Water Level Transmitter Within a Tank or Reactor
Water Level Transmitter and Controls Within a Tank or Reactor
One of the most common applications of a level transmitter (LT) is measuring the level of water, or another liquid, in a tank. The LT will usually be mounted to a nozzle or bulk head fitting where it can be flanged or threaded into the top of the tank. This top mounting is ideal as it means the LT itself will not have to come into contact with the medium it is measuring (although some have probes that will contact the medium).
For information on common types of level transmitters please see my article: What Are Level Transmitters Used For?.
Automatic Filling Using a Level Transmitter
Using a transmitter for level indication has the added benefit of allowing for automated filling and emptying procedures. You can set up these controls using a stand alone controller or a programmable logic controller (PLC) where you can specify a level setpoint. Using the LT, and with the help of an actuated valve (otherwise known as a control valve, or in this case a fill valve or inlet valve) and a pump starter, you can setup the system to automatically stop filling once it hits the specified level setpoint.
Usually the sequence for automatic filling would be: enter your LT setpoint in either height, volume, or percentage full. Then hit start to start the fill procedure. The automated fill valve would open, then the pump would start. The LT will continuously measure the liquid level in the tank, and, once it hits the specified set point, it will stop the pump then close the automated fill valve.
Automatic Emptying Using a Level Transmitter
Similarly, level controls can be used to stop a pump once a tank has been emptied. This is especially important if the pump you use to empty your tank will be damaged if it is run with no liquid in it, this is called “run dry”. The damage from running a pump dry comes from the pump overheating. Since the liquid the pump is pumping keeps the pump itself cool, it can rapidly overheat of free spinning in air. This is of particular concern with many magnetic drive pumps that run on elecricity. Where it isn’t really much of a concern is with an air driven diaphragm pump and sometimes not much of a concern for some other types of progressive cavity pumps.
An automatic safety, further to that provided by a low water lever cutoff from an LT, can be setup to ensure a pump isn’t run dry. This typically involves a current sensor that is tied into the controls for the pump. When a pump is operating it draws electrical current and uses it to spin the pump shaft. The current draw is associated wtih the energy it takes for the pump to move, or “pump”, the water or other liquid from one location to another. If you set up a current sensor, which senses the current as the amperage the pump is drawing, you can tell if the pump has stopped pumping liquid if the measured amps fall too low. You can set a minimum amp draw the pump must use, and if the current sensor determines that the pump has fallen below the minimum amp draw, it will shut the pump down to protect it from overheating and being damaged.
Additionally, having controls in place to automatically stop a pump once all the liquid has been moved will save on energy costs since the pump will not needlessly run when the tank is already empty.
Level Control For Overfill Protection
Another useful application for a level transmitter is overfill protection when using a manually operated pump when adding liquids to tanks. The level transmitter can be set up to close a valve on the tank’s fill line when the tank reaches it maximum safe volume. This would be an upset condition, typically you would not fill a tank to its max level unless an error from the person who is operating the pump occurrs and they add too much liquid.
There are different rules on overfill protection depending on the tank contents. For instance, if the tank contains a flammable solvent, like ethanol, methanol, acetone, isopropyl alcohol, etc., it must have some form of level control that prevents overfilling. There are other ways to achieve this, like dedicated level switches that trip on high level and close a fill valve. In most cases, level controls are a good idea even if not working with flammable solvents.
Water level transmitters, along with some additonal controls, can not only save money in the long run but can provide safer filling and emptying sequences than manual controls.
There are some up front costs for these items along with the required controls wiring and programming, but when a tank is going to be in service for a few years there is a chance of a mishap. Depending on the volume and worth of the tank contents, having a few, or even one overfill scenario may pay for the cost of the controls.
Another great reason to control level using a level transmitter is to keep the tank outlet pump from running dry and being damaged.
The full cost of the downtime required to clean up a spill or repair a pump failure should be considered when budgeting to purchase level controls. Things to take into account would be:
Loss of revenue, loss of raw materials, reduction in plant capacity for that month or year, safety incident reporting, clean up, maintenance time, possible injury, damage to equipment and the list goes on. Depending on the scenario, these costs can add up quickly.
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All the best,,