Guide to Level Instruments Principles & Application

Level monitoring and control is a fundamental requirement in any process plant. You will find level instrumentation installed at even the simplest treatment plants for pump control, chemical storage tanks and process level controls or alarms.

Though the P&IDs or other documents provided as part of the FEED or during detail engineering generally indicate the type of instruments still it is necessary to verify the type even in case of an existing specification provided as part of the contract documents.

Since these level instruments are mounted on Mechanical equipment which are finalized during early phase of detail engineering the engineering activity requires very close coordination with Process, Mechanical and Piping group. 

The purpose of this post is to outline minimum requirements that need to be considered for the selection of level instruments, developing the specification and associated engineering. 

1. Type of level sensing principle

Following are the type of level sensing principle generally in use in the projects:

(i) Sensing by float:

A float moves up and down along with the level and thus represents the level or height of a liquid.

Type of level instruments:

• Magnetic Level gauge.
• Auto tank gauges (Float and tape type level indicators).
• Float operated Level switches.

(ii) Sensing by displacer:

A partially submerged displacer loses weight in proportion to the extent of submersion.

Thus variation of level is proportional to the change of level.

Type of level instruments:

• Displacer operated level transmitters.
• Servo operated tank gauges.
• Displacer operated Level switches.

(iii) Sensing by pressure:

The head or pressure exerted by a liquid is proportional to its height or level. Differential pressure transmitters are used in this kind of measurement.

Type of level sensors/transmitters:

• Diaphragm sensing differential pressure transmitters.

(iv) Sensing by change of electrical characteristics:

A partially submerged resistor will have its resistance varying with the level change of a liquid if the liquid is electrically conductive since the portion submerged in liquid will shorted by the conductive liquid.

A partially submerged capacitor will change its capacitance with change of level as the

vapour space and liquid space having different dielectric values change with liquid level.

(v) Sensing by transit time of wave:

Ultrasonic transducers use sound wave and Radar transducers use electromagnetic wave.

The wave emanates from the transducer (which contains both the source and detector) and is reflected from the level of liquid. Lower the liquid level the more time is taken by the reflected wave to reach back at the detector. This kind of measurement can be noncontact type as well as contact (with liquid) type.

(vi) Sensing by radioactive method:

Radiation is absorbed more by liquid and less by the vapour. This is used for detection of level. Usually the radioactive transducers and detectors are installed on the external surface of the equipment and not required to be installed inside.

(vii) Sensing by weight (by load cells).

 (viii) Viewing level through transparent/refractive medium like glass, called Level

Gauge Glass.

2. Application, Selection Criteria and Installation

The following are the brief application types:

(i) Detection of level in process equipment which sees a dynamic process, for example a two phase or three phase Separator or a flash vessel. In such applications level needs to be detected with fast response and continuously to allow online monitoring and control of the level in the equipment.

Typical instruments used are:

• Level Transmitters based on sensing liquid head measurement, DP type
• Radar type Level Transmitters (contact and non-contact type and guided wave type)

• Displacer type Level Transmitters.
• Nucleonic gauges (Sometimes used in Separators)

(ii) Measurement of level in a storage tank. In such applications level measurement is required to maintain inventory. Tank gauging is a more elaborate storage application where the materials stored in a number of tanks are controlled for inventory by level instruments and a centralized computing system.

Typical instruments used are:

• Auto tank gauges, for atmospheric or for tanks with low pressure and non-toxic fluids
• Servo operated displacer type transmitter for pressurized tanks, bullets and for Tank Gauging requiring high accuracy.
• Radar type Level Transmitter
• Ultrasonic type Level Transmitter
• Level Transmitters based on sensing liquid head measurement, DP type

(iii) Interface level measurement is required in equipment like Separators where water is separated from Oil (Hydrocarbon liquid).

Typical instruments used are:

• Radar type Level Transmitter
• Displacer type Level Transmitter
• Capacitance type Level Transmitter
• DP transmitters

(iv) Sometime Level measurement or detection is required to signal the malfunction of equipment. For example Level instruments are used in Instrument air receivers to detect any water level. Water vapour may condense in the air receiver if the instrument air-dryer does not function well.

Typical instruments used are:

• Displacer type Level Transmitters.
• Capacitance type Level Transmitters.
• Local gauge like a transparent level gauge glass.