The centrifugal pump in the engine room was clogged again. This is the third time this month that it has been demolished and 

repaired. The second engineer squatted in the bilge with his hands full of oil, cursing - you may have seen this scene before. 

Searching for "frequent bilge pump failures" on shipping forums will turn up hundreds of similar complaints. And the answer 

to the question may be simpler than you think.

If you have ever been on a ship, or have talked with veterans on the ship, you should know a common phenomenon: the 

centrifugal pump used in the bilge water system, almost every ship does not fail due to this.

block. Self-priming failed. Cavitation. The motor was overloaded and burned out. If you practice twice a month, you are considered 

lucky, and if you practice three times a month, it is normal.

In recent years, more and more new ships and refitted ships have begun to turn their attention to a relatively "new" thing in the 

domestic shipping circle - the pneumatic double diaphragm pump (AODD) in terms of bilge drainage, fuel transportation, and 

ballast water. Wilden is the brand that has been doing this for the longest time. It invented this thing in 1955, and it has been

 almost 70 years now.

Below, I will explain to you exactly what is good about this thing, where it can be used on the boat, and how to avoid pitfalls.

1. Why do the pumps on the ship always break down?

To be honest, it's not that the quality of the pump itself is poor, it's that the work on the ship is too "dirty".

Let’s do it one by one:

The media is too messy, and all kinds of messy stuff gets in there. How can there be any "clean water" in bilge water? Sea water,

 lubricating oil, diesel, rust slag, sediment, cleaning agent residue...all mixed together. The impeller of the centrifugal pump rotates 

at high speed to throw water out. If it encounters hard particles, it will be beaten. Over time, the impeller will be chipped and the

 volute will be worn out, and the efficiency will drop rapidly. If the slag is bigger, the flow channel will be blocked directly.

The self-priming ability is not enough, and when you inhale the air, it will cause numbness. The self-priming ability of the centrifugal 

pump is very weak, and it will not be able to suck in even a little air from the inlet pipe. The bilge water is pumped intermittently, the

 liquid level fluctuates high and low, and air intake is a common occurrence. As soon as the air is taken in, cavitation occurs, the 

impeller is pitted, and the flow rate drops.

Electric explosion-proof is expensive and troublesome. The fuel areas of oil tankers and engine rooms must use explosion-proof 

equipment according to regulations. Electric explosion-proof pumps are not impossible, but how much does an explosion-proof 

motor cost? The sealing of the junction box, the armoring of the cables, the grounding system... Not only will the purchase cost 

double, but the annual inspection will also have to be dedicated to it.

Space on a ship is valuable, and repairing it once can cost lives. The pipes in the engine room are as dense as a spider web. To 

dismantle a pump, you often have to remove the pipes next to it first, and then you can squeeze in by leaning sideways. It takes

 less than half a day to disassemble an ordinary centrifugal pump. Seals, bearings, and mechanical seals are all wearing parts, and

 a bunch of models must be kept in the warehouse.

The working conditions change and the pump cannot bear it. Water is not pumped at full capacity all the time. Sometimes it is 

pumped for a short period of time, and sometimes it is operated intermittently. The efficiency of the centrifugal pump is extremely

 low under such variable working conditions, and frequent starts and stops cause great losses to the motor and contactor.

Did you find out - most of these problems are caused by the combination of "electric + rotation". If "electricity" is replaced by

 "gas" and "rotating impeller" is replaced by "reciprocating diaphragm", many problems will not occur at all.

2. Why does Wilton’s set of things work so well on a ship?

Wilton's AODD pump is not complicated in structure:

Compressed air goes in and pushes a connecting rod to move back and forth, with a diaphragm connected to each end of the

 connecting rod. When the left chamber takes in air, the left diaphragm bulges to squeeze out the liquid, and at the same time, 

the right chamber draws in new liquid - just like this, moving back and forth from left to right, the liquid is "sucked in and pushed out".

The structure is that simple. But it is precisely this "simplicity" that becomes a huge advantage on board.

The first one: no electricity, naturally explosion-proof. The only power source is compressed air, which is what the engine room

 on the ship is most indispensable for. Without a motor, there is no spark, and there is no need for explosion-proof wiring, isolation 

barriers, or intrinsically safe certification. There is ATEX certification, but that is a problem with the grounding of the material of the 

pump itself, and has nothing to do with your power distribution system.

Actual case: An oil tanker installed a Wilton TZ series metal pump in the cargo pump room for bilge water collection. The chief 

engineer's exact words were - "In the past, electric submersible pumps were used, and each time before entering the pump room, 

they had to be ventilated and tested for explosions for half an hour. Now, the pneumatic pump is put in and started directly, which 

saves a lot of trouble."

The second one: It is naturally capable of self-priming and idling. The suction principle of the diaphragm pump is to rely on the 

diaphragm to bulge to generate negative pressure, rather than relying on the high-speed rotation of the impeller to generate 

centrifugal force. So as long as the diaphragm can move, the suction is there. The dry suction height can reach 7 meters - completely 

covering the bilge height on the ship. And the most cruel thing is: there is no water in the pump, and it will not break even if it is just

 idling. There is no problem of seals being burnt by dry grinding.

The third one: scumbag? Just let it go. There is no narrow gap between the impeller and the volute inside the Wilton pump, and the

 fluid flows through a large cavity. The maximum particle passing capacity of the T15 series three-inch pump can be 25 mm - you heard 

it clearly, it is two and a half centimeters. The bits of rust, welding slag, and rags in the bilge were nothing to it.

Fourth: The pipeline is blocked? The pump stops on its own without competing with you. This is one of the most underrated features

 of air-operated diaphragm pumps. As soon as the outlet valve is closed and the pressure in the pump chamber is suppressed, the air 

distribution system will automatically switch to a "balanced" state, and the diaphragm will hardly move - which is equivalent to the 

pump itself "stopping". But the motor didn't burn, and no parts were holding up the pressure. Open the valve and the pump immediately

 resumes full flow. This is simply tailor-made for it in the scenario of intermittent drainage of the bilge.

Fifth: The flow rate and head can be adjusted easily by two valves. The inlet pressure is adjusted for "power" (lift), and the inlet flow 

is adjusted for "speed" (flow rate). No frequency converter is required, and no complex control system is required. In a scenario where

 working conditions change every day on a ship, the operator can fix the problem by simply turning the valve.

Sixth: No special tools are required for disassembly and assembly. With Wilton's clamp structure, the pump body can be disassembled

 with an ordinary wrench. The diaphragm is a wearing part, but a skilled worker can replace one diaphragm in 20 minutes, and it can

 be replaced on site without having to dismantle the entire pump and send it to the workshop.

I met a second-tube drummer on a bulk carrier who changed the diaphragm by himself. He said something that I still remember: 

"I am sweating when repairing a centrifugal pump, but I feel light when replacing this one."

For more information, please see the following article.

https://www.dfinsewagepump.com/Why-are-experienced-engineers-starting-to-replace-centrifugal-pumps-with-those-used-for-transporting-bilge-water-and-other-fluids-on-ships-2-n.html