Balancing Valves and Impeller Trimming

I often hear the comment, that constant flow pumping obviously does not require any variable speed drives. While this to some extend is true, there are many constant flow applications, which would still benefit from variable speed drives.

First of all many constant flow applications are trimmed using balancing valves. In many installations I have seen a trim to 80% of the pumps nominal flow. The simplest calculation on this is to use the cube law and 80%^3 = 51.2% energy usage, which means that a drive would be quickly paid for. Even taking changes in motor efficiency and added efficiency loss from the drive into account, the saving would still pay for a drive in less than a year, especially since a trim of 80% with a valve would also reduce the efficiency on the pump.

Then people say, that it is cheaper to imply trim the impeller to the load and thereby avoid the investment in the drive. Well firstly trimming an impeller is not free (although cheaper than a drive off course). Secondly there is an efficiency loss in the pump, when the impeller is trimmed, which even in cases of limited trimming could pay for the investment in the drive.

If you then take the added advantages of the modern variable speed drives, such as electronic monitoring, built-in motor protection, end of curve detection and no flow detection, then you might actually get a very good deal by adding a drive.

I am however short on actual cases where drives have been selected over impeller trimming and would greatly appreciate any comments in this forum, which could help with a more quantifiable argument against impeller trimming.

For the pump manufacturers, the advantage would also be a vast reduction in variants in the product program.