Get the Most out of Variable-Speed Pumps
To do this successfully, pumps and their associated components need to selected and controlled correctly. Gil Avery, P.E., discussed product pros and cons in “Selecting Valves and Piping Coils in Variable Speed Pumping Systems.”
Avery is director of systems research with The Kele Companies, Memphis, Tenn. Low delta T problems, he pointed out, often are caused by inadequate control valves. “We must use high-performance valves on hydronic-only systems.”
VALVE CHARACTERISTICSAvery pointed out the critical requirements for two-way valves on variable-flow systems. Rangeability, he said, is the ratio of the maximum flow through the wide-open valve to the minimum flow. “Valves with high rangeability must be used in variable-flow systems.”
Critical requirements for two-way valves in variable-flow systems, he said, are a closeoff of 1.5, dynamic of 1.5, and rangeability of 200. (“Closeoff” is the differential pressure the valve/actuator combination can overcome in order to seat the valve and stop flow completely. “Dynamic” is the maximum differential the valve’s wetted parts can withstand. “Rangeability” is the ratio of the maximum flow with the valve wide open to the minimum controllable flow.)
Avery then looked at the characteristics of some common mechanical system valves.
The standard globe valve, he said, has the benefits of being readily available and serviceable. Among its disadvantages, he said, are its short packing life, low rangeability, low dynamic rating, low coefficient (Cv), high stem forces, it’s noisy, and there is no flow-measuring capability.
The double-seated globe valve has the benefits of low stem forces, field serviceability, and “It’s OK for primary variable-flow systems,” Avery said. Among its disadvantages, the valve is expensive, it does not provide a tight shutoff, it has a short packing life, and you have to special order it.
Butterfly valves, he said, are readily available, inexpensive, and OK for water bypass and chiller isolation systems. Its disadvantage is its very low range, Avery said. It’s not designed for primary-only-variable-flow (POVF) systems.
The common ball valve is inexpensive below 2 inches. Its disadvantages include its low range, low dynamic rating, and low Cv. It’s not designed for POVF systems, it offers no flow measuring, and it’s not field serviceable.
Pressure-independent ball valves, Avery said, are readily available; however, they are also expensive, with a low range, low dynamic rating, and low Cv. They are not field serviceable and they have a high pressure drop, which Avery said is its biggest disadvantage. “You must add 5-10 percent to the pump hp,” he said. There is no flow-measuring capability.
OUTSIDER SOLUTIONThe segmented spherical valve, Avery said, can be bought with any type of actuator. This rotary valve has an American National Standards Institute (ANSI) leakage rating of 6, which Avery said is the lowest of any valve - less than 1 bubble per minute on a 1-inch valve. It has a range of 300:1.
“Another nice thing about these valves,” Avery said, “is that they publish the flow coefficients. This type of valve actually gives you flow verifications as well as flow control. We know how much flow we have through the valve.”
Its high Cv means that “When the valve is open, it’s almost a straight shot through the valve.” It is serviceable and quiet, with a high range, medium price, good flow measuring, and a five-year warranty. Its main disadvantage is that it’s new to the HVAC industry. Previous applications have included food processing, pharmaceutical, and petrochemical industries.
SIMPLE GUIDELINESTorben Kynde Nielsen, Grundfos Management A/S, Bjerringbro, Denmark, discussed “How to Select and Control Variable Speed Pumps.”
“If you go back 20 years, it was a question of flow and head and nothing else,” Nielsen said. “Now we have to take into consideration variable flow.” He asked, “Is it better to have more pumps than only one? Do we need to have pump speed control with variable frequency, or can it be mixed with constant speed?
“Go back and ask, what is the load profile of the system; where should we set the limits? My recommendation is you cannot set a limit,” he said. “You have to look at how the system is behaving. The best thing you can do is use a system with the lowest life-cycle cost and a high level of reliability.”
Publication Date: 08/06/2007