Achieving flow control of centrifugal pumps

Centrifugal pumps are often operated over a wide range of conditions. For example, many cooling systems experience variable loads caused by changes in ambient conditions, occupancy, and production demands. Boiler feed pumps have to adapt water flow to the changing demand of steam for power generation. There are five methods for controlling flow through a system or its branches: • Bypass lines • Throttle valves • Multiple pump arrangements • Impeller trimming • Pump speed adjustments

This article contains excerpts from the paper, "Superimposing planetary gears as variable speed drives for rotating equipment" by Martin Tilscher of Voith Turbo, Bernd Lauter of Voith Digital Solutions and Jochen Lindenmaier of Voith Digital Solutions at the 2017 Turbomachinery Symposium.

The appropriate flow control method depends on the system size and layout, fluid properties, the shape of the pump power curve, the system load, and the system’s sensitivity to flow rate changes. Bypass lines allow fluid to flow around a system component. They provide accurate flow control while avoiding the danger of “deadheading” a pump. Deadheading is the condition in which a pump’s flow is completely choked off by closed downstream valves.

A major drawback of bypass valves is their detrimental impact on system efficiency. The power used to pump the bypassed fluid is wasted. In static-head-dominated systems, however, bypass valves could be more efficient than throttle valves or systems with variable speed drives (VSDs).

A throttle valve chokes fluid flow so that less fluid can move through the valve, creating a pressure drop across it. Throttle valves are usually more efficient than bypass valves, because as they are shut, upstream pressure is maintained and can help push fluid through parallel branches of the system. Throttle valves provide flow control in two ways: by increasing the upstream backpressure, which reduces pump flow, and by directly dissipating fluid energy. By increasing the backpressure on a pump, throttle valves make a pumping system less efficient. In low-static-head systems, variable speed operation allows the pump to run near its best efficiency point (BEP) for a given head or flow as shown.

Multiple pump arrangements typically consist of pumps placed in parallel in one of two basic configurations: a large pump/small pump configuration or a series of identical pumps placed in parallel. In the large pump/small pump case, the small pump, often called the “pony pump,” operates during normal conditions. The large pump is used during periods of high demand. Because the pony pump is sized for normal system operation, this configuration operates more efficiently than a system would that relies on the large pump to handle loads far below its optimum capacity. Some of the advantages of multiple pump arrangements are flexibility, redundancy, and the ability to meet changing flow needs efficiently in systems with high static head components. In systems with high-friction components, alternatives such as adjustable speed motors tend to be more efficient solutions to variable demand requirements.

With a series of identical pumps placed in parallel, the number of operating pumps can be changed according to system demands. Because the pumps are the same size they can operate together, serving the same discharge header. If the pumps were different sizes, the larger pumps would tend to dominate the smaller pumps and could cause them to be inefficient. If the proper pumps are selected, each pump can operate closer to its highest efficiency point. An added flow control benefit of parallel pumps is that a system curve remains the same whether one or several pumps are operating; what changes is the operating point along this system curve.

Multiple pumps in parallel are well suited for systems with high static head. Another advantage is system redundancy; one pump can fail or be taken off line for maintenance while the other pumps support system operation. When identical parallel pumps are used, the pump curves should remain matched; therefore, operating hours should be the same for each pump, and reconditioning should be done at the same time for all of them.

When a smaller impeller is not available or the performance of the next size is insufficient, impeller trimming can be an alternative. Impeller trimming reduces the impeller diameter—and thus the impeller tip speed—so that the same constant-speed pump motor can be used. Since the head generated by a pump is a function of its tip speed, impeller trimming shifts the entire performance curve of the pump downward and to the left. Impeller trimming can be a useful correction to pumps that, through overly conservative design practices or changes in system loads, are oversized for their application. Impeller trimming reduces tip speed, which in turn directly reduces the amount of energy imparted to the system fluid and lowers both the flow and pressure generated by the pump.

Pump speed adjustments are the most efficient means of controlling pump flow. Reducing the pump speed means less energy is imparted to the fluid and less energy needs to be throttled or by-passed. Variable speed drives (VSDs) allow pump speed adjustments to be made over a continuous range, avoiding the need to jump from speed to speed. VSDs control pump speeds using several different types of mechanical, hydrodynamical and electrical systems. Mechanical VSDs include adjustable belts and pulleys, hydraulic systems include fluid couplings, geared fluid couplings and hydrodynamically controlled superimposing planetary gears (HSPG) and electrical VSDs include eddy current clutches, wound-rotor motor controllers and VFDs. VFDs adjust the electrical frequency of the power supplied to a motor to change the motor’s rotational speed. The VFDs themselves do also introduce efficiency losses.

If normal operation is far below the full load rating of the motor for long operating periods, the cost of these losses can be considerable. A VFD can also introduce harmonics in the motor windings, which increases the winding temperature. Over an extended period of time, this increase in the motor winding temperature accelerates the breakdown of insulation. Hydraulic couplings with or without gears are used to transmit power through a fluid. Speed is regulated by changing the oil level in the coupling using a scoop tube. A more efficient way is the use of a superimposing planetary gear (SPG) with a torque converter generating control power. Only a small part of input power is taken out, controlled hydrodynamically and fed back to the main line. This principle has been developed and successfully established in markets, where reliability and long life time under harsh ambient conditions are crucial.