Why don’t many reciprocating pumps have a return line? What is the reason behind this?

Adjustment of reciprocating pumps: The flow rate of a reciprocating pump depends only on the pump’s geometric dimensions and the number of reciprocations per unit time, and not on the pump’s head pressure. Regardless of the head under which it operates, with just one reciprocation, the pump discharges a certain volume of liquid. Therefore, a reciprocating pump cannot be started with the outlet valve closed, otherwise the liquid drawn into the pump body has no way to escape. The greater the pressure applied to the piston, the higher the pressure becomes, to the point where it may damage the pump body and burn out the motor. Reciprocating pumps cannot simply adjust flow by changing the valve opening. Based on the characteristics of reciprocating pumps, the flow rate can be adjusted using the following methods: 1. Change the speed of the prime mover to adjust the number of piston reciprocations. For transporting flammable and explosive liquids, a steam-driven reciprocating pump is used; by adjusting the amount of steam supplied, the number of reciprocations of the piston can be increased or decreased, thereby regulating the flow rate. For reciprocating pumps driven by electric motors, a speed control device is required to adjust the speed. 2. A bypass is installed at the outlet of the reciprocating pump, allowing a portion of the liquid to return to the inlet. In this way, by installing a control valve on the bypass and adjusting the degree to which the control valve is open, the flow rate can be regulated. This method is simple and convenient, so it is often used in production. The safety valve in the diagram opens automatically when the pressure downstream exceeds a certain limit, to ensure the safe operation of the system. Therefore, reciprocating pumps can be equipped with bypass regulation.

A bypass line needs to be installed; otherwise, high pressure can easily cause the pump to stall or the safety valve to activate. All the reciprocating pumps we produce come with bypasses.

Generally, reciprocating pumps are equipped with bypasses

I work in the water treatment industry, where dosing metering pumps are frequently used. The most common type is the diaphragm pump, which belongs to the category of reciprocating pumps or positive displacement pumps. In actual design, no bypass is provided; only a safety valve is used. Flow regulation is achieved by adjusting the speed of the motor or the stroke (most diaphragm pumps used in this industry have a stroke adjustment function). Of course, it’s because the industries are different, resulting in significant differences; moreover, the water treatment industry is less standardized compared to the chemical and petroleum industries. For reference only.

Reciprocating pumps should generally be equipped with a bypass to regulate flow; otherwise, the pump’s flow rate can only be at its maximum level and cannot be adjusted.

This post was last edited by forus on 2010-7-7 at 10:18. Pumps with bypass systems are common, while those without such systems are relatively rare; the pumps without a bypass that I’ve seen seem to be small pumps. I’ve never seen reciprocating pumps use a bypass to regulate flow. Bypass systems are generally used during pump startup and switching, and if there is no need for them, then they don’t need to be installed. But a safety valve must be installed

What is the purpose of having a bypass for the pump? 1. Control flow rate; 2. Control pressure. Firstly, pressure is controlled using a circuit on the reciprocating pump, but manual adjustment is slow and not very effective. It’s better to use a safety valve, as the pressure of the reciprocating pump is designed to be fixed anyway. Secondly, to control the flow rate, a metering pump (a type of reciprocating pump) can be used, or frequency conversion and adjustment of the stroke can be employed; there is no need to install a bypass. Finally, I think this is a design issue – if the design pressure is appropriate, and if there are safety valves as well as reliable flow control devices, then the circuit is unnecessary

Reply to 8# 397430: It seems that the pressure of a reciprocating pump cannot be determined; by closing the outlet valve, the outlet pressure will keep rising until the safety valve activates

Reply to 9# yangjie007: A reciprocating pump is a type of positive displacement pump. In general, positive displacement pumps regulate the pressure and flow rate at their outlet by installing return lines along with control valves on those lines. However, this approach of using control valves and altering pipeline characteristics results in increased equipment costs as well as higher energy consumption. In the following situations, it is possible to omit the return pipeline in order to save energy and reduce costs: first, the pipeline from the volume pump’s outlet to the downstream user is straight, which prevents pressure buildup at the pump outlet; second, the downstream user does not require frequent adjustments to the flow rate at the pump outlet, and a stable flow rate can be maintained over an extended period by setting the initial stroke length. The three downstream pipelines are in direct connection, and flow regulation can be achieved by adjusting the frequency of the motor driving the pump. These are merely superficial opinions; I earnestly ask for your guidance, expert.

Adding a circuit may increase the risk of internal leakage, thereby reducing pump efficiency and leading to negative consequences.