Your questions answered: Efficient pump selection and control

Several questions about variable speed pumps, paralleling pumps, and controls are covered in these responses.

By Reece Robinson, Grundfos Pumps Corp. April 13, 2015

Reece Robinson, senior technical trainer at Grundfos Pumps Corp., tackled unanswered questions from the March 31, 2015, webcast on Efficient pump selection and control.

Question: How we can install and control variable speed pumps in an old hydronic circuit with constant speed pump?

Answer: In most cases the answer is yes. The existing pumps are operating according to their performance curve with higher head output as flow approaches zero. A new variable speed pump can be installed to maintain either a constant differential pressure, a proportional differential pressure at the pump or a constant differential pressure at a remotely installed sensor.

Question: What about the net positive suction head (NSPH) of option 4?

Answer: Yes, good catch! In this example the most efficient option also had the highest NPSH requirement at the DESIGN flow. If this pump were to be considered for a closed loop installation (hot or chilled water), the NPSH requirement of 27 ft would not be of major concern. If the pump were to be considered for a cooling tower or other application with a flooded suction, care would have to be taken before using this pump.

Question: How do you create a curve for parallel connected pumps?

Answer: This can be done with a simple spreadsheet file. Most pump performance curves can be very well displayed with 5 points. Figure 1 shows 5 points for flow and head. For parallel pumps you simply multiply the flow by two, using the same value for head.

Question: Efficiency sequencing: Does this mean a flowmeter and kW board are now required for my controller?

Answer: Not necessarily. Advanced controllers today can have pump performance data loaded into the controller. The controller will need some feedback, usually the differential pressure of the pump and flow from a flow sensor. Modern controllers will use a calculated flow rate and/or pump head as well. For example, if you know the pump head differential, the flow can be calculated if the pump curve information is loaded into the control. Power can be measured directly off of the variable frequency drives (VFDs) or it can be measured using current transformers and voltmeters.

Question: For a typical hot or chilled water HVAC application, do you see designs incorporating three pumps, two operating together to maximize efficiency instead of the typical to (one running, one redundant)?

Answer: Yes, especially when part load conditions are evaluated and also when central plants are installed with future loads are to be added later on down the road.

Question: Why is the design point on the system curve below the design point on the pump curve?

Answer: The design point is what the designer calculated. The duty point (actual operating point) is where the pump would be running unthrottled. Usually what happens is that there will be a multi-function valve and/or balancing valve that will add artificial head to the pump and the actual pump flow will match the design flow.

Question: When running pumps in parallel typically how slow can you safely run two pumps in parallel before switching in one of the pumps?

Answer: Centrifugal pumps are variable torque machines such that operation down to and even below 25% speed will not be harmful to either the pumps or motors. Pump hydraulic efficiency can be high even at speeds lower than 25%. From a practical standpoint Figure 2 shows what happens with two pumps are running at 25% speed. You will see that the head is so low that in most real-world applications the pump will not be able to run this slow and still meet the objective. Usually there’s a minimum static head or control valve head that has to be overcome thus making the pump minimum speed a very low concern.

Question: In your experience, has the use of buffer tanks and secondary pump plants with VFDs for high-rise buildings been an efficient alternative to purchasing large enough pumps to do all that variable and static work from the ground?

Answer: When looking at high-rise building design, the more energy-efficient designs have more sets of pumps. The main reason is the number of pressure regulator valves required. A good example would be the John Hancock building in Chicago. Midway up the building is a large water storage tank. A set of pumps at or near the street level pump water into the tank. The tank then serves the bottom half of the building via gravity flow. The lower floors still need pressure-release valves (PRVs) due to the head that is gained from water being 50 floors above. A second pump set adjacent to the storage tank serves the upper floors. PRVs are required for some of the upper floors because the pressure leaving the pump station is high in order to get water to the top of the building. This could have been a three- or four-tier pump design with the intent on reducing the total number of PRVs. The buffer tank adds installation costs and also may bring water treatment concerns into the picture. All in all, the more pumps sets, the less energy wasted going through PRVs.

Question: What determines the control curve, particularly the left end point?

Answer: The left end point of a control curve would be the minimum head required to circulate the liquid or to get the liquid to where it needs go. For example if we had a hydronic heating system requiring 200 gpm at 85 ft of head, both pumps would need to overcome that 85 ft of head at full flow. If the furthest coil from the pump system required 40 ft of head and a differential pressure sensor was installed there, the system would have to at least produce 40 ft of head to get the liquid across that coil. Essentially that 40 ft of head becomes the minimum control head on the control curve. If the sensor were to be mounted at the pump station across the pumps and/or headers that 40 ft would have been calculated at the minimum control head.

Question: How do we produce combined efficiency curve for parallel pumps?

Answer: If we use the same spreadsheet file for the two pumps shown in Figure 1, we can add a column for pump efficiency shown in Figure 3. You can now see where the efficiency curves intersect to determine when it is most efficient to start pump 2.

Question: I have centrifugal pumps running all the time just to provide water whenever it is"desired by a person," but wear and energy consumption is problems. Is there any pump control I may use to turn on/off pump to reduce energy use and wear?

Answer: Yes, these controls exist. To be most effective, the use of a diaphragm tank is also recommended. The use of energy saving no flow shut-downs in discussed in the webcast ASHRAE 90.1 – What this means to the plumbing engineer.

Question: Is efficiency sequencing beneficial for smaller pumps, 5 hp or less, or only for larger pumps?

Answer: Efficiency sequencing can be beneficial at any size, regardless of horsepower. To achieve this level of control might require a higher investment cost, which may not result in a quick return on investment. Staging on calculated efficiency (i.e. pressure measured with flow calculated) can be an affordable option these days.

Question: Is that a bypass across the control valve for the dp sensor? Would dp control at that point work if there was no bypass?

Answer: Yes, differential control will work if there was no bypass at that location. The purpose of the bypass is to avoid pumps from running against a closed valve. Usually at some point in a closed loop system there will be a bypass, sometimes located at the pumps.