Finding the right frequency
Hidrostal's Derek Jackson reveals the advantages and disadvantages of using variable frequency drives in sewage pumping stations
The recent publication by Europump and Hydraulic Institute Variable Speed Pumping - A guide to successful applications is a comprehensive document and should be essential reading for all engineers involved in the design and operation of pumping equipment. Correctly applied variable frequency drives (VFDs) offer the potential for significant energy savings as well as optimising the many processes into which pumping equipment is installed, all of which amply illustrated in the Europump guide.
In the specific case of wastewater, unscreened raw sewage and sludge, which can be expected to contain rags and fibrous material as well as grit, sand and other gross solids, VFDs can be the cause of pump blockages, leading to substantial unscheduled maintenance costs and temporary loss of pumping capability.
Variable speed drives (VSDs) are often used to control roto-dynamic pumps at terminal and inlet sewage pumping stations where incoming flow must be delivered continuously to the treatment works, rather than intermittently by fixed speed pumps with level controls.
Avoidance of loss of pumping capability due to blockages is essential on these critical pump stations. However, problems can occur when speeds are reduced to achieve low minimum flows. When a pump operates too far left of its best efficiency point (BEP), fluid recirculation within the impeller, volute and suction can cause rags and fibrous material to accumulate, blocking the impeller. Take a typical example - a pump selected for a maximum flow of 260 l/s at 25m total differential head but with a minimum flow requirement of 50 l/s. A VFD is used to adjust the pump speed to vary the flow. Whether or not this is a practical solution depends on where the system resistance curve intersects the pump performance curve at the minimum speed.
How far to the left of the BEP the pump is required to operate as the speed is reduced is the essential consideration. Figure 1 shows the pump performance curve with four system curves super-imposed upon it. Each is based on the duty point of 260 l/s at 25m, but each has a different combination of static head and frictional loss.
Table 1 summarises data from Figure 1, for the minimum flowrate of 50 l/s. This shows that when the static head accounts for a large part of the total head, the pump is required to operate too far to the left of its BEP as its speed is reduced. Consequences are:
In this example, only on system curve D is it acceptable to turn the flow down to 50 l/s (56% BEP). For raw sewage pumps a sensible practical lower limit is 50% of BEP bearing in mind the pump should only run at the lower limit for a small percentage of the time. If it is known that low flows will predominate an alternative solution should be found in a smaller pump so the pump runs close to BEP.
When considering operation at minimum flows it is important not to forget the resultant velocity in the discharge main. It is generally accepted for sewage applications rising main velocities of 0.75m/s are required to prevent solid settlement. Operating at too low a flow can also cause pipeline and pump blockages. Costs caused by blocked pumps can exceed the combined energy and capital costs of the pumps. Reducing the risk of blockages should therefore have a high priority when selecting pumps for raw sewage pumping stations.
While this article draws attention to the increased risk of blockages occurring when a roto-dynamic pump is run too far to the left of BEP it is important to point out reducing speeds even when running in the region of BEP can increase the risk of blockages.
Reducing the speed to obtain a low output results in low motor torque and insufficient fluid energy to sweep the offending material through the pump. While there are no hard and fast rules on this matter, advice should be sought from the pump supplier, who will have the necessary experience to evaluate each situation based on the facts available. The use of VFDs and the ease at which pump speed and performance can be adjusted can lead to engineers failing to establish accurate system head curves. Such failure can result in incorrect pump selections, which can cause additional operating costs to be embedded in the project. These will remain with the user throughout the life of the plant.
Pumps do not have a minimum speed for acceptable operation. The minimum speed in each case is determined by the relationship between the duty point and BEP - and this is governed by the system curve characteristics. On systems where the range of flows fall outside of the permitted pump operating envelope, which occurs more frequently on high-static systems, it is important appropriate solutions are found if high operational costs are to be eliminated. Solutions include:
The prospects of avoiding blockage problems even at BEP are further improved if the impeller free passage progressively increases with pump discharge diameter and more importantly uses a single spiral vane impeller with a sickle shaped inlet edge which is automatically swept clean of any rags and fibre tending to accumulate by the incoming flow. The Hidrostal screw centrifugal impeller was invented and developed to provide highly efficient and blockage free pumping of unscreened raw sewage and sludge