Tank (Bridle) Level Calibrations

Field Calibrations for Tank levelling Systems can be somewhat challenging if you have not completed one before. There are several considerations that need to be taken into account to achieve best results, as not all installations are identical.

This installation is a Nakakita NS287 level switch that consists of a simple float with dual magnetic pickup switches. After calibrating several types of level transmitters installed to bridle applications, I find the below works well for me, but every system is a little different and you will need to assess what works best for your situation.

Things I consider when attempting Bridle Calibrations

• * Is the Calibration to be performed while online or during an outage.
• * What are the process connections needed to tap or connect to the bridle.
• * What are the manufactures recommendations for calibration, adjustments or servicing.
• * What is the material within the tank, and what medium is needed to calibrate the system.
• * What are the default level switching targets.
• * What are the Ranges, Alarms, Trips and Setpoints of the system.
• * Is the area a hazardous or non hazardous area.
• * Is the bridle equipped with its own site glass

Equipment I Use

• * Twine Buckets large enough to fill the bridle to the required height.
• * Clear or Transparent hose to read the filling level.
• * Two needle type isolating valves for controlling the inlet flow.
• * Two Tee Pipe Fittings or Connections.
• * Various Pipe fittings pending on bridle connections.
• * Stop Watch or Timer to time the filling and emptying rates.
• * Multimeter for measuring voltages and resistance.

The medium within this tank is cooling water for a rotating machine, for this reason I’m using Demonised Water as a calibration medium. If the tank was filled with a medium such as oil, I would use the same oil to perform the calibration. This is important as the level floats are weighted for the type of medium or density used. If I was to calibrate the switch using a different medium to the tank process, this would result in an error and the levels would read incorrect when the equipment is returned to service.

Sometimes this is not always the case. Previously I have had to use different types of medium for calibration due to the vessel medium being too hazardous to engage with or not being available. In all cases this was when calibrating pneumatic level controllers. Most Pneumatic Level Controllers allow for a density adjustment where I could adjust the controller density to water, test and calibrate the instrument with water and then returning the transmitters density back to the process medium. After drying out the bridle the system is returned to service and level testing is checked again with minor adjustment while online. Not the best type of scenario, but touch wood it has delivered results.

My Calibration Procedure

The below procedure is a guide only and all efforts should be considered with relation to your own safety and your own site requirements.

• * Obtain all Site access requirements before proceeding to work on any equipment.
• * Isolate the bridle from the Process Drain and vent the Bridle to atmosphere.
• * Install all Calibration components, Pipework, Hoses, Buckets, Valves Etc.
• * Open the level switch terminal encloser, check for any voltages.
• * Attached the multimeter across the the switching and set to ohms.
• * Begin to fill the bridle and set fill rates as per manufactures recommendations.
• * Continue filling bridle until magnetic contacts change state while recording level.
• * Slowly drain the bridle at a rate recommended by the manufacture and record the reset.
• * Continue filling and draining while recording contact operation several

I will normally complete the filling and emptying procedure a few times to check the repeatability of the instrument. If the instrument needs some adjustment I refer back to the manufactures manuals for recommendations as there are some adjustments they recommend you do not adjust. With the case of the Nakakita Level Switch some small adjustments of the level floats where required to achieve the correct switching of the instrument.

To find the required switching or level spans for any instruments, I always check all available documentation such as P&IDs, Data Sheets, Management of Change, Previous Calibration Records, DCS Name Plates and talk with Operators or Site Technicians. On the rear occasion I find some sites don’t keep records of field changes and adjustments are made on the fly to get a system back up online as soon as possible. This creates problems when a calibration to equipment is completed under previous documentation and the units is returned to service.