Pressure Propagation in Gas Lines: A dynamic simulation study using HYSYS
Pressure Propagation in Gas Lines
Pressure propagation in gas lines is an event that, if not properly controlled, can cause severe damage to the pipes and equipment in any facility. The most common cause for a pressure spike is the sudden opening or closing of a valve at the receiving end of the process or even in-between the process that may either be planned or unexpected behaviour in response to an emergency.
In the below sketch, failure of the heater temperature control. If there was a heater between the high and low-pressure letdown
Dynamic simulation is an essential tool to quantify the pressure surge in the system in these cases as it enables modeling of the control system, the dynamic valve behaviour, and mass accumulation (i.e., pressure buildup). For further information on dynamic simulation, see our article, Dynamic Process Simulation: When do we really need it?
In particular, it is important to check the pressure propagation of in-plant piping for gas lines where the difference between the operating pressure and the maximum allowable operating pressure (MAOP) is small. This evaluation can ensure that the sudden closure of a downstream valve will not lead to overpressure and frequent emergency shut-down scenarios in the facility. The results from a line packing dynamic study are required to design safety and process control strategies, including the determination of set points, and the tuning of controller parameters.
In this article, we present the results of a recent pressure surge case study conducted for an oil sands operator in Western Canada.
Gas Boiler Line Packing Simulation Study
In this Simulation Study, natural gas is distributed to three boilers for heating purposes via a medium pressure buried pipeline. The pipeline is fed by a high-pressure let-down facility. Each of the boilers has its own inlet pressure and flow control valves along with an emergency shutdown valve to isolate the boiler in case of an emergency. A sketch of this configuration is shown in Figure 1.
Figure 1: Process Configuration
A dynamic model was built in Aspen HYSYS to determine whether maximum pressure in the gas piping downstream of the high-pressure letdown valve would exceed MAOP. A worst-case scenario where all the boilers trip at the same time and reject the incoming gas flow was considered. This is a classic case of line packing. See our article, Simulating Pressure Profile in HYSYS to learn more.
In the Aspen HYSYS dynamic simulation, once a steady-state condition is achieved, the boilers are tripped by slamming shut their inlet isolation valves at the same time. The sudden closure of the boiler isolation valves results in a pressure buildup in the upstream lines which in turn leads to the closing of the boiler pressure control valves as the pressure rises above the set point.
The high-pressure control valve in the letdown station tries to maintain the pressure in the downstream gas line to its predefined set point and will start closing according to its controller parameters. In this case, even with the action of the high-pressure valve controller, eventually, the pressure rises to the high pressure (PAH).
In this facility, as soon as PAH is detected, a signal will be sent to the controller of the high-pressure valve to de-energize its solenoid and the valve will close in one second. The results of this study are shown in Figure 2.
Figure 2: Simulation Results
In this case, due to the quick closing time of the high-pressure control valve, the surge pressure in the letdown station remains below the MAOP thus avoiding a high pressure (PAHH) trip.
Other studies were conducted to study the partial shutdown of the boilers where only a subset of the boilers start rejecting the flow. In these cases, it is important to prevent PAH in the letdown station to allow for continued flow to the boilers that are still operating. Using the dynamic simulation, the controller parameters were tuned to ensure the PAH is avoided.
The results of this study were used to verify the safety of the process as well as the required controller parameters tuning to ensure the reliable operation of the facility in the case of a partial shutdown.
A dynamic study is crucial and provides a more in-depth analysis of the process that provides answers and insights that would not be possible with a steady-state simulation.
For more information or queries about simulation studies, feel free to contact us.