control systems – one to add liquid into the tank, and another to remove it from the tank and into the process. The main requirement for this application is to monitor the fluid level, automatically refill the vessel, and prevent it from overflowing or becoming dry.
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Technology
Ultrasonic sound waves with a high frequency but a short duration, are pulsed up to four times per second from the face of the transducer. The sound waves reflect off the liquid’s surface and return to the transducer. The time of flight between the sound generation and receipt are measured by the level sensor and translated into the distance between the face of the transduce and surface of the liquid. The distance is then deduced into a percentage of measured span and outputted as a proportional 4 to 20 mA signal.
Best Practices
The implemented level sensor must possess a clear view of the liquid surface, meaning that there should be no obstructions in measurement space below the level sensor, including pipes, apparatus or walls inside the tank. Ultrapure water storage tanks are always enclosed and are often located indoors. Water is released into the vessel from above and removed from the bottom. This creates an agitated liquid surface in the vessel that is being filled and a smooth surface during the emptying process. Some designs can recirculate water internally to prevent the growth of biological matter. Recirculation systems range from a trickle for the prevention of a static surface or a spray ball. If a spray ball is present, a level sensor with a longer and more powerful measurement range is required. The position of the level sensor or spray ball is crucial and the level sensor and the measurement space beneath the level sensor should not be within the spray pattern. If that’s not feasible, then the level sensor needs to be installed in a stand-pipe to isolate the spray from the point of measurement. If that’s not desirable, a pulse radar level sensor should be considered.
Installation
There are multiple ways to mount a level sensor for this application. The internal area of an ultrapure water storage tank usually possesses few obstructions. The vessel top can either be flat, domed, round or angled. Locate a mounting position where the level sensor has a clear view of the liquid throughout the whole measurement span. The location must be flat, at liquid level and easily accessible. The level sensor can be installed using the following equipment.
Tank Adapter
Tank adapters are recommended for use when the tank’s mounting position is level and not on an incline. It is recommended that a tank adapter used should be slip x thread, and thread x thread adapters should be avoided. Do not mount tank adapters upside down.
Coupling
Shorter half couplings are preferred over taller full couplings. It is recommended to use a coupling that is slip x thread, thread x thread couplings should be avoided. If you choose to use a full coupling, it must adhere to the height and diameter restrictions described under Riser with Flange.
Stand-Pipe
You can install level sensors in a stand-pipe to isolate the level sensor from the spray. The stand-pipe must be a single continuous section of smooth pipe with no breaks or transitions present throughout. The inner diameter of the pipe needs to be equal to or greater than the beam width of the level, with larger diameter pipes being recommended. Installation of the level sensor requires the mounting of a low-profile threaded coupling onto the top of the pipe. Under the coupling, and within the dead band of the level sensor, drill two quarter-inch vent holes on opposite sides of the pipe. The pipe should reach the bottom of the tank, if not, it is required to be below the level sensor’s measurement span. Cut a 45° angle on the bottom of the pipe. Finally, maintain the level above the 45° cut, ensuring that there’s always liquid in the pipe.
Riser with Flange
Long, narrow risers, that extend within fiberglass tanks by a few inches from the top can affect the acoustic transmission and receipt. The internal surface of the riser needs be smooth in nature and free from ridges, especially in the region below the face of the transducer. It is highly recommended that risers with 3” diameters or greater should be used. If your only option is a 2” diameter riser, then the height of the riser and any mounting connections must not exceed 5”. Caution should be ensured with any riser heights that are greater than 8”, and it is also not permissible to use tee connections within the riser structure.
Center of Dome Top Tank
Whenever possible, you should avoid installing a level sensor at the center of a dome top tank. The dome top behaves like a parabolic reflector and can amplify acoustic energy that may cause the level sensor to become ineffective at certain tank levels.
Electrical
Storage tanks are sometimes located near large pumps, motors or variable frequency drives which are known to potentially produce substantial EMI or RFI noise. You need to ensure that such devices are grounded to earth, and that the level sensor and associated electrical equipment are grounded to the same earth-ground connection. Some areas may be susceptible to regular lightning strikes or possess unreliable power sources. Whenever this is the case, it is recommended that efficient surge protection and filtering is in place.
Span
The level sensor outputs a 4-20 mA current signal that’s proportional to the measurement span within a storage tank. Users normally set the 4 mA to either empty or the lowest level measured; and 20 mA to either full or the highest level measured. The placement of the 4 mA or 20 mA span setpoints at or near levels where pumps, valves or alarms may actuate should be avoided.
Interface
The 4-20 mA current signal in a level sensor is often connected to a local controller or centralized control system. These devices can include a PLC, SCADA, DSC or stand-alone level controller. Either is usable, so long as it accepts a 4-20 mA current signal. The operational range of the controller is then required to be programmed as to match the measurement span of the level sensor, whilst taking into account that the level sensors 4 mA setpoint is usually placed above the empty tank condition. Once the operational range of the controller has been set to the correct levels and engineering units, the relay setpoints are then applied to the pump, valve or alarm automation. It is useful to remember that the primary control in this application involves filling of the water storage tank before it becomes empty, in an effort to avoid the process shutting down due to a lack of supply. This is usually performed with a pump or valve. The fill process should begin with the low-level pump on, or valve open setpoint, and terminate at a high-level pump off, or valve closed setpoint. A low-level alarm or shut-off setpoint should be situated under the pump on or valve open setpoint. You should always use an independent high-level alarm or safety shut off system with the primary system, and an independent low-level alarm or safety shut-off system is recommended for the protection of the pump and the process.
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.