Variable Flow and Head Systems

Published On: February 19, 2024By Categories: Engineering Your Business, Pumps and Water Systems

Part 1. Introducing the systems used for variable flow.

By Ed Butts, PE, CPI

We will kick off this month a multi-part series on variable flow and head systems, but largely dedicate it to variable speed and frequency drives.

Figure 1. Hydraulic horsepower reduction with flow and head change

Variable flow and head pumping systems, specifically adjustable speed or variable frequency drives (VFDs), are rapidly becoming popular for use in all types and sizes of water systems requiring variable delivery rates of flow and head (pressure). This is due in no small measure to the recent advances in electronic circuitry, flexibility, and their general adherence to the affinity laws for pumps.

Unfortunately, many in the water well industry now view VFDs as a panacea that can fix or solve any inherent problem associated with a pumping system—but this often steadfast belief couldn’t be further from reality. In some cases, VFDs are now automatically considered as the go-to device for providing flow and pressure modulation without ever considering the many other available options.

This month, we will begin to explore the various systems that are currently used for variable flow and head pumping applications in the first column as part of a six-part series.

Variable Flow and Head

Most pumping systems nearly always require some type of variation in the delivered flow rate and head, often in the form of pressure. Common examples for needed flow changes include the daily demand cycle in the consumption and use of drinking water; the varying demand for process water in industrial settings; variable irrigation demands; or seasonal heating or cooling demands for boilers, cooling towers, or water source heat pumps.

However, the variation required may also be needed in pump head such as cyclical changes in system pressure or head when pumping to closed-loop water systems or reservoirs, or from dewatering wells with a variable or undulating water level or cone of depression.

Fortunately, there are several different methods available to match the delivery flow or pressure to the precise system requirements. Common flow control methods for pumps use flow or pressure types of inline throttling or pressure-regulating control valves. These bypass or vent fluid to release excessive capacity or pressure from the system, conventional on-off or cycling pressure control, and variable or adjustable speed drive control methods.

The relative power consumption of the different control methods can be fundamentally estimated from the area between the x- and y-axes within the corresponding operating points on a pump curve, determined by using the basic relationship of:

P = Q × H

where:

P = Input power

Q = Flow rate

H = Total head.

Based on the affinity laws, the corresponding ranges of potential reduction of hydraulic horsepower with associated reductions in flow rate and head are shown in the graphic in Figure 1.

As an example, displaying a theoretical system head and pump curve in Figure 2 with an original matching base flow rate and head non-dimensional condition of 10, a power output of 100 (10[Q] × 10[H] = 100), and a desired corrected flow rate ratio of 7, or 70% of the original flow rate, the use of inline throttling, while meeting the desired corrected flow percentage of 70%, will generate a corrected head ratio of 12.7,
translating to a power correction of 89% of the base conditions.

The next choice, bypassing or a diversion of excess flow to an offline destination, while decreasing the head ratio to 6.6, increases the flow ratio to 12.4, resulting in a power correction to just 82% of the base conditions.

Next, on-off pump control or cycling, while not actually reducing the output or head, results in a power reduction to 70% due to the application of intermittent operation of the pump and driver.

Figure 2: Typical methods of variable flow and head control.

Variable Speed

Variable speed control results in both a proportional flow rate and head reduction with a commensurate power reduction to 45% of the base conditions, the lowest reduction of all displayed options. This basic discrepancy between the four methods underscores why variable speed drives are fundamentally preferable for power savings if not efficiency.

However, variable speed drives are not always the solution to a pump design or application mismatch. In some cases where the duty requirements don’t vary, resizing the pump, changing the impellers’ diameter, or even replacing the pump with a correctly sized unit may result in greater lifetime cost savings at a lower initial cost than using a variable speed drive.

For purposes of this column, variable frequency drives, inverters, eddy, magnetic, or hydraulic coupling drives are all referred to simply as variable or adjustable speed drives. Whatever they may be called, the terms are sure to cause apprehension in the minds of many of those unfamiliar with the design, application, and operation of these devices. Not to fear, this series of columns will hopefully dispel some of the myths and concerns associated with their application and make you feel more comfortable in the use of these devices.

Actually, the term variable speed drive encompasses just four basic methods of speed regulation for typical water pumping applications, each with their own individual advantages and disadvantages. These are:

  • Electrical AC variable speed drives
  • Electrical DC variable speed drives
  • Mechanical variable speed drives
  • Hydraulic coupling variable speed drives.

It should be noted that reduced voltage starting methods such as solid-state soft, wye-delta, or autotransformer starters are not included in these columns.

Potentially saving pumping energy by using variable speed or flow devices on a pump relies on basic physical and hydraulic relationships, as the pump speed ultimately refers to the rotational speed of the pump shaft, even though the actual reduction in speed may initially occur at the driver or between the driver and pump. The pump shaft is connected to a single impeller or stack of impellers that adds an appropriate amount of energy to the water. Slowing the rotation or restricting the flow of the pump reduces the energy that is transferred to the water, and thereby the input power requirement of the pump and in turn the driver.

It is common knowledge that use of variable speed drives can significantly increase pump life as generally a slower speed pump will outlive a faster rotating pump with otherwise identical installation and construction features.

Typically, the wear rate is proportional to somewhere between the square and cube of the speed ratio. As a result, a pump rotating twice as fast will typically wear at four to eight times the rate of the slower unit. This can be readily observed between a 3600 RPM and 1800 RPM unit.

Figure 3. Variable speed device (VSD) vs. control valve.

Generally, a review of the response of a basic pumping system suggests that pump speed control is much more of an energy-efficient approach to controlling flow rate than inline flow throttling or offline bypass methods.

In a closed piping loop example, such as a pressure boosting pump, with adherence to the affinity laws, flow varies directly with pump speed, head with the square of the pump speed, and the horsepower requirement with the cube of the pump speed. In this case, the relationships are generally based upon a situation in which the pump head is composed entirely of frictional and delivery head, as fixed elevational or static head is not a factor.

Pump and driver speed can be controlled and varied using many methods, including:

  • Mechanical (fixed speed driveline or pump directly connected to a variable-speed internal-combustion engine)
  • Hydraulic (fixed speed motor through a variable-speed hydraulic drive coupling to directly drive a pump)
  • Variable-speed pulley arrangements (automatic or manual; atypical for most potable water applications)
  • Modifiable gearboxes or drive (constant-speed input with variable-speed output; also not commonly used)
  • Right-angle geardrive (variable-speed engine connected to a fixed ratio geardrive; common use of an engine)
  • Magnetic couplings (constant-speed input from the driver with variable-speed output to the pump)
  • Eddy-current couplings (constant-speed input from the motor with variable-speed output to the pump)
  • Wound rotor motors with liquid rheostat drives (variable-speed motor directly drives the pump)
  • Electrical (variable-speed induction motor using a variable frequency drive that drives the pump).

Variable Flow and Pressure

Figure 4a. Pressure reducing valve schematic and materials.

In addition to variable speed devices (VSDs), there are also many types of variable flow, inline, pressure-regulating devices that can lower the output flow and pressure, which in specific cases can also decrease the energy input to the motor or driver, provided the flow drops proportionally lower than the resultant increase in delivered pump head.

Since this specific column is intended to entirely cover all energy-reducing devices, its title is “Variable Flow and Head Systems” rather than “Variable Speed Pumping” to reflect the possible use of control-regulating valves and other devices as an element of this goal.

The primary difference between a control valve and a VSD is illustrated in Figure 3. This full speed pump and system head (load) curve shown on the left is reflective of a pump capable of an original design point of 200 GPM at 130 feet of head. However, the required service condition is 100 GPM at 50 feet of head.

As seen on the right curve, the VSD lowers the speed, which also lowers the output capacity and head, and therefore input horsepower, while the control valve lowers the capacity but with a resultant increase in head to 175 feet.

Assuming an efficiency of 75% at 200 GPM (8.75 BHP) and 60% at 100 GPM results in a power reduction of only 1.38 brake horsepower (BHP), or just 15.8% for the control valve, whereas the VSD application will result in a drop of almost 6.7 BHP or around 75%.

Figure 4b. Pressure relief valve schematic and materials.

In actuality, the power reduction will likely be greater as the pump’s efficiency will generally follow the speed reduction proportionally. Therefore, it is apparent the application must be carefully weighed against the pump curve characteristics before deciding on the method of flow control.

Each method of flow restriction and control has distinct advantages and disadvantages based on various elements and applications. The information in Table 1 outlines and summarizes the chief factors with each of the methods outlined.

Pressure Control and Regulation

While neither of the following two systems can be classified as variable speed controls, the common and viable use of these methods warrants discussion. Both control valve methods of providing constant pressure control use hydraulically (typically water) controlled valves consisting of a rubber diaphragm or flexible rubber sleeve control surface controlled by a pilot-operated control system.

The inline throttling method consists of a pressure reducing valve (Figure 4a) to regulate the downstream or delivery pressure. The bypass arrangement simply uses an offline pressure relief valve (Figure 4b) to bypass water back to the suction supply or vent to atmospheric discharge.

A typical deep well vertical turbine pump configuration using a pump control valve for pump activation and deactivation with an inline pressure reducing valve and offline pressure sustaining or relief valve for pressure and flow control and modulation is illustrated in Figure 5.

Figure 5. Typical deep well pump installation.

While simple in operation and generally reliable for pressure control, both methods do not usually save much if any in net energy costs on deep well applications and are prone to constant adjustment and system pressure fluctuations due to repeated valve cycling, and in many cases, frequent maintenance and internal component rebuild from rapid internal velocities that occur over the seat when the valve is barely open.

The inline pressure reducing valve method of constant pressure regulation also has several other inherent restrictions, as the device does not automatically provide inherent energy savings unless a pump with a reasonably flat curve and wide efficiency window is selected. Most well pumps operate with a steep curve and do not fall under this characteristic, and therefore electrical costs are not necessarily or automatically
reduced using these devices.

This method of pressure control must also have some method of controlling flow under extremely low-flow conditions as this situation may cause valve hunting or rapid surging due to water flowing over the valve seat at a nearly closed condition. This may require the use of a smaller bypass valve for the lower flows or a special trim on the seat and disc, such as stainless steel, to resist damage from this high velocity.

However, the inline method of pressure regulation has been in considerable use for several decades, and when properly applied, is an effective and low-cost method of providing stable pressure control.

Control Valves

Although there are as many types of control valves as there are applications, for the purposes of this discussion we will limit it to pilot-controlled, hydraulically operated, diaphragm type, pressure/flow control or modulating valves. This is not intended to slight or ignore other types of valves such as actuator- controlled butterfly, sleeve, or ball valves used for fluid or process control but rather to compare the type of control valve most often compared to the function of variable frequency or speed drives.

Control valves usually represent a viable operational alternative to VFDs in virtually all pump applications where pressure or flow control is the primary or only desired function. However, they have definite limitations as well.

Flow-regulating or throttling valves operate on the principle of progressive closure (throttling) to restrict and thereby limit the output flow or head by applying additional operating head against the pump. Referring again to the sample pump/system head curve shown in Figure 2, this valve action will lower the output flow rate from a fully open or unthrottled valve position by backing the pump on its curve to the left towards or approaching shutoff conditions.

However, as seen in the same graphic, although the flow rate has been reduced, the generated pump discharge head is now much higher than needed, a primary disadvantage with using this method. Although both capacity and head possess equal numerical value in determining brake horsepower, the percentile and dynamic relationship of capacity to head is generally much higher. Therefore, reducing the output flow rate will normally demonstrate a more profound impact on the pump’s horsepower than head.

Control valves when properly applied are effective in providing pressure regulation as well as control of the hydraulic surges associated with the starting and stopping of pumps. Numerous options are available that can control the opening or closing speed of the valve as well as electrically operate the valve in many cases.

When used with a pump with a relatively flat curve, control valves can even act as an energy-conserving device, although that is generally not their primary design function. In many cases, the decision whether to use a VFD or a control valve for a specific project is based on initial cost, specific application, pump curve characteristics, and designer preference.

These decisions must be made on a case-by-case basis, but there are a few guidelines and recommendations as to the proper application and pitfalls associated with using pressure-regulating control valves.

Control valves are typically less expensive to purchase, install, and maintain than VFDs, but the associated piping changes and options needed to perform surge control and slow opening or closing speeds greatly complicates the valve and can raise the cost to rival a VFD, especially in smaller sizes.

A debate has been circulating in the industry for some time in which many proponents of control valves believe the simple restriction of flow alone will automatically lead to the same, if not more, energy conservation and savings than a variable frequency or speed drive.

In reality, the truth is much more complicated. Any standard induction motor depends on part of the motor current just to create the magnetizing field necessary to cause rotation of the motor. This is simplistically called the power factor. In most three-phase motors, this value can be as much as 20% to 25% of the motor’s full load current. Because this current is required regardless of load, throttling a pump alone cannot simply counteract this inherent and required electrical demand.

In addition, it must be remembered that the input power (what you pay for) is a function or product of the pump’s output flow rate and head plus the pump and motor’s efficiency. This means lowering the pump’s flow rate alone will not necessarily lower the input power if the head rises appreciably or by the same approximate value as the flow diminishes.

Also, restricting a pump or motor’s output can have a detrimental impact on its efficiency, further increasing the input power demand.

For example, a typical 6-inch-diameter submersible pump has the capability of producing 350 GPM at 250 feet TDH at 72% efficiency, which equates to 30.688 brake horsepower (BHP). This same unit, at a reduced flow rate of 100 GPM, or 35% of the design flow rate, will now generate 400 feet of head, but the pump’s efficiency has now fallen to 39%.

The resulting brake horsepower of this unit becomes 25.90 BHP, a difference of only 4.788 BHP or just 15.6% less than the rated condition at 350 GPM, even though the flow rate has dropped by 71.4%. This coincides with the graph shown in Figure 1, which illustrates a hydraulic horsepower reduction range of just 0-30% under these same conditions.

Fundamentally, if 100 GPM at 250 feet of head is the actual required service condition, this duty should be able to be performed with only 10 to 11 BHP with a different selection, far less than the 25.9 BHP required by the example pump in a throttled condition.

This is just one example, but a steep curve is a common characteristic of many multistage submersible and vertical turbine pumps, which demonstrates why sound judgment must be employed when applying a control valve to any pump with a steep or moderately rising curve.

To be fair, this performance example does not automatically apply to all flat curve pumps. Many pump curves that exhibit a flat or baseline performance curve, including many single stage centrifugal pumps, fall off on head much more rapidly than capacity, creating a Q-H product that is often 20%-25% of the original COS value, so there is often real potential for energy savings in these cases.

In addition, pump efficiency, as a component of brake horsepower, is also critical when attempting to use a control valve for energy conservation. As a rule of thumb, the more you can retain a majority of the pump’s COS efficiency as the pump curve moves to the left towards shutoff head, the lower the resulting BHP, and therefore the greater the energy savings.

In summary, always remember that the combination of pump brake horsepower along with motor efficiency, not simply amperage and voltage, is a function of a pump’s flow rate, the total dynamic head, and the pump and motor’s combined efficiency.

Lowering just one of these elements will not necessarily lower the horsepower by a commensurate amount if the other elements increase (or decrease in the case of efficiency) at the same or comparable rate. Applying a relatively flat type of pump curve will usually provide the greatest degree of energy savings when used with a control valve.

In addition, the pump efficiency is often the wild card in these applications and must also be considered and factored at the lower operating flow rate if a control valve is to be used primarily in an energy conservation role.

______________________________________________

This concludes this first installment of our series. We will continue next month with an introduction to variable speed drive pumping systems.

Until then, work safe and smart.

Learn How to Engineer Success for Your Business
 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. Click here for more information.

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.

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