B.8 - CHART 8 - Junction Box (manhole) 90° Deflection - Lateral Coefficient

A.

Square Manhole at 90° Deflection - Figure B.8 Chart 8

Pressure change coefficients are presented in this chart for use in determining the elevation of the pressure line of an upstream pipe connected by means of a square manhole to an outfall pipe at a 90° angle. The manhole conditions covered by this chart do not involve an upstream pipe in-line with the outfall pipe. For this and other manhole charts, the lateral pipe is designated by the subscript L irrespective of its right-hand or left-hand position. The coefficients given by the chart apply directly to manholes having a square-edged entrance to the outfall pipe. Coefficients for a rounded entrance are obtained by reduction of the chart values as stated below. The design of manholes with deflector devices is discussed separately.

To use the chart:

1.

Determine the outfall pipe pressure line elevation - Gen. Instr. 1.

2.

Calculate the velocity head in the outfall - Gen. Instr. 2.

3.

Calculate the ratios D _L /D _O and B/D _O - Gen. Instr. 3.

4.

Enter the lower graph of Figure B.8 Chart 8 at the pipe size ratio D _L /D _O and read  at the curve or interpolated curve for the manhole size ratio B/D _O . For all flow from a lateral, K _L =   .

5.

For a rounded outfall pipe entrance or one formed by a pipe socket reduce the chart value of  by 0.3 as defined by Gen. Instr. 6.

6.

Calculate the charge of pressure  (always positive for 90° deflections).

7.

Add h _L to the elevation of the outfall pressure line at the branch point to obtain the elevation of the lateral pipe pressure line at this point.

8.

The water surface elevation in the manhole will be above the lateral pipe pressure line. To determine the water-surface elevation use Figure B.9 Chart 9 as instructed in steps (12) through (18) of the instructions for a square manhole at the junction of a 90° lateral with a through main.

9.

Check to be sure the water surface elevation is above the pipe crowns to justify using these charts and that it is sufficiently below the top of the manhole to indicate safety from overflow.

B.

Round Manhole at 90° Deflection - Figure B.8 Chart 8

Pressure change coefficients may also be obtained from this chart for use in determining the elevation of the pressure line of an upstream pipe connected by means of a round manhole to an outfall pipe at a 90° angle.

To use the chart:

1.

Proceed as instructed in steps (1) through (4) for a square manhole at a 90° deflection to obtain a base value of  for the particular values of D _L /D _O and B/D _O .

2.

To provide for the effects of the round manhole cross section, reduce   in accordance with the following table:

Reductions of 

The reduced values apply for a sharp-edged entrance to the outfall pipe.

3.

With a well-rounded entrance to the outfall pipe from a round manhole, reduce  of step (1) by 0.3 with no further reduction for manhole cross section shape.

4.

Follow steps (6) through (9) as detailed for square manholes at a 90° deflection.

C.

Deflectors in Square or Round Manholes at 90° Deflection - Figure 7.2.6.6.1.8 Chart 8

Pressure change coefficients are presented in this chart for use in determining the elevation of the pressure line of an upstream pipe connected to an outfall pipe at a 90° angle by means of a square or round manhole modified by flow deflectors. Deflectors in a manhole effectively eliminate the effects related to the shape of the manhole. The basic types of deflector walls which may be constructed in square or round manholes to effect a reduction of the pressure loss are detailed and described in the comprehensive report of the investigation.

The deflectors which are most easily constructed and are as effective as more complex types provide a vertical wall to guide the flow toward the outfall pipe. The wall need not be higher than the outfall pipe diameter and must fill in that part of the manhole opposite the lateral pipe exit so that it is flush with the side of the outfall pipe. Three basic types of such deflector walls are possible and are included in the curves of Figure B.8 Chart 8. These three are (1) walls parallel to the outfall pipe centerline or 0° walls, (2) inclined walls, limited to an angle of about 15° to the outfall centerline if an upstream in-line pipe is to be used, and (3) walls at 45° to both the lateral and outfall pipes, or walls curved on a radius of about the manhole dimension extending from lateral to outfall, and therefore to be used only when no upstream in-line pipe is involved. Rounding of the corner formed between the deflector wall and the manhole floor is not required, and may be detrimental in some cases.

To use the chart:

1.

Determine the outfall pipe pressure line elevation - Gen. Instr. 1.

2.

Calculate the velocity head in the outfall - Gen. Instr. 2.

3.

Classify the type of deflector used:

a.

Parallel wall - 0°

b.

Inclined wall - 5° to 15°

c.

45° or curved wall.

4.

Calculate the ratios D _L /D _O and B/D _O . No distinction between square and round manholes is necessary.

5.

If B/D _O is 1.5 or less, enter the lower graph of the chart at the ratio D _L /D _O and read at the curve for the appropriate deflector type. In the case of a parallel wall, use the curve for B/D _O = 1.00.

6.

If B/D _O is more than 1.5 and less than 2.0, use the same dashed curve for 45° or curved deflectors, use the curve for B/D _O = 1.10 for 5° to 15° angle deflectors, and use the curve for B/D _O = 1.20 for 0° angle deflectors.

7.

A rounded entrance to the outfall pipe or one formed by a pipe socket is less effective in reducing the pressure change with deflectors than when deflectors are not used. A reduction of  by 0.l may be justified.

8.

Calculate the change of pressure:   

9.

Add h _L to the elevation of the outfall pressure line at the branch point to obtain the elevation of the lateral pipe pressure line at this point.

10.

The water-surface elevation in the manhole will be above the lateral pipe pressure line. To determine the water surface elevation use Figure B.9 Chart 9 as instructed in steps (2) through (8) for deflectors in a manhole at the junction of a 90° lateral with a through main.

11.

Check to be sure the water surface elevation is above the pipe crowns to justify using these charts and that it is sufficiently below the top of the manhole to indicate safety from overflow.

Figure B.8 Chart 8 - Square or Round Manhole At 90° Deflection Or On Through Pipeline At Junction Of 90° Lateral Pipe (Lateral Coefficient)
(Source University of Missouri E.S.B. #41)