How does a diaphragm pump work? Without an impeller, how can it pump water?

People who disassemble a diaphragm pump for the first time may feel a little confused.

I dismantled a pneumatic diaphragm pump for the first time in a shipyard. I originally thought there were some precision 

rotors, gears or impellers inside. When I opened it, I found two rubber diaphragms, an intermediate shaft, a few small balls, 

and a few O-rings, that's all. The structure is so simple that it doesn't look like an industrial pump.

But this thing can pump out sewage, oil, and acid and alkali. Even if the suction port is exposed to the liquid surface, it will

 still be spinning there without burning. How on earth does it pump the liquid up? Let me explain the principles in the order

 I saw when I dismantled the pump.

Remember one sentence first: the diaphragm pump does not rely on "sucking", but relies on "squeezing"

Many people think that when a pump pumps water, the pump generates suction inside and draws the water up. Not really.

Diaphragm pumps essentially rely on two things to pump water:

The pump chamber becomes larger for a while and smaller for a while;

Four one-way valves only allow liquid to go in one direction.

When the pump chamber becomes larger, the pressure inside decreases, and the atmospheric pressure outside presses the 

liquid in from the suction tube. As the pump chamber becomes smaller, the liquid is squeezed out. The "heart" of a diaphragm 

pump is not the impeller, but the diaphragm.

You can think of it as two syringes connected with a stick in the middle. One syringe is pulled out and the other is pushed back. 

The diaphragm pump allows the two syringes to be pulled and pushed back and forth.

Two diaphragms, one shaft, four one-way valves

Taking apart a double diaphragm pump, the core components are probably these:

Two diaphragms: one in the left pump chamber and one in the right pump chamber;

A central axis: connects two diaphragms together, with one side bulging outward and the other side retracting inward;

Four one-way valves: each pump chamber has an inlet valve and an outlet valve, four in total;

Air distribution valve: installed in the middle, responsible for sending compressed air to the left and right air chambers in turn;

Valve ball and valve seat: Many pneumatic diaphragm pumps use ball valves. The ball sits on the valve seat, opens forward for 

Forward flow, and compresses for reverse flow.

The diaphragm does not move by itself, it is pushed by compressed air. The central axis is not a decoration. It ensures that when

 the left diaphragm moves out, the right diaphragm must be retracted.

How to make compressed air move

The working process of the pneumatic double diaphragm pump can be viewed in four steps.

Step 1: Enter compressed air into the left air chamber.

The air distribution valve sends air to the left air chamber, and the air pressure pushes the left diaphragm outward. The volume

 of the left pump chamber increases and the pressure decreases.

Step 2: Aspirate the liquid on the left side.

When the pressure in the left pump chamber is low, the atmospheric pressure outside presses the liquid in from the suction tube. 

At this time, the inlet valve on the left is pushed open and the outlet valve is pressed tightly, so the liquid can only enter but not exit.

Step 3: Drain the liquid on the right side.

The left diaphragm moves outward, and the right diaphragm is pulled toward the center through the central axis. The volume of 

the right pump chamber becomes smaller and the liquid inside is squeezed. At this time, the inlet valve on the right is closed tightly, 

the outlet valve is opened, and the liquid is discharged from the outlet.

Step 4: Reverse the valve.

After the left side is sucked and the right side is discharged, the air distribution valve automatically changes direction. The compressed 

air is diverted to the right air chamber, the right diaphragm goes out, and the left diaphragm goes back in. So the right side sucks the 

liquid and the left side drains the liquid.

In this way, the left and right sides suck and row, push and pull, and the cycle repeats. The liquid flows continuously from the inlet to 

the outlet.

The secret to water absorption is actually that atmospheric pressure helps

Diaphragm pumps can self-prime not because they have a vacuum pump inside. It's because when the pump chamber becomes larger,

 a negative pressure is formed inside, and the atmospheric pressure outside forces the liquid in along the suction tube.

Therefore, there is an upper limit to the height of suction. Theoretically, one standard atmospheric pressure can press water up to about 

10.3 meters high. In fact, due to pipeline losses, liquid vaporization, and loose sealing, it is pretty good to be able to dry-suck for five to

 six meters. Wet absorption will be higher, but not infinitely high.

If the suction pipe leaks, the flange gasket is broken, and the suction inlet filter is blocked, the pump chamber will not be able to suck 

up no matter how large it is. Many people say that "the diaphragm pump is not self-priming". In fact, it is not that the pump is not 

working, but that the suction tube is leaking.

Why is the water flowing out of the outlet flowing in one stream?

A double diaphragm pump has two pump chambers that work alternately, but the outlet will still pulsate. Because there is a switching

 process between left chamber drainage and right chamber drainage, it is not completely smooth.

Where stable flow is required, a pulsation damper or pressure stabilizing tank is usually added to the outlet. Some sewage oil and water

 transfers on ships don't care much about pulsation and just discharge them directly. For chemical dosing and precision transportation,

 dampers must be added.

In addition, the flow rate and outlet pressure of a pneumatic diaphragm pump are two different things:

The higher the air source pressure, the higher the outlet pressure;

The greater the air source flow rate and the faster the reversal frequency, the greater the flow rate;

The higher the outlet back pressure, the lower the flow rate.

If you close the outlet valve, the pressure will increase and the flow rate will decrease. When closed to a certain extent, the pump will

 reach pressure balance and the diaphragm will not move much. So don't leave the pump closed for a long time, as the diaphragm 

will tire easily.