A pressurised LPG bullet or sphere holds a boiling liquid under its own vapour, so an overfilled vessel can lift its relief valves or worse. Reliable level gauges, independent high level switches and disciplined filling limits keep that energy safely inside the steel.
LPG is stored as a pressurised boiling liquid, which makes level gauging harder than in an ordinary atmospheric tank. Good LPG level measurement combines two independent gauges, an independent high level switch and a clear 85 percent filling discipline.
What Is LPG Level Measurement?
LPG level measurement is the continuous gauging of liquefied petroleum gas, a mix of propane and butane, stored under pressure in horizontal bullets, mounded bullets and spheres. The level reading feeds inventory, custody records and, most importantly, overfill protection, so it is a core part of automatic tank gauging.
At ambient temperature LPG stays liquid only because the vessel holds it at its vapour pressure, typically several bar. Any drop in pressure makes the liquid boil, which produces a turbulent, foaming surface for any gauge to read.
VEGA describes its LPG sphere application with measuring ranges up to 20 m and notes that the low dielectric constant of LPG makes accurate measurement difficult. It recommends radar for level with a separate pressure transmitter, and a cut off valve so the sensor can be replaced during operation.
Why LPG Level Measurement Is Hard in Bullets and Spheres
Emerson tank gauging expert Tomas Hasselgren told Control Global that boiling and foam appear during rapid depressurisation, temperature swings and filling. He also pointed out that product density changes with composition and that pressurised tanks cannot be entered for maintenance.
The liquid dielectric constant is low, so radar echoes are weak, and the dense vapour above the liquid slows microwaves slightly. Density depends on temperature and the propane to butane ratio, which upsets any method based on DP level with varying density.
Microwaves travel measurably slower through dense, pressurised propane vapour than through air. High accuracy LPG radar gauges therefore use reference pins in the still pipe to measure the vapour effect and correct the level reading continuously.
6 Essential LPG Level Measurement Methods
A displacer on a measuring wire follows the surface, driven by a servo motor and drum.
Non contact radar measures inside a vertical pipe with a ball valve on top.
Pulses travel down a rod or coaxial probe to the surface.
A float with a magnet rides on a probe tube with a waveguide.
Hydrostatic head plus vapour pressure correction.
A float in a chamber flips coloured flaps on the outside.
A servo level gauge weighs a displacer hanging on a fine wire and keeps it balanced at the surface. It can also measure density and interface, but it has moving parts and needs a calibration chamber with an isolation valve for maintenance under pressure.
Radar inside a still pipe is today the most common LPG level measurement choice for large spheres. The pipe guides the signal, calms the boiling surface and holds verification pins, and the ball valve on top lets the gauge head be removed without depressurising the vessel, as discussed in non contact versus guided wave radar selection.
A guided wave radar works well on smaller bullets, while magnetostrictive probes give very fine resolution on long horizontal vessels. The float must be designed for the low LPG density, roughly half that of water, or it will not ride on the surface correctly.
Differential Pressure and Its Limits
DP level looks attractive because transmitters are cheap and familiar, as shown in DP transmitter level measurement. In LPG level measurement, however, density varies with temperature and composition, so a fixed density calibration gives a drifting level.
The vapour leg is another problem, since LPG vapour can condense in the impulse line and add a false head. OISD 144 also limits the vessel to a single bottom nozzle with a remote operated valve, so extra bottom tappings for DP transmitters are not normally allowed.
If a DP transmitter is used as a backup on an existing vessel, record the density used in its calibration on the loop sheet. Compare it with the radar or servo reading every shift, and investigate any steady divergence.
Overfill Protection and OISD 144 Rules
OISD 144 states that every LPG storage vessel shall have minimum two independent level indicators and one independent high level switch. It adds that the two indicators should be of different type, which protects against a common mode failure of one technology.
The standard also says high level alarms shall be set at not more than 85 percent of the volumetric capacity of the vessel. On actuation of the high level alarm, audio visual indication is given at the local panel and control room, and the remote operated valves of that vessel shall close.
In practice this high level trip is a safety instrumented function, so the switch, logic and valve need a target SIL from the hazard study. The switch must be tested regularly, as described in proof test interval and coverage.
Level to Volume Calculator for an LPG Bullet
Because a bullet is a horizontal cylinder, 80 percent of the diameter is not 80 percent of the volume. The segment formula below is the same one used in tank volume from level measurement, ignoring the dished ends for simplicity.
Liquid volume = Segment area × L
Example:
D = 3 m, so r = 1.5 m, L = 20 m, h = 2.4 m
acos(minus 0.6) = 2.2143 rad, so r² term = 2.25 × 2.2143 = 4.9822
√(7.2 minus 5.76) = 1.2, so second term = minus 0.9 × 1.2 = minus 1.08
Area = 4.9822 + 1.08 = 6.0622 m², full circle = 7.0686 m²
Volume = 121.24 m³ of 141.37 m³, fill = 85.76 percent
The answer shows why the high alarm level in a bullet must be calculated from the strapping table, not guessed from a percentage of the height. Here a level of only 80 percent of the diameter already crosses the 85 percent volume limit.
Second Worked Example: LPG Sphere
For a sphere of radius R, the liquid volume at height h is π × h² × (3R minus h) ÷ 3. A 15 m sphere has R = 7.5 m and a total volume of 1767.1 m³.
At a level of 11.2 m, the liquid volume is π × 125.44 × 11.3 ÷ 3 = 1484.4 m³, which is 84.0 percent of capacity. So in this sphere, about 75 percent of the height already corresponds to 84 percent of the volume.
Emerson states that its 2 in 1 radar gauge gives two independent measurements through a single tank opening. This lets older spheres with limited nozzles add an independent overfill channel without hot work on the vessel.
Selecting LPG Level Measurement Gauges for Indian Terminals
LPG falls in gas group IIA, but always confirm the site classification using hazardous area classification and gas groups IIA, IIB and IIC. Average product temperature from a multipoint temperature sensor is needed to convert observed volume into standard volume.
If the vessel supports custody transfer, the gauge must meet the legal metrology and custody transfer accuracy requirements. Emerson states that its LPG tank gauging system meets OIML R85:2008 custody transfer accuracy and carries SIL 2 and SIL 3 certification to IEC 61508.
- Radar in a still pipe has no moving parts.
- Two different principles cover common mode failures.
- Remote verification avoids opening the vessel.
- Level, temperature and pressure give accurate mass inventory.
- Still pipes and ball valves add cost on new vessels.
- Servo gauges need periodic mechanical maintenance.
- DP readings drift with density changes.
- Retrofitting nozzles on old vessels is difficult.
Commissioning and Maintenance Checklist
- Verify gauge reference height against the vessel datum drawing.
- Check that both level indicators agree within the agreed tolerance.
- Test the independent high level switch and the ROV closing action.
- Confirm the high alarm is set at not more than 85 percent volume.
- Check vapour compensation and reference pin readings on radar.
- Inspect still pipe ball valve and servo calibration chamber.
- Record proof test results in the safety management system.
When the two gauges disagree, never average them and carry on filling. Stop the transfer, check each gauge against a manual reference or verification pin, and only then decide which one is wrong.
Applications of LPG Level Measurement
For an overview of all level technologies, compare types of level transmitters before finalising any LPG level measurement design.
Rosemount Liquefied Pressurized Gas Flyer
Remote Proof Test of a Radar Gauge Video
LPG Level Measurement FAQ
It is the continuous gauging of liquefied petroleum gas stored under pressure in bullets and spheres. The reading is used for inventory, custody records and overfill protection.
Because the liquid boils whenever pressure drops, the surface is turbulent and the vapour is dense. Special gauges such as radar in a still pipe or servo gauges are therefore preferred.
Non contact radar mounted on a still pipe with a ball valve is the most common choice for large spheres. It has no moving parts and can be verified with reference pins.
Servo level gauges are also widely used, especially where density measurement is wanted. Many terminals combine one radar and one servo gauge to meet the rule of different principles.
OISD 144 says every storage vessel shall have minimum two independent level indicators and one independent high level switch. The two indicators should use different measuring principles.
This arrangement guards against a common mode failure of one LPG level measurement technology. It also allows the shift operator to compare two separate readings before each tanker or pipeline transfer begins.
LPG expands strongly when it warms, so space must remain for the liquid to grow. OISD 144 limits the high alarm to not more than 85 percent of volumetric capacity.
A full vessel with no vapour space can build hydraulic pressure very quickly. That pressure may lift the relief valves or damage the vessel shell.
LPG density changes with temperature and with the ratio of propane to butane in the product. A fixed density calibration therefore makes the level reading drift through the year.
Vapour can also condense in impulse lines and add a false head. In addition, OISD 144 limits the vessel to a single bottom nozzle for its connections.
OISD 144 requires audio visual indication at the local panel and in the control room. The remote operated valves of the affected vessel shall also close automatically.
Filling then stops until the cause is found and corrected. The switch, logic and valve together form a safety function that must be proof tested at regular intervals.
High accuracy gauges use verification or reference pins fixed inside the still pipe at known heights. The gauge compares their echoes with the known positions.
Emerson offers remote proof testing of its radar gauges from the control room while the tank stays in service. This saves time and avoids exposing technicians to a pressurised vessel.
Related Articles
- Servo Level Gauge Working Principle
- Automatic Tank Gauging Explained
- Radar Level Selection, Non Contact vs Guided Wave
- Tank Volume From Level Measurement
- Safety Instrumented Function Design
External References
- Rosemount Tank Gauging System for Liquefied Pressurized Gas, Emerson
- The Challenges of LPG Tank Gauging and How to Overcome Them, Control Global
- Liquefied Petroleum Gas, Wikipedia
What We Learn Today
- LPG level measurement is difficult because the stored liquid boils, has a low dielectric constant and changes density with temperature and composition.
- Radar in a still pipe, servo gauges, guided wave radar and magnetostrictive probes are preferred, while DP is mainly a backup method.
- OISD 144 requires two independent level indicators, one independent high level switch and a high alarm at not more than 85 percent volume.
