In last month’s column (read it here), we looked at the design process for a small waste collection system. Based on the expected plant operation, the system was designed to handle a maximum flow rate of 400 gpm of 80 F water. These system requirements along with various rules of thumb were used to size the items making up the system.
Now we will look for ways to reduce the total life cycle costs of the system. This is accomplished by looking for ways to reduce the system’s capital, operating and maintenance costs and will require a closer look at the system requirements.
Process Flow Rates
The process fluid is water at 80 F, with a design flow rate based on an expected maximum flow of 400 gpm. After a deeper investigation of the operation was conducted and compared to similar facilities, an average operation table was developed. Table 1 shows the typical range of flow rate and percent operating for waste collection systems.
Looking at the operating profile of typical waste collecting systems, the design flow rate of 400 gpm is only required for five percent of the system operation. Most of the time the flow rate through the system is between 150 and 200 gpm. We can also see that when the plant is shut down, the collection system remains in operation with a flow rate of 80 gpm. Using this revised operating data we can look for ways to improve our design.
Since all piping systems are composed of process, control and pump elements, we need to evaluate the combined effect of the system elements.
The process elements found in our system consist of our collection and distribution tanks as well as the connecting pipeline.
Tanks and Vessels
The elevation of the system tanks is often thought of as a fixed value that cannot be changed. If the waste collection system is being installed in an existing facility, the elevation of the tanks may indeed be fixed. If a system is being installed in a new facility the tank elevations can still be adjusted.
In the example system design, the difference in the tank elevations resulted in a static head of 20 feet. If the difference in tank elevations was reduced by five feet, the static head would be reduced to 15 feet, resulting in a permanent reduction of the transfer pump’s total head required by 5 feet. This may result in a large decrease in pumping costs over the
The size of tanks and vessels are often based on the system design flow rate into the tank. The location of the tank within the plant can also affect the tank size. A smaller waste collection tank results in a smaller tank footprint in the plant facility and will cost less to fabricate and install. The volume of the smaller tank will cause the tank level to change rapidly, which could cause an excessive number of starts on the collection pump. A large waste collection tank results in a larger footprint within the facility and will cost more to fabricate and install. The large tank results in a slower rate of level change reducing the number of starts required by the waste collection tank.
In our system, the tank was sized for the maximum flow rate of 400 gpm, but it operates at this flow rate only 5 percent of the time. Since the normal flow rate through our system is 200 gpm or less, one could consider reducing the size of the waste collection tank. This would result in a lower cost for building and installing the tank.
For the same flow rate through a pipe, a smaller pipe diameter results in a higher fluid velocity than a larger pipe. A smaller pipe diameter typically costs less to manufacture and install but results in a higher head loss. Increasing the pipe diameter results in less head loss in the pipelines but increases construction cost.
The number and type of valves and fittings used in a pipeline are also affected by the pipe diameter. As with pipe size, valves and fittings of larger diameter cost more to purchase and install, but similar to pipe diameter, smaller diameter valves and fittings increase their head loss. The geometry of valves and fittings influence the head loss. Globe valves with multiple changes of direction and smaller inside diameter have a higher head loss than ball valves with a larger internal flow passage and no change of fluid direction within
the valve body.
It is common practice to size a pipeline based on achieving an optimum fluid velocity. The optimum fluid velocities for a pipeline are based on the fluid being transferred, the pipe material used, international standards, pipe routing, and are typically called out in customer specifications documents.