People, products or equipment can all be negatively affected by contaminants in water including organic or inorganic compounds, bacteria, toxins, particulates, and gases. For this reason, a tight set of specifications was developed for ultrapure or high purity water. Ultrapure water is widely used in the semiconductor, electronic, medical, pharmaceutical, biotech and food industries.
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Two of the main water purification processes, which are wholly different in their operation, are reverse osmosis and deionized water systems. Reverse osmosis uses a membrane to filter out molecules and ions from water as it passes through. Deionization is a chemical method that uses specialty resins to exchange hydrogen and hydroxide ions for ions from minerals dissolved in the water and then recombines the two to form purified water.
Both of these techniques produce ultrapure water that then has to be safely stored for use in high purity applications. There are two basic control systems in the storage tanks, one polices the addition of liquid into the tank, and the other controls its removal from the tank into the process. The primary requirement for this application is to monitor the liquid level, automatically refill the tank, and prevent it from overflowing or running dry.
Technology
The ultrasonic level sensor works by pulsing a short duration, high-frequency ultrasonic sound wave at the face of the transducer up to four times per second. Once the sound wave reaches the surface of the liquid it reflects off and returns to the transducer.
The time it takes between sound generation and receipt is measured by the level sensor, which then uses the known speed to convert the time into the distance between the transducer face and liquid surface. The distance is then converted into a percentage of measured span and output as a proportional 4 to 20 mA signal.
Best Practices
To ensure the best operation of the level sensor it must have a clear view of the liquid surface once installed. For this reason, there should be no obstructions in the measurement space beneath the level sensor including pipes, apparatus or walls inside the tank. Ultrapure water storage tanks are always enclosed and usually placed indoors.
To create an agitated surface when filling, the tank is filled from the top. It is then removed from the bottom for a smooth liquid surface when emptying. There are some types of tank that recirculate the water around the tank to prevent biological growth. These recirculation systems range from a light flow to prevent a static surface or a spray ball that affects a wide area. If the tank uses the spray ball method, the sensor chose must have a longer more powerful measurement range, with the level sensor or spray ball positioned so that the level sensor and the measurement space beneath the level sensor is not within the spray pattern.
If there isn’t place where this can be avoided, install the level sensor in a stand-pipe to shield the point of measurement from the spray. If that’s still not workable, consider using a pulse radar level sensor.
Installation
There are a number of places where a level sensor may be mounted for this type of application. The three criteria for the location must be flat, level to the liquid and accessible. The inside of an ultrapure water storage tank usually has few obstructions.
The top of the tank may be flat, domed, round or angled. The ideal mounting location is one where the level sensor has a direct view of the liquid throughout the entire measurement span. The following is a list of equipment that can be used to install the level sensor.
Tank Adapter
If the tanks mounting location is level and not on a slope then a tank adapter is recommended. The adapter should be slip x thread, avoiding the use of thread x thread adapters. If a tank adapter has been mounted upside down, do not use it and change its orientation.
Coupling
A shorter half coupling is preferred over a taller full coupling. As with the adapters use a coupling that is slip x thread, and avoid thread x thread couplings. If a full coupling is used, it must follow the height and diameter restrictions described under Riser with Flange.
Stand-Pipe
In order to prevent the level sensors being obstructed by spray, it can be installed in a stand-pipe. The stand-pipe must be one continuous section of smooth pipe without any breaks or transitions, with an inner diameter equal to or greater than the level sensors beam width, and larger diameter pipes are recommended.
To mount the level sensor securely, fix a low-profile threaded coupling on top of the pipe. Immediately underneath the coupling, and within the level sensors dead band, drill two quarter-inch vent holes on either side of the pipe. The pipe must run the height of the tank, or at least below the level sensor’s measurement span. To ensure water gets into the pipe, cut a 45º angle at the bottom of the pipe, with the water level being maintained above the 45º cut, so there’s always liquid in the pipe.
Riser With Flange
Long, narrow risers, can affect acoustic transmission and receipt. In fiberglass tanks these risers may also extend a few inches inside the tank top. The inner surface of the riser must be smooth and without ridges, in particular the area below the installed transducer face. The recommended diameter of the riser should be 3”, and if only 2” diameter risers are available, the height of the riser and any mounting connections above it must not exceed 5”. If any risers exceed 8” in height, caution should be excised, and the use of tee connections within the riser structure is not permissible.
Center Of Dome Top Tank
If possible, do not install the level sensor in the center of a dome top tank. The curved surface of the dome can act like a parabolic reflector to amplify acoustic energy and can cause the performance of the level sensor to drop in and out at certain tank levels.
Electrical
Some storage tanks locations may be adjacent to large pumps, motors or variable frequency drives that can generate substantial EMI or RFI noise. If this is the case, make sure that such devices are grounded to earth, and then ground the level sensor and accompanying electrical equipment to the same earth-ground as these devices. Some areas may be subject to frequent lightning strikes or have unreliable power. If this is the case, it is recommended the sensor is surge protected and filtered.
Span
The current signal of the level sensor outputs at 4-20 mA, which is proportionate to the measurement span within the storage tank. The typical set up is to assign the 4 mA as the empty or the lowest measured level, and the 20 mA signal to full or the highest measured level. Avoid placing the 4 mA or 20 mA span setpoints at or near levels where pumps, valves or alarms may actuate.
Interface
The level sensors 4-20 mA current signal is normally connected to a local controller or centralized control system that may include a PLC, SCADA, DSC or stand-alone level controller. Either of these devices is suitable as long as it is compatible with a 4-20 mA current signal.
The controller must then be programmed so that its operational range matches that of the measurement span of the level sensor, taking into account that the 4 mA setpoint of the level sensor is normally positioned above the empty tank condition.
After the levels and engineering units of the controllers operational range has been correctly configured, then the relay setpoints are applied for pump, valve or alarm automation. It is important to recall that the primary control for this application involves bringing the water levels in the tank back up before it empties, avoiding process shutdown due to lack of supply.
This is typically done with a pump or valve. The fill process should begin at a low-level pump on or valve open setpoint and end at a high-level pump off or valve closed setpoint. A low-level alarm or shut-off setpoint should be placed under the pump on or valve open setpoint. An independent high-level alarm or safety shut off system should always be setup in as well as the primary system, and an independent low-level alarm or safety shut-off system is recommended for pump or process protection.
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.