Positive displacement pumps and vortex pumps must be equipped with a bypass valve between the inlet and outlet valves of the pump

By \"connection,\" it means there must be a closed loop; when the pump is started, the loop must remain fully open to prevent overload of the motor current.

Shouldn’t it be turned off after the pump starts working normally? Why are such connection pipes hardly seen on field positive displacement pumps?

Once it’s turned on, it can’t be turned off – it stays on continuously; that’s the case with the vortex pumps I’ve seen.

One of our previous vortex pumps was equipped with a backflow valve between the inlet and outlet valves, primarily to prevent backflow when the flow rate was low. Pump fans of the solvent type have the lowest outlet current when in vent mode. One of our Roots fans had a pressure-controlled valve on its vent line, but for some media venting is not allowed; in such cases, the flow must be directed back to the inlet or to the equipment located before the inlet. The vortex pump example mentioned earlier was an acid water return pump, whose control parameter was the liquid level in front of the pump inlet. Since the flow rate of the acid water was very low and unstable, it often caused the pump to go into vacuum condition when the fluid returned to the pump inlet. Later, I changed the return line to the inlet of the acid water return tank located ahead.

Is there another pipeline that can be used as a bypass? Bypass regulation is also commonly used for flow control in positive displacement pumps

But it seems that the specification states this not because of the minimum flow rate; otherwise, it would have emphasized that every volumetric pump must be installed as well

This is used for flow regulation; it’s not needed for those that don’t require regulation

When starting up, the outlet valve is usually opened; a safety valve must be installed. If the bypass isn’t adjusted, I don’t think it’s very useful. In fact, specific instructions for operating the pump are required

Oh, I forgot to mention it – this connection is needed while driving; it might also be a temporary circuit added for driving purposes

For positive displacement pumps, the flow rate is constant, except when variable frequency control or stroke adjustment (in the case of reciprocating pumps) is used; therefore, a circulation line at the pump’s inlet and outlet is required to regulate the flow rate at the pump outlet. Clause 11.10 of the SH/T3122 standard specifies that a flow indicator should also be installed. In my opinion, whether a flow indicator is necessary depends on the process requirements – if strict control over the flow rate is needed, then a flow control valve should be installed on the circulation pipe； Additionally, a circulation line at the pump’s inlet and outlet is also required when starting a positive displacement pump; its main purpose is to prevent the pressure in the pump’s outlet pipe from rising immediately after the pump starts up.