Drainage

Question 1

What is black water in a sanitary pipework installation

Answer 1

Black water is the waste containing faeces, urine and waste food

Question 2

What is Grey water in a sanitary pipework installation

Answer 2

The water is contaminated but will not contain significant amounts of faecal matter (mainly water from showers washbasins etc)

Question 3

What are some of the health and safety reason for internal pipework within a building?

Answer 3

Under BS EN 12056-1:2000 sanitary systems must ensure users are not put at risk from:

1. leakage
2. flooding
3. toxic or noxious smells and odours
4. contamination of portable water
5. Excessive noise
6. Spread of fire or smoke

Question 4

What protection must be in place for sanitary pipework systems?

Answer 4

Mechanical failures
Corrosion
Frost
Condensation

Question 5

Describe what a trap is?

Answer 5

An appliance shall discharge through a trap to prevent foul air from entering the building by retain water acting as a seal. Often incorporating within the appliance such as a bottle trap in a WC and P and S traps placed under sinks.

Question 6

How can trap seals be lost?

Answer 6

Through evaporation, typically at a rate of 2.5 mm per week depending on the relative humidity in the room.

Question 7

Describe the equation for flow in branch discharge pipes?

Answer 7

As most systems are gravity, the volume flow rate of water is determined by the vertical drop as well as the resistance to flow.

A modification of Bernoulli Equation allows for a simple analysis between any two points in a pipe that is full bore flow.

Question 8

What is self siphonage?

Answer 8

Self siphonage is when water flows backwards up the pipe due to a partial vacuum being created. When this happens it is likely that foul smells will enter a living space.

Question 9

What is induced siphonage?

Answer 9

induced siphonage is when more than one sanitary fitting is connected to a branch discharge pipe. The partial vacuum being created in one pipe by one fitting discharging may be enough to unseat the other fittings.

Question 10

How can self and induced siphonage be prevented?

Answer 10

In order to prevent self and induced siphonage minimum gradients, pipe lengths and diameters plus connections to a single discharge branch pipe must be established. Also increasing branch pipe diameter can assist in prevent self and induced siphonage.

Question 11

Describe the Darcy Equation

Answer 11

Deltapressure losses (friction) = fL/d x1/2 rho x velocity^2

Question 12

Describe the equation for shock losses

Answer 12

Deltapressure losses (shock)= zeta x 1/2 x rho x velocity^2

Question 13

What is the equation for total pressure loss?

Answer 13

DeltaPlosses= Ru^2

R= Resistance 
u= velocity 

Resistance is found:
R=1/2 x rho (fL/D + Sigma zeta)

Question 14

Give the equation for water velocity

Answer 14

u= square root (Ps1-Ps2 + rho x gravity (z1-z2)/R

Question 15

What is the minimum gradient specified in Part H of the building regulations

Answer 15

18mm in all cases

Question 16

What happens to flow in the discharge stack?

Answer 16

The upper part of the discharge stack has below atmospheric pressure and the base has above atmospheric pressure inducing more air into the stack

Question 17

What happens to branch pipes as a result of pressure in the discharge stack

Answer 17

As a result of the pressures above and below the atmospheric pressure it can result in losing the water trap leading to the following limitations:

1. Maximum simultaneous flow rate that a discharge stack of a particular diameter can carry so as t limit pressure fluctuations
2. Limit for system of 5 storeys to prevent excessive positive pressure at the lowest floor

Question 18

What is meant by wavering out

Answer 18

Wavering out occurs when high velocity winds pass over the top of the stack resulting in a drop in static pressure drawing out air in the stack. The gusts of winds cause the water to oscillate gradually causing the water to fall into the discharge branch until water is lost.

Cowls should be placed on the top of the high rise building to prevent this.

Question 19

What is the formula for capacities of stacks in tall vertical stacks?

Answer 19

q= Kd^8/3

q = discharge capacity 
d = diameter of stack 
K= a constant

Question 20

Describe what is meant by a primary ventilated stack?

Answer 20

used to be called a single stack. All the wash basins and WCs are connected to it so should have a minimum diameter of 100mm. Its either open to the atmosphere to the top of the building or in an air admittance valve area. connected via a swept bend. Stack should finish at least 900mm above any window or opening and be at least 3m away in a horizontal distance.

Question 21

What does Approved Document H 2010 say about terminations of discharge stacks?

Answer 21

Discharge stack termination should have a domical cage (wire balloon) which should not unduly restrict airflow. Alternatively a air admittance valve that is British Board of Approval certificated be used. Any buildings above 5 storeys should have a sub stack from the first floor because it will cause positive pressure at the base and cause pressure fluctuations. Best to have two stacks one serving WCs and the wash basins and showers.

Question 22

What is secondary ventilated (ventilated stack)

Answer 22

Secondary ventilation stacks are added to the discharge stack and are there to provide air only to prevent the induced siphonage

Question 23

Where can you find dimensions gradients and diameters for sanitary pipework?

Answer 23

Approved document H and BS EN 12506:2000 Part 2

Question 24

How do we size sanitary pipe work?

Answer 24

Use Binominal Probability Theorem.

Pn,r= Cn,r p^r(1-p)^n-r where Cn,r = n!/r!(n-r)!

What we need to know is the probability of any number of fittings up to r discharging simultaneously (Pn

Question 25

Describe the other equation that will determine number of fitting discharging simultaneously?

Answer 25

m =(approx) np + x (2np (1-p))^0.5

figures for x found in CIBSE guide B6

Question 26

How is design flow rate determined?

Answer 26

Once the number of sanitary fittings that are discharging simultaneously has been decided, the design flow rate is simply applying a design discharge flow rate for the particular fitting

Question 27

What is the equation for the volume flow rate for the total number of discharge units?

Answer 27

Q=K square root sigma DU

Q= simultaneous design volume flow rate 
K= frequecy of use (usage ratio) 
DU = Demand Units

Question 28

how is rainfall rate usually quoted?

Answer 28

Rainfall rate is normally quote in mm over a stated period or L/s m^2

Question 29

what are rainfall drainage systems?

Answer 29

Rainwater drainage systems are there to remove water from a set place and to decrease the risk of damage and leakages to the building and cause disturbance for the staff.

Question 30

Describe design rainfall intensity?

Answer 30

takes into account risk, the frequency with which rainfall rate will be exceeded and the impact of such an event.

Question 31

What is the effective catchment area?

Answer 31

when the surface area is not horizontal the effective catchment area needs to be calculated. Rainfalls at an angles of 26.5 degrees measured from vertical.

Question 32

Flat roofs equation?

Answer 32

equal to plan area of the surface

Question 33

Sloping roofs equation?

Answer 33

Ae=Ah + (Av/2)

Ae=effective catchment area
Ah= Plan area
Av=Vertical height

Question 34

Valley gutter equation?

Answer 34

Ae = Ah1 + Ah2 + 0.5 (Av2 - Av1)

Question 35

How do you calculate run off rate?

Answer 35

Q= Ae.r

Q= is in L/s
Ae= effective catchment area 
r = design rainfall intensity