The diagram shows the main instrumentation and accessories installed on a typical storage/process tank, covering level, pressure, temperature, alarms, venting, and draining.
Main tank instruments
| No. | Instrument | Main Function |
|---|---|---|
| 1 | Vent / Breather (PCV) | Allows the tank to breathe and prevents excessive pressure/vacuum. |
| 2 | Pressure-Vacuum Relief Valve (PVRV) | Protects the tank against overpressure and vacuum. |
| 3 | Pressure Transmitter (PT) | Continuously measures tank vapor-space pressure. |
| 4 | Local Pressure Gauge | Provides local/manual pressure indication. |
| 5 | Temperature Transmitter (TT) | Measures tank/product temperature. |
| 6 | High Level Switch (HLS) | Gives high-level alarm/trip when the liquid reaches a predetermined level. |
| 7 | Magnetic Level Gauge (MLG) | Provides local visual indication of liquid level. |
| 8 | Level Transmitter (LT) | Provides continuous level measurement—e.g., Radar, Guided Wave Radar or Ultrasonic. |
| 9 | Displacer / Float Type LT | Measures level using float/displacer movement. |
| 10 | Low Level Switch (LLS) | Gives low-level alarm/trip. |
| 11 | Bottom Drain Valve | Used for draining, sampling and maintenance. |
| 12 | Bottom Temperature Indicator (TI) | Measures/indicates temperature near the tank bottom. |
How the level instruments work
1. Radar Level Transmitter (LT)
A radar transmitter sends electromagnetic waves toward the liquid surface. The reflected signal is used to calculate the liquid level.
2. Guided Wave Radar (GWR)
Radar pulses travel along a probe inserted into the tank. It is particularly useful where there are obstructions, vapors or difficult process conditions.
3. Ultrasonic Level Transmitter
The transmitter sends an ultrasonic signal toward the liquid surface and measures the reflected signal. It is a non-contact measurement.
4. Magnetic Level Gauge (MLG)
A float inside a chamber follows the liquid level and drives an external magnetic indicator, allowing operators to see the level locally.
5. Displacer/Float Level Transmitter
A float or displacer responds to changes in liquid level and converts the movement/buoyancy change into a level signal.
Typical control philosophy
A tank may use several instruments together:
LT → DCS/PLC → Level indication & control
LIT → DCS/PLC → Continuous level monitoring
HLS → ESD/PLC → High-level alarm/trip
LLS → ESD/PLC → Low-level alarm/trip
For a critical hydrocarbon tank, redundant level measurement and independent high-high/low-low level switches may be provided for safety.
Important: The actual instrument arrangement depends on the tank type, fluid, pressure, temperature, API/company standards and project specifications. For example, API 650/API 653 are commonly relevant to atmospheric storage tanks, while project/company specifications determine the final instrumentation details.
