Since there are so many different types of pumps and mechanisms, it can be a little confusing to keep track of them and how they all work. To make things a bit easier to digest, we’re going to cover 9 of the most common PD pumps in this post. All types of positive displacement pumps involve a mechanism that falls into two categories depending on the type of motion it utilizes. To make it a bit easier to understand, we have broken down the most common positive displacement pumps into nine basic types based on their mechanics. These two categories of pump movement type are called reciprocating and rotary.
Reciprocating pumps work on the principle of a chamber expanding and contracting, or reciprocating, to change the pressure within the pump to draw fluids in, and then direct it out the discharge, utilizing a series of check valves to ensure the liquid flows in one direction.
Diaphragm Pump – Instead of driving a rigid plunger or piston to displace fluid, a diaphragm pump works by utilizing a flexible elastomer cover or diaphragm. When the diaphragm expands it draws fluid into the pump and when it expands, sends the fluid out of the discharge side of the pump, utilizing check valves to ensure a one-way flow. One feature in diaphragm pumps is that it does not require a seal because the wet and dry sections of the pump are completely separate from each other.
Piston Pump (Axial Piston Pump) – The mechanism consists of a reciprocating piston inside of a cylinder with check valves at both the intake and discharge. When the piston is on the suction stroke, the chamber inside of the cylinder expands, the pressure drops, the pressure valve on the suction side is opened, allowing fluid through the intake. By definition, check valves permit flow in one direction only, which greatly helps prevent backflow. As the valve on the suction side opens, the valve on the discharge side closes. On the discharge stroke, the piston pushes on the fluid in the cylinder, raising the pressure, closing the suction valve and opening the discharge valve, pushing the fluid out. The benefits of a piston pump are good efficiency and capable of generating a high amount of pressure.
Plunger Pump – Plunger pumps look very similar to the piston pumps but operate a bit differently. A plunger pump works differently from a piston pump as the piston fits loosely in the cylinder, as opposed to a tight-fitting seal that a piston pump provides. The mass of the piston displaces the fluid in the cylinder as it is lowered, forcing it out the discharge side.
Rotary pumps all utilize a range of rotating or moving chamber that traps fluid and moves it from the intake side to the discharge side of the pump.
Screw Pump – Screw pumps can have a single shaft with large threads on it, but it is more common to see two or three. The center shaft drives the other shafts like a screw gear. As the shaft rotates in the housing, fluid trapped in the gaps between the screw threads progresses along the length of the housing, then delivered to the discharge outlet. This pump looks and operates similarly to an auger, which you may have seen before.
Peristaltic Pump – Peristaltic pumps contain fluid trapped in a flexible hose. The hose is pinched between the rollers and the inside wall of the housing. As the rollers revolve around a central axis, they squeeze the fluid through the tubes towards the discharge outlet, similar to what happens when you squeeze a tube of toothpaste at one end, and it squirts out the opening. As with diaphragm pumps, peristaltic pumps do not require a seal because the fluid is always contained in the hose. These types of pumps are excellent at moving high viscosity or solids content.
Vane Pump – A vane pump rotor contains a series of slots for sliding vanes that press against the wall of the housing by centrifugal force when the rotor spins, forming a sealed chamber. Since the rotor is offset within the housing, the vanes extend and retract during rotation. When fluid is introduced, it is captured between the vanes as they are rotated, and directed to the discharge.
Progressive Cavity Pump – While looking similar to screw pumps, the progressive cavity pumps do, in fact, function differently. The helix-shaped rotor fits into the housing which is molded into the shape of a double helix. As the rotor rotates, it begins to oscillate within the housing, forming a cavity which progresses towards the discharge end of the pump. Any fluid trapped in the progressive cavity will eventually discharge out of the pump during operation.
Gear Pump – A gear pump utilizes a pair of rotating gears inside a chamber (one gear driving the other) that move and pushes the fluid out towards the other end of the pump. As the gears spin, it traps fluid between the gear teeth and inside wall of the pump. The trapped fluid is then pushed in the direction the gear is spinning, eventually releasing out of the other side. Gear pumps can generally be built two different ways; an external gear pump and an internal gear pump.
Lobe Pump – Lobe pumps look and function quite similar to external gear pumps. In lobe pumps, the rotors are driven independently by external timing motors, instead of one rotor driving both of them like you’d find in gear pumps. The rotors have two or more lobes which rotate and trap fluid between the lobes, forcing fluid through the discharge outlet on each rotation.
That we a brief introduction to 9 of the most common types of PD or positive displacement pumps found around the world in many different applications. As you can see, they operate much differently than the more common centrifugal pumps. It’s good to familiarize yourself with how some of these types of pumps operated in case you ever run into one on your own.