LNG Sampling System that Helps CTCI Support Energy Transition

— Yuda Zeng, Lead Engineer at CTCI Corporation, Group Engineering Business 

In response to global carbon reduction trends, Taiwan has also introduced an "increase natural gas, reduce coal-fired" energy policy. As countries worldwide replace coal-fired power generation with natural gas, the demand for liquefied natural gas (LNG) receiving terminal construction is also experiencing continuous growth. CTCI, aspiring to be a "Guardian of Sustainable Earth," has undertaken LNG receiving terminal projects both domestically and internationally. Notable examples include: CPC Corporation's Taichung Phase I and Phase II Receiving Terminals, as well as the recent CPC Third LNG Receiving Terminal in Taiwan; overseas projects include India's Dhamra and Thailand's PTT LNG receiving terminals .

In a LNG receiving terminal, the LNG sampling system plays an important role; through it, the total calorific-content quantities of the LNG unloading from the carrier (determined by the liquid volume, liquid density, and gross calorific value of the LNG) can be confirmed to ensure the quality of the natural gas. Therefore, the establishment of this system is crucial.

This article will introduce the various sampling characteristics and main components of the LNG sampling system.

Introduction to the LNG Sampling System

According to ISO 8943 Refrigerated light hydrocarbon fluids: Sampling of liquefied natural gas (Continuous and intermittent methods), LNG sampling systems can be divided into two types: continuous sampling and intermittent sampling (also known as discontinuous sampling). The following content introduces the technical and equipment differences between the two types.

1. Continuous Sampling

During the process of LNG unloading, the LNG is continuously collected from the pipeline. The regasified LNG from vaporizer is continuously fed to the gas sample holder with constant flow rate which is maintained by the gas sample holder inlet valve, while the pressure in the sampling line is controlled by pressure regulator. The gas collected in gas sample containers is used for offline analysis of its total calorific-content. The gas sample holder is divided into two types: water seal type and waterless seal type.

A water seal type gas sample holder requires the sealing water to be saturated with gas bubbling of regasified LNG before collecting the natural gas. On the other hand, a waterless seal type gas sample holder typically uses an expandable/contractible, transformable rubber membrane along with a vacuum pump.

2. Intermittent Sampling (Discontinuous Sampling)

Similar to continuous sampling, this method also involves the continuous collection of LNG from the pipeline during the unloading process. The difference lies in that a portion of regasified LNG from the vaporizer is directed to a gas chromatograph for online analysis, while another portion is directed to the constant pressure floating piston (CP/FP) sample container.

The constant flow rate into the CP/FP sample container is maintained by the inlet valve of the CP/FP sample container, while the gas pressure in the sampling line is controlled by a pressure regulator. The CP/FP sample container is used to maintain constant pressure during the sampling from the LNG pipeline to the gas cylinder. The gas collected in this container will be used for offline analysis of the total calorific-content quantities of the natural gas.

Main Components of the LNG Sampling System

1.Sample Probe

This is a device inserted into the LNG pipeline, used to collect the LNG samples required by the LNG sampling system.

To ensure the accuracy of sampling, the sampling probe must be inserted at a location in the LNG pipeline where the LNG is in a sub-cooled condition. The sub-cooled condition refers to the liquid's temperature below its normal boiling point. If the location of the sample probe is improperly chosen, the sample gas may not accurately reflect the composition of the LNG, thereby affecting the analysis results. Therefore, the design of the sample probe must also consider fluid dynamics and the distribution characteristics of LNG. Additionally, the sample probe should be installed vertically on the LNG pipeline, taking into account the issue of cold displacement.

2.Vaporizer

This is a device used to vaporize LNG.

The design of the vaporizer must avoid fractionation and ensure its heat exchange capacity is sufficient to vaporize all the collected liquefied natural gas samples. Structurally, it must ensure that heavier components of LNG do not remain in the sampling vaporizer. Vaporizers typically use electric heating, but there are other types, such as those heated by steam-heated water or by steam directly.

3.Gas Sample Holder

This is a device typically used for continuous sampling to retain gas samples and transfer them to analyzing instruments.

The capacity of the gas sample holder should be greater than the total sum volume required for charging gas sample containers and additional volume should be considered to allow for purging the sampling line between the gas sample holder and the gas sample containers.

There are two types of gas sample holders. One is a water seal type gas sample holder, designed with an inner tank submerged in seal water, which can completely discharge the gas inside. The regasified LNG can be bubbled through the seal water to prevent sample contamination by atmospheric gases dissolved in the water. The other type is a waterless seal type gas sample holder, which is designed to easily discharge any residual gas.

4.Constant Pressure Floating Piston (CP/FP) Container

This device is typically used for intermittent sampling and can maintain constant pressure as the gas from the process pipeline into the gas cylinder.

CP/FP sample containers have a free-floating internal piston that divides the container into two separate components (sample and carrier gas). Additionally, this container is typically a stainless steel cylinder with stainless steel needle valves and pressure gauges at both ends, capable of withstanding the designed pressure. Generally, the container is designed to hold a gas volume between 500 cm³ and 1000 cm³.

5.Gas Chromatograph

This is a gas analyzer directly connected to the pipeline or sampling device, which confirms the composition of regasified LNG in real-time through online analysis.

The design of gas chromatograph must take into account its performance (cycle time, repeatability, etc.) and provide flow selection function. According to sample gas requirements, not only does it require to confirm instrument air, carrier gas, and calibration gases, but also requires to confirm the sample injection measurement conditions (gas samples measured in volume). To achieve continuous online analysis, it is typically designed to have two carrier gas cylinders with automatic changeover function. Gas chromatograph is usually installed in closed and temperate housing to avoid sunlight, heat sources, or air currents.

Conclusion

CTCI, aspiring to be a Guardian of Sustainable Earth, actively engages in sustainable development through its core business. Green engineering projects are carried out worldwide that balance environmental protection and economic development needs, allowing CTCI to work together with clients to achieve the goal of net-zero emissions. In particular, the construction of LNG receiving terminals is one of the crucial infrastructure in advancing energy transition.

From what was discussed in this article, it is evident that the LNG sampling system is a highly specialized system in the construction of LNG receiving terminals. Its design must consider the unique properties of LNG, including extremely low temperatures and high-pressure conditions. The LNG sampling system must have precise temperature control, pressure control, and vaporization capabilities, while also preventing sample gas contamination and fractionation, requiring significant technical expertise. The LNG sampling system not only plays an important role in the production, storage, and transportation of LNG but also plays a crucial role in LNG commercial trading and environmental monitoring. In the future, CTCI will continue to advance its technology and expand its track record in the LNG receiving terminal field, providing global customers with the most reliable engineering services.

Reference: Custody Transfer Handbook 6th edition (2021)