In this post, I want to share you typical LNG tank instrumentation. This post is related to my old post: rollover in LNG storage tank. As explained previously, rollover in LNG storage tank begins with stratification which is LNG layer formation due to density differences. To monitor stratification, good and necessary instrumentation should be installed thus prevents rollover.
This is signal of presence of stratification for review:
- Decrease of boil-off gas rate
- Increase temperature of bottom part in LNG tank
Required LNG tank instrumentation based on API 625
API 625 is applicable for tanks having storage volume of 800 m3 with minimum design temperature is -198 deg C and maximum design internal pressure 50 kPAg. I think this is applicable for flat bottom LNG storage tank. The standard explains instrumentation required in refrigerated tanks. The instrumentation include level gauge, leak detection, temperature, rollover prevention, and pressure.
Level Gauges and Overfill Protection
- Equipped with two independent liquid level gauges, which account for possible variables in liquid density
- Level gauge shall include high level alarms indicating start of process shutdown. Alarm shall be audible to personnel controlling tank filling
- A separate, liquid level alarm and cutoff device is also required, set at design liquid level
- All level instruments shall be designed and installed so that they can be maintained during operating condition
- Primary liquid container leak detecting systems shall be provided for all double wall and full containment system
- Leak detection system may be based on temperature change, gas detection, or differential pressure measurement
- Temperature monitoring device for primary container shall be provided to:
- Assist in controlling cool down
- Assist foundation heating
- Monitor vapor and liquid temperature during operation
- Detect leak
- Assist in rollover management
- For controlling cool down, temperature element shall be located on the inner tank bottom and in vertical array near or on the inner tank shell
- If possible, rollover shall be prevented by active LNG store management
- Density measurement system shall monitor the density over the full liquid height and give alarm when predicted rollover conditions are approached
- Active store management include:
- Monitoring temperatures and densities
- Mixing the liquid by top and bottom filling or by recirculation
- Two pressure elements are required to monitor and control tank pressure
- Two pressure elements shall be connected to the space above design liquid level
Figure below shows typical instrumentation in LNG tank.
LTD (Level, Temperature, and Density Gauge)
LTD is designed to collect temperature and density over the entire depth of liquid. This is accomplished by traversing a single, multifunctional probe through the height of liquid and recording temperature and density at present interval. This LTD is different from that stated in API 625 (“Equipped with two independent liquid level gauges, which account for possible variables in liquid density”).
This is good illustration from Emerson the concept of LNG tank gauging.
I suggest to check more about this topic in: example of tank gauging configuration in several LNG projects.
Vapor Withdrawal Rate
BOG flowrate is usually measured by turbine meter or orifice plates gas flow meter. The flow meter must withstand and measure high vapor-evolution during LNG tank rollover.
Type of LNG level gauge used are float gauges, displacement gauges, and radar gauge.
- LNG filling rate
- LNG withdrawal rate
- LNG recirculation rate
- LNG composition (obtained from gas chromatography)
- Tank pressure