| Larry Bachus
(a.k.a. “Pump Guy”)
I attended a course on pumps presented by you awhile ago, and it really helped me. I have a follow-up question though: The eccentric reducer in the suction pipe of a pump must, according to your book, be sloping upward to the pump to prevent air accumulation. However, some people at my plant now have the opinion that if the fluid being pumped is a slurry/sludge, it should be sloping downward to the pump in contrast to the norm. What is your opinion?
Thanks for writing. Yes, my pump text shows the eccentric reducer sloping upward (reducing) from the bottom and coming straight across the top as the pipe enters into the suction nozzle of the pump.
|Eccentric pipe reducer on the suction nozzle of this pump.|
This is good piping practice for clean liquids without solids or sludge. There are other considerations if the pumped liquid is slurry with solids, crystals, or sediment. Each situation is different.
Before I explain further, let me create a verbal analogy. Everyone knows it is best to drive an automobile modestly (i.e., accelerate and brake slowly) to conserve energy (fuel) and extend the service life of your automobile. Now, imagine your house were burning. Imagine you had suffered an injury and were bleeding profusely. Would you want the firemen or ambulance driver to drive modestly and conservatively to your house? Of course not!
You’d want them to drive fast and furious. The goal now isn’t fuel conservation. The goal is high velocity to arrive quickly and save a house or save someone’s life. For fire trucks and ambulances, the emergency overrides the interests of conservation.
Pipe Reducers & Energy Conservation
There is no law or rule that states you must use a pipe reducer at the suction nozzle of a pump. “Energy conservation” is the real reason to use a pipe reducer at the pump suction nozzle. I’ll explain.
Let’s say I have an operation that requires 1,000 GPM of flow. The pump has a six-inch suction nozzle. I could use six-inch suction piping and fittings leading to this pump. Or, I could use larger diameter suction piping and fittings leading to the pump, and install a pipe reducer to mate the larger pipe with the six-inch suction nozzle.
Now, let’s consider the pipe system between the suction source or vessel and the pump. Let’s say the suction piping system contains:
• 100-feet of schedule 40 carbon steel pipe, with
• Nine elbows of 90-degrees each, and
• One check valve, and
• Four gate valves.
Let’s calculate the energy hit (friction losses, Hf) of a six-inch suction piping system and compare with a 10-inch suction piping system.
Who would have imagined the friction energy of a six-inch suction piping system (15.6-ft) is almost seven times greater than the same piping arrangement using slightly larger diameter pipe and fittings (2.3-ft)? And if you negate the elbows and valves and consider just the pipe friction, the energy hit is 12 times greater with 6-inch pipe (6.17) compared to 10-inch pipe (0.5). You can see this on the graph.
This graphical example demonstrates the real reason to use larger pipe with reducers. Get the energy into the pump or into the process where you need the energy. Don’t lose the energy (and company profits) as friction in the pipe.
When pumping a clean liquid, it is best to use larger diameter pipe with a reducer to mate to the pump or other equipment. With slurry, the liquid velocity overrides the economic concerns. Let me mention a few typical examples:
Crude oil is pumped from the ground with high concentrations of dirt and rocks.
Water collects in the bottom of all mines. Dewatering pumps lift this water with rocks, sand and dirt.
Many pharmaceutical products begin as liquids that precipitate solids or crystals in a reaction. Eventually, the mixture assumes the consistency of mud or wet cement. Further evaporation yields a dry powder that is then compressed into pills and capsules.
Like driving an emergency vehicle, the goal is different when pumping solids, slurry or sludge. With solids in the pumped liquid, the goal is to maintain a certain minimum velocity in the pipes so the solids don’t settle and form a dam inside the pipe.
With a sludge or slurry, it is best to specify the pipe diameter so the desired quantity of slurry moves at the specified velocity. Then you would specify the pump with a suction nozzle at the same diameter so the pump properly receives the high-velocity flow through the suction pipe.
If the liquid contains solids, and especially if the liquid contains solids with gas or air bubbles, I’d use a pump with a suction nozzle diameter the same as my suction pipe diameter. No Reducer.
I am a maintenance practitioner, and I stay in my territory. I’d meet with a process engineer. I’d ask the process engineer what velocity we need in the pipe – for horizontal and even vertical runs – to keep rocks and gravel (or whatever slurry) moving through the pipe without settling and forming a dam. The process engineer might say the slurry needs 9-ft., or 13-ft., or even 22-ft. per second velocity to keep the solids in motion through the pipe.
It may not be economical. But economy isn’t the goal with a slurry or sludge. The goal is to maintain a minimum specified fluid velocity to avoid settling, not energy conservation.
Hey, look, at the risk of getting a little gross, let me provide a more human-oriented example. When you have a cold and your nose is filled with snot, the body’s natural reaction is to sneeze. Simple breathing doesn’t generate the velocity to blow the snot (sludge) from your nose. The sneeze generates the velocity. DUH!!
If this makes sense, then I’m sure you understand that it is rather like “sweeping sand off the beach with a broom” to discuss whether the reducer’s slope should be up or down. When pumping slurry, use pipe and nozzles that support the required velocity. NO Reducers!!!
Pipe reducers (and increasers) are recommended for clean or mostly clean liquids. They provide significant energy savings with modest cost. Larger diameter pipe consumes less energy because the fluid velocity goes down with larger pipe. (See the energy of velocity (Hv) on the graph above.) The pump and liquid’s energy stays in the liquid and is not lost as friction against the internal pipe walls.
I always urge purchasing agents to come to my lectures on pumps. Purchasing agents must make big decisions with limited information. Consider pipe.
I called two local pipe distributors in Nashville and got quotes:
• 100-ft. of 6-inch carbon steel schedule 40 pipe sells for $12.60/ft.
• 100-ft. of 10-inch carbon steel schedule 40 pipe sells for $32.70/ft.
|No pipe reducer on this pump suction nozzle.|
So 10-in. pipe sells for more than 2.5 times the price of 6-in. pipe. 6-in pipe saves money. Not so fast, Bunky! Using clean water and 10-cents/Kwh electricity rate:
• 100-ft of 6-in pipe will burn $1,226 per year in profit as friction energy.
• 100-ft. of 10-in pipe will burn only $96 per year in energy as friction losses.
And that’s just a hundred feet of pipe, people. Put that in your pipe and smoke it!
• A beer brewery could have 200 MILES of pipe.
• An oil refinery could have 1,000 MILES of pipe.
• A municipal water department could have 30,000 MILES of pipe.
The whole argument of “Larger pipe costs too much money!” is another attempt at sweeping sand from the beach with a broom. Stop the insanity! (Forward this e-mail to the purchasing agent and tell him to attend the next Pump Guy Seminar. Purchasing agents need this information to make better decisions.)
However, if the slurry needs 18-ft. per second velocity in the pipe to avoid settling, I’ll use pipe diameter that supports this velocity leading to and away from my pump. And I’ll use a pump with suction and discharge nozzles that can mate with this pipe and support this liquid velocity. No reducers or increasers.
Dennis, I hope this answers your question. Stay in touch.
The Pump Guy
Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at email@example.com.