When selecting a wastewater pump, there are two factors that must be determined. The first of these is the flow demand (Gal Per Min) and the second is the total resistance that the pump must overcome while pumping at that flow rate (Total Dynamic Head or TDH). The proper pump can then be determined by examining the relationship between these factors as represented on the pump curve.
The TDH is determined by adding the Static ( or vertical) Head to the Friction Head. While this is usually a straight forward process, there are some factors in the system design that can serious affect the performance of the pump and may even cause premature failure.
Problems can arise if we ignore the implications as shown in the pump curve for our selection. One of the requirements for UL certification is that the pump must not overload at any point on the published pump curve. If we examine most pump curves we see that the right hand end of the curve almost always corresponds to a TDH value that is something other than 0 ft. The most common starting point is 5 feet, which means that the pump will not overload at any value of TDH that is equal to or greater than 5 feet. If the TDH is less than 5 feet for these pumps, then the pump will produce more flow than the motor was designed to handle, the pump will draw higher than the design amps and will either trip the safety overloads in the panel or damage the motor.
If you look at this family of pump curves, you will see that 4 of the pumps have a minimum TDH of 5 feet. The fifth pump ( model 185) has a minimum head of 30ft. If this pump is operated at a TDH of less than 30ft, it will overload the motor.
Another point to remember is that the Full Load Amp rating ( FLA) for a pump is taken at the minimum TDH for that model. This is assumed to be the highest work load allowable for that pump/ motor combination, and the maximum AMP draw for that pump. For most pumps with a normal curve, the observed amp draw will decrease and the TDH increases ( as the pump GPM decreases). This fact should be taken into consideration when using AMP readings to determine proper rotation on 3 phase pumps. The base line AMP reading for proper rotation could be lower than the listed FLA for the pump and it may be necessary to take a reading after changing the rotation and then using the wiring configuration that has the lower of the two values as the proper rotation.
TDH must be considered when laying out a pumping system. There are three situations where a system configuration can result in shorter pump life.
1. NEGATIVE NET ELEVATION IN THE DISCHARGE LINE: This most often occurs in systems with long discharge lines which have little or no rise over the total distance. When lines must run under a road or stream, without a corresponding rise, the net elevation change in the system can actually be a negative number. In this case, it is difficult to maintain a TDH that will be within the non-overloading section of the pump curve. The problem can be eased if the flow rate through the discharge line is high enough to generate significant friction loss. This can be difficult when the line is of a significant diameter.
2. PUMPING FROM A RETENTION POND : In systems where the pump is evacuating a vessel and pumps down to a fixed level, we define the vertical head as “the distance from the discharge of the pump to the highest point in the discharge line measured vertically”. The actual definition of vertical head is “ the distance from the pumping level of the water to the highest point in the discharge line measured vertically.” When pumping from a pond or other body of water where the water level does not drop, the vertical head must be measured from the surface of the pond and not from the discharge of the pump. In many of these situations, there is not sufficient vertical head to maintain the minimum TDH for the pump.
3. SYSTEM CONFIGURATION RESULTS IN A SIPHON : We see this happen in applications where the pumped discharge runs up to a high ceiling, a long horizontal run and then drops back down to floor level. If this line becomes full, the water running away from the lift station can start a siphon. The siphon has the effect of lowering the TDH to zero ( or even below zero).
There are some solutions to a negative head situation.
· If the liquid being pumped does not contain solids, a valve can be used to throttle the rate. This has the effect of raising the TDH. If this is done using and AMP meter, the flow can be adjusted to yield the maximum flow for the system without overloading the pump. The pump will operate at its rated amp draw, so there is no harmful effect to operating a pump under this condition.
· When designing a system that could result in a siphon, the section of piping that drops back down to floor level should be increased by one pipe size. The larger pipe will not be filled with liquid and a siphon will not be formed. An Air Admittance valve can be installed at the point where the piping drops back down to the lower level. This will allow air into the system if the piping starts to draw a vacuum and siphoning is starting to occur.
· When pumping from a pond, a deep basin can be used as a pump chamber. The starting level of the liquid in the basin should be kept at a level that results in a Static head that is less than the minimum head for the pump. The pump should be sized to maintain this lower level and keep the liquid level from rising above the start level.
· When choosing a discharge line size in a situation where the static head is minimal, the diameter of the discharge line should be kept as small as possible. The size should be large enough to allow a pump to reach its maximum flow, but still small enough to generate friction loss.
If the factor of minimum TDH is addressed at design, time money and equipment can be saved over the operational life of a system.