Financial justification:Capital cost savings

Capital cost savings

When designing and installing a new pump system, the capital cost of the VSD can often be off-set by eliminating control valves, by-pass lines and conventional starters, as explained in this Chapter.

Elimination of control valves

Control valves are used to adjust pump output to suit varying system requirements. Usually a constant speed pump is pumping against a control valve, which is partially closed for most of the time. Even at maximum flow conditions, a control valve is normally designed to be 10% shut, for control purposes, and hence a considerable frictional resistance is applied. Energy is therefore wasted overcoming the added frictional loss through the valve.

By installing a variable speed drive, the output of the pump can be varied to match the system requirements without throttling the pump. The losses associated with a throttling valve can therefore be eliminated together with the valve itself. In many cases, where the system losses are difficult to calculate, safety margins will lead to over-sizing of pumps. By using variable speed drives in a system, such over-sizing can be compensated for, by reducing the speed of the pump instead of throttling the flow.

The cost of the mechanical installation of the valve, the associated pipe­ work requirements and the electrical wiring, will be replaced by the cost of purchasing and installing the VSD. It is generally accepted that the instal­lation costs of a control valve can be at least twice the purchase cost of the valve; hence considerable sums can be saved if this item is not required.

If the liquid handled requires special materials, the cost of the control valve can be even more substantial.

Elimination of by-pass lines

All fixed speed centrifugal pumps have a minimum flow requirement. If the pump is operated at flow rates below the minimum for extended periods of time, various mechanical problems can occur. If the flow requirements in a system can drop below this minimum flow capacity, it is necessary to install a constant or switched bypass in order to protect the pump.

With a constantly open bypass line, excess energy is absorbed in con­ tinuously pumping the minimum flow down the bypass even though bypass flow is only needed in cases of low system flow requirements.

In a switched bypass, the bypass line opens, usually by use of a solenoid valve, when the system requirement is below minimum flow conditions. In this case, excess energy is used pumping the liquid down the bypass only when the bypass is in operation.

If a pump is controlled by a variable speed drive the need for the bypass may be eliminated by using an intelligent control system, or at least the bypass flow may be greatly reduced.

Capital cost savings are therefore possible if the bypass line is not required. In addition to the piping, savings will also be made on switched bypass systems, by eliminating the need for solenoid valves, with associated pressure or flow monitoring devices and wiring.

Elimination of starters

Fixed speed pumps always require an electrical starter. However, most variable speed drive units work without a starter and will also provide a soft start.

Therefore the capital cost of a starter and the associated wiring can also often be saved. Again it is generally accepted that the installation cost of a starter is at least twice the capital cost of the item. Hence the capital and installation cost saving of the starter can be offset against the cost of the VFD.

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