Hydraulic Analysis Ltd. undertake flow assurance studies and surge analysis studies using VariSim and the following videos show some of the study output in order to demonstrate typical VariSim operation, study objectives and pipeline hydraulic reaction.
This video shows a controlled pump start-up and subsequent end of line valve closure transient event. Following pump start-up, the pressure rises downstream of the pump allowing flow to develop throughout the pipeline. Pressure and flow oscillations continue until a steady operating regime is established. At this point, the end of line valve is closed generating a pressure rise upstream of the valve as the flow through it is restricted and ceases. Again, the pressure and flow oscillations are observed between the pump and the closed valve with high locked in pressures generated as the pump operates at it’s flow head.
This video shows VariSim simulating a single pipeline. The Pumping station is at the start of the line with a Surge Vessel 1.5km and Air Valves at 3.7km & 9.5km along the pipe length. At the top left of the screen is a time based graph showing flow from the Pumping Station & the water level in Surge Vessel. At the top right of screen is the schematic showing the system components changing in colour during simulation
At the bottom of screen is the pipeline longitudinal profile illustrating the head during the simulation together with the maximum & minimum heads for the simulation. The simulation is of a Power Failure at the Pumping Station. The surge vessel provides some attenuation of pressure but it is too small and so the air valves open. 1 minute 20 seconds after power failure closure of the Air Valve at 3.7km leads to further pressure surges.
This video shows the nett transient effect of rapid closure of the turbine inlet valves and extended opening of the turbine bypass valves, in response to a steam turbine power failure event. At a time of 0.1s into the simulation the turbine inlet valves commence closure with a stroke time of 90ms, whilst at the same time the turbine bypass valves commence opening with a stroke time of 2.50s.
The steam hammer system extends from the outlet of the boiler superheater to the inlet of the steam turbine. The superheated steam exits the boiler at a temperature of 550 degC and a pressure of 260 bar abs and passes through the system at 2400 t/hr and a density of 85 kg/m3. In sections of the system the steam velocity is as high as 50m/s.
It can be seen that there is a pronounced Joukowsky-type peak pressure rise on the upstream side of the turbine inlet valves consequent to their rapid closure, which is subsequently followed by further peak pressure rises due to linepack in the pipeline as the boiler continues to input steam into the system. Due to the extended opening stroke time of the turbine bypass valves they offer little in the way of surge relief for this system. Their main function is to limit the effects of continued linepack in the pipeline and therefore prevent excessive over-pressurisation.
Simulation Summary: Key Transient Information
The following demonstrations are for the same scheme with and without surge suppression equipment sized and installed.
The first video shows a power failure event tripping all main pumping station pumps. After a short delay (7s), the associated booster pumping station is instructed to shutdown and its dedicated bypass valve simultaneously opens using a stroke time of 10s.
It can be seen that even allowing for the operation of the booster pumping station bypass valve, significant air valve activity is observed in this unsuppressed system which is unacceptable and could lead to potential contamination of the water supply.
Simulation Summary: Key Transient Events
This second video shows a power failure event tripping all main pumping station pumps with surge suppression in the form of a surge vessel installed at the main pumping station and in-line vessel 1900m downstream of the booster pumping station. After a short delay (7s), the associated booster pumping station is instructed to shutdown and its dedicated bypass valve simultaneously opens using a stroke time of 10s.
It can be seen that by installing surge vessels at these two locations no air valve activity is observed throughout the pipeline as pressures are maintained above atmospheric, hence preventing the possibility of potential contamination of the water supply.
Simulation Summary: Key Transient Events
