When performing a time simulation of a piping system operation in Overtime, the program takes into account how the boundary conditions change over time. For example if a pump is pumping one tank down, and filling another tank, the changing tank levels have an effect on the flow rate through the pump. The changing tank levels are the boundary conditions for the system.

Let's say you want to see how long it will take Pump 1a to lower the tank level in the Supply Tank from 10 ft to 5 ft. This is a time simulation type of problem. As the Supply Tank is pumped down the Storage Tank is filled. This change in elevation of both the Supply and Storage tanks causes the static head to increase. This increase in static head causes the pump to run back on its curve resulting in a reduced flow rate through the pump as the Supply Tank is pumped down.

The capacity for the Supply Tank is 10,000 gallons with 10 ft of working level, resulting in 1,000 gallons per ft of tank level. The Storage tank has a capacity of 20,000 gallons with a working level of 10 ft, resulting in 2,000 gallons per ft of tank level.

The starting condition for the Supply tank is 10 ft of level, and 5 ft of level in the Storage tank. The pressure is controlled in the Storage tank to a constant 20 psig regardless of tank level.

Now we run a time simulation on this piping system using Overtime. Running a time simulation is as easy as specifying the duration and frequency of the simulation.

Here we have set the simulation to run for 2 hours with 360 calculations per hour (one calculation every 10 seconds).

We can observe the progress of the simulation in the Log Window. Notice that the simulation ends prematurely at 1 hour and 11.5 minutes. The reasons are stated in the Log Window: the Supply Tank would have emptied, and the Storage Tank would have overflowed on the next iteration.

With Overtime there is the ability to go into VCR mode with PIPE-FLO in the background.In this mode you can fast-forward and rewind throughout the simulation.While it is running, you can observe the changes occurring in the piping system.

First, we will run the simulation for 1 hour at 3600 calculations per hour (one calculation every second).This is a fairly high frequency simulation which takes a little longer to run, but can pinpoint the time when certain events happen.

Here is a graph for the pump's flow rate and total head.

Here is a graph that shows the levels in both the Supply Tank and the Storage Tank.Notice the trends as you go from the time 0:00 to time 1:00.

Next we will calculate the amount of time it will take for Pump 1a to drain the Supply Tank down to 5 ft of liquid level. To do this, we will instruct Overtime to run the simulation until the Supply Tank reaches a liquid level of 5 ft, and then end the simulation.

Notice in the Log Window that the simulation ended at 27 minutes and 51 seconds because the Supply Tank reached a level of 5 ft.

So the amount of time required for this system to drain the Supply Tank down to 5 ft is 27 minutes and 51 seconds. Look once again at the results graphs and note that the Supply Tank gradually drops to 5 ft, and then the simulation ends.

This PIPE-FLO system is about as simple as you can make. Remember, Overtime is able to run a time simulation on any working PIPE-FLO system, no matter the size. With the Event builder, you can turn pumps on and off, open and close pipelines, change valve settings, vary the flow rates and basically have complete control over the operation of your system. Any lineup setting changes that you can make in PIPE-FLO, can be programmed into Overtime. And, as we have seen in this example, Overtime can be a useful tool in determining tank residence times.