Smart Pumping Systems: The Time Is Now (Part 1)

About the author:

Stavale is the Principal Engineer, ITT Corporation, Fluid Technology Industrial & BioPharm Group.

Anthony Edward Stavale P.E.

The pump industry has begun to incorporate computer technology to operate, control and protect pumps and their systems. Smart pumping systems can match pump output exactly to system conditions and can detect and protect against unusual operating conditions. Through the use of a smart variable speed controller, these systems can significantly reduce pump-operating costs by eliminating the use of energy consuming control valves, as well as reducing life cycle costs. All of the major components of life cycle cost, such as operation, maintenance and installation, need to be evaluated when comparing smart systems to conventional systems.

What is a smart system?
The pump industry has started to use computer technology to operate, control and protect pumps and their systems. These smart pump controllers incorporate microprocessors as part of their normal function.

A smart pumping system must be capable of knowing when to adjust the process variable in response to system changes without manual intervention. The system must also be fault tolerant, to enable the system to recognize and safeguard itself from operating under conditions that may reduce its life. Adverse conditions like dry running, operation below the minimum flow, deadheading, runout condition and cavitation must all be recognized and reacted to before damage occurs. The system must also be capable of understanding when the system transient or unusual operating condition has cleared, thereby allowing normal pump operation to resume.

A smart pumping system consists of a pump, variable speed drive or controller, instrumentation (when required), microprocessor and special software. The pump can be any standard centrifugal pump fitted with instrumentation to measure a process variable. More common applications control discharge pressure, flow or process level (either suction side or discharge side). However, there are advanced products available that can measure and control the process variable without the need for external instrumentation. The pump control software enables the controller to sense pump and process conditions and react accordingly. These systems can be designed to maintain constant values of speed, capacity, pressure, level or other process variables, and can be controlled either locally or through a distributive control system (DCS).

The value to the customer in using Smart-Pumping Systems lies in the reduced life cycle costs. The major components of life cycle cost are initial cost , operation, maintenance and installation.

System Curve. A system curve is comprised of a static component and a dynamic component. The static component of the system curve does not change with flow rate. The dynamic component is essentially proportional to the square of the rate of flow. It is also a function of other variables such as pipe configuration/size, surface roughness, quantity and type of fittings/valves and fluid viscosity. These can be represented by a single system constant and the dynamic or frictional head can be expressed as: Hf = K Q2

The dynamic head constant K is a constant for a given system. However, if a control valve position changes in the system, then the constant K will also change.

Conventional System. Figure 1 shows a typical control scheme for a conventional pumping system. Note the system curve includes a static component of 6.1 meters. This is the change in elevation between the suction and discharge source. In this type of system the pump operates at a fixed speed and the pump performance curve is based on an impeller diameter pre-selected to match the system requirements as closely as possible. It is common practice to add a safety margin to the design point when it is difficult to accurately define system losses. This can result in an oversized pump that runs out too far on the curve and absorbs too much power. The total system head curve intersects the pump head-capacity curve at point “A” with the control valve wide open. The design flow for this system is 638 m3/hr so the friction head (point “B”) must be increased to 37 meters in order to operate at this point.

The most common method of varying the capacity in a conventional system is to introduce a variable resistance device that will alter the system friction curve; this is the main function of a control valve. Control valves are throttling devices, which use some of the available pumping energy to control the process. The amount of consumed energy will vary depending on the method of control, valve sizing and the operating point. In the U.S., a common control valve standard (PIP PCECV001) specifies that the control valve shall be 50-80% open at design flow, at least 10% open at minimum flow and no more than 90% open at maximum flow. Other methods base the amount of pressure drop on past plant practice or rules of thumb.

Variable Speed System. In a variable speed system the controller will match the pump output to system head requirements without the need for throttling a control valve. Safety factors and pressure margins typically built into dynamic head systems can be eliminated and in some cases result in a lower-cost pump selection. Since the smart controller can adjust the pump speed to suit the required system conditions, only one impeller diameter needs to be stocked, thus lowering inventory costs. In variable speed systems the design point no longer needs to be based on a fixed speed. This yields a larger number of selections over a given pump range with a better chance of operating at or near the best efficiency flow. Variable speed control is most effective and efficient in all friction head systems. The effectiveness diminishes somewhat for applications having high static head and low dynamic head curves, since the intersection of the pump and system curve moves further to the left of the best efficiency flow(1).

Systems with flat head curves can result in unstable flow, making control difficult. Advanced products control the process variable by adjusting pump torque rather than speed. This provides a steeper, easier–to-control performance curve.

Figure 2 shows a variable speed system with system curve “A” identical to that shown in Figure 1. Head-Capacity curves are shown at various speeds. If the desired operating flow is 638 m3/hr it is shown that the pump can operate at a substantially lower speed and head. In this example, the savings of the variable speed system over a conventional system is represented by the difference in head between points “B” (Figure 1) and “C” (Figure 2).

Life cycle operating cost
This difference in system head requirements can often translate to thousands of dollars in energy savings over the life of a pump. Table 1 exemplifies these savings and is based on a cooling tower installation(2) represented by Figures 1 and 2. Note that the total pump head has been reduced from 37 meters to 23 meters. As a result, the pump power has dropped nearly 30 kW and operating speed has been lowered by over 300 rpm.

Table 1 shows the difference in energy costs between a conventional system and a variable speed system represents a 35.5% decrease in operating costs over the life of the pump.

References

1. Casada, D., 1999; “Energy and Reliability Considerations for Adjustable Speed Driven Pumps”, Industrial Energy Technology Conference, Houston, TX, pp. 53-62.
2. Kratowicz, R., 2000; “Less is More”, Fluid Handling Systems Magazine, pp.30-33.