Pit alignment and pressure change coefficient charts
Procedures for determining pit pressure change coefficients involve the alignment of the outgoing pipe to the incoming flows.
With the QUDM Charts procedure, orientations and relative angles of pipes are determined from the system drawing, which is assumed to be more or less to scale. It is also necessary to classify pits as being aligned or misaligned in the QUDM page of the property sheets for the pits in the system.
In the QUDM procedure, several cases must be considered, following rules set out in in QUDM.
The first case is the topmost pit of a drainage line, considered to be misaligned if the change of orientation between the approaching flowrate and the outgoing pipe is more than 15 degrees. This is to be determined visually by the user and entered into the pit property sheet. The pressure change coefficient, Kg, for this case is estimated from one of the two curves shown below (Chart A2-3 in the 2013 QUDM):
Curve A applies to aligned pits and Curve B to misaligned ones.
Other cases relate to varying numbers and orientations of incoming pipes relative to the outgoing pipe. Various charts apply to different ranges of horizontal angles between pipes. A typical chart (QUDM Chart A2-10) is shown below:
The Ku value dependents on the orientation of the pipes, the proportion of grate flow entering from the surface and the submergence ratio S/Do defined by the hydraulic grade line (HGL). The pipe orientation is automatically taken from the DRAINS system drawing and the other factors from a trial run of the model that determines pipe diameters, flowrates and grade lines.
In these cases an aligned pit is one through which water will pass easily, such as those where incoming jets of water are directed straight into the outgoing pipe. A misaligned pit has highly mixed and turbulent flows due to incoming jets being directed onto pit walls. The diagram below shows two pits with a 45o difference in pipe orientations.
The first pit would be classed aligned and the second as misaligned in the pit property sheet. In the first case the pipe centrelines intersect at the downstream face of the pit; in the second, the intersection point is on the upstream face.
No detailed coverage of vertical alignments (drops) is given in QUDM or DRAINS. Allowance is made for this by considering flows from higher incoming pipes as grate flows. Chart A2-32 and equations by Hare and O'Loughlin are used in more complex cases.
Many complications come into estimating Ku factors, such as pits with two outlets. Various types of misalignment can be defined. In the end, many decisions about Ku coefficients will need to be made subjectively, and probably conservatively.
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