Ventilation of pumping stations is required to prevent the accumulation of high levels of potentially hazardous chemicals, and ensure that working conditions meet health and safety requirements. UK occupational exposure limit (OEL) concentrationsl
for hydrogen sulphide and other gases associated with septic conditions are given in section 1.6 of this manual.
Typical ventilation rates for odour containment in pumping stations used in current operational practice in Doha are given in Table 2.23.1.
Table 2.23.1 – Typical Ventilation Rates for Odour Control in Pumping
Stations
Air changes per hour
Pumping station
(no man access)
One for local covers
12 for pumping
stations extracted
from close to the
sump and process
units
Pumping station
working area
(current
practice)
20 during man
access (initiated by
light switch)
Dry wells
(current
practice)
12
Separate screen
chamber
Passive ventilation
through carbon filter
(where there is no
other route for odour
escape)
Ventilation systems should be designed so that in the event of a fire being detected in any area, all the air conditioning equipment and ventilation systems are shut down. All supply and exhaust ventilation louvers should shut automatically to
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compartmentalise the buildings and below ground chambers
.
This restricts the spread of the fire and smoke, and ensures effective use of automatic fire extinguishing systems.Other points to consider include:
The air conditioning systems, ventilation fans and odour control equipment should be run
simultaneously and ventilation fan louvers should shut, when the fan stops;
Louvers should be sized to keep the air velocity through them below 0.5m/s;
Air ducts should be designed to ensure the velocity through them does exceed 10m/s in occupied areas; Materials should be selected to limit the corrosion effects of hydrogen sulphide (H2S)
.
Ventilation of Pump Rooms and Dry
Wells
Air supply should be provided by either two or three duty fans and one standby fan, depending on the size of the pump room.
Exhaust air should be removed by either two or three duty fans and one standby fan, depending on the size of the pump room.
The exhaust fans should have approximately 5% less flow capacity than the air supply fans to keep the building at a slight positive air pressure. This is to avoid drawing unfiltered dust laden air into the pump room which can drastically shorten the equipment life.
Pump rooms and dry wells should typically have 12 air changes an hour for normal operation, increasing to 16 air changes an hour during man entry. The cable basement should be ventilated as part of the pump room ventilation system
.
Ventilation of Wet Areas - Pump Sumps & Screen Chambers
Wet areas should normally be ventilated by air extraction only, with a natural air supply to keep the wet area under slightly negative pressure and avoid releasing odours to the atmosphere.
Exhaust air should be removed by duty/standby fans, the number and configuration depending on
the size of the wet areas. Each fan should have a two-speed motor.
During man entry, the additional air supply should be provided by the fans running at high speed. The fans should be sized so that with all fans running at high speed, the required air changes per hour for man entry are achieved.
Ventilation rates should be designed to ensure a maximum of 3ppm of H2S in the wet areas. The
system should be designed to achieve this with only one fan operating.
Wet areas should typically have 12 air changes an hour for normal operation, increasing to 20 air changes an hour during man entry.
2.23.2
Odour Control
Air vented from pumping stations will in most cases require odour treatment. In most cases, a two bed (duty/standby) system using carbon regenerated using alkali (caustic soda or potash) is preferred. At larger pumping stations consideration may be given to pre-treatment of strong sources using catalytic iron filters.
Further details of requirements are given in Volume 5 Section 1.5. Reference should also be made to Section 1.6 of this Volume
Typical conditions to be considered in the design of the odour control unit are given in the table below. Table 2.23.2 – Conditions to be Considered in
Odour Control Unit Design Sewage temperature 25 – 35oC
Ambient temperature 0–50oC
Relative humidity Up to 100% Temperature of air
vented from the sewerage system to an Odour Control Unit
Up to 30oC
Radiating surfaces temperature 85
oC maximum
Hydrogen sulphide from
Hydrogen sulphide with
workplace air 10ppm
2.23.3
Air Conditioning
The required air conditioning systems and ventilation capacities are shown in the tables below. Table 2.23.3 - Air Conditioning (AC) Systems
Location Air Condition system
Electric Switch Gear Dual Split AC unit system Control Room Split AC unit system
Table 2.23.4 - Ventilation Capacities
Location Ventilation (l/s) per person Ventilation (l/s) per sq.m. Approximate air changes per hour. * Electric Switchgear Room - 0.8 1 Control Room 10 1.3 2 Kitchen and Toilet - 10 8
Note: Figures extracted from BS 5720, Table 1. *Depending on the dimensions of the rooms.
The designer shall assess the potential for corrosion of A/C units, particularly from H2S, and ensure that
they are appropriately designed and located. Air Conditioning of Electrical Switch Gear Rooms
Electrical switchgear rooms should be completely isolated from the remainder of the building for the following reasons:
• The thermal loads are higher than elsewhere in the building;
• In the event of a fire being detected the air conditioning should be switched off to allow the fire suppression equipment to operate effectively.
Two split AC units working independently (mechanically and electrically) of each other should be used to air condition the room, with air diffusers discharging horizontally towards the panels. Return air should be sucked back by the split unit, via receiving air diffusers located at evenly placed points between the supply air diffusers, and fixed to the ceiling.
Each split AC units should be rated at 50% above the required capacity (i.e. 150% total), so that should one unit fail, the other unit will provide 75% of the required air conditioning capacity.
The required thermal load should be calculated on the basis of peak conditions.
The required quantity of exhaust air should be removed from electrical switchgear rooms to atmosphere by a fan with an actuated louver. Air inlet should be by natural supply through a filtered and actuated louver.
In the event of a fire, the electrically actuated louvers should be closed to seal electrical switchgear rooms during the use of any fire extinguishing system.
Air Conditioning of Control Rooms, Kitchens and Toilets
A single split AC unit should be provided for air conditioning the control room. No air conditioning should be provided for the kitchen or toilet.
The kitchen and toilet areas should be air conditioned by exhausting part of the control room air through them.
Exhaust air in the kitchen and toilet areas should be discharged outside the building. The fans should be run continuously for the following reasons:
• To provide the required air changes for the control room and kitchen;
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Air louvers should be fitted in the bottom of kitchen and toilet doors.
2.24
Structural Design
General Design Requirements
Unless local design standards dictate otherwise, in general, he design of concrete structures shall be in accordance with BS 8110-1 “Structural Use of Concrete”li and BS8007 “Design of Concrete
Structures for Retaining Aqueous Liquids”lii.
Likewise, the design of steel structures shall be in accordance with BS5950-1 “Structural Use of Steelwork in Buildings”. Local standards shall govern if any conflict arises. All structures shall be designed based on a ‘limit-states’ philosophy. Unless required otherwise, all structures shall be designed for a minimum service life of 60 years. The designer shall prepare calculations for each design package, including as a minimum the following information:
• Description of the structure and design methodology adopted;
• All assumptions made for design (i.e. geotechnical parameters, loadings, etc); • Standards, guidelines and specifications used
for design;
• Input and output from software where appropriate.
2.24.1
Substructures
2.24.1.1 Thermal Crack Control
Requirements
Calculation of the reinforcement requirements for control of early-age thermal cracking shall be in accordance with BS 8007lii.
For the calculation of the likely maximum crack spacing and the reinforcement ratio the following formula shall be used:
(
1 2)
max maxT
T
R
S
+
=
α
ϖ
Equation 2.24.1 Equation 2.24.2 Were:ωmax = allowable crack width (0.2mm maximum)
Smax = likely crack spacing (mm)
R = restraint factor (0<R<0.5; to be taken as 0.5 for most structures)
α = co-efficient of thermal expansion (varies between 10x10-6/oC – 12x10-6/oC)
T1 = fall in temperature between the
hydration peak and the ambient (oC)
T2 = ambient placing temperature (oC)
ρbar = reinforcement ratio (ρmin = 0.0035)
φbar = reinforcement diameter (mm)
Where the section thickness exceeds 500m, only the outermost 250m of each face shall be used in calculating reinforcement areas; however, the design temperature T1 shall still be based on the
entire element thickness.
h<500mm h/2
For h < 500mm assume each reinforcement face controls h/2 depth of concrete
h ≥ 500mm 250mm
250mm
For h ≥ 500mm assume each reinforcement face controls the outer 250mm depth of concrete. Ignore any central core beyond these surface zones. Given that thermal crack control requirements determine the minimum limit of reinforcement, particular care should be given to the adopted values of T1 and T2. Factors including local site
conditions, concrete mix design, formwork type, seasonal variations in ambient temperature, distance from plant to site, etc shall all be taken into account.
Considering the relatively high ambient temperatures that may be encountered in the Qatar region, consideration shall be given to limiting the concrete placing temperature T2 to a value ranging
between 15oC and 30oC. Designers are referred to max
2
67
.
0
S
bar barφ
ρ
=
CIRIA Report No’s 91liii and 135liv for further
information on this subject. Ground Investigation & Flotation
The designer shall have, at a minimum, an understanding of the basic ground conditions likely to be encountered on site, either from historical data or a desk-top study. Preferably, the designer shall obtain a Ground Investigative Report (GIR) from suitably competent geotechnical engineers giving more precise values and ground conditions. Data to be considered includes ground level (GL), ground water level (GWL), soil types, classification and properties, allowable bearing capacities and a soil chemical analysis.
Depending on the GWL and GL conditions, buoyancy (or flotation) of the structure may govern the section thickness. Flotation of all structures shall be checked in accordance with BS 8007lii against
the anticipated GWL. In considering the flotation calculations, the following methodology is recommended:
• Calculate the volume of water displaced based on external dimensions of the structure and the GWL;
• Calculate the mass of the structure taking into account construction assumptions (e.g. does the site need to be de-watered until after the roof has been placed? does the site need to be de-watered until any mass concrete benching has been placed?);
• Calculate the factor of safety to obtain 1.10 as a minimum;
• Re-size any element thicknesses as required (ensuring that structural requirements are still maintained).
A factor of safety of 1.10 shall be achieved for both temporary and permanent conditions. For the flotation calculations the following concrete unit weights are recommended:
Minimum Maximum In-situ RC 22.5kN/m3 23.5kN/m3
Unreinforced 21.6kN/m3 22.5kN/m3
2.24.1.2 Structural Analysis
Loading
All liquid retaining structures are to be designed for both the full and empty conditions, with the load combinations arranged to give the most severe combination likely to happen.
Both serviceability (SLS) and ultimate (ULS) load conditions shall be considered. The following load factors shall be adopted (unless local design codes specify more onerous load factors) as per Table 2.24.1.
Table 2.24.1 – Serviceability (SL) and Ultimate (ULS) Load Factors
Load SLS Factor ULS
Self Weight 1.0 1.4 Dead Loading 1.0 1.4 Retained Liquids 1.0 1.4 Retained Soils 1.0 1.4 Live Loads (incl.
surcharges)
1.0 1.6
In general the walls and base shall be checked against the following load combination (where appropriate):
• Internal hydrostatic pressure only (water- tightness test before backfilling);
• External soil pressure only (backfilled soil but no water);
• Hydrostatic uplift on base; • Base ‘soft-spot’ capacity;
• Hydrostatic + soil pressure + uplift (normal working conditions);
• Roof loading.
Where required, the structure shall be designed for an appropriate wheel/vehicle live load. Vehicle live loads shall be in accordance with local standards and engineering judgement (where local standards do not cater to vehicle loads then loading shall be in accordance with BS 5400-2lv and BS6399-1lvi). A
minimum live load of 5kN/m2 shall be adopted
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Elements shall be analysed in accordance with BS 8007lii, BS 8110-1li and established engineering
principles. Depending on the slab arrangement (i.e. degree of restraint, span ratio’s, etc), bases and walls may be considered as either one-way or two- way spanning.
Where appropriate seismic loading shall be considered in accordance with local design codes. Base Slabs
Base slabs designed as one-way spanning shall be designed for flexure in accordance with engineering principles and the following formulae:
156
.
0
2≤
=
cu ULSf
bd
M
K
Equation 2.24.3d
K
d
z
0.95
9
.
0
25
.
0
5
.
0
≤
−
+
=
Equation 2.24.4 andz
f
M
A
sy ULS st95
.
0
=
Equation 2.24.5 where:MULS = design ultimate moment (kNm)
b = width of section (mm - typically taken as 1 m)
d = effective depth (mm) fcu = concrete strength (N/mm2)
z = lever arm (mm)
Ast = area of required tension reinforcement
(mm2)
fsy = reinforcement strength (N/mm2)
With K ≤ 0.156, compression reinforcement is not required. Designers are referred to BS8110-1li for
cases where compression reinforcement is required. Base slabs designed as one-way spanning shall be designed for shear in accordance with engineering principles and the following formula:
(
cu)
vf
d
b
V
8
.
0
,
N/mm
5
2≤
=
υ
Equation 2.24.6 where:υ = design ultimate shear stress (N/mm2)
V = design ultimate shear force (kN)
bv = width of section (mm - typically taken as
1m)
d = effective depth (mm) fcu = concrete strength (N/mm2)
Table 2.24.2 – Shear Stress and Rebar to be provided Shear Stress υυυυ Form of shear rebar to be provided Area of shear rebar to be provided υ<0.5υc None Required - 0.5υc < υ<(υc + 0.4) Minimum links in
areas where υ<υc Asv ≥ 0.4bsv/0.95fsyv
(υc + 0.4) < υ<5 or 0.8√fcu Links in any combination Asv ≥ bsv(υ- υc)/0.95fsyv
Shear reinforcement shall be provided based on the following:
• The critical shear stress uc shall be determined
in accordance with BS 8110-1li;
• Base slabs designed as two-way spanning shall be designed for flexure in accordance with engineering principles and the following formula: 2 x sx sx
nl
m
=β
&m
sy=β
synl
x2 Equation 2.24.7 Values of βsx and βsy shall be obtained fromTable 2.24.3.
• Base slabs designed as two-way spanning shall be designed for shear in accordance with engineering principles and the following formulae: x vx vx
β
nl
υ
=
&
υ
vy=β
vynl
x Equation 2.24.7Values of βvx and βvy shall be obtained from
Table 2.24.4.
A nominal ‘soft spot’ diameter shall be assumed in the subgrade (unless local conditions preclude this from occurring) and the base checked accordingly.
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Table 2.24.3 – Base Slab Flexure Coefficients
Edge Condition
Short Span Co-efficient (ββββsx) Long Span Co-
efficient (ββββsy) for
all values of Ly/Lx
Values of Ly/Lx
1.0 1.1 1.2 1.3 1.4 1.5 1.75 ≥2.0
1. Four edges continuous 0.024 0.028 0.032 0.035 0.037 0.040 0.044 0.048 0.024 2. 1 short edge discontinuous 0.028 0.032 0.036 0.038 0.041 0.043 0.047 0.050 0.028 3. 1 long edge discontinuous 0.028 0.035 0.041 0.046 0.050 0.054 0.061 0.066 0.028 4. 2 short edges
discontinuous 0.034 0.038 0.040 0.043 0.045 0.047 0.050 0.053 0.034 5. 2 long edges discontinuous 0.034 0.046 0.056 0.065 0.072 0.078 0.091 0.100 0.034 6. 2 adjacent edges
discontinuous 0.035 0.041 0.046 0.051 0.055 0.058 0.065 0.070 0.035 7. 3 edges discontinuous
(1 long edge continuous) 0.043 0.049 0.053 0.057 0.061 0.064 0.069 0.074 0.043 8. 3 edges discontinuous
(1 short edge continuous) 0.043 0.054 0.064 0.072 0.078 0.084 0.096 0.105 0.043 9. 4 edges discontinuous 0.056 0.066 0.074 0.081 0.087 0.093 0.103 0.111 0.056
Table 2.24.4 – Base Slab Shear Coefficients
Edge Condition
Short Span Co-efficient (ββββvx) Long Span Co-
efficient (ββββvy) for
all values of Ly/Lx
Values of Ly/Lx
1.0 1.1 1.2 1.3 1.4 1.5 1.75 ≥2.0
1. Four edges continuous 0.33 0.36 0.39 0.41 0.43 0.45 0.48 0.50 0.33 2. 1 short edge discontinuous 0.36 0.39 0.42 0.44 0.45 0.47 0.50 0.52 0.36 3. 1 long edge discontinuous 0.36 0.40 0.44 0.47 0.49 0.51 0.55 0.59 0.36 4. 2 short edges
discontinuous 0.40 0.43 0.45 0.47 0.48 0.49 0.52 0.54 0.26
5. 2 long edges discontinuous 0.26 0.30 0.33 0.36 0.38 0.40 0.44 0.47 0.40 6. 2 adjacent edges
discontinuous 0.40 0.44 0.47 0.50 0.52 0.54 0.57 0.60 0.40
7. 3 edges discontinuous (1 long edge continuous)
0.45 0.48 0.51 0.53 0.55 0.57 0.60 0.63 0.29
8. 3 edges discontinuous (1 short edge continuous)
0.29 0.33 0.36 0.38 0.40 0.42 0.45 0.48 0.45
Walls
Walls may adopt vertical, horizontal or two-way spanning action. Walls may be analysed by first principles, design charts or software. Earth pressures shall be calculated using Rankine’s theory. At-rest earth pressures shall be used for structural design. The value of ko will vary according
to site conditions but a minimum value of ko = 0.5
shall be adopted. Surface surcharging shall be allowed for (typical values range between 5- 10kN/m2), as shall construction and permanent live
loads.
φ
φ
sin
1
sin
1
+
−
=
ak
Active
Equation 2.24.9
φ
φ
sin
1
sin
1
−
+
=
pk
Passive
Equation 2.24.10)
5
.
0
(
sin
1
fordesign
k
rest
At−
o=
−
φ
=
Equation 2.24.11 Su rc ha rg e C om pa ct io n H yd ro st at ic So il kqsurch kqcomp γwHw kγsH s ¦--- where required ---¦Where the structural arrangement calls for internal walls, these walls shall be checked for a full hydrostatic head against one side only (representing a full chamber on one side, an empty chamber on the other).
Where designed as vertical cantilevers, walls shall be checked for deflection in accordance with BS8110-1li span-depth criteria.
Roof Slabs
Roof slabs shall generally be designed in a similar fashion to base slabs, however, they should be considered as simply supported with limited fixity (and hence moment transfer) at the supports. Particular care shall be given to roofs subject to vehicle loading.
Design Software
Slab and wall elements may also be designed using appropriate commercial software (e.g. ROBOT Millennium, STRAND 7, STAADPro, Microstran V8, etc), either as 2D, or preferably 3D, models. Appropriate spring elements shall be used to represent the soil stiffness. Designers should refer to the program user manuals for assistance with design software.
Foundations and Settlement
Where an interface between a structure (be it above ground, partially buried or completely buried) and the underlying ground exists, there is said to be soil- structure interaction. The actual behaviour of structures and soil-structure interaction is complex and leads to some simplification of assumptions in order to obtain a design.
A fundamental design concept is the selection of either a rigid structure or a flexible structure. A flexible structure will be able to tolerate a degree of differential settlement by the basic arrangement of the structure, the nature of its materials and by the inclusions of movement joints. Conversely, a rigid structure is designed to neglect any differential settlement by having sufficient strength to span across any loss of ground support. Factors to consider include the relative settlements likely to occur (i.e. immediate and long-term), any history of previous soil loading (i.e. over- consolidation) and the non-homogenous content of most soils.
The support given by the subgrade is often modelled as springs of varying stiffness (with the stiffness based on geotechnical parameters), and base slabs may occasionally be designed as beams on elastic foundations. This is a time-consuming and complicated procedure, and many design software programs are ideally suited to this task (although it should be remembered that any software output is only as good as its input).
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As stated, the analysis and consideration of any soil- structure interaction is a complex affair, and in part depends on a degree of experience. Designers are strongly recommended to consult geotechnical engineers and to refer to specialist literature such as “Soil-structure interaction – The real behaviour of structures”lvii for further information on this subject.
Ground movement leading to differential settlement can cause severe cracking and leakage from liquid retaining structures, and as a general rule they should be designed as rigid structures. Where appropriate the design bearing pressure shall be calculated and checked against the allowable bearing capacity. If required, measures shall be taken to provide suitable foundations such as piling or other ground improvement techniques - consultation with suitably competent geotechnical engineers is strongly recommended. A maximum differential settlement value of 20–25mm should be adopted.
Where piled foundations are required, the design ultimate resistance of a single end-bearing pile shall