Ah, the Good Life… Before COVID! I was called to a Caribbean Island in February to look at a sick pump in a recently renovated 5-Star vacation resort. Five hours after leaving the snow and ice, I was poolside in the tropics, sipping a sweet, rum-laced concoction. A little paper umbrella, stabbed into a cherry and nestling on the rim of my plastic cup, tickled my nose as I sipped-away.
The suntanned vacationers looked down and snickered when I removed my shirt and displayed my pasty paunch, punctuated with a pink appendectomy scar and a shock of grey hair circling my navel like muddy water racing down a drain hole. “What’s the joke?”, I thought to myself.
You see, the other people had worked all year, maybe in jobs they despised, just to spend $2,000 per day on a vacation in paradise. I was being paid to be there and staying free in a suite at the resort hotel with the sick pump.
The pump was mostly new. It had been installed two months before into a 200,000-gallon pool with an elaborate water slide. The pump was designed to push pool water up 68 ft. from a sump to the entrance of the fiberglass toboggan slide.
The resort manager had summoned the local authorized pump dealer to help with the pump. But the dealer wouldn’t touch the alien pump. The local pump dealer was upset that their exclusive distribution territory had been invaded. You see, the resort had sneaked the pump into paradise from a Georgia supply house that was not authorized to export pumps into an international territory.
The application (a pump lifting pool water to the top of a water slide) was not unique. And the pump was not that expensive. But suites at the resort were very expensive. The resort’s promotional literature showcased the twisting, looping water slide through an imaginary cave with an internal light show before bursting forth in the shallow end of the enormous pool. The resort didn’t need any bad publicity from disgruntled guests. That’s why the resort called me. I carry a cheater bar. I’m the Pump Guy.
The resort maintenance manager escorted me to the pool and started the pump. I noticed a rumbling sound of vibrations. We climbed the spiral stairs to the top of the slide. I could hear gurgling in the pipe. But there was no water at the top of the slide.
The resort manager greeted me in the hotel lobby as the sun sank below the horizon. He asked if I was ready to work. I said, “Yes!” He gave me 40 plastic coins to squander in the resort casino. OKAY!
The maintenance manager said the pump is off the curve. I thought, “Aren’t they all!” He said the pump appears to work but won’t deliver the water up to the slide. The slide was dry and hot under the midday Caribbean sun. The tourists got red cheeks on the way down the slide. The lifeguard closed the toboggan after a millennial exited the slide with a wedgie that pained me just to witness it.
The next morning, I started earning my consulting fee. I inspected the pump and motor. Immediately, I knew there were three problems. First, I had left my house key in the front door lock back in Nashville as I left for the airport the day before. Second, the name tag on the pump indicated 1,750-rpm. And third, the electricity on the island was at 50-hertz (Hz).
The local pump supply house had quoted a 50-Hz centrifugal pump rated for 75-ft of head. The purchasing agent at the resort hotel thought the quoted price, with freight and import tax, was too expensive. He went onto the Internet and saw a pump for less money. He bought the pump through a pump distributor in Savannah, Georgia. The Savannah pump had similar performance stats. The Savannah pump dealer had little export experience. They simply took a credit card and shipped a pump without asking questions.
The Affinity Laws are a set of formulas that predict pump behavior with different rotational speeds. Sometimes they’re called the Laws of Similarity. The laws apply to fixed-speed pumps, adjustable speed pumps and variable speed pumps.
In North America, and parts of the Caribbean and South America, electricity undulates in waves at 60-cycles per second. This is called 60-Hz electricity. A 4-pole electric motor rotates at 1,800-revolutions per minute (rpm) under no load. The rpm is slightly less with load and slip.
In most other countries around the globe, the electricity moves in waves at 50 cycles/second. This is called 50-Hz electricity. A standard 4-pole electric motor rotates at 1,500-rpm when under no load. A change in frequency brings about a change in the speed (rpm). As the speed changes, the pump’s head, flow and power requirements change by the Affinity Laws.
Regarding speed, the Affinity Laws indicate:
1. The change in flow (gpm) is directly proportional to the change in speed (rpm). Twice the speed will produce twice the gpm. 80% rpm produces 80% gpm.
2. The change in the head (pressure) varies by the square of the change in speed. At 2-times the speed, the pump will develop 4-times (2 2 ) the head. At half the speed, the pump will develop one fourth the head [(½) 2 = ¼].
3. The power (horsepower or kilowatts) varies by the cube of the change in speed. At twice the speed, the pump requires 8-times (2 3 ) the horsepower or kilowatts. At half the speed, the pump requires one eighth [(½) 3 = ⅛] the power.
The Affinity Laws are even more important with variable speed pumps. I first saw a variable speed AC electric motor (VSM) about 35 years ago. Some people refer to the variable speed motor as variable speed drive. Others say variable frequency drive. By varying the motor’s frequency, and thus the rpm, the pump’s performance is also variable by the Affinity Laws.
An automobile is a form of variable speed drive. Your foot on the accelerator pedal governs the speed of the auto. No one would drive their car by setting the cruise control at 100 mph, and then govern the actual speed of the car by applying the emergency brake or feathering the clutch pedal. Yet, it’s the way we’ve operated pumps in the past. We’d start the pump, mated to a fixed-speed electric motor (at say 1,750 rpm) and then govern the flow through the pipes by opening and closing valves. Now, that’s really stupid!!! It was never important with cheap energy. It’s important now, and will be more important in the future. Every day there are more and more variable speed motors (VSMs). Most everyone says the same thing. VSMs are replacing older fixed-speed motors, and the purchase prices are dropping.
VSMs are definitely good for pumps, the economy and the environment. At 50% speed, the pump delivers ½ design flow at ⅛ the power consumption. You must understand the Affinity Laws and work with them, or the laws will work against you. VSMs can be mostly operated over their entire range when mated to a pump in a flow (gpm) application. Flow and speed are directly proportional.
Variable speed can be used with pumps in pressure or head applications, but the range of speeds might be restricted depending on the profile of the system curve. The operator may not have complete freedom with his pump mated to a VSM in a head application because at 50% speed, the pump will only develop 25% of design head. Many people in production make this mistake because production types generally only consider flow (gpm). But I digress.
In the case of the imported pump designed for 75-ft of head on a 60-Hz motor, this pump was only developing 52-ft of head, mated to a 50-Hz motor. And this was the reason the pool water was deadheaded in the vertical pipe, the water slide was dry, and the reason the pump was vibrating. Problem solved! They should all be that easy.
A pump, rated at 75-ft on a 50-Hz motor, as originally quoted by the local distributor, was the right pump for the water slide. The resort maintenance manager and I drove a pink Mini Moke with a lime green cloth canopy to the local pump dealer. We begged forgiveness in an effort to repair relations between the resort and the local pump house. The maintenance manager bought the local pump.
I talked with the resort maintenance manager and the purchasing agent about the Affinity Laws and benefits of working with the local suppliers. The local guys delivered their pump the next day and installed it into the water slide. The local pump shop accepted the alien imported pump as a trade-in. The red cheeks, brown streaks and wedgies went away.
After the installation, I went out on the ocean in a complimentary sunfish (a little sailboat) and remembered my days as a machinist mate in the Navy at Great Lakes. The next morning, I stopped at the resort pool to inspect the toboggan pump just to be sure it was performing properly. It was. The previously-mentioned millennial and two friends sauntered up the spiral stairs to the top of the water slide, and I smiled to myself as they went down. Then, I borrowed the pink Mini Moke and drove around the island on the other side of the road. I stopped in a little fishing village and ate a fried shark fritter sandwich for lunch. The resort van dropped me at the airport, and I napped on my afternoon flight back through Atlanta to Nashville.
I later learned that the local pump supply house had modified and re-sold the imported pump to the municipal government. That pump with a smaller impeller is doing duty in the fountain at the public square in Bridgetown. Ah, The Good Life!
I always spend plenty of time on the Affinity Laws in my lectures. I want everyone to completely dominate these concepts as variable speed pumps replace fixed speed pumps. Maybe you can attend my seminar, live or virtual, later this year. There will be announcements on my website and through Empowering Pumps.
About the Author: Larry Bachus is a pump consultant, lecturer and inventor, based in Nashville, Tennessee. He’s a member of ASME, and lectures in both English and Spanish. Send e-mails to firstname.lastname@example.org. Phone 615-361-7295.