How fast can you drain a bath?

[octane75]

This is a particularly odd question:

Assuming its 3-4 mins under its own water pressure does anyone know how to calculate how quickly a typical bath full of water with standard waste fittings could be drained if the drain was pumped?

 

[ChrisR]

Duh it depends on the pump

 

[octane75]

but surely there is a maximum flowrate of the plug outlet and wate pipe. How do I work out flow rate based on the pipe diameter?

(assume pump is as powerful as a needed)

 

[kevplumb]

(assume pump is as powerful as a needed)

as chris said it depends on the pump

if you fit a pump that can shift 6lpm

or if you fit a pump that will shift 20 lpm

 

[chrishutt]

For practical purposes (assuming the question has some practical basis) you would be better off looking at increasing size of trap and waste pipe from bath waste outlet. Using a powerful pump would throw up all sorts of problems, not least noise.

 

[octane75]

Okay, thanks guys, this is good input.

(the situation does have a practical application and noise is not an issue)

So am I correct in thinking now that maximum flow rate through a pipe is not dependent on pipe diameter?

 

[bathjobby]

So am I correct in thinking now that maximum flow rate through a pipe is not dependent on pipe diameter?

Correct, otherwise why would pumping it shift the water any quicker than gravity, which you have already understood by your original question. Therefore, not much more of a mental leap to understand that the more pressure the pump applies, the more water will be moved - but hang onto your compression fittings !
For all our efforts to help you here, at least let us into what this practical application is !

 

[chrishutt]

So what is the practical application?

I think you need to study fluid dynamics if you're really serious about this. Basically the flow rate of water through a pipe will depend on the resistance to flow (mainly down to size of pipe) and pressure applied to overcome such resistance.

A bath empties because atmospheric pressure bears down on the bath water, overcoming the resistance of waste outlet, trap, pipe and bends. Atmospheric pressure also applies at the outlet end of the pipework so the net pressure differential depends on height difference between bath water level and the point at which discharge pipework becomes subject to atmospheric pressure.

And that's a simplified explanation. To answer your specific question

am I correct in thinking now that maximum flow rate through a pipe is not dependent on pipe diameter?

No, you (and bathjobby?) are wrong. Flow rate is very dependent on pipe diameter.

 

[octane75]

Thankyou this is kind of what I thought originally, that that the waste pipe diameter does provide a fixed resitance to the flow rate.

Now I just need a equation to work it out so that I can specify the correct pressure of pump to empty it in the required time.

You're right looks like its fluid dynamics 101 for me.

 

[rumplestiltskin]

The pipe size, bends, elbows, etc will act as a resistance to the flow, lets call this the overall resistance to flow. This can be calculated or measured as a differential pressure under flow conditions. What is important is that this differential pressure increase is squared (DP X DP) with increased velocity.
What this means is that if you fit a pump twice as powerful you will not get anything near twice the flow rate as the effective resistances will now be much higher due to the increased velocity.
I suspect this application is nothing to do with a bath?

 

[chrishutt]

The whole thing is further complicated because you're dealing with an "open" (to atmosphere) system. As soon as a powerful suction is applied to the waste outlet, a vortex will form (something to do with the Coriolis effect?) and air will be drawn down the waste, displacing water (and spoiling all your calculations) and probably buggering up your pump.

Have fun. I hope you have lots of time and money to waste on this.

 

[octane75]

This is great info. guys, thanks. I'll keep you updated.

 

[chrishutt]

I think octane75 is a school student who's been set some questions to research over summer holidays. He's using us to do his homework for him....and we fell for it!

 

[kevplumb]

methinks you may be right

 

[ChrisR]

A bath empties because atmospheric pressure bears down on the bath water.

Come on Chris that's total, utter, complete, consummate and unforgivable *******s. You're better than that! I suggest, er,