Narrowing down a section of waste pipe - what's the effect?

[Doug]

The short version of the question... what will be the effect of introducing a short section of 32mm pipe into a 40mm waste pipe run from a sink? The narrowed length of pipe will only be about 15% the total length of run but will also constitute an elb0w.

The slightly longer version of the question... I'm planning a keg cleaning system that needs to redirect used liquid waste into two different tanks stored under the "sink". I intend to use an "L"-ported 3-way actuated ball valve to perform this task (so the task can be automated). The waste from the cleaning system will be 40mm but there is a big price hike in the ball valve when you go up to this size. So, unfortunately, the orifice in the ball valve will have a 32mm bore.

Will this have a significant detrimental effect on the flow rate of the waste back into the tanks? Is it worth me going for 40mm pipe at all? Or should I really increase the size of the ball valve and to hell with the price?

Cheers!

 

[Stigster]

In short it will probably completely fill the pipe with water in the restricted section and will pull water from the sink trap which exposes a pathway for foul gasses to enter the room. This is known as self syphonage. A trap with a built in air admittance valve will likely work. I have to set up scenarios for students to fault find and one of them is like your situation, a 32mm section added to a 40mm run from a basin and it syphons the trap.

It's best to design it properly rather than add more complexity but by adding a trap with an air admittance valve does stop it gurgling and pulling the trap.

 

[Doug]

Nice one, Stigster, thanks. Thing is, this isn't a sink - it's a cask / keg washer and there's no danger of foul smells because it's not connected directly to the sewer so there won't be a trap involved at all. Perhaps my analogy of a sink wasn't entirely accurate. The waste will drop directly out of the "sink" or, more accurately, drainage trough, through the waste pipe with minimal resistance and into the tank it originally came from so it can be pumped round again. With this in mind do you think it will affect the flow rate?

 

[Stigster]

How many litres over how much time are we talking about? How many litres per minute will your pump be outputting?

 

[Doug]

Not huge. The tanks underneath will be approximately 25 litres which the pump may be able to drain in about 1 to 2 minutes, so the waste flow back into the tank will need to exceed that, otherwise the pump could run dry. (However, the cycle time may only be a couple of minutes (5 mins max) so as long as the tank doesn't empty in that time then it should be ok). These figures are guesses at this stage.

 

[Stigster]

So worst case scenario is 25 lpm or 0.42litres per second. I am not actually sure if 32mm will do that. It's going to be marginal I reckon. Anyone else here know the maximum flow rate for say a 1m long 32mm pipe under ideal conditions?

 

[Stigster]

OK, I've used this calculator Free Online Manning Pipe Flow Calculator and put in figures for a 32mm pipe, vertical fall (100%) and 100% full pipe, using the Manning's roughness of PVC tube of 0.009 and get a figure of 3.57 l/s.