Good Drain Design - Part 2

The Essentials Of Good Drain Design

Plumbing Problems

Find a properly trained plumbing tradesman

It is well said that a good plumber is worth his weight in plastic pipe as a neighbour of mine recently found out. Many of us are aware of the shortage of good electricians however we are less aware that there is a greater shortage of good plumbers.

Typical mistakes

There is a house close to my own that was only completed about 4 months ago. But then, only a couple of months after completion the hammering started - you know the sound. That unmistakable sound of someone removing expensive tiles from newly completed bathroom walls.

I was just a little intrigued, I had to see what was going on.

Between hammer blows and the shattering of expensive ceramic the tukang explained that the joints in the water pipes throughout the brand new house were leaking.

The problem ‐ the worker had used glue. In plumbing you should never use glue for PVC pipes, you use solvent cement. Glue merely sticks things while solvent cement melts and welds the pipes together. A simple but very expensive mistake for a non trained worker to make especially considering that in many places glue is being sold marked as being suitable for PVC pipes.

The aspect of plumbing we know as drainage where we carry water away (the water can be rainwater, grey water from showers, washing machines or washbasins and black water from toilets) is a field little understood by mere mortals. Installed when a building is constructed, it is underground, out of sight and out of mind and only becomes an issue when something unexpected happens such as flooding, overflowing toilets and washbasins, bad smells or perhaps a septic tank that mysteriously needs to be emptied every couple of months. By this time no one has any idea where the pipework goes or where it discharges its contents.

Good Drainage Design

Basic Drain Requirements

When designing and installing drainage systems it is important to consider several factors:

1. How much water they will have to carry at peak times (after a heavy thunderstorm) this will determine the size of the pipe needed to do the job.
2. The slope of the pipe, at least 2% (a fall of 2cms per meter) is advised (SNI 03-6481-2000) but more is better.
3. How to stop unwanted items (leaves, twigs, sand, earth, building cement, food scraps, garbage and female hygiene products) entering the pipe.
4. Where the pipe delivers its water. There are strict laws regarding where what sort of water can be discharged.

How to avoid blocked drains

Drainage systems also need to be designed so that they are unlikely to become blocked and, if they do become blocked, can be cleaned out easily. To achieve this we do the following:

1. 1. Use round pipes rather than square or rectangular pipes.
2. Use long straight runs wherever possible to ease both the flow and cleaning.
3. Design pipework so that the diameter progressively increases along the pipe as the amount of water collected increases.
4. Keep bends to a minimum.
5. Do not put constrictions (such as valves or narrow connections) in pipes. Constrictions will slow the water flow, cause turbulence and may "capture" things that are passing.
6. Avoid right angled bends in pipes, cleaning rods and coils can move through 45 degree bends but can be stopped by 90 degree bends. If a 90 degree bend is used this should be a "slow" bend not a sharp elbow.
7. In rainwater drains install manholes at critical points to allow access for cleaning and unblocking.
8. In black and grey water pipes install sealed "cleanouts", these allow access to the pipe for cleaning while being gastight so they do not smell.
9. Make sure drainage designs are documented with detailed drawings indicating where pipe systems go. These should be made available to building owners and/or residents.

Self Cleaning Drains

Drains should always have round bottoms. There is a simple reason for this - the circular profile makes them "self cleaning". After a heavy storm there is a good water flow and anything in the pipe gets carried along by the water. A drain with a flat wide bottom needs a lot of water to keep it clear and if the flow decreases the water starts to meander around in the bottom of the drain, the flow of water slows down and any detritus in the water drops to the bottom and starts to collect. If we make the bottom of the pipe round, as the water flow decreases, the circular profile concentrates the flow in the bottom of the drain, the water keeps flowing quickly and so helps to keep the pipe clean.

It is good to note that people in some country areas of Bali seem to have caught on to this simple concept and in a number of areas half pipes are being installed for the bottom of roadside drains improving water flow and keeping them clean.

Elliptical pipes

In some countries across the world cities are taking this concept a stage further by installing elliptically shaped main sewer pipes. These are installed with the ellipse in the vertical direction thereby increasing the concentration of water flow in the bottom of the pipe.

Generally speaking in many parts of the world the concept of self cleaning drains is not recognised and flat bottomed "gotts" (roadside drains) will continue to provide annual work for those poor folk who have to crawl into foul, cramped spaces to dig them out every year.