A vortex is a flow field that has both an axial and a rotational flow component, similar to a tornado. Vortices can form on any sump boundary, including the water surface, sump floor, and sump walls. When a vortex enters a pump, the impeller blade encounters the low pressure core and the loading on the impeller changes. The impeller receives one of the loading discontinuities each time a blade encounters a vortex core. These impulses cause the impeller to vibrate at the vane pass frequency (or multiples thereof for more than one vortex).
Free Surface Vortices
Classic free-surface vortices entrain air into pumps, causing them to rumble and vibrate. The vibration results from the fluctuating loads on the impeller blades as they encounter the varying densities of air and water.
Free-surface vortices usually form when there is insufficient submergence. Approximately one foot of submergence is required for every foot/second of fluid velocity existing at the entrance to the suction bell for flows less than 150,000 gpm and 1.5 feet of submergence for every foot/second of fluid velocity for flows greater than 150,000 gpm.
The entrance velocity can be calculated from the equation below, where “d” is the suction bell diameter. It is recommended that suction bell velocities be kept below 7 ft/sec.
vbell = velocity @ suction bell (ft/sec)
Q = flow (gpm)
d = suction bell diameter (in)
The figure below provides more accurate data regarding minimum required submergence as a function of flow.
Surface vortices may be detected as follows:Surface vortices may be detected as follows:
- Visual observation of the sump free surface – Increased vibration amplitudes as vane pass frequency
- Dye-injection in a model sump test
Observation of the water surface in a sump may reveal the presence of surface vortices and surface flow patterns indicitave of pre-rotating flow entering the pumps. Observations may require auxiliary lighting in the sump. If lighting is adequate, video recording of the water surface should be collected. The observation should be made over a long enough period of time, as flow patterns may require as long as 15 minutes or more to fully establish.
Certain site conditions preclude the capability of providing the minimum submergence required. There have been other instances where surface vortex formation exists even with “proper” submergence. Alternative solutions to this problem include:
- Reduce the suction bell entrance velocity. This can be achieved by increasing the suction bell diameter through the use of a suction umbrella.
- Provide a grating to dissipate vortices. Some form of vortex suppressor is required in most sumps to dissipate surface vortices. Grating acts as such a device. The grating should be located at least one (1) foot below the minimum sump level based on all operating conditions. The aspect ratio (depth of the bars to the center-to-center spacing of the bars) must be at least 1:1 to straighten the flow. Periodic maintenance is required to ensure that the grating is free of FME. Grating should only be considered in very clean fluid with little chance of clogging.
- Provide a curtain wall to dissipate the vortex. A curtain wall is another method for dissipating surface vortices. Positioned approximately one (1) bell diameter upstream of the pump and extending three (3) feet below the minimum sump level, the curtain wall usually dissipates surface vortices by setting up a horizontal wave action at the surface. Curtain walls are constructed using reinforced concrete or steel plate attached to the existing sump walls. It is a good idea to provide grating (to the same specifications described above) to dissipate any vortices that could shift upstream of the curtain wall.
Vortices that form at places other than the water surface are called “submerged” or “subsurface” vortices. Sub- surface vortices do not carry air into a pump as do free surface vortices.
The kinetic energy of the fluid increases near the core of a vortex, resulting in a decrease in pressure (as shown in Bernoulli’s equation). The decrease in pressure can be enough to cause the core of the vortex to vaporize. This low pressure core affects the impeller in much the same way as entrained air.
Sub-surface vortices are usually formed in sump dead zones where sufficient area exists for the fluid to collect (at low velocities) and begin to swirl. The compatibility of the pump to the sump is essential to ensure pump reliability and life. An improperly designed sump can result in inefficient pump operation, excessive downtime, pump failure, and expensive retro-fitting of the sump structure.
The ideal sump is the lowest cost structure that will provide steady and uniform flow conditions with minimum head loss for all operating conditions.
Hydraulic Institute provides recommended clearances from teh pump to the floor, back-wall, and side-walls, as shown below, to preclude these dead zones. The “D” in this figure refers to suction bell diameter.
In reality, approach flow fields are seldom uniform. Even if the pumped flow rate andthe flow cross-section upstream of the pump result in an average velocity that satisfies the design criterion, the approach channel direction, layout, and numbers/sequence of operating pumps may result in a velocity distribution and/or velocity direction not parallel to the sump axis.
Sub-surface vortices can be detected as follows:
- Increased vibration amplitudes at vane pass frequency
- Model sump test
The following sump design upgrades have proven effective in eliminating sub-surface vortices by providing a more gradual change in the flow direction and smoother acceleration of flow into the pump suction bell:
- Cones: These devices are used under pump bells to eliminate the formation of a vortex from the floor. For suction bells that do not house a bottom bearing, the cone is normally designed with a height that is between 0.5 and 0.7 of the floor clearance and with a diameter equal to the suction bell diameter. For bottom bearing configurations, the cone has a truncated top and a height equal to 0.26 of the floor clearance.
- Splitters: These devices are used under the pump bells to reduce pre-rotation of flow from the back wall to eliminate the formation of back wall vortices. The most effective floor splitter is triangular in shape. The back wall splitter is usually shaped as a 45 degree isosceles triangle.
- Fillets: Fillets are installed along the side walls and sometimes along the back walls to fill in dead zones where vortices can form. Typically, they have a triangular cross- section and can be of constant or tapered width.
- Formed Inlets: Formed inlets are elbow shaped and guide the flow into the pump. Formed inlets are effective when the sump arrangement does not allow for other types of modifications or when the approach flow velocities are high. Excellent applications for formed inlets are for smaller pumps that co-exist in sumps with much larger condenser cooling water pumps.