Valve closure often causes the highest surge pressures on systems with a low static head - in comparison, pump start-up and trip are usually less severe. And this means that many oil and petrochemical systems are particularly susceptible from closure of ESD valves, control valves and isolating valves. The deceleration of flow through the closing valve causes a corresponding pressure rise and both the magnitude of this pressure and its rate of change are important - high pressures can potentially lead to over-pressure, whilst a rapid pressure rise can cause pipe movement. Both can cause downtime and loss of revenue.
Overall, this means that the closure time of the valve is a significant factor in many schemes and so the opportunity to investigate this at design stage - by Pipeline Simulation - is cost effective. The closure time of non-crucial valves can be extended to ensure that pressure problems are inherently avoided; alternatively, surge relief or HIPS can be customised for ESD valves where fast action is imperative.
No simulation program has any credibility unless it gives good correlation against ordinary site tests and so our program is vigorously validated against several independent sources. Our policy is to discuss the examples from the public domain (rather than release proprietary information about industrial shutdown incidents) and so this example is taken from a laboratory test at the University of Michigan. However, this does not limit the usefulness of the case - both on-site studies and fundamental hydraulics confirm that surge pressure changes are independent of scale.
The test case demonstrated the viability of adopting a design philosophy which contains surge pressures (by selecting a suitable valve closure time) if the operation of the system is studied in full. The theory was validated by comparing measurements from a 4000ft long test pipe with computerised predictions. And we have repeated that test, comparing the output from our simulation program with those original test results - both demonstrating the accuracy of our program and underlining the effectiveness of this approach in the design stage of any piping system.
Program validation is part of our company policy and the process ensures good accuracy.
