Ray Hardee is a principal founder of Engineered Software, creators of PIPE-FLO and PUMP-FLO software. At Engineered Software, he helped develop two training courses and teaches these courses internationally. He may be reached at firstname.lastname@example.org.
The process elements consist of waste collection sumps, the pipeline details and the facility’s waste treatment tank. The individual waste collection sumps provide the inlet system boundary.
The information needed on each sump includes the tank’s base elevation and operating level. Other information can include the normal inflow along with the inflow into the tank and its dimensions.
The information needed for the pipelines includes the pipe length, diameter and corresponding pipe roughness.
The type and number of valves and fittings in the pipeline will also affect the minor losses associated with these devices.
The system is controlled by the level in each tank. The waste collection pump is started on a high-level control, and the pump is stopped on a low-tank level.
Additional system operating information is needed for an accurate simulation. We must know the properties of the process fluid, specifically its fluid type and temperature. Since the fluid in the waste collection system is predominantly water, that information is easily available.
The flow rate through the pump is based on the level in the waste collection sump, along with the pump’s outlet pressure.
For example, when pumping from a tank, the flow rate through the pump decreases as the tank is pumped down. As additional pumps are started in the system, the pressure in the discharge header increases, causing the flow rate in each individual pump to decrease.
Using the Simulation Results
Once the model is completed with the above information, the simulation software will calculate the operation of the entire piping system. Since the system operation is based on a level control and the tank levels and pumps affect each other, we will model the system showing the interaction over a period of time.
When comparing the actual system operation with the model, the results provide insight about why the waste collection tanks overflow.
While running the model, it was discovered that when a pump in any of the new buildings switched on, a significant decrease in the flow rates from the existing collection sumps resulted.
Under conditions with more than three existing pumps in operation at any given time, starting a pump in the new buildings caused the flow rate from the tank to be less that the flow rate into the existing tanks, causing the overflow.
In looking at the design point supplied to the pump manufacturer, it appeared that the head required was overstated, and the pump supplier selected a pump that caused the sump pump to pump down too quickly.
Another result of the simulation was the design flow rate into the sumps of the existing building was greater than that called out in the pump specification.
Over time, the flow rate into the existing building waste collection sumps increased, resulting in a longer time to run the waste collection pumps. Some of the increase in flow rate was due to greater waste generation load from the building and some was of it was due to leaking water into the system.
Using the system simulation results along with the operation of the actual piping system, plant management was able to discover the system problems and look at ways to improve the system to meet actual system conditions.