Is this design approach of using only variable-frequency pumps to control flow without regulating valves now mature?

I don’t know; anyway, it seems that the pumps in our company don’t use this technology

This can be done for those with low traffic control accuracy

Precise control of flow isn’t possible. Constant-pressure variable-frequency control for transportation is feasible. Design a small system – a constant-pressure variable-frequency control system; this type is suitable for continuous transportation, as intermittent operation is not appropriate.

This post was last edited by HEJIYUER on 2021-10-9 at 22:54. VFD variable frequency drive technology helps save energy by: 1. The required flow load decreases, and the flow rate is reduced by varying the speed. 2. With a reduced flow rate, the pressure loss in the pipeline decreases; once again, the pump’s speed is adjusted to lower the outlet pressure, thereby compensating for that pressure loss. If the pipeline pressure drop is not high, such as 60 KPA, whether a control valve or a VFD is used, the adjustable flow range is limited; flow control is ultimately achieved through the hydraulic system (via changes in pressure or pressure drop), rather than through the speed of the motor. Taking a centrifugal pump with a required flow rate variation of 3:1 as an example: 1. FV changes the flow area S; S can cause a change in the pipeline pressure drop that is proportional to the square of this value. It can be said that the control over S is effective, and the control valve should be capable of achieving a 3:1 flow rate control for the centrifugal pump. 2. For VSD, the flow rate ratio equals the speed ratio, and the head ratio equals the square of the speed ratio; however, what we actually control is the speed. If a 10% reduction in flow rate is desired, then the rotation speed also needs to be reduced by 10% ； At the same time, if the head needs to be reduced by 20%, the rotational speed also needs to be decreased by 10%. Overall, the range over which flow rate can be adjusted by changing the rotational speed is smaller than that in the FV case. The relationship between speed and flow is similar to that of valves with a quick-opening characteristic; especially in the low-flow range, changes in speed result in significant changes in flow, which makes stable control difficult, even when using closed-loop control with a FIC-VFD. Summary: Using VSD can help save energy; to determine whether it also saves money, it is necessary to compare the investment costs with the recovery costs. More importantly, it is essential to examine the pump performance curves at different speeds, in order to assess whether this system that replaces the control valve can maintain stable operation within the flow rate variations required by the process. Otherwise, the benefits will not outweigh the costs. Many years ago, variable-frequency energy saving was a hot topic, but it ultimately did not become a trend. I believe that for any new technology, it is necessary to consider its application status and scope – it is actual use that matters most. Attached are reference documents comparing VFDs and control valves; apologies for the electronic version in format E.

There is no issue with control itself; the main considerations are the type of pump and whether it operates continuously.

This post was last edited by myface on 2021-10-11 09:39. It mainly depends on the design of the pump; specialized metering pumps are designed for exactly this purpose. As long as the metering pump meets your accuracy requirements, it is suitable; otherwise, use a regular pump along with a control valve. It’s not possible to use both together, and using them together makes control difficult.

A metering pump is a positive-displacement pump; there is no FV in the main outlet line, but a PV can be installed in the bypass line. For precise flow control, the FIC-VFD control method can also be employed, such as for catalyst injection of a few kilograms per hour. The pipeline curves for centrifugal/volumetric pumps are different: the centrifugal flow rate is reduced by half, while the outlet pressure increases by approximately 5-10%; the volumetric flow rate is also reduced by half, and if no bypass is used, the pressure rises sharply.

The main difference between control using a control valve and frequency conversion control for the flow rate in the centrifugal pump’s outlet pipeline lies in the characteristics of the relationship between pump outlet pressure and flow rate. When a control valve is used, the pump outlet pressure remains above the rated value at low flow rates, so there is no need to worry about the material not being delivered ； With variable frequency control, low flow rates correspond to low rotational speeds and relatively low pump outlet pressures. If the simulation analysis of the pump’s operation under variable frequency conditions is not taken into account during selection, it may result in the pump outlet pressure being well below the rated value at low flow rates, thereby preventing the material from being delivered. Therefore, the range of variable frequency regulation is relatively limited; it is constrained not only by the accuracy of flow control but also, to a greater extent, by the pressure at the pump outlet.

Variable frequency only changes the pump power; as pressure changes upstream and downstream, the flow rate will change accordingly. Not advisable!