The company uses dynamic modeling to analyze causes for HIPPS activation; Evaluates options to circumvent HIPPS activation
The ever-increasing flow rates in combination with the environmental constraint shave initiated a widespread and rapid acceptance of HIPPS as the ultimate protection system, in the last decade or so.
A High Integrity Pressure Protection System (HIPPS) is a type of safety instrumented system (SIS) designed to prevent over-pressurization of a plant. The HIPPS will shut-off the source of the high pressure before the design pressure of the system is exceeded thereby protecting the downstream equipment.
RasGas is operating a natural gas processing plant at Ras Laffan, Qatar. The plant is equipped with a HIPPS system, as shown in Fig. 1. The frequent tripping of HIPPS resulting in plant shut-down was eroding the revenue substantially. This forced the engineers and management to carry out a detailed root-cause analysis and development of solution strategies using dynamic simulation.
Modeling the unit
The starting point was to build an accurate dynamic (base) model considering the equipment, valve stroke speed, actuator characteristics, Cause & Effect Logics, Controller tuning parameters, control strategy, layout, and piping isometrics.
The model was tuned to reproduce the conditions that led to the most recent HIPPS activation. Fortunately, a detailed time snapshot of process conditions was available from the DCS historian, which was used to reconstruct the conditions leading to HIPPS activation.
The (base model) validation phase focused on matching time taken in the plant for HIPPS activation in the model. This was necessary not only to build the client’s confidence in the model but also to build confidence in the predictive capability of the model. Some of the interesting scenarios, which were analyzed, were:–
Effect of sudden opening/closing of
- Pressure Let Down Station valves
- ESDVs’
- Flow Control Valves
The study recommended the following set of changes to meet the objective of circumventing HIPPS activation in the future, which were implemented in the plant, to achieve the desired objectives:
- Anew control scheme
- Better controller tuning parameters
- Advice on operating conditions
Challenge: An LNG facility faced repeated HIPPS activations, leading to plant trips, causing significant production loss. The challenges were:
- To understand the plant’s transient behavior, in a startup, shutdown, ESD, and normal operating conditions
- To analyze the causes leading to HIPPS activation
- To evaluate options to avoid HIPPS activation in future
Solution:
Build a rigorous dynamic model of the complete system, taking into account the actual pipeline layout & other details.
Benefits:
- A rigorous dynamic model was built from the plant data
- An actual plant occurrence of HIPPS activation was replicated on the model
