Condition monitoring of LNG plants

Industry maintenance requirements

Due to the projected growth in demand, the pace of constructing new LNG plants and extending existing ones is unrelenting. Although LNG is one of the more expensive forms of energy produced, there have been a number of successful efforts to drive down these costs.  These initiatives include increasing the size of the trains, reduction of over-design margins, and grouping of critical machine components on the same shaft to name a few. Over the years, the net sum of these improvements has significantly reduced the cost of producing LNG, while concurrently increasing the risk of interrupting a larger portion of production in the case of a single component breakdown. Subsequently, effective machine condition and performance monitoring has become even more critical in ensuring the reliability and productivity of these production facilities.

Machines in an LNG plant, especially those in the liquefaction process, are subject to extreme operating conditions. The thermal expansion of a refrigerant compressor, pump or hydraulic turbine between start up and full production at cryogenic temperatures is intense, thereby subjecting the bearings and other machine components to severe loads during start-ups and shut-downs. Even small variations in the composition of the natural gas being processed can have significant effects on the overall loading of the machines. Furthermore, many machines are operated at variable speeds and loads; thereby rendering the mean time before failure for the machine components both variable and unpredictable.
Machine trains are becoming larger, over-design margins are diminishing and critical machine components are being grouped on the same shaft. These initiatives were launched over the years to save production costs but they also consequently increase the risk of lost production if a single critical machine is down. This is further compounded by efforts to downsize at the plants and the very competitive nature of the industry.

Despite the increased risk of a larger portion of production being stopped because of a critical machine failure, downtime is tolerated less than ever. In the case of a machine trip in the liquefaction portion of an LNG plant, you only have a few hours to determine the cause and to get the machine up and running again before losing cooling capacity. Failure to do so can result in extended downtime of the entire production train.

As in any sector of the petrochemical industry, ensuring the reliable operation of these machines is not a trivial task – even less so when considering the reduced maintenance staff and specialists at the plants and the very competitive nature of the industry.

In general, the machine monitoring requirements in the Petrochemical and Oil & Gas industry are demanding and especially so when monitoring an LNG plant. The sensors themselves have to be able to withstand the extreme operational temperatures. Add to this the fact that there are a number of potential failure modes that are not found on similar machines in other industries, which need to be detected and diagnosed both reliably and at an early stage. Furthermore, there is little tolerance for unplanned shut downs either for inspection or for maintenance activities.   For example, in the case of a machine trip in the liquefaction portion of an LNG plant, you only have a few hours to determine the cause and to get the machine up and running again before losing cooling capacity. Failure to do so can result in extended downtime of the entire production train.

Automated early detection of incipient faults is crucial to enable sufficient lead time to plan the necessary remedial maintenance actions. If the machines have different duty cycles, and are operating at different speeds and loads, the vibration signature and performance parameters will also differ. Therefore, the alarm limits need to be tailored to the different operating conditions to avoid false alarms. This adaptive monitoring concept, widely used in the petrochemical industry, is vital in the monitoring of LNG plants.

As a result of the operation and maintenance requirements of the machines and the increased risks involved with the industry cost-saving initiatives, condition monitoring plays a vital role in safeguarding the company’s competitiveness.

Monitoring Philosophy

Within the LNG industry as in all other industries, Brüel & Kjær Vibro is completely focused on providing the necessary system, diagnosis and detailed supporting information necessary to help avoid unplanned machine downtime; no matter where the customer is located or what level of technical expertise and resources they have on-site. Brüel & Kjær Vibro helps minimize downtime by preventing machine failures, rather than just understanding what happened after the event! This is based on a dedicated monitoring philosophy that has been developed and refined over the years to ensure optimum results are achieved right from the start-up of machinery.

Our monitoring philosophy is based on:

• Fully automated monitoring. This ensures data is captured and analysed for all operating conditions
• Early fault detection. Monitoring techniques are implemented to detect potential failure modes in their early phase of development, so action can be taken before a machine trips unexpectedly. Post mortem tools are of course essential, but Brüel & Kjær Vibro’s monitoring concept helps to avoid the machines failing in the first place
• High-resolution event monitoring. Brüel & Kjær Vibro records detailed data with extremely high resolution for up to 15 minutes around a user-defined event
• Remote diagnostics. Our remote diagnostic team stands by to assist with all levels of diagnostics and analysis support

Three Steps to Optimization

1. Machine Safety Monitoring
   Protect your critical machines from catastrophic failures and the subsequent production losses, unplanned downtime and additional maintenance and repair costs
2. Machine Condition Monitoring
   Optimise your machine operation by avoiding unnecessary maintenance, predicting maintenance requirements and potential failures, scheduling long lead time parts and service with confidence. Minimise machine failure, production losses and maintenance costs.
3. Machine Performance Monitoring
   Optimise machine efficiency. Make maintenance and operation decisions based on maximizing the economic impact of machine performance. Ensure the maximum performance and productivity of your critical machines.

Dedicated monitoring strategy for LNG plants

Machines in an LNG plant, especially those in the liquefaction process, are subject to extreme operating conditions. The thermal expansion of a refrigerant compressor, pump or hydraulic turbine between start up and full production at cryogenic temperatures is intense, thereby subjecting the bearings and other machine components to severe loads during start-ups and shut-downs. Moreover, even small variations in the composition of the natural gas being processed can have significant effects on the overall loading of the machines. Furthermore, many machines are operated at variable speeds and loads; thereby rendering the mean time before failure for the machine components both variable and unpredictable.
The typical LNG plant includes several trains and processes, as shown below. We are currently monitoring many of the machines in each of these processes in a number of different LNG plants around the world. The Liquefaction process is one of the primary processes in the LNG plant.

Centrifugal and axial compressors (propane, mixed refrigerant, end flash)

Of all the machines used in the oil & gas industry, the centrifugal compressor is one of the most widely utilized. There are numerous compressor processes in the oil & gas industry, and equally numerous machine configurations to meet these process requirements. The monitoring requirements are also individualized for each specific process application. The compressor plays an equally important role in the LNG industry. Both axial and centrifugal compressors, sometimes with many side streams, are used for compressing low temperature refrigerant gases in the liquefaction process. The monitoring strategy example described in the images below is based on a typical propane, mixed refrigerant and end flash gas compressor application.

Gas turbines (single axel)

The gas turbine is widely used in the oil & gas industry as a prime mover. Although there is not a lot of maintenance needed for operating a gas turbine in comparison to other machines, effective monitoring is crucial to ensure optimal uptime and reliability. As power output and specific fuel consumption have a huge impact on the lifetime operating costs of the gas turbine, plant performance also has to be carefully monitored. The monitoring strategy example described in the images below is based on a single-axel industrial application.

Liquid expander

The variable-speed hydraulic turbine liquid expander is used for isentropically expanding high-pressure heavy mixed refrigerant or LNG across a turbine to further reduce the temperature of the fluid while at the same time to generate power. The generator and bearings are cryogenically submerged in LNG at a temperature of around -160° C. The thermal stresses on the bearings and other components are high, so maintenance is critical. The monitoring strategy is described in the images below.

Cryogenic pump

The cryogenic pump, like the liquid expander, has the motor and bearings cryogenically submerged in LNG at a temperature of around -162° C. The thermal stresses on the bearings and other components are high, so maintenance is critical. The monitoring strategy is described in the images bellow.

Motor, Generator

The electric motor and generator are extensively used in many industrial processes. They come in all sizes, so the specific monitoring strategy used depends on the application. The monitoring strategy example described in the images below is for a large motor used for the end flash compressor.

Gearbox

The gearbox is not a machine in itself, but a machine component for converting the rotational energy of the prime mover at one speed to the driven machine at another speed. The gearbox is extensively used in many industrial processes, and the specific monitoring strategy used depends on the application. The monitoring strategy example described in the images below is for the end flash compressor train gearbox.

Reciprocating compressors

These maintenance-intensive machines are often monitored less closely than centrifugal compressors. With our contemporary monitoring strategies, you can monitor reciprocating compressors efficiently and effectively.

Dedicated monitoring system for LNG plants

Our products for machine monitoring can be easily interfaced to the user’s process control system (DCS), enterprise and maintenance management systems, emergency shutdown system, operator work stations and even other pre-existing monitoring systems. In many of these applications remote monitoring with secure communications over long distances is a requirement. The image below is an example for the machines in the LNG liquefaction plant.

LNG Plant Products