This final part of our series on intake design covers three short topics; first, trench type intake designs for clear liquids; second, intake designs for solids bearing liquids and finally, suction manifold design.
Trench type intake design (clear liquids)
Depending upon the site layout, it might be convenient to arrange the pumps in an open rectangular trapezoidal shaped channel referred to as trench type design.
Trench type differs from the rectangular intake by the abrupt change in geometry used to form the transition between the dimensions of the inlet pipe (or channel) and the sump itself. There are not many model tests on this type of design, however, there are successful installations with an individual pump capacity of up to 4,900L/s and combined installation capacity up to 14,000L/s for centrifugal pumps. Axial and mixed flow applications of trench type wet wells include an individual pump capacity up to 2,900L/s and total capacity up to 12,000L/s.
A model study would be required for any installation with an individual capacity exceeding 2,500 L/s and total station capacity as 6,300L/s.
Basic recommended dimensions are shown in the Fig.13.9.1.
Intake design for solids bearing liquids
Wet wells for a liquid handling solids require special consideration to allow for the removal of floating and settling solids. The common use is in stormwater and wastewater applications.
Circular pit design for solids bearing liquids
Most of the design proportions are the same as clear liquid design as discussed in Part 3 of this series. Additionally, the bottom of the pit should have the sloped surface around the inlet bell mouth or pumps as shown in Figures 13.10.1.1 and 13.10.1.2. These figures illustrate the design concept. It is advisable to consult a pump supplier for the optimum dimensions and arrangement.
Open trench design for solids bearing liquids
The main idea is to minimise the horizontal surface in the wet well anywhere but directly within the influence of the pump inlets, thereby directing all solids to a location where they can be removed by pumps.
Vertical or steeply sloped sides should be provided for the transition from upstream conduits or channels to the pump inlets. The bottom of the trench should have sloped surfaces as shown for circular pits in Figures 13.10.1.1 and 13.10.1.2 in the previous section of this article.
The basic design may vary based on whether the pumping station is fixed speed or variable speed. It is advisable to seek the help of a hydraulics consultant or the supplier’s experience with this type of arrangement.
Suction manifold design
A suction header or manifold is used when two or more pumps are fed through a common suction intake. It is important to arrange the takeoffs carefully to provide the required flow to all the pumps.
Take-offs directly opposite each pump are not recommended. The maximum velocity should be limited to 2.4m/s in the suction header.
Summary
This final part of our intake design series covers trench type intakes, intakes handling liquids with a solid content and suction manifolds. As previously noted for the trench and solids bearing liquid intakes, it is important that if there is any doubt about the intake’s ability to handle the job, model tests should be undertaken.
The contents of this series on intake design are a guide to the design of pump intake structures. For a more detailed and comprehensive analysis of pump intake design we refer you to ANSI/HI 9.8-1998 ‘Pump Intake Design’.
