Submergence is a commonly overlooked pump suction side issue that can create mechanical and hydraulic performance problems. Insufficient submergence can lead to increased vibration, a broken shaft, shortened mechanical seal life, premature bearing failure, lower flow rates and diminished discharge heads (pressure) for the pump. In the worst cases, these issues will cause surging and will actually stall (air block) the pump.
Submergence (simple submergence) is defined as the distance (D) measured vertically from the surface of the liquid to the centerline of the inlet suction pipe. A more important term is required submergence, also known as minimum or critical submergence (SC). Required submergence is the vertical distance—from the fluid surface to the pump inlet—required to prevent fluid vortexing and fluid rotation (swirling and or pre-swirl). Vortexing will introduce unwanted air and non-condensable gases, which can cause pump damage and reduce pump performance. A centrifugal pump is not a compressor, and performance is greatly affected when pumping dual and/or multi-phase fluids (gas and air entrainment in the fluid).
A common misconception is that inadequate submergence issues are found only on vertical and/or very large pumps. This issue, however, can and will occur on small and/or horizontal pumps.
When a centrifugal pump operates at a given flow rate (Q), there is a corresponding fluid velocity in the suction line (V). You can calculate this velocity easily by using Equation 1 or by simply looking it up in references such as Cameron Hydraulic Data (Chapter 3) or Crane Technical Paper No. 410, Flow of Fluids.
The velocity of the fluid is an important value to know because it will determine the correct submergence required to prevent the formation of vortices.
Figure 1 shows two separate cases; both are for a flow rate of 300 gallons per minute (gpm), but the suction pipe diameter is 4 inches in one case and 6 inches in the other.
Using Equation 1 and Figure 1, we can see that the 4-inch pipe yields a velocity of 7.7 ft/s, and the 6-inch pipe yields a velocity of 3.4 ft/s. Note the difference in velocity between 4- and 6-inch piping for a flow rate of 300 gpm. Figure 1 indicates that the required minimum submergence for the 4-inch pipe at a velocity of 7.7 ft/sec will be about 4.5 feet. The minimum submergence for the 6-inch pipe at a velocity of 3.4 ft/sec will be less than 2 feet. In other words, for a given flow rate of 300 gpm and a suction pipe diameter of 4 inches, a minimum of 4.5 feet of liquid above the inlet pipe will be required at all times to prevent the risk of air entrainment due to vortex formation. But if the suction line diameter is 6 inches, 2 feet of submergence will be required to preclude vortex formation. That is a difference of more than 2.5 feet.
If you have the 4-inch diameter suction pipe at 300 gpm but cannot raise the suction side fluid level to mitigate air entrainment, you can add a 6-inch suction bell to the 4-inch pipe inlet to reduce the velocity at the entrance and, in essence, fool the system to prevent vortexing. Addition of the 6-inch suction bell will reduce inlet velocity to 3.4 ft/sec and reduce the minimum required submergence to less than 2 feet. If a suction bell cannot be added, then a barrier (anti-vortex device such as floating or submerged rafts or fixed baffle plates) that presents a torturous path may also be installed.
Note that, while the barrier may preclude vortices, swirl and turbulence in the fluid stream can still present unbalance issues with the impeller. The size of the barrier can be calculated by knowing the added submergence required and then sizing the barrier to make the fluid take that same length of travel. Proper placement and location of the barrier is important; if you are not comfortable or experienced with this subject, please consult someone who is. I am reminded of an incident at a nuclear power plant 36 years ago when I, as the factory field engineer for the pump manufacturer, advised the user not to place a 12-foot I-beam near the suction of the vertical pump. The user determined that I was wrong and sought to block any vortices while measuring flow. He securely attached the flow meter to the large beam and placed it near the pump suction. Less than 3 seconds after the pump was started, the beam became an integral part of the suction bell and the impeller.
Besides inadequate submergence issues, air and non-condensable gases can enter the fluid in other ways. Liquid can fall from above the suction tank/sump (e.g. cooling tower), or agitation can occur in the upstream process. Sometimes air or gases are introduced on purpose as part of a process requirement (e.g. pulp slurry recycling). An additional issue is having a suction tank/sump that is not sized properly. A properly sized tank/sump will, by design and with respect to volume and geometry, allow for sufficient transient time (hold time) for the fluid to facilitate the escape of entrained air prior to entering the pump suction.