Many pumps require minimum flow through them to prevent damage. There are a variety of methods to protect the pump to insure there is sufficient minimum flow. They include:
- Continuous bypass
- Multi-component control valve system
- Variable frequency drives
- Automatic recirculation valves
- Pressure sensors
The continuous bypass has the lowest initial cost, requiring only piping and a balancing orifice. But it requires additional pumping capacity, which increases the pump's operating cost. In this article we will compare the annual operating cost of a continuous minimum flow bypass and a controlled minimum flow bypass.
A single centrifugal pump is drawing suction from a Supply tank open to atmosphere with a fluid surface level of 105 ft above sea level. The water is pumped to the Destination tank with a tank surface level of 125 ft above sea level; the destination tank is pressurized to 50 psig. The system is designed to pump 500 gpm into the destination tank (See Figure 1 below).
Figure 1 – Simple Piping System with Process Pipe and Minimum Flow Recirculation Line Open
A pump was selected from our sample catalogs. It is an ESP 4x3-13 pump running at 1780 rpm with a 12.375 inch diameter. The minimum flow rate for this pump as specified by the manufacturers is 168 gpm (red line on the graphs).
Figure 2 – Pump curve for the selected pump - the design point is selected at 668 gpm to accommodate the pump's minimum flow requirements
It was determined that the pump would be operated with the minimum flow line constantly open. As a result the pump had to be sized for 668 gpm, 500 gpm to the destination tank and 168 gpm continually flowing in the minimum flow recirculation line. The pump will be operating at the design condition of 668 gpm for 6,000 hrs / year. The system runs with a minimum flow of 168 gpm for 100 hrs per year. The cost of power is $.10 USD per kWh.
Figure 3 – Annual Operating Costs with Continuous Minimum Flow Bypass Open
The annual operating cost for the pump with the continuous minimum flow bypass open is $17,598 USD per year as calculated by PIPE-FLO.
If the same pump was originally sized to pump 500 gpm instead of the 668 gpm, a 12 inch diameter impeller could be used to develop the same pump head at the lower flow rate. When the pump with the 12 inch diameter is operated so the minimum flow valve only opens when required, the annual operation cost is calculated as follows:
500 gpm for 6000 hrs/year
168 gpm for 100 hrs/year
Figure 4 – Pump curve for the select pump at a design point of 500 gpm
Figure 5 – Annual Operating Costs without the Minimum Flow Bypass Always Open
The annual operating cost for the pump with an on-demand minimum flow bypass is $13,682 USD per year. This represents an annual savings of about $3,920. A variety of options can be used to close the minimum flow line when it is not needed. One should consider the total life cycle cost when considering which minimum flow option to employ.