(“The Pump Guy”)
Editor’s Note: This is Part I in a two-part series on best practices for specifying pumps into a pipe system. Part II will run in the July 2015 issue of Flow Control magazine.
Racecar drivers circle the Indianapolis Motor Speedway at more than 225 mph. One driver wins. Only the best drivers can even compete in the Indianapolis 500 race. But, most people can drive to work every day. Five-star chefs are rare. However, most people can grill some meat and steam veggies for the family.
A frequent question before the Pump Guy is, “How do I specify the correct pump into a given service?” The answer is simple. It’s like riding a bicycle. If you can master a few concepts, specifying the correct pump into a system is really quite easy.
Another frequent question before the Pump Guy is, “How can I reduce pump maintenance and extend the service life of my problematic pumps?” The answer is the same. Master a few concepts, and most of your pumps will calm down and offer many years of trouble-free service.
When the pipe system, the liquid and the pump are correctly mated, most process pumps give many years of trouble-free service. You can expect your seals, bearings, lubricants, couplings and other components to obtain their projected service life.
In any given population of process pumps, there are some pumps that run for years with no problems. Other pumps are constant maintenance headaches.
Some pump maintenance is the result of incorrect maintenance practices. But, remember the same maintenance technicians rebuild all the pumps in the plant, the behaving pumps and the problematic pumps. The mechanics apply the same procedures, use the same tools, and install the same seals, bearings and lubricants on the behaving pumps and the misbehaving pumps. So, other factors outside maintenance are at the root of runaway pump failures.
Just as some automobile accidents are caused by reckless driving, some pump maintenance is the result of reckless pump operation. But remember, the same operators control all the pumps in the plant, the behaving pumps and the problematic pumps. The operators apply the same procedures, manipulate the same valves, and monitor the performance of the behaving pumps and the misbehaving pumps. So, factors other than pump operation are the roots of runaway pump failure.
Incorrect Design and Engineering
Some cars crash and some airplanes fall from the sky due to inadequate design. Some pump maintenance is the result of incorrect design. A pump is a simple machine compared to a passenger jet or automobile. The pump performs one simple duty. It adds energy to a liquid. We use that energy to elevate, accelerate, or pressurize a liquid through a system of pipes and fittings.
In aviation, energy, expressed as speed and lift (up), must be in balance with resistance and gravity (down). With pumps, the energy added to the liquid must be in balance with the energy contained in the pipe system.
Think of it this way: The flame from a charcoal or gas grill contains the energy needed to cook some steaks. You can’t cook steaks with a forest fire. You can’t cook steaks with the flame of a match. The energy (heat and flame) would be out of balance with the task (four pounds of raw meat) at hand.
Therefore, the principal distinction between the pumps that suffer continual and repetitive maintenance and the “low-maintenance” pumps is the way the pump is mated into its system. You would apply the same information in the remainder of this article for a new pump into a system, or to correct a problematic pump.
When the pump is properly mated into its system, the pump’s energy and the system’s energy are in balance. The energy expresses itself as pressure and flow into the pipes for many years with minimal maintenance and problems.
When a pump is not properly mated into its system, the energy imbalance expresses itself as runaway vibrations, heat, component distortion and excessive noise. Vibrations, component distortion, heat and noise are all forms of energy, diverted from the pump’s principal mission, which is pressure and flow out the discharge nozzle. The symptoms of the energy imbalance are repetitive maintenance, premature seal and bearing failure.
Begin with the System
Don’t start with the vibration meter. Don’t start with the synthetic lubricant. Don’t start with the Plan #53—a modified seal pot. Start with the pipe system.
Maybe you want to push hot water into a boiler to make steam. Maybe you want to transfer white base paint from a holding tank into a mixing vat to add color. It really doesn’t matter. You would follow the same steps to specify a new pump, or to resolve a problematic pump.
You determine that you need to complete this operation at 800 gallons per minute. Then you need a pump with best efficiency flow of 800 gpm.
Next, you determine the energy contained in the pipe system. This is called the total dynamic head. There are up to four energies in a pipe system.
1. The energy to overcome any elevation differential.
2. The energy to overcome any pressure differential.
3. The energy to maintain the liquid velocity as it moves through the pipes.
4. The energy to overcome friction between the liquid and the internal surfaces of the pipes, elbows, fittings and process devices (control valves, heat exchangers, strainers, etc.).
Let’s consider each element.
1. Elevation Differential
Parts of New Orleans are below the level of the Mississippi River. When it rains, the stormwater must be pumped up and over the levies and into the river. If this vertical distance is 8 feet, then the pump that performs this duty must develop at least 8 feet of elevation.
Let’s say your cooling tower is 35 feet high, measured from the water reservoir pan below the tower, to the spray (or release) nozzles at the top of the tower. The pump that lifts the water to the top of the cooling tower must develop 35 feet of elevation differential.
2. Pressure Differential
Not all systems contain pressure differential. But, if it exists, pressure differential is added to the elevation differential. Here is an example: Let’s say you work for the New England Patriots football team and you want to inflate a regulation football with 10 PSI of air pressure so the quarterback has a better grip. The air pressure inside the football is 10 PSI higher than the air pressure outside the football. Uh, maybe this isn’t the best example. I’ll save this material for an article on Engineering Ethics.
OK! Consider a boiler. The boiler feed-water pump receives treated water from the deaerator tank and injects the feed-water into the pressurized boiler. The deaerator tank is at atmospheric pressure, or sometimes at low positive pressure. Higher pressure is inside the boiler. This is the pressure differential. The boiler feed-water pump must develop additional energy to overcome this pressure differential.
3. Velocity and Friction Energy
These energies work in concert and only exist when there is flow or movement through the pipes. If there is no velocity or movement through the pipes, there is no friction. If one exists, the other exists.
Velocity and friction will remove or absorb energy from the liquid. Some of the liquid’s energy is absorbed to maintain the liquid velocity through the pipes. Some of the liquid’s energy is lost as friction between the liquid and the internal surfaces of the pipes and process equipment.
Liquid velocity and friction losses can be measured and reported accurately with pressure gauges. Or you can estimate these losses with some pipe resistance tables, or a computer program. The pump must replace this lost energy, or generate additional energy to compensate for these losses.
Here is an example: I need a pump to complete some operation at 800 gallons per minute. My pipe system elevates the liquid into a tank 65 feet above the liquid level in the suction vessel. This is 65 feet of elevation differential.
The suction tank and the elevated discharge tank are both exposed to atmospheric pressure. There is no (0 feet) pressure differential to consider.
Velocity and friction losses are 25 feet through the pipe system at 800 gpm. Total head is 90 feet (65 ft. + 0 ft. + 25 ft.). This pipe system requires a pump with best efficiency coordinates at 90 feet of head at 800 gpm.
My pipe system contains 90 units of energy (called feet). My pump develops 90 units of energy (called feet) at 800 gpm. The pump’s energy is balanced with the system’s energy. This pump, the seal and bearings will provide many years of trouble-free service in this system with minimal maintenance.
Mating the best pump into a system, and correcting a mismatched pump and system is easy once you dominate the basics. It requires practice, like working a Sudoku puzzle or riding a bicycle. The more you do it, the better you get at it.
I’ll write more on this for next month’s Pump Guy entry. So commit this article to memory, or stash this edition of Flow Control in your locker.
P.S. The next Flow Control Pump Guy Seminar will be in Indianapolis, Indiana, June 16-18, 2015. Contact Matt Migliore at email@example.com for specifics. For my full schedule of upcoming pump trainings, visit www.bachusinc.com/seminars.html.
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, Tennessee. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. Contact him at firstname.lastname@example.org.