The mixture carrying system of a cutter suction dredge, is more than just a cutter and a pump in a pontoon. In the dredge system, there are many valves and fittings, that make the system work. One question I was asked, what these extra valves do. Actually, there are several valves, that are worth mentioning. The sniffer valve, the vacuum relief valve and the non-return valve. For working in the designed operating point, you don’t need them. But, to get there and back, they can be quite useful.
The first valve is affectionally called a sniffer valve. A more descriptive name would be ‘discharge line de aerator valve’. Usually, it consists of a floating ball in a cage with a seat at the top, that can be closed by the ball. Provided the ladder is already under water and starting up the dredge pump from a fresh situation, probably air is in the high onboard discharge pipe sections on deck. Behind the dredge, the line goes down again and the air is basically trapped, preventing the dredge pump from properly priming. The sniffer valve allows the air to escape and the water to enter the floating discharge pipe at the water line. Problem solved.
On the other hand, when the discharge pressure falls, the ball floats down with the receding water level. This opens the top and allows air to enter the pipe again. Which is no problem as it can be expelled again through the same sniffer valve. When the water can flow away from the high section on board, this will break the water volume in the system. When opening the pump, only the small section between pump outlet and bulk head passage will fall in the pump well. Or, if properly executed: need to be drained and discharged.
A last function of the sniffer valve is in case there is a blockage of the suction pipe. There will be no new mixture flowing in, but the mixture in the discharge line still has a lot of momentum. For a 1 km, 500ø mm pipe, the mixture has the equivalent momentum of a 75 ton truck barrelling down the pipe at 80 km/h. You don’t stop that in an instant either. The mixture keeps flowing and draws a vacuum. The sniffer ball drops and allows air to enter the pipe.
A better way to prevent the vacuum, is to install a vacuum valve in the suction line. That will allow water in and enable you to clean the discharge line without a cavitating pump. Once the suction block is removed, the relief valve opens again and mixture can be inserted in the dredge line system.
In case there is a high discharge height, the mixture mass will not be broken by the sniffer valve. The geodetic pressure will close the sniffer and all of the mixture volume wants to return through the system out of the suction mouth; or open pump, swamping the dredge. To prevent this from happening, a non-return valve can be mounted in the onboard discharge line.
These fittings will cover most operational situations. There might be even more for exceptional situations, depending on the design choices by the manufacturer1 and to the taste of the owner.2 e.g. We provide a suction deaerating valve. Any ideas about such a provision?
I did receive the right comment about to the purpose of the suction deaerator valve. It is indeed for letting out the air trapped in the suction pipe when the ladder is being lowered. It could be argued that the air will also leave through the sniffer valve at the back of the dredge. If somehow, the air would have trouble escaping all the way to the back, the pump will be very slow in priming itself. Providing a deaerator on the local high at the bulkhead passage, the suction line can purge the air there and the pump starts quicker.