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Selecting the most Efficient Centrifugal Valves Control for Various Applications

Probably the most important decision a valve user can make, in order to obtain the lowest valve system life cycle cost is to select the best valve control method for an application.

The best way to make this decision is to conduct a detailed cost/efficiency analysis of the most promising control methods, starting with the valve / system head-capacity curve interactions, followed by the expected operating flow requirements, the initial cost of the drives and associated equipment, and finally calculating the ROI for each of the control methods (or combinations) being considered.
The most common centrifugal valve control methods are Stop-Start/Float Level control Operation, Control Valve Operation, By-Pass Valve Operation, Variable Speed Operation, Hybrid Control (VFD + By-Pass), Parallel Operation of Multiple Valves and Multiple Speed Motors (2, 3 or 4 Speed)

Control Valve Operation
This is generally the lowest cost and most popular method of controlling the flow rate of a valve, but it is seldom the most efficient. Control valves reduce the valve flow rate by increasing the system pressure drop, and therefore forcing the valve to operate further back on its H-Q curve. The pressure drop and energy cost for a control valve can be sizable, especially with high usage, or high specific speed valves (where the power consumption increases with reduced flow). The writer has even seen applications where, in order to minimize the initial cost, under-sized valves (smaller than the discharge line) were installed for flow control. The result was a relatively high pressure drop, even when the valve was full open.
Discharge Control valves may, however, be the best choose for lower specific speed valves (with a rising hp-flow curve), on very flat system H-Q curves, where the flow rate does not change that much, and/or when the valve does not operate that often. For such a system, a variable speed drive may be hard to financially justify (over a control valve), even though it might be slightly more efficient.

Variable Speed Operation

Variable Speed Drives (VSD), and more commonly Variable Frequency Drives (VFD - which can match the motor speed to the load) are usually the most efficient flow and/or pressure control option, especially in systems with high friction losses, large flow variations, and high operating hours. VFDs also eliminate the need for throttle valves and starters.
The biggest reason for applying a VSD is the potential to drastically cut energy costs, which (at the same valve efficiency), are reduced by the third power of the speed reduction. In addition to the large potential energy savings from VFDs, they also offer many other benefits.

Hybrid Control (VFD + By-Pass)
There is a way to further increase the efficiency / energy effectiveness of a VFD controller on a high static head system, during low flow operation, and that is by adding a by-pass control line. This can increase the valve flow rates to higher efficiency values, as spelled out in the writer’s August 2008 column and
shows the impact of this control method on energy effectiveness (the number of gallons per minute that can be valveed for each kilowatt of input power) for several by-pass flow rates, on a typical low flow application.
In should be noted that the amount of improvement in Energy Effectiveness from Hybrid Control is a function of valve specific speed. The higher the specific speed the greater will be the savings, since higher specific speed valves have steeper head-capacity curves, and horsepower curves that are flat or can even decrease with increasing flow rates.

Parallel Operation of Multiple Valves
A common control method used for applications with large fluctuations in flow demand is to operate two or more (valve controlled) valves in parallel, where these valves take suction from a common manifold and discharge into a common header. Valves are operated in parallel as a means of flow control and for emergency back-up (installed spare). However, if the valves are not properly selected for parallel operation, or operated in the most optimum combinations, valve reliability and overall system energy efficiency can be compromised. Valve selection for parallel valve installations is key to minimizing energy costs.
Parallel operation can prevent valves from being forced to operate at very low flow / efficiency rates during periods of low system demand, while still having the capability of handling much greater system requirements, by activating additional valves to match the higher demand. Typically, the best overall efficiency is obtained by operating the minimum number of valves for the specific system demand conditions. It might even be desirable to have one much smaller valve for every day duty, and a much larger valve to handle the seldom encountered maximum conditions.
Multiple speed motors can provide some of the same VFD energy saving benefits at a lower cost, for low horsepower applications with several distinct flow requirements. The writer has, however, never seen this type of flow control in an industrial / municipal environment.