Granted that the temperature of cool water is lower as is the viscosity, so obviously longer running at reduced flow is possible, and lubricity must also be greater.
Temperature is not the only concern. Low flowrates in most pumps do hurt, by causing increased bearing wear due to unbalanced hydraulic forces. Flows less than 60% of BEP are not recommended by API, although with a VSD, the pump would not produce such proportionally high unbalanced forces, since discharge pressure is reduced.
Each pump having its own control valve does not ease the situation. The pressure of the header (assumed to equal the highest discharge pressure of any connected pump) must be reached by any other pump in order for any other pump to discharge into that header. A pump producing less pressure than the header pressure must have its own discharge check valve in order to avoid being spun backwards by fluid attempting to enter from the higher pressure header. Placing a control valve betwen a pump discharging at a pressure lower than the header will still not improve that situation. You could only close it in an attempt to avoid backspin. Flow from the higher pressure header would enter the even lower pressure at the control valve's outlet and still attempt to backflow into the pump.
Additionally, when a centrifugal pump is deadheaded its discharge pressure is deadheaded at shutoff pressure, which is the same pressure, if it has a discharge control valve or not. Deadhead pressure with a control valve is not reduced and is in fact usually the maximum pressure that a centrifugal pump can produce. And, power is not being expended on the fluid, except for minor internal recirculation flows and all the rest of the power used by the pump to overcome internal fluid, bearing and stuffing box friction is being converted to heat. There is no possibility for improvement by deadheading a pump at a lower pressure or any other pressure other than its shutoff pressure,... without a VSD.
On the other hand, a VSD driven pump, especially with recirculation, could be used to lower the shutoff head corresponding to a lesser rpm, and reducing the heat load. Even a VSD driven pump without recirculation would produce less pressure and less heat. Total power consumption does drop off with lesser flow at reduced discharge head, and since head drops with the rpm^2, and flow drop is linear, a VSD w/ recirculation can improve the heat load.
True there is less power consumption at lower flows, with or without a control valve on a non VSD equipped pump, but most all power delivered is converted to heat at the lesser efficiency, so a lesser power consumption is paid for with increased resulting temperature load to the pump.
It also don't think that power consumption with a VSD drops, as you say, almost the same as if flow is restricted with a valve (on a pump without a VSD). Power consumption drops with the cube root of the pump speed on a VSD equipped pump, but power consumption on a pump without a VSD looks like that on any typical centrifugal pump curve with reducing flows, which is an inverted curve that depends mostly on the pump's hydraulic efficiency at flows away from BEP Q. Power consumption with a VSD dropping with the cube root of rpm and with only a very slight change in efficiency at different rpms and the same change in efficiency with flowrate as a non-VSD equipped pump, would appear as a cubic curve and power consumption drops very fast. I think those curves are very different, however the power consumption of a VSD equipped pump with a control valve, or a VSD equipped pump without a control valve would indeed be equivalent. The control valve has no effect on power consumption of the pump, as it mearly increases or decreases resisting head and consequently only changes the power required to flow into the system at any given control valve setting.
BigInch
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