Mechanical Technology September 2015

⎪ Pump systems, pipes, valves and seals ⎪

In this pump guy article, Larry Bachus defends the statements: “the term head is the constant for the pump manufacturer”; “a pump that generates 30 m of head can elevate any liquid to 30 m above the surface level of the liquid’s source”; and “if I use the word ‘head’, the liquid’s density is not important”. Total head and density

The Pump Guy

M any engineers, technicians, pump users and even pump company employees mis‑ understand the concept of head. The pump industry is guilty of not properly explaining this concept to the pump users, while engineering textbooks and professors of fluid mechanics are guilty of leaving too many engineering students with doubts regarding the exact meaning of ‘head’. This is what happens after industry downsizes: older engineers are let go and mentoring as a teaching tool is abolished. Pump companies rate their pumps in metres of head because the pump com‑ pany normally doesn’t know the liquid that will move through the pumps. And what the pump company calls ‘metres of head’ is a measure of the specific energy available to the pump, ie, the energy per unit weight of fluid displaced. And this

be about 257 kPa (24 m ×10×1.07 = 257 kPa). The pump manufacturer in Joburg doesn’t know two pumps are in Durban moving milk. Another sales rep sells three pumps to the petrochemical plant in Durban to pump sulphuric acid. Sulphuric acid has a specific gravity of 2.0. The pumps will elevate sulphuric acid 24 m, or overcome 24 m of resistance in the pipes. The differential pressure across the pumps will be about 480 kPa (24 m ×10×2.0=480 kPa). With a specific gravity of 2.0, these pumps will require a motor with twice the power (kW) rating. The technician will mate and align larger motors to these pumps before shipping the pumps to the customer. In Joburg, the pump manufac‑ turer might never know that three pumps are moving sulphuric acid in Durban. Another sales rep sells a pump to the local paint factory to pump paint thinner. The paint thinner has a specific gravity of 0.87. The pump will elevate the paint thinner 24 m, or overcome 24 m of resistance in the pipes. The differential pressure across the pump at best efficiency will be about 209 kPa (24 m×10×0.87=209 kPa). The pump manufacturer in Joburg will never know this, unless there is a problem while the pump is under warranty. Let’s return to the dairy in Durban. The pumps were bought to pump milk and whole milk has a specific gravity of 1.07. However, skim milk, chocolate milk, half and half, evaporated milk, coffee cream and ice cream mixes all have different specific gravities. If a pump moves more than one liquid, the pres‑ sures and the motor’s power (to drive the pump) will vary according to the specific gravity of the liquid. But 24 m of specific energy is always 24 m. And frequently, this is all the pump manufacturer knows. So, the pump manufacturer prints a curve that shows metres of head. I ask those of you in doubt to review your university ‘Fluid mechanics’ text‑ book. Also, review some recent pump

specific energy is, therefore, independent of density. Let me explain this with an example: A pump manufacturer in Joburg sells 20 centrifugal pump models to their distributor in Durban. The 20 pumps are rated for the chemical process industry. Durban has many applications for mid- frame, back pullout chemical process pumps. The 20 pumps accommodate impel‑ lers up to 250 mm in diameter. The 20 pumps each have a shaft that measures 50 mm in diameter and bearings that can handle the loads generated by the shaft and impeller. The performance curve for each pump is rated at 1 450 rpm and indicates best efficiency at 78%, when pumping 24 m at 125 m 3 /hr. The sales manager at the pump distributorship in Durban wants to give good service to the customers. He put

these pumps on his shelf for immediate delivery. The sales reps are under instructions to promote these pumps as much as possible. One sales rep sells three pumps to the water bottling plant in Durban to pump potable water. Water has a specific gravity of 1.0. The pumps will elevate potable water 24 m, or overcome 24 m of resistance in the pipes. At best efficiency, the differential pressure across the pumps will be about 240 kPa (24 m ×10×1.0=240 kPa). The pump manufacturer in Joburg doesn’t know three pumps were sold to pump water in Durban. Another sales rep in Durban sells two pumps to a dairy to pump milk. Whole milk has a specific gravity of 1.07. The pumps will elevate milk 24 m, or overcome 24 m. of resistance in the pipes. The differential pressure across the pumps at best efficiency will

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Mechanical Technology — September 2015