Relief valve/swan neck puzzle

[alibaba 83]

I've recently changed boats [downsized to fit my wallet!], and the new boat has something I've never seen before. It appears to have a

plastic swan neck and relief valve in the piping between the raw water pump [impeller] and the heat exchanger. The engine is a three cylinder

Nanni.

I can't see what it's there for, but before I remove it and simplify the system, does anyone else have any ideas?

[CarpeDiem 518]

12 minutes ago, alibaba said:

I've recently changed boats [downsized to fit my wallet!], and the new boat has something I've never seen before. It appears to have a

plastic swan neck and relief valve in the piping between the raw water pump [impeller] and the heat exchanger. The engine is a three cylinder

Nanni.

I can't see what it's there for, but before I remove it and simplify the system, does anyone else have any ideas?

It an anti-siphon/vacuum breaker valve. As the engine cools a vacuum is created and this vacuum can be strong enough to start a siphoning effect. The release valve stops the engine from back siphoning the sea water, filling the exhaust and hydro locking the engine.

I would certainly not remove it without a detailed study to determine if it's necessary which can be done by consulting the engine installation manual and the position of the exhaust mixer and lift muffler in relation to the water line. 

They need maintenance. Open the valve on the top and clean out any salt crystals.

[alibaba 83]

Thanks Carpe Diem, I can understand the idea to prevent back siphoning of water, but there is a bloody big swan neck between the exhaust muffler and the exit to the rear of the boat. That's the usual arrangement I've known. That would prevent any possibility of back siphoning I think, as apart from the  height of it, the volume of the piping and the muffler must be at least 5liters. Do you think I would need both swan necks?

[CarpeDiem 518]

16 minutes ago, alibaba said:

Thanks Carpe Diem, I can understand the idea to prevent back siphoning of water, but there is a bloody big swan neck between the exhaust muffler and the exit to the rear of the boat. That's the usual arrangement I've known. That would prevent any possibility of back siphoning I think, as apart from the  height of it, the volume of the piping and the muffler must be at least 5liters. Do you think I would need both swan necks?

The riser in the wet exhaust line helps to prevent the lift muffler from filling due to waves smacking into the exhaust outlet. 

If the high point of your wet exhaust pipe is above the exhaust mixer which most yachts are, then the system is inherently unsafe and a vacuum breaker is good insurance although that won't save you if the exhaust line fills. 

When you stop the engine any water in the wet exhaust riser runs back into the lift muffler. If the lift muffler becomes full enough and seals the muffler then a vacuum can pull water from your water intake, past the impellor and flood the engine. 

If your wet exhaust, for its entirety, is a good 200mm below the exhaust injection point at all angles of heel then the vacuum release valve is likely unnecessary. 

If at anytime your exhaust becomes blocked, eg a following sea, a wave, heeled over. Then there's a chance a vacuum could form. 

[waikiore 462]

It does sound as if the install is fairly low in the boat compared with static waterline , in which case the anti syphon is a good move. A large waterlock fitted as low as possible after the injection elbow is essential.