The plastic trigger sprayer head with cover is a very useful device, it is an excellent example of the fundamental principle of pipe pumping. The head of the plastic pump head with a cover consists of only a few parts. It has a trigger which can trigger a small water pump. The water pump is connected to a plastic tube, which sucks the cleaning liquid up from the bottom of the reservoir. The pump presses the liquid into a narrow cavity and ejects them from the small hole in the spray nozzle. This hole, or a nozzle, can bring the moving liquid together and turn it into a concentrated stream of water. The only complex in this plan is a hydraulic pump, but its layout is also very simple. Its primary movable element is a piston, located in a cylindrical hydraulic chamber. In the hydraulic chamber, there is a small tension spring. In order to make the hydraulic pump work, you must first pull the wrench back, and then push the piston into the hydraulic chamber. The moving piston tightens the tension spring so that when you loosen the wrench, the piston is pushed out of the hydraulic chamber again. The two strokes of the piston in and out of the hydraulic chamber constitute a complete pump cycle.

During the downward stroke, the piston is pushed inward, shrinking the volume of the hydraulic chamber, and then squeezing the liquid out of the pump. In the upstroke, the tension spring pushes the piston outwards, expanding the volume of the hydraulic chamber, and then sucks the liquid into the pump. When using a plastic pump head with a cover, you need to suck up the cleaning liquid from the lower reservoir, and then knead it out of the upper cavity. In order to get all the liquid that flows into the cavity, the pump can only push the liquid upwards—not the liquid back into the reservoir. In other words, the liquid can only flow through the pump in one direction. The equipment that accomplishes this function is called a one-way valve. The plastic pump head with cover has two one-way valves in its pumping system: one is located between the pump and the reservoir, and the other is located between the pump and the nozzle. Generally, the one-way valve located between the pump and the reservoir includes a small rubber ball, which is lightly placed in a small sealed cylinder. The edge of the sealed tube is sharp, so the rubber ball will not fall from the sealed tube. According to the plan, when you are not pumping water, the ball relies on its own gravity or the elastic force of a small tension spring to press against the sealed cylinder, and then block the water passage. When the piston moves outward (when you loosen the wrench), the expanded hydraulic chamber will suck the small rubber ball away from the sealing cylinder and suck up the liquid from below. Because the ball is lifted, the liquid can flow unimpeded from the reservoir. When you press down the wrench, the external force of the movable liquid pushes the ball back to the sealing cylinder, blocking the passage between the pump and the reservoir.

Therefore, the pressurized liquid can only flow to the upper cavity. The one-way valve located between the pump head and the nozzle is a cup-shaped device, which is placed at the end of the cavity. In the upstroke, because the pressure in the pump is lower than the outside air pressure, the cup-shaped device is pulled tightly against the cavity, so air cannot flow in from the nozzle. In the downward stroke, the liquid being pressed out slightly pushes the cup-shaped device away from the cavity, and then flows out of the nozzle. If there is no second one-way valve, the pump system will not be able to suck up the liquid from the reservoir due to lack of suction (no air pressure drop). At this moment, the upstroke does not drop the air pressure in the pump; it only sucks in more air to maintain the pressure inside the pump. The second plastic pump head check valve can also be used as a shut-off system. When you tighten the nozzle, the cup valve will be pressed tightly on the cavity so that no liquid can be pressed out. When you unscrew the nozzle, there will be enough space for the cup valve to move back and forth.