Local level indication should be provided for all columns, vessels and drums to determine total and interface (if any) level.
• Gage glasses. Gage glasses are preferred for local level indication, with the following exceptions:
– At pressures above 900 psig, except for steam or water service
– Where they are unsuitable for the process fluid (dirty stocks that will coat the glass, etc.)
– Determine if a gage glass for an elevated vessel will be readable from grade and, if not, include an additional indicator at grade
• Displacer-type level transmitters. When level glasses cannot be used, include a displacer-type level transmitter with a local receiver gage. Usually, any level alarm should be taken from this transmitted signal. This transmitter should be separate from the level controller loop
• Differential pressure level transmitters. Use with a flange-mounted diaphragm capsule when neither a gage glass nor a torque-tube displacement type instrument is suitable
• Pyrometer-type level sensors (ram’s horns). Use for heavy oil columns (e.g., atmospheric and vacuum columns) if approved by the operations representative. No fewer than five should be used
• Automatic tank gages. Tanks used for inventory control should have automatic tank gages readable from the ground, and level transmitters that display in the central control house (if there is one). Heated tanks and tanks storing product at above-ambient temperature should have remote readout of spot tank temperatures. If there is an existing tank gaging system, a project decision should determine whether automatic tank gages should read out on it
