The actual cause of the reduced flow measurement was a leaking check valve that allowed part of the flow to return to the wet well through the other pump. When the pressure was measured at the valve box and corrected for the elevation difference between the water level and the gauge, the operating head was nearly 48 feet. It turns out that neither was the culprit. Is this reduced flow and increased head due to a blockage in the discharge pipe, or was the original design point incorrect? Based on the performance curve, the pump is operating at 55 feet instead of the design point of 48 feet. Suppose we perform several draw down tests and compute an average flow of 350 gpm. When possible, we like to use one foot because little change in flow occurs for most pumps over such a small elevation change. Dividing the volume by the time in minutes provides us the flow rate in gallons per minute. In a standard draw down test, the wet well volume per foot is calculated and the time required to lower the water level one foot is recorded. In this example, any friction in the discharge piping will be ignored. The red, horizontal line is the system head at the pump-start level in the wet well. Pump curve for a duplex lift stationįigure 1 shows the pump curve for a duplex lift station that was designed to pump 400 gallons per minute (gpm) into a gravity main against a static head of 48 feet. Pressure must also be measured to assess the true performance of the station.įigure 1. However, flow rate alone may not provide all the needed information. Often, a draw down test is used to measure pump flow rate, and then flow rate is used to determine where the pump is running on the H/Q curve. This testing method can be reliable as long as the data collected is accurate and the test is repeated several times to ensure reliability. In the Northwest, the vast majority of these stations use draw down as the standard pump test technique. Although it accommodates several different pump designs, wastewater pumps were a major focus.įlow meters are becoming more popular in many wastewater applications, but justifying their cost in smaller lift stations and those that are not connected to a SCADA network is often difficult. In February 2011, I wrote an article on my Field Test Calculator. In 2010, this column discussed unbalanced radial thrust and how the wide vane impellers used in wastewater pumps are extremely vulnerable to the damage it can cause.

Last month, I revisited suction recirculation and its potential effects on wastewater pumps due to the rather large eye diameter to impeller diameter ratio. Based on several of my previous Pumps & Systems articles, you probably know that I have major concerns about the off best efficiency point (BEP) operation of wastewater pumps.

4 hour well drawdown test