mailbag: Homework Help

Hello Christian,

I hope all is well in Canada. If your son is anything like my children, I bet we are doing your son’s homework assignment. Or maybe, we’re laying the foundation for his teacher’s doctoral dissertation. Don’t worry! I’ve done this before. I try not to, but sometimes I can’t resist. I talk about this very issue in my pump lectures. Here goes.
There are two easy methods to determine power consumption. Pick one:

One way: You can look at most pump curves and see the BHP or kW consumption at shutoff head or zero flow. The curve is based on water. If your pumped liquid is not water, you can extrapolate the power consumption at zero flow by factoring the liquid’s specific gravity.

Another way: You can put an amp meter onto the pump’s motor when the pump is operating against a shut valve, record the amps, and convert the amps to kW. (kW = (amps x volts x 1.73 x power factor) / 1,000). This will indicate the kW consumption at shutoff head for whatever liquid is in the pump.

Next, we must determine the volume of liquid. In the Imperial system, you need gallons. For metrics, you need kilograms or liters. So, you need to know how much liquid is in the pump at any instant in time. There are two ways to do this:

One way: Although somewhat impractical, you can take a new or rebuilt pump with all assembled parts. Then, block and seal the entrance and exit nozzles and all points of leakage. Fill the pump with water to the top. Then drain the pump, collect and measure the volume of liquid that fits in the pump.

Another way: We can consider how much liquid moves through the pump with every revolution of the motor, knowing that more liquid will enter and leave the pump with the next revolution. Let’s say the motor spins at 1,750 RPM and the pump develops 800 GPM flow. Then you know approximately how much liquid is in the pump with every revolution of the motor. This is the approximate volume of liquid exposed to the kilowatts or horsepower at any moment in time.

If the pump is operating at zero flow (deadheaded), then this same volume of liquid remains inside the pump churning and exposed to the kilowatts or horsepower at zero flow.

BTUs are the energy to raise the temperature of one pound of water one degree F. Calories are the energy to raise the temperature of one KG of water one degree C.

Then you can convert BHP to BTUs. Or, you can convert kW to calories.

Two factors (dissipated heat and volumetric efficiency) might be out of control and difficult to determine. Dissipated heat is the heat that radiates through the pump casing as it heats. Insulation can reduce or contain this.

Volumetric efficiency is one element of a pump’s total efficiency. The other elements are hydraulic efficiency and mechanical efficiency. A pump’s volumetric efficiency is not 100 percent because some liquid recirculates inside the pump with each revolution of the shaft assembly rather than moving through the pump. However, at zero flow with no leaks leaving the pump, we could say the volumetric efficiency is 100 percent. And, all heat accumulation would result from mechanical and hydraulic losses.

With this information you can ask, “With a 100-Hp (75-kW) motor burning ‘X’ kilowatts at shutoff head, if I go to lunch and leave this cold water pump deadheaded (at zero flow), how long before the water boils inside the pump while I am on my lunch break?” The answer will probably compute to between six and 12 minutes, factoring (estimating) the dissipated heat.
Then, we’ll know the source of cavitation damage, mysterious seal and bearing failures, and never-ending pump maintenance.

We always think that a deadheaded pump is a pump that operates against a closed discharge valve. But a pump is also deadheaded when it operates against too much resistance, or too much elevation differential, or an over-pressurized downstream header, or a check valve installed backward in the pipe, or a clogged downstream filter.
Of course deadheaded pumps will vibrate. The vibrations cause the heat. Without gauges, no one knows the source of the vibrations.

Reliability engineers rarely consider this. Undetermined runaway vibrations in process pumps are considered one isolated, unconnected incident that requires immediate attention. Clogged filters are considered another isolated, unconnected incident that we’ll address sometime in the next week or two.

Pumps and filters without gauges are considered another isolated, unconnected incident that no one has addressed in the past 15 years. The Pump Guy says, “They’re all the same connected incident beginning with no gauges to interpret.”

And finally you asked: Is a large split-case pump with one impeller more susceptible to “feel” heat buildup than a multi-stage pump for the same application (flow and head)?

I’d say the heat generation is the same if the efficiencies, power input, discharge head and flow are the same for both types of pumps. A larger split-case pump with one impeller might dissipate more heat because it has a larger surface area to radiate the accumulating energy. So, the multi-stage pump might “feel” hotter if you touch the pump.

Where was the Pump Guy when I was a university student back in 1972?

Cheers,

Larry

Flow Control will bring the Pump Guy Seminar Series to Orlando in December, Chicago in March, and New Orleans in June. For more details, please visit FlowControlNetwork.com/PumpGuy. Come if you can!

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 protected].

EDITOR”S NOTE: This article was updated to indicate that BTUs are the energy to raise the temperature of one “pound” of water one degree F rather than one “gallon” of water. We apologize for this error.