drain with pipe outlet under the waterline

[NuckChorris]

Hi guys, first post so a quick intro: I'm an jr. marine engineer in the Netherlands working for a shipyard building superyachts. My field of experience is mostly HVAC and propulsion, but of course I get all kinds of fun little projects here. As I'm eager to become more experienced at all the different marine engineering areas I try to read up and sometimes ask some questions here at the forum. My fluid mechanics and pump knowledge can use some freshing up, so thanks in advance!

Problem: We have swimmingpools and whirlpools on our yachts and they have an emergency drain which must drain the (whirl)pool within 3 minutes. I figured out the maths for a normal drain (diameter) which has the outlet on the side of the ship just above the waterline. But now they ask for an outlet under the waterline so they can rout the (rather big) pipe straight down through the ship with the outlet at the bottom of the ship.

Question: How does a pipe outlet approx. 3 meters under the waterline decrease the time of draining for an 'open water tank' (whirlpool).

My thoughts: you will have an additional backpressure at outlet (3m under water - 3kPa extra backpressure), but I'm not so sure and I can't seem to figure out how much it will decrease. I only need to have an indication, as there can be all kinds of unpredictable turbulent flows and of course we take a safety factor, and the end result will only be the choice for a pipe diameter.

 

[BigInch]

The backpressure added by the 3m depth of water outside the hull is canceled by the new 3m of liquid draining from the inside, so basically that's simply balanced. Except for one thing, it adds 3 meters to the pipe, so flow losses are at least that much greater.

I think I would be more concerned with dynamic pressure variations from changes to the water level outside. That will tend to force water up into the drain as the immersion depth increases, or "suck" it out when the water immersion depth decreases. The first might be helped with a check valve, although draining in progress would be temporarily stopped, but the second might suck the pool dry. Do you have a vacuum breaker or vent?

Careful with the changes, or complete reversals of velocity that might happen in those flow direction change circumstances. You might also have some trouble with placement of the drain on the hull. One place might have velocity induced venturi pressure changes, such as pitot tube pressures picked up if the drain outlet faced somewhat forward into and catching the oncoming water, or negative pressures that might result from water slipstreams past a flush-mounted drain.

Reaction to change doesn't stop it

 

[LittleInch]

Agree with BI.

Only other point is that I think the outlet discharge coefficient is different for a fully filled liquid pipe into air than a pipe into water. Might not be significant and probably dwarfed by the kinetic effects BI is talking about, not to mention potential fouling / marine growth inside your nozzle.

Curious as to where three minutes comes from as this creates your issue. I assume you're only looking at gravity draining so you won't have a lot of drive force, hence your rather big pipe.

I would have thought that sucking someone to the outlet would be more dangerous....

Can you use more than one pipe?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[NuckChorris]

Thanks for the quick respons mate!
The extra flow losses because of the extra pipe length is already taken into account. I took the height from bottom of the pool to the waterline now, so that's fine right?

In practice the pipe will be partly filled with seawater (up to the waterline) so you might have some trouble with locking some air during draining (3,5m^3 through a DN165) creating bubbles and some other hydrodynamic processes, but I think these can be ignored, as we built in safety by selecting one or two steps higher DN.
There is no vacuum breaker or vent no. There is an automated emergency valve all the way up in the drain line which will only be opened in case of emergency.. and the process will (hopefully) not be used very often.

I added a simple overview:

When class society comes timing the (3 minutes) draining, this is done in harbour condition, so there will be no problem with ship velocity inducing venturi pressure changes, am I right?

 

[NuckChorris]

@LittleInch: The 3 minutes is a demand from the classification society. This drain is only for emergency, when for example the pool is being filled with rain or deck waves the pool must be drained quickly so there won't be any stability problems. There is also another drain system without these requirements for normal draining (to a pool tank) and for the water treatment which can be used when people are in the pool, so nobody will be sucked into the drain hehe...

More than one pipe is possible yes.

 

[bimr]

Regarding: so nobody will be sucked into the drain hehe...

This is not a laughing matter and probably the biggest issue with what you are designing. There have been many cases where people have gotten their hair stuck in the pool drain from suction and have their skalp ripped off or drowned. The plumbing code calls for specific design stndards for pool drains.

 

[NuckChorris]

@bimr you're right. That's not funny at all, damn. But as I said, the emergency drain is only for moments when the pool is absolutely not in use.

 

[cvg]

anti-vortex covers and dual drains are required on all public pools in Arizona and other areas, due to drownings caused by the suction on single drains in swimming pools.

http://www.clean-pool-and-spa.com/swimming-pool-drains.html

 

[BigInch]

As well as sitting in the wrong place can have the potential to remove your intestines.

Reaction to change doesn't stop it