DESIGN: DEAR MR PLUMBER
17
Art of knowing
They say a good engineer is one who knows what he doesn’ t know, but one who knows where to find the information that he doesn’ t know.
By
Vollie Brink, Pr Eng
One important element is still necessary: selfstudy, also called research.
I don’ t think many plumbers and engineers know the‘ power’ that they have available to seek and find information.
I received a call from a person who wanted to talk to me about SANS 10252-1. This person is an architect and he is involved with lecturing, but said that he finds SANS 10252-1 difficult to understand. I told him that SANS 10252-1 was not intended as a document for architects; it was developed by engineers as a design manual for engineers.
I also told him that we register engineers in terms of the Engineering Profession Act, and that our Code of Conduct does not allow us to venture into the work of other professions; therefore, we don’ t do architectural design and so forth. However, we agreed that you don’ t need an engineer to design the services of a house or for straightforward installations— for these, the plumber should be able to do the design and installation, for which his or her training and competency should be sufficient.
The deem-to-satisfy rules have been developed since 1977 to be the guide for the plumber to do the design and installation. The problem is that the boundary between the deem-to-satisfy rules and the engineering design has been eroded, and with the last few revisions, it has become even vaguer, such as that a deem-to-satisfy design shall not allow a gradient of less than 1:60.
The latest revisions still to be approved and promulgated will hopefully solve the boundary issue and serve the plumber and the architect with better information in relation to the deem-to-satisfy design.
In another case, I was contacted for advice on the installation of a few small pumping systems in a building that is being refurbished. The new layout required new toilets and kitchens in positions that are not near to a services duct. Therefore, the solution was to provide small pump systems with a pump discharge pipe from the pump to the ceiling and then horizontally in the ceiling to an existing stack pipe, which was on a lower floor.
The pump had to‘ lift’ the effluent to the ceiling, run horizontally, and then drop again to a connection. The total horizontal length of the discharge pipe, which we call a pumping main or a rising main, is about 30 – 40m and the supplier of the small pump system presented the diameter of this pipe as 50mm diameter.
Another experienced plumber argued that the pipe size is wrong and that the 50mm diameter pipe must be enlarged to 100mm diameter after 6m from the pump.
I thought that this is a good example from which to learn. The 6m length is irrelevant in this case: the 6m refers to the maximum length of a branch pipe that does not require ventilation and is only relevant in the case of a gravity drainage system— not a pumping main.
In a pumping system, the discharge pipe has the same diameter all the way from the pump right up to the position where the pumping main discharges into a sump or a stilling chamber before it connects to a manhole and discharges into the sanitary drainage system.
The pipe system designer must calculate the duty point of the pump, which includes the static head( lift) and all the friction losses along the pumping main( the diameter must be calculated to suit the flow rate of the pump), right up to the discharge. This will be the hydraulic head, and the duty point can then be plotted on a pump flow chart and the appropriate pump selected.
The pipe diameter will be the same from the pump to the discharge point and the 6m has no bearing on this situation. The design of a sanitary pumping system is fully described in SANS 10252-2. I just want to caution my plumber friends to be careful when installing a pumping system that must pump soil water( raw sewage) in a building that does not have a sump as prescribed in SANS 10252-2, but which has a soil pipe containing sewage in a ceiling that runs above the workers in an office. I would not recommend it for sewage, but it could be considered for grey water. Happy plumbing. PA
Vollie Brink
Vollie Brink is one of the industry’ s longest serving wet service engineers. He continues to serve on SABS committees and has been involved in the Green Building Council Star rating system. Brink continues to consult for various organisations while enjoying a wellearned retirement.
One important element is still necessary: selfstudy, also called research.
www. plumbingafrica. co. za June 2017 Volume 23 I Number 4