Feature article
Automated cool-down of main cryogenic heat exchanger improves LNG plant reliability and efficiency
16/07/2008
Maria Parra-Calvache and Kees den Bakker, Shell Global Solutions International BV; Clive Beeby, Shell Technology India Pvt Ltd; and Junaiza Bt Abdul Ramman, Malaysia LNG Sdn Bhd
With energy demand forecast to double in the first half of this century and conventional sources diminishing, the production of liquefied natural gas (LNG) continues to make an important contribution to meeting the energy challenge.
The days of the so-called easy oil and gas are numbered. Increasingly, producers will need to access fossil fuel resources in more challenging environments and in more distant and remote locations.
As the distances between resources and markets increase, LNG is vital to the security of energy supply. Not surprisingly, therefore, demand for LNG remains strong. Last year, LNG accounted for about 7% of global gas consumption. Currently, LNG demand is growing at around 8 to 10% per annum – four times the global annual growth rate of natural gas overall.
LNG offers advantages over other energy sources:
- Like other forms of natural gas, LNG offers cleaner energy than oil or coal to a world increasingly concerned about environmental and health issues.
- LNG is now cost competitive with other forms of gas delivery, mainly thanks to advances in technology.
- LNG has a commendable track record of reliability and safety. This track record, spanning more than 40 years, underpins the crucial role of LNG for security of supply in the future.
- It offers flexibility; unlike pipeline gas, LNG can be sourced from several supply points by buyers.
- When demand fluctuates unexpectedly, clauses within long-term contracts can allow partners to divert volumes to third parties. This provides greater liquidity and rebalances the market to match supplies to demand more accurately.
LNG requires developers and operators to make long-term investments and commitments. The sector’s tradition of long-term partnerships of trust and “sanctity of contract” has led to its strength today. Maintaining this tradition is vital to LNG’s continuing success. The challenge for suppliers will be to bring on new capacity when the market needs it in a world where the costs of new production capacity are rising relentlessly.
New infrastructure and production capacity are needed to help address potential long-term supply–demand imbalances. Through further investment, LNG suppliers can contribute more to the energy security that will underpin global and regional economic growth. By investing in technology, producers will be able to unlock new resources and ensure continued reliability and efficiency in existing and planned LNG operations.
Targeting plant reliability
Shell has developed a new technology, automated cool-down of main cryogenic heat exchangers (MCHE), that specifically targets LNG plant reliability. The development aims to alleviate a problem LNG sites sometimes experience: bundle tube leaks in MCHEs. Such leaks can and do cause unexpected unit shutdowns. The technology reduces the risk of leak formation during start-up by automatically cooling down the MCHE in a controlled way and at the most efficient average rate. It leads to rapid start-up and less flaring or venting, while operating closer to performance limits. Therefore, it increases reliability and availability of the LNG train.
The MCHE is the heart of the LNG plant. It consists of over 1000 km of tubes in an aluminum shell. At start-up, the MCHE has to be cooled down from ambient temperature to around –160C. The main objective of the cool-down process is to achieve a predefined temperature profile in the MCHE after a shutdown or trip. The cool-down procedure is complex and has previously been a manual operation. The process also responds differently as it progresses, which may result in the rate of temperature change (the rate of cooling) or the difference in temperature between tube and shell side exceeding the recommended levels (see Figure 1), thus increasing the temperature shocks on the tubes and the risk of leaks developing.
Figure 1: Temperature rate of change (TROC) data for cool-downs during the past five years from a selection of LNG plants.
In some start-ups, these limits have been exceeded for 20–30% of the total cooling time. MCHE units typically have 2 to 8 cool-downs a year. Exceeding the recommended limits will increase the risk of leaks from the tubes, which can cause unplanned shutdowns for repair work and, consequently, significant loss of revenue.
Shell’s automated cool-down procedure has been developed to help improve the control of the cool-down process for MCHEs. The design of the procedure takes into account best operator experience, process and equipment expertise, analyses of existing procedures and historical data from previous MCHE cool-downs.
The procedure handles tasks that previously were undertaken by an operator and comprises several software modules, each performing a specific task, for example, manipulating a valve. The modules have been designed to operate independently, which gives operators the opportunity to switch off an individual module or to take over when required. The automatic cool-down procedure is an interactive process with the crucial decisions still being made by the operator.
A key performance indicator (KPI) has been defined that quantifies each cool-down to evaluate its success. This KPI considers the time required to cool down the MCHE and the temperature gradients experienced (see Figures 2 and 3). A perfect score means that cooling occurred as fast as possible without exceeding the maximum rate of temperature change constraint.
Figure 2: Results of implementing automated cool-down at Malaysia LNG Dua.
Figure 3: The temperature profile of LNG exiting an MCHE in manual and automated operations.
The procedure has been designed and implemented in a logical sequential programming environment that communicates with the current distributed control system (DCS). The automation platform used is Exapilot†, which was developed by Yokogawa Electric Corporation. The operator can interact with the procedure via a DCS interface screen and is assisted by special system graphics.
Reaping the benefits
There are many advantages to an optimized and automated cool-down procedure:
- Automation reduces the complexity of the process for operators and gives them more time to concentrate on the non-automated aspects. A more consistent cool-down procedure will be robust to changing operator shifts or different levels of operator experience and enhances the efficiency of the operation.
- More accurate control of cool-down significantly reduces the risk of cracking exchanger tubes because of an overly rapid contraction of the metal. Consequently, unplanned plant shutdowns due to leaking tubes are fewer and lost revenue is reduced. The repair time for leaking tubes can be of the order of one week. In general, a one-week shutdown corresponds to a 2% LNG production loss per unavailable train.
- The cool-down time is reduced, which results in faster LNG production recovery and consequently less flaring or venting and the associated environmental and economic benefits.
The automated cool-down procedure is independent of liquefaction technology and can be implemented at most LNG plants. It should benefit LNG plants anywhere in the world, both on- and offshore, and at any scale. The development opens up opportunities for automating other, similar, complex start-up procedures and is a step toward a fully automated LNG plant. A patent application for the automated cool-down process has been filed.
Automated cool-down in action
The technology has already been implemented at the Malaysia LNG Dua Sdn Bhd plant. The process resulted in a 60% reduction in the severity of excursions outside the recommended ranges for rate of temperature change and temperature difference. This lessened the chance of tube leaks caused by temperature shocks. Repairing tube leaks entails a plant shutdown of around one week, which can cause a production loss of about 2%.
The automated cool-down was also 2–3 h faster than the manual procedure. This means that, with the start-up of the MCHE having a delay time approximately 60% that of the previous average delay time, LNG production was quicker and, consequently, the amount of venting during shutdown was reduced. Cool-down delay is costly: each additional hour more than the ideal cooling time represents a significant production loss. Five-year LNG plant data shows an average cool-down delay time of more than 3 h when manually controlled.
†Exapilot is a registered trademark of Yokogawa Electric Corporation