by Brian James P.E., Business Development Manager and Roger Weldon, Service Engineer, Grundfos Pumps Corporation
Packaged pump systems are gaining in popularity because they include benefits such as space-savings design, simplified installation, single-source responsibility and advanced control options with the ability to communicate with other systems.
The term “packaged” means that all the components needed for the pump system are pre-engineered and mounted on a skid or base, making installation simple. In many cases, the installation consists of merely connecting the system up to the suction and discharge piping and providing power to the packaged pump system.
Most manufacturers test the complete package pump system before it leaves the factory to ensure it is ready for installation. These tests include a hydrostatic test to check for possible leaks in the system and a performance or functionality test to determine if the system will operate correctly when installed. This will minimize the common problems associated with built-up pump(s) installations that can not be tested as a complete system until time of commissioning.
VARIABLE FLOW REQUIREMENTS
Domestic water supply systems typically have variable flow demands and can save energy with VFD controlled pump systems. To address these needs, most systems use multiple pumps to meet maximum usage conditions, and use fewer, or even a single pump, to meet minimum usage rates. There is an opportunity for managers of existing pump installations to discover the “true” flow demand and energy consumption of their application by performing a comprehensive pump audit. This information is extremely beneficial because it will determine actual required flow profile and current energy usage during normal operation. Wouldn’t it be nice to have a pump or a pump system that is tailored to a specific application? Acquiring a flow profile for an existing pumping installation provides the precise data required to design/select the optimum pumping system for the application.
New installations typically use plumbing fixture counts and Hunter’s Curves for sizing the pump system as recommended or required by inspectors or local codes. Sizing based on this method can lead to oversized pumps or pump systems. Additionally, this method does not address the variable flow requirement for the domestic water supply.
Determining a flow profile, whether it is an estimate or actual empirical data, will greatly enhance the prospect of designing a pump system that will be the most efficient, resulting in the least cost of ownership.
Figure 1 is a typical domestic water consumption flow profile.
The flow profile in Figure 1 shows the demand increases in the morning and again in the afternoon to the maximum flow of 190 gallons (719 liters) per minute. Then the flow demand gradually drops off until it stops in the night hours. The peak flow is required for only about three hours of the day. The majority of the time the flow is less then 50% of peak flow. A pump system with two pumps would better match the flow profile compared to a single 100% pump. Two 50% pumps with VFD control ensure that the pump system operates near each pump’s peak efficiency the majority of the time compared to a single pump. One 100% pump would run near peak efficiency for less than half of the time. This example shows relatively small flow requirements, but the same variable flow requirements hold true for larger applications, where even more energy can be saved.
The Affinity Laws state how the performance of a pump changes with changing pump speed (RPM).
The brake horsepower reduces by a factor of the speed cubed, with speed reduction in the pump. That means small reductions in pump speed can have large reductions in brake horsepower.
Consider a constant speed pump and a PRV to maintain the required pressure (116 ft). The pump builds 167 ft of pressure at the required flow rate where only 116 ft is needed and consumes 5.7 BHP. The extra pressure is reduced by the PRV. This leftover energy is not being used, and is bled off as surplus pressure. The PRV wastes energy and requires periodic maintenance that can be eliminated with the use of VFD-controlled pumps. The difference in brake horsepower between the fixed speed pump and VFD-controlled pump is: 5.71 – 3.77 HP = 1.94 hp
If the reduced operating point was required for 50% of the time and operating 7-days a week; using the cost of energy of $0.10 KWh results in a savings of $750 a year based on an overall motor/VFD efficiency of 85%. This illustrates how a pump with relatively low speed reduction (15% speed reduction) can save significant energy.
The energy savings through use of VFD-controlled pumps is one benefit for system engineers; other considerations are the ability to eliminate PRVs and their inherent energy losses and maintenance costs. VFD-controlled pumps and pump systems allow for precise control of the discharge pressure and have other advantages including:
- Flexibility to electronically “trim” pump(s) when oversized;
- Soft starting of motors so frequent motor starting (motor cycling) is not an issue;
- Smooth and controlled performance ramp up and down; and
- Stop pumps at zero or low flow. Best way to save money when no flow is to turn the pump(s) off.
CHOOSING THE RIGHT PUMP(S)
Pump selection for variable flow applications is a little different than choosing a pump for a single duty point. Variable flow applications are just as the name describes – variable flow. Selecting a pump for a single duty point is more defined, in that the pump selected should meet the duty point at, or very near, the best efficiency point (BEP). Pumps are “happiest” when they run near their BEP. They are the most efficient and have their highest life expectancy when they can run near their BEP. But what about variable flow applications? There is no single duty point for the pump(s), but rather, a range of duty points. Selecting pump(s) for variable flow service is more of an art, in that some considerations must be taken into account:
- NPSH available
- Possibility of the pump system to be oversized or undersized
- What type of pump – single impeller or multiple impeller
When selecting a pump for variable flow service, the duty point used to select the pump value should be to the right of the BEP. The BEP is typically near the center of the pump curve with efficiency dropping as you look to the left and right of the BEP along the pump curve. Selecting a pump to the right of the BEP will allow the pump(s) to operate longer in the best efficiency range with the variable flow demand.
Selecting a pump to the right of the BEP is generally in an area of the pump curve where the pump has an increased NPSH requirement. For this reason not every pump selection for variable flow should be selected to the right of the BEP.
The type of pump is another consideration. Single impeller pumps typically have a flatter curve compared to multiple impeller pumps, and single impeller pumps are less forgiving in situations where conditions change and the pumps are undersized. An oversized pump with VFD control can be electronically “trimmed,” where the maximum speed is reduced to eliminate the possibility of the pump running off its curve. The steeper curve of a multiple impeller pump will allow for a greater opportunity for speed reduction at lower flows and help increase the pump(s) flexibility to meet a changing design pressure requirement. Multistage pumps are more service-friendly than single impeller, end suction pumps. A multistage pump paired together with VFD control, is an ideal choice for variable flow pressure boosting.
PROPORTIONAL PRESSURE CONTROL
Municipal domestic water pump systems tend to have a substantial amount of head requirement dedicated to pipe friction loss. Combine VFD pump control and proportional pressure control (pipe friction loss compensation) and large savings can be realized. Proportional pressure control provides better efficiency throughout the flow range, and can help reduce problems associated with high pressure, which can happen when flow rates are low relative to the design conditions.
Pump systems are typically oversized from the start, and when safety margins are included, most pump systems are larger than what is required. Municipal pump systems with large pipe networks are often purposely oversized to be prepared for future growth. The head requirement for the pump system is calculated by the future high flow design conditions, (i.e., future flow capacity and head at the furthest user or highest pressure drop scenario). Then a pipe friction loss calculation is made using the future growth requirement and the pipe friction head is added to the design condition. This conventional process can result in a large pump system using more energy than necessary.
Let’s look at a four-pump system with a maximum design condition of 2,200 gpm at 115 psi boost or 266 ft. If the design pipe friction loss of the system is 20% of the total required boost pressure (266 ft), using proportional control, the pressure boost at 400 gpm is 212 ft.
Proportional pressure control can decrease the pressure at lower flows and increase it at higher flows where it is needed to compensate for pipe friction loss as shown above.
If the pump system had the lower flow rate duty of 400 gpm for 50% of the time and operated 7-days a week; using the cost of energy of $0.10 KWh, it results in a savings of $2,592 a year based on an overall motor/VFD efficiency of 88%. In addition to energy savings, proportional pressure reduces leaks and saves water at low flow periods by reducing the system pressure.
VFD-controlled pump systems offer smooth pressure control with energy savings and the ability to match the flow profile to multiple pumps without waste. Systems with multiple pumps deliver variable flow requirements at an increased efficiency over the entire flow range. This is why it is important to understand the specific flow consumption profile of the installation. For example, if the flow rate is constant, then a single-service duty pump operating at its BEP can be used, and VFD-controlled pumps would not be the most efficient control, but may be desirable for other reasons. VFD-controlled pumps and pump systems may not be the best for all applications, but for variable flow applications such as domestic water pressure boosting, they really do have the ability to save energy and decrease maintenance costs.
CASE IN POINT
The city of Cottonwood, Ariz., nearly doubled its population between 1990 and 2010, which put significant strain on the community’s aging water delivery system.
Residents were faced with frequent water outages that would last a day or two, as well as inconsistent pressure and continual water hammer noises in their homes.
The majority of pumps in the city’s existing distribution system were constant speed, across-the-line pumps that were either off, or running at top speed. The resulting pressure surges subjected the pipes to a 25 psi pressure swing, which stressed and prematurely aged the lines (some dating back to 1930), causing significant leakage and capital repair costs.
The city replaced the old, inefficient pumps with the Grundfos Hydro MPC BoosterpaQ. These integrated pumping systems utilize an advanced controller to adjust pump speed and to stage additional pumps to meet fluctuating system demand.
Overall, eliminating pressure surges in the system has slashed the number of pipe breaks and leaks for Cottonwood by 30% or roughly $38,000 in capital repairs. Moreover, the proportional pressure setting lowers water pressure/volume during off-peak demand cycles, such as overnight, which translates into less wear-and-tear on the community’s pipe infrastructure, as well as less water lost to leaks.
Moreover, the level of “unaccounted water” — which refers to the difference in the amount of water extracted versus the amount of water billed to customers — has dropped to 11% from 40%, helping to bridge the water crisis facing the southwest.
Brian James P.E., is a business development manager and Roger Weldon is service engineer for Grundfos Pumps Corporation. Grundfos is the world’s largest pump manufacturer. For more information, visit www.grundfos.us.