Why can’t a reciprocating pump use an outlet valve to regulate flow?

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A reciprocating pump is a type of positive-displacement pump that is widely used. It relies on the back-and-forth movement of the piston to sequentially open the intake valve and the discharge valve, thereby drawing in and discharging liquid. Reciprocating pumps are suitable for transporting liquids with high head pressure, low flow rates, and high viscosity, but they are not appropriate for transporting corrosive liquids. 1. Structure and working principle of reciprocating pumps. The main components of a reciprocating pump include the pump cylinder, piston, piston rod, suction valve, and discharge valve (all of which are check valves). The piston rod is connected to the transmission mechanism, which drives the piston to move back and forth within the pump cylinder. In the structure of a reciprocating pump, as the piston moves from left to right, the volume of the working chamber increases, creating a low pressure that allows the liquid in the reservoir to be drawn into the pump cylinder through the suction valve. When sucking in liquid, the discharge valve closes due to the pressure of the liquid in the discharge pipe. When the piston moves to the right endpoint, the volume of the working chamber is at its maximum, and the amount of liquid drawn in is also the greatest. Thereafter, the piston begins to move from right to left; the liquid inside the pump cylinder is compressed, resulting in an increase in pressure. This causes the suction valve to close while pushing the discharge valve open to expel the liquid. After the piston moves to the left endpoint, the liquid is drained, completing one working cycle. Thereafter, the piston moves to the right again, starting another working cycle. Therefore, a reciprocating pump does work on the liquid by changing its working volume. During one working cycle, liquid suction and discharge occur alternately once each, resulting in discontinuous fluid transport. The piston moves back and forth at non-constant speeds, so the flow rate fluctuates. Based on the working principle of reciprocating pumps, the low pressure inside such pumps is created by the expansion of the working chamber; therefore, it is not necessary to fill the pump with liquid before starting it, meaning that reciprocating pumps have self-priming capability. However, just like centrifugal pumps, reciprocating pumps also have a certain limit on their suction height. This is because reciprocating pumps draw in liquid by means of the pressure difference between the outside environment and inside the pump; therefore, the suction height varies depending on the atmospheric pressure at the location where the pump is installed, as well as the properties and temperature of the liquid being transported. 2. Why cannot reciprocating pumps use outlet valves to regulate flow? Unlike centrifugal pumps, reciprocating pumps cannot rely on outlet valves for flow control, as this would result in energy waste and even overload the drive mechanism of the reciprocating pump. Reciprocating pumps generally use the method of opening a bypass control valve to regulate flow. When the flow rate provided by the pump exceeds the flow rate required by the pipeline, and a portion of the fluid needs to be returned to the inlet of the reciprocating pump, a bypass pipe for the reciprocating pump must be installed. By adjusting the opening degree of the bypass valve, the fluid can be sent back to the suction pipe, thereby regulating the flow rate in the pump’s discharge pipe. A safety valve should be installed on electric reciprocating pumps; it opens when the pressure in the discharge pipe becomes too high, thereby preventing the pump and the prime mover from being overloaded. The flow rate is adjusted using a bypass valve; the pump’s flow volume remains unchanged, but part of the liquid that has been pumped out is returned to the reservoir, thereby changing the flow rate in the main pipe. However, this adjustment method is very inefficient and is only suitable for regular adjustments where the flow rate changes are minor. The flow rate can be adjusted by changing the crank speed. Since the motor is connected to the reciprocating pump through a reduction gear, changing the transmission ratio of this reduction gear allows for easy adjustment of the crank speed, which in turn changes the frequency of the piston’s reciprocating motion and thus enables regulation of the flow rate. In addition, reciprocating pumps can also have their flow rate adjusted by other methods; for example, by changing the stroke of the pump’s piston or plunger, or by altering the eccentricity, the clearance of the plunger, as well as the length and position of the connecting rods in the linkage mechanism, in order to change the stroke of the piston or plunger and thus adjust the flow rate of the reciprocating pump ； By forcing the suction valve open with the reciprocating pump, the liquid in the suction cylinder returns to the suction pipe through the suction valve during the return stroke, thereby reducing the discharge flow rate of the reciprocating pump.