N ATIONAL D IPLOMA M ECHANICAL AND E LECTRICAL S ERVICES C ONSTRUCTION Drainage Design

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Aims – To understand the process of calculating the correct diameter of pipe for a drainage system. Objectives To understand the methods of calculations used. To investigate the drainage requirements in a domestic situation. To complete a calculation based on a domestic drainage situation

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION We can calculate the size of drains required for Domestic and commercial use in two ways. 1. We can use the formula Q = V x A 2. Alternatively we can estimate the drain and sewer flow by adding up the number of discharge units being used. For the purpose of this unit, we will be investigating the second alternative, and estimating the number of discharge units being used. For the purposes of this calculation we are assuming all the units will discharge into one discharge pipe before going out to the Soil and Ventilating Pipe, (SVP)

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Working Example. If all the internal appliances for a house empty into one discharge pipe before going out to the SVP, find out the following information. 1. What should the internal Diameter of the discharge pipe be? 2. What Gradient should the pipe be at to maximise the flow? 3. What should the internal diameter of the SVP be? In the house the following appliances can be found. 3 WC’S. 3 Washing Machines. 5 Washbasins. 2 Sinks 3 Baths

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION The first thing we need to establish is the Total number of Discharge units for our house. To do this we need to refer to the chart in the next slide. In this chart we will be referring to the figure in the right hand column, which is giving us a No of units in respect of the frequency of use of our appliances. The chart also refers to Domestic, commercial and public, for some of the appliances. Domestic = houses and flats. Commercial = offices, factories, hotels, schools and hospitals. Public = cinemas, theatres, stadia, sports centres. If we multiply this figure by the number of appliances in our house, we can then determine the total discharge units.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION ApplianceSituationNo of Units WCDomestic7 Commercial14 Public28 BasinDomestic1 Commercial3 Public6 BathDomestic7 Commercial18 SinkDomestic6 Commercial14 Public27 ShowerDomestic1 Commercial2 Urinal0.3 Washing Machine4-7 Dishwasher4-7 Waste Disposal Unit7

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION WC’S.3 x 7 = 21 Washing Machines.3 x 4 = 12 Washbasins.5 x 1 = 5 Sinks2 x 6 = 12 Baths 3 x 7 = 21 Total Discharge Units = 71 Now that we know the total number of discharge units, we can now determine the internal diameter of the discharge pipe. And again we can refer to a chart to help us do this.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Discharge Stack nom I.D (mm) Max No of Discharge Units From our calculation, we can see that a discharge stack of 65mm internal diameter is adequate for our total of 71 discharge units.

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION To calculate the slope or inclination of a drain or sewer, we use Maguire's rule of thumb. Maguire established a measure to provide an adequate fall depending on the internal diameter of the pipe. The formula is simply X = Pipe Diameter in mm ÷ 2.5 Our pipe diameter is 65mm Therefore 65mm ÷ 2.5 = 26. The minimum gradient of our discharge pipe is therefore 1 in 26

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION All we have to do now is calculate the size of our SVP. If our discharge pipe is 65mm, then it would make sense to have the SVP at least the same size if not bigger. Therefore we would recommend an SVP of 75mm Internal Diameter. (This is the nearest size of SVP to 65mm diameter)

M ECHANICAL AND E LECTRICAL S ERVICES IN C ONSTRUCTION Conclusion. To calculate the Internal diameter of a discharge pipe, first we find the total discharge units of the appliances. By using this total, we can establish the internal diameter from the Table supplied in slide 8. By using Maguire’s rule of thumb, we were able to calculate the minimum gradient of our pipe. Finally we can then establish the diameter required for our Soil and Ventilation Pipe.