For oil pipeline in mountain areas, high hydrostatic pressure in the pipeline may cause error-opening of pressure relief valves, and oil is discharged from the pipeline to the pressure relief tanks, bringing spilling-over risk of the pressure relief tanks. Therefore, simulating the error-opening situations of the pressure relief valves and investigating the oil discharge process are necessary for checking the possibility of the spilling-over accident and then proposing measures to improve the pressure relief system. This research focuses on a continuous undulating oil pipeline with large elevation difference and a station along this pipeline, which is named B station in this paper, is studied. By OLGA software, simulation model of the pressure relief system of B station is established, and the accuracy of the model is verified by reconstructing a real accident and making a comparison with the actual accident data. The maximum discharge rate reached 8284 m3/h when the pressure relief valve was opened by mistake in the inlet and outlet of the station. The accumulated filling time of the two pressure relief tanks is 200 s, which is in good agreement with the accident data. On this basis, for error-opening of the pressure relief valves at the inlet and outlet of B station, simulation is performed to investigate variations of the discharge velocity, discharge flow rate, accumulated discharge volume and ventilation volume of the vent valve. The discharge velocity is found to be over the maximum velocity allowed for safety consideration. According to the accumulated discharge volume, it is inferred that spilling over of the pressure relief tanks will be caused once error-opening of the pressure relief valve occurs. Also it is judged that the existing breathing valve can not satisfy the ventilation requirement in case of failure of the pressure relief valves. From these simulation results, it is proposed that increasing the number of vent valves, replacing the manual valves with electrically operated valves, and employing security control interlock protection program are improvement measures to guarantee safe, efficient and reliable operation of the pressure relief system at B station.
For an oil pipeline going through complex landform areas with mountains, hills or rivers, there is acute topographic undulation along the way, leading to a large number of pipe sections with large elevation difference and thus high hydrostatic pressure in the pipeline. In case of emergency situations such as pump shutdown and process switch, water hammer may occur, which leads to excessive pressure in the pipeline, and thus possibly causes pipeline failure, property damage and even casualties [
For a given continuous undulating oil pipeline with large elevation difference, the pressure relief system at the stations is consisted of pressure relief valves, discharge pipes, pressure relief tanks, manual valves and field instruments. The performance of a pressure relief valve is related with design parameters, system parameters and operation parameters as well as experience of the decision-makers. By now, there have been extensive researches conducted on pressure relief valves or systems, mostly on the dynamic behavior and performance optimization [
In the continuous undulating oil pipeline with large elevation difference, pilot-operated pressure relief valves [
The remaining of this paper is organized as follows.
Two-fluid model is applied to simulate the multi-phase flow. The governing equations include the mass conservation equation, momentum conservation equation and energy conservation equation. The equations are given as follows.
(1) The mass conservation equation
For the gas phase, we have,
For the liquid phase near the pipe wall which is a liquid film, we have,
For liquid droplets, we have,
In the equations,
(2) The momentum conservation equation
For the gas phase and liquid droplets, the momentum equation is as follows:
For the liquid film adhere to the pipe wall,
where
(3) The energy conservation equation
For the whole system, we have,
where
As with the boundary conditions, for the upstream, we have given flow rate as the boundary condition, while for the outlet, the pressure is known.
The continuous undulating oil pipeline of large elevation difference applies airtight transportation to transport oil under normal temperature. A, B and C stations are stations along this pipeline, as presented in
Oil type | Density at 20°C (kg/m3) | Kinematic viscosity (mm2/s) | Flash point (°C) | ||
---|---|---|---|---|---|
10°C | 20°C | 30°C | |||
Saudi light oil | 856.5 | 12.53 | 7.04 | 3.99 | 150 |
Pressure relief system | Diameter of pressure relief valve (mm) | Flow rate (m3) | Discharge pipe diameter (mm) | Volume × |
Diameter × |
Set-pressure (MPa) |
---|---|---|---|---|---|---|
At the inlet | 200 | 1800 | 323.9 | 200 × 2 | 250 × 2 | 7.7 |
At the outlet | 200 | 1800 | 323.9 | 13.2 |
As is depicted by
Based on the pressure relief systems of B station, a simulation model is established, which sets A as the starting point, and C as the ending point. As
A real over-spilling accident of the pressure relief tanks at B station is used to validate the established model. Before the accident, the inlet pressure of B station was 4.86 MPa and the outlet pressure of B station was 12.8 MPa. The throughput of B station was 1000 m3/h, and both the pressure relief tanks at B station were empty. As the outlet pressure of B station was over 12.4 MPa,
Data | Pressure relief valve opening pressure (MPa) | Delayed opening time of relief valve (s) | Convergence parameter (MPa) | Running time (s) | CPU |
---|---|---|---|---|---|
Simulation data | 12 | 200 | Main line pressure = 5 | 2000 | ≤85 |
The simulation result of this real accident is presented in
Data | Maximum discharge flow rate (m3/h) | Average discharge flow rate (m3/h) | Time required for filling 400 m3 pressure relief tank (s) |
---|---|---|---|
Accident data | – | 8000 | 180 |
Simulation data | 7930 | 7422 | 194 |
While the real accident data shows that 3 min (180 s) was cost to fulfill the two pressure relief tanks (which have a total volume capacity of 400 m3). According to the simulation data, the time required for filling the 400 m3 pressure relief tanks is 194 s, meaning a 7.78% deviation from the accident data. Then comparing the average discharge volume of the accident data and the simulation data within 180 s indicates a deviation of 7.22%. The simulation accuracy is considered to be acceptable for engineering analysis of field accidents.
The medium pressure acting on the relief valve equals the static pressure of the medium caused by elevation plus the dynamic pressure of the medium in the pipeline. Therefore, under the same conditions, the higher the elevation, the greater the pressure is imposed on the relief valve, and the smaller is the flow rate in the main pipe.
During normal operation of the pipeline, if pressure relief valve error-opens and fails to be closed, an alarm signal is sent out. Then the station staff can report the situation and apply the permission for manually closing the manual valve in front of the pressure relief valve. Also the pressure relief tank is equipped with a liquid level indicator of Safety Integrity Level 2 certification, and when the liquid level in the pressure relief tank reaches the alarm set-level, the advance shutdown control program of the whole pipeline is triggered against water hammer.
However, error-opening of the pressure relief valves may occur after the pump of B station is shutdown, then the discharge pipe can only be closed through the manual valve in front of the pressure relief valve. If there is a delay in manually closing the manual valve, spilling over of the pressure relief tanks happens. To avoid the above illustrated spilling over accident of the pressure relief tank, simulating and analyzing the error-opening situations of the inlet and outlet pressure relief valves after the pump of B station is shutdown is necessary.
At 600 s, error-opening of the inlet pressure relief valve occurs, and the simulation results are shown in
Inlet or outlet? | Discharge pipe diameter (mm) | Maximum discharge flow rate (m3/h) | Maximum discharge velocity (m/s) | Accumulated discharge volume (m3) |
---|---|---|---|---|
Inlet | 323.9 | 5129 | 20.16 | 192 |
Outlet | 323.9 | 8284 | 32.55 | – |
Inlet or outlet? | Time required for discharge of 400 m3 (s) | Valve No. | Pressure class | Set pressure |
Inlet | – | 0116 | CLASS600 | 7.7 |
Outlet | 200 | 0119 | CLASS600 | 13.2 |
For error-opening of the pressure relief valve at the outlet of B station, the simulation results are given in
There was also an accident occurred during a test of putting the pipeline into commission using water. According to the test, totally 5 min (300 s) was cost from the beginning of relieving pressure to manually closing manual valves. In the 5 min, 2 min was spent for applying the permission of operation and receiving the permission. It is inferred that, manually closing manual valves required 3 min. While as with the simulation, 200 s is cost for an accumulated discharge of 400 m3. It is known only 80 s is left for the field staff to close the manual valves, which is not enough according to the test data, and thus spilling over of the pressure relief tank occurs.
In error-opening condition of the inlet and outlet pressure relief valves, the discharge flow rate is up to 8284 m3/h. According to
Also, during error-opening of the pressure relief valves at both the inlet and outlet of B station, the discharge velocity exceeds the allowed maximum velocity even with the static electricity eliminator. It is suggested to shutdown the discharge pipes to avoid the accumulation of static electricity. Thus, changes to the pressure relief pipes are not required.
The time cost for fulfilling the two pressure relief ranks is as short as 200 s in case of error-opening of the pressure relief valves at B station. If error-opening of the pressure relief valves happens during normal commissioning of the pipeline, 80 s is not enough for the on-site staff to manually close the manual valves before spilling over of the pressure relief tanks. Thus, it is suggested to replace the two manual valves in front and behind the pressure relief valve with electrically operated valves, which take as least as 20 s to be closed.
Based on the above suggestions, the pressure relief systems at the inlet and outlet of B station can be improved. Below, error-opening of the improved pressure relief system at the outlet of B station is simulated. It is assumed that closing the electrically operated valve in front of the pressure relief valve begins 120 s after error-opening of the pressure relief valve, and ends 20 s later. The simulation result is shown in
Due to complex elevation variations and high hydrostatic pressure in the continuous undulating pipeline of large elevation differences, there is high pressure remaining in the pipeline after pipeline shutdown. Therefore, error-opening of the pressure relief valves may happen, causing discharge of oil to the pressure relief valves for a long time. And spilling over of the pressure relief tanks may occur. This research simulates the error-opening situations of the pressure relief systems of a station (named Station B) in a continuous undulating pipeline of large elevation difference. OLGA software is used for the simulation. The main conclusions are provided as follows:
(1) Once error-opening of the pressure relief valve occurs, oil is discharged to the pressure relief tanks, with the maximum discharge flow rate respectively being 5129 m3/h and 8284 m3/h for error-opening of the pressure relief valve respectively at the inlet and outlet of B station. The discharge velocity of the oil is over the upper limit of security allowance, which might cause safety accidents due to increasing accumulation of static electricity. To prevent such accidents, the discharge pipe should be closed as early as possible to quit oil discharge.
(2) After error-opening of the pressure relief valves, the ventilation volume required by the pressure relief tank is larger than the provided ventilation ability. To avoid tank inflation or tank tearing, the number of the vent valves should be increased according to the simulation result.
(3) For the discharge pipe, replacing manual valves with electrically operated valves could reduce the time for closing valves in case of accident situations, and cease the static electricity accumulation during the discharge process, therefore reducing risks of tank spilling, combustion and explosion.
(4) The pressure in the pipeline is still at a high level after the pipeline is shut down. When the pressure relief valve is opened by mistake, a large amount of oil is released continuously.
The authors would like to extend their sincere gratitude to the editors and reviewers of