Do you understand the savings that variable speed drives for pumping equipment can offer your plant?

Variable speed drives for pumping equipment have many advantages compared to butterfly valves when it comes to flow regulation. That is to say, they offer good flow control, great energy savings, and reduced operating costs if applied correctly in all pump systems with a high relative static head. 

However, the growing popularity of the variable speed drive has brought with it a host of incorrect applications. In these instances, the end-user receives higher energy bills instead of savings as a result of installing a variable speed drive. 

This is due to inadequate design and selection of the pumping system. Achieving energy efficiency in pumping equipment begins with the correct selection of the pump with variable speed drives. In this case, the first thing to do is ask about the process demand that is expected to be met. For example:

1. Do we need to regulate the flow continuously or in steps? 
2. Can we use batch pumping?
3. What are the necessary flow rates and how are they distributed over time?

The recommended approach is to create a duration diagram, which encapsulates the answers to these questions. In this diagram, each point on the solid curve is interpreted differently, indicating how many hours per year the flow rate exceeds the value on the axis. This diagram is fundamental for understanding your plant's pumping needs most of the time without installing variable speed drives for pumping equipment.

With this information, you can proceed with the pipework design. If, for instance, the maximum flow rate only occurs for short periods of time, it may not be practical to install a large diameter pipe. On the other hand, if you are operating at maximum flow rates for long periods of time, you must consider this fact when specifying the pipe diameter.

After the design of the pipe system is completed, the system curve must be calculated. This curve indicates the amount of head or pressure a pump needs to drive a specific flow through the pipe system. Head has two components:

• The static head indicates how far the fluid must be lifted, or, if pumping to a pressurised vessel, the static pressure differential across the pump.
• The dynamic head is the friction force and other point losses that must be overcome to move the fluid at the desired flow rate.

Now, to see where on the system curve the operation will take place and for how long, one must return to the head-flow curve, specifying the pump or pumps, plus the appropriate means for flow regulation if required. Furthermore, it is now useful to understand how a pump operates with variable speed drives for pumping equipment.

In static head systems, the system curve follows lines of constant efficiency and the pump efficiency remains essentially constant as the pump speed changes.

Whereas, in systems with static head, the system curve does not start from the origin but at a non-zero value on the axis, therefore, the system curve does not follow the constant efficiency curve, but intersects it. Which means that the pump efficiency changes when the pump speed changes. 

The operating point on the reduced speed curve moves relatively higher and higher up the pump curve as the speed is reduced. Eventually, the pump operates at shut-off head if the speed is reduced too much with variable speed drives for pumping equipment.

Difficulties that arise in the system if variable speed drives are not implemented for pumping equipment

1. The pump efficiency trends towards zero as the operating point moves towards the shut-off head.
2. The pump then operates in a flow regime in which it should not be operated. Does serious damage occur to the pump if it is operated near the shut-off head for prolonged periods? even if this takes place at a reduced speed.

It is important to understand that these difficulties always occur in a system with static head. The speed at which specific energy rapidly increases is a function of the amount of static head. In systems with low relative static load, there may still be opportunities to save energy by reducing the pump speed. 

In systems with high static head (over 50 percent of total head), there's a clear possibility that using variable speed drives for pumping equipment will increase net energy usage, compared to on/off pumping at full speed. 

In most cases, pumps are sized and selected to deliver the maximum flow rate with an upper margin. Therefore, as duration curves resemble the continuous curve, it is common for the normal flow rate to be approximately 50 per cent or less of the nominal flow.

If the system curve shows a considerable amount of static head and the pump is oversized for most pumping duties, then problems can arise. It is not uncommon in such systems for the pumping cost to increase considerably compared to on-off pumping at maximum speed, when variable speed drives are used for pumping equipment. 

The fact that the combined efficiency of a variable speed drive motor package can fall considerably as the load decreases does not improve the situation. The flow acceleration, however, is even worse! 

Our recommendation is that you consider installing variable speed drives for pumping equipment in a system with static head, as the useful range of this equipment can be extended if the pump is chosen so that the intersection of its system curve and the full-speed pump curve is to the right of the pump's best efficiency point.

This extends the area in which the variable speed drive is useful, as the pump's efficiency first increases before starting to decrease, as the speed is reduced. For a better understanding, you must recognise your plant's pumping needs and discover how Saving costs on pump energy consumption with shaft alignment.

We invite you to discover How to improve the energy efficiency of centrifugal pumps subscribing to the Technology for Industry Newsletter, with the most comprehensive content on new industrial technologies, manufacturing innovations, equipment, and automation trends.