(153c) LNG Plant Inlet Facility - Design, Safety & Operability

Abstract

In Onshore Gas processing Inlet Facility of LNG or Gas Plant the feed gas or mixture of gas & condensate are received by means of high pressure pipelines. The condensate or heavies if any are separated and feed gas is spilt into multiple trains for processing. The capacities of these trains are based on project specific considerations, available gas supply and future sales demand.

In each train the pressure is letdown in single or multiple pressure control valves (PCV) or by turbo expanders depending on the process involved for gas to liquid conversion. The train through put is controlled by a combination of pressure and flow control as shown below. Dynamic simulation of inlet facility helps to solve key engineering problems with safer design and operation flexibility as listed below.

In recent years, there is an increase in applying dynamic simulation during the engineering phase of a project in addition to its use during plant startup, shutdown, revamp and troubleshooting.

Control System Verification:

Multiple control configurations arise during design phase of a project and multiple trains make the control system more complex owing to various operating scenarios of the plant. Generally the PCV are fully open in normal operation as the pipeline pressure will be less and flow through the train is controlled by flow controller. During packed mode operation the pipeline pressure will be higher and PCVs letdown the pressure and provides steady inlet pressure to the flow control valves which will set the flow through the train.

Dynamic behavior and controllability of pressure and flow controllers can be evaluated for various transient scenarios like shut down of one gas train and its impact on the other train, Loss of pipeline pressure and turn down of a gas train.

Determination of relief load:

When the pressure is let down across the control valve there is a design pressure rating change downstream of PCV in order to reduce the design pressure of the gas treating unit and associated piping. Any blocked outlet condition downstream of PCV or when PCV fail open, will pressurize the train and calls for protection. General engineering practice is to size the relief valve for the maximum train capacity. Generally the relief event is dynamic in nature. The relief rates are higher in the beginning and decreases as the system pressure settles out as in the case of PCV fail open scenario. This is not the case in the event of blocked outlet downstream of PCV as the relief event is continuous. These scenarios can be carried out by means of dynamic simulation and to verify the safety of the system.

High Integrity Protection System (HIPS) Evaluation:

In line with the relief scenarios dynamic simulation helps to make engineering decision or judgments to identify the necessity of High Integrity Protection System (HIPS) valves to mitigate the scenario. Further dynamic model can be extended to finalize High High Pressure trip set point, HIPS valve closing times and their response.

Operation Flexibility or time:

In some inlet facilities the feed gas is sourced from a single pipeline or several pipelines which gets dry feed gas from one or more upstream gas treating units (GTU).  GTU's removes water from wet gas obtained from one or more wells usually referred as Well Heads.

Any disturbance in inlet facility will affect upstream operation and vice versa. For example shut down of train will pressurize the pipeline as the gas utilization rate is less than the export rate. Due to the physical or larger volume of the pipeline the rate of pressure increase may be dampened. Dynamic simulation can provide the increase in pipeline pressure profile with respect to time which helps to decide when to trip the Wells. On the other hand, dynamic analysis can be carried out for sudden loss in pipeline pressure when few wells get tripped.