In last months blog I stated that the most common communication problem in piping system design is the failure to accurately state the process requirements when selecting a pump. In this month’s blog I will add commentary of what could be done to improve the communications. (Italics indicate quote from last month’s blog.)
Sizing the Pump
Every system needs a pressure gain device or pump to move the fluid through the piping system. Choosing the right pump, motor sizing, correctly sizing pipes, and determining the best efficiency point all depend on knowing the capacity requirements for the system.
“The owner of the system design provides a capacity requirement based on future system needs knowing full well that the system will be operating at a lower capacity for an extended period of time until the market need catches up with capacity.”The owner should clearly state in the design documents that the pump system is expected to be operating at 500 gpm for the first 5 years then increase to 1,000 gpm once product sales ramp up. Full disclosure on the system’s intended design and purpose should be stated from the beginning.
“The engineer designing the system takes the capacity provided by the process group and adds a 20% design margins of flow (to allow for future capacity increases).”Assuming that the owner has clearly stated their design requirements in the design documents, the engineer now knows from the beginning that the system will be operating at 500 gpm for the foreseeable future, and the possible future requirements of 1,000 gpm capacity. There is no need for the engineer to factor the additional 20% design margin increase in pump selection. The engineer can size the pipelines to meet both the immediate needs AND have a design flexible enough to meet the future needs as well. Once the pipelines are sized, the head loss in the system can be determined for both 500 gpm and 1,000 gpm. Based on this information, the total head for the pump can be determined for both conditions but selected from the current condition of 500 gpm.
Avoiding the Snowball Effect
It may start as a small over estimate but it can get compounded when others on the project try to compensate for the lack of specificity in the beginning.
“In addition, a design margin for head is added (to account for system uncertainties during the design process) when specifying the equipment. As a result the pump design point is 1,200 gpm and 200 ft of head.
The individual selecting the pump chooses a pump with the design point left of the pumps Best Efficiency Point (BEP) to allow the pump to better accommodate future system capacity increases.”Since engineering is now initially designing the system to operate at 500 gpm, there is no need to add extra capacity for the future at this time. When it comes time to add the future capacity, the owner will have a better understanding of the actual production needs and can factor that into the selection when the increased process loads are known. The engineer submitting the pump specification to the supplier can provide information on the current needs as well as the future needs.
The options available to the engineer and customer can be:
- Select a pump that can meet the future needs with a variable speed drive. This allows the pump to meet both today’s needs (at a slower speed) as well as the projected future requirements (at a higher speed).
- Select a pump that has a sufficient range of impeller diameters to meet the 500 gpm flow rate, with sufficient additional diameter to meet the expected future flow. A new impeller will need to be purchased to meet the future requirements but the savings in pumping cost will more than make up for the difference.
- Design the system with multiple pumps in parallel. To meet the initial 500 gpm one pump can be operated, and to meet the future needs a second pump can be operated providing the proper head and flow for the future capacity.
The next step is for the engineer to submit a pump specification to the pump supplier. Since the system owner provided a detailed description of how the system will operate, the design engineer for the system is able to provide the known information in the pump specifications. Armed with the full details of how the system will be operated throughout the next 5+ years, the pump supplier is able to select the best pump meeting the stated requirements.
By being upfront with the design intentions and explaining the current needs and future vision in detail, each group in the process knows how the system will be operating in the foreseeable future along with the projected future capacity. No assumptions needed. If that information is shared with each individual, every step of the way, the design margins will reflect how the pumps needs to be run, not how it will be run in the future, and less of a chance of a snowball effect.
I have offered one example of a common area of miscommunication in the piping system design process. Please feel free to add your experiences to the comments below or offer advice to anyone starting out on their first design project. I appreciate all of your feedback.
