(a.k.a. “Pump Guy”)
Imagine a spectrum of automobile design and operation. The economical vehicle is on one end of the spectrum; on the other end is the high-performance vehicle. The dilemma facing drivers is performance versus economy. Drivers want high-performance, fast-accelerating cars. Drivers also want fuel-sipping, high miles-per-gallon, economical autos. Most auto manufacturers make high-performance cars and economical models too, but you can’t have both features in the same car.
There is no fast-accelerating, high-performance, yet economical car with today’s technology. (Maybe in the future.) If you want economy with a high performance vehicle, you must:
• Drive it conservatively;
• Accelerate slowly;
• Respect preventive maintenance schedules (oil change, tire pressure, etc.);
• Understand and interpret the gauges on the dashboard;
• Turn off all energy-eating accessories; and
• Practice economical driving habits until you really need a blast of high power.
If you’re willing to make these sacrifices, perhaps you can drive a high-performance car that is relatively economical as well. Now let’s consider a similar dilemma in process pump design and operation. To begin, new piping systems are designed so that when all valves are wide open, all pipe is new and free of scale, and all filter cartridges and strainer baskets are new and clean, the pump will perform at or close to best efficiency. Design engineers study pipe schedules, friction tables, Reynolds numbers, and the formulas of Hazen-Williams and Darcy-Weisbach as they learn to properly mate pumps into piping systems.
New pumps and pipe systems are designed this way so the pump will operate at, or close to, best efficiency. Consistent or extended pump operation to the left or right of best efficiency causes stress to the pump. To understand this stress, imagine driving a car in first gear at 60 mph. This would stress the motor and transmission. Imagine operating a car in fourth gear at 10 mph. This would make the car jerk and shudder, leading to damage.
Operating a process pump away from the best efficiency zone leads to mysterious, premature pump, seal and bearing failure. It causes unplanned pump maintenance and reduced production as MTBF stalls against a 20- or 30-month ceiling. This is the complaint of too many reliability engineers.
Pumps are employed to overcome the resistance in the piping system. Energy is required to push a liquid through a filter screen. There is more resistance through a dirty filter and less resistance through a clean filter screen. A process pump migrates on its curve as the filter clogs.
Small-diameter pipe offers more resistance. Larger-diameter pipe offers less resistance. A throttled valve offers more resistance. A totally open valve offers less resistance to the pump. The pump constantly migrates on its curve in a dynamic system.
High resistance drags the pump to the left of best efficiency. With less resistance, the pump migrates to the right of best efficiency. Engineers use the term “system upset” to describe an abrupt change in resistance.
A system upset (loss of resistance) can drag a pump to the right of best efficiency on the performance curve. This type of upset is relatively easy to resolve. An astute pump operator or engineer can increase the artificial resistance on the system. For example, the operator can throttle a downstream open valve. You can add artificial pressure to a downstream header. (This is the reason process gases exist.) The increased artificial resistance will push the pump back into best efficiency.
A good suction energy (NPSHa) margin over NPSHr will reduce the stress on a pump that consistently operates or migrates to the right of best efficiency. Pump designers can increase the diameter of the eye of the impeller to reduce the NPSHr and increase the margin to avoid cavitation. The impeller eye receives the liquid arriving into the pump through the suction nozzle.
However, other system upsets (too much resistance) can push the pump to the left of best efficiency. These upsets are difficult, time-consuming, or even dangerous to resolve. For example, how can you further open an already wide-open downstream valve? Can you stop production now and install larger diameter discharge piping on the pump? Can you vent and relieve a downstream header or tank containing hydrofluoric acid, or di-methyl formamide, or methyl iso-cyanate? OUCH!!! Probably not!
The options are limited to resolve a system upset that consistently drags the pump to the left of best efficiency. Therefore, it is a good idea to design the pump to operate to the left of best efficiency with less stress. To do this, pump designers can reduce the diameter of the eye of the impeller.
An impeller with a reduced diameter eye can better withstand pump operation to the left of best efficiency with low stress. Engineers want this feature when the system drags and holds the pump to the left of best efficiency for extended periods.
An impeller with a larger diameter eye reduces the energy required (NPSHr) by the pump. Engineers frequently specify pumps with low NPSHr to avoid cavitation. However, the impeller’s large diameter eye (with low NPSHr) restricts the pump’s ability to operate with minimal stress to the left of best efficiency.
This is the basis of the Suction-Specific Speed (Nss). Consider the formula:
• RPM is a function of the driver speed.
• Flow (gpm) is a requirement of production to meet marketing and sales projections.
• NPSHr is a pump design consideration.
The NPSHr is the denominator in the equation. The Nss rises as the NPSHr reduces. The Nss drops as the NPSHr rises. Low Nss reduces the pump’s stress when operated to the left of best efficiency. Low NPSHr is desirable to avoid cavitation. Herein lies the dilemma.
A plant engineer can specify a pump with low Nss for extended operation to the left of best efficiency. Or, the engineer can specify a pump with low NPSHr to avoid cavitation to the right of best efficiency.
Do you want low suction-specific speed? Or, do you want cavitation resistance? These two designs are mutually exclusive. You can have one, or the other, but not both in one pump with today’s technology. The Nss formula can indicate a trade-off for a modest balance of the two features.
Some specification engineers have latched onto Nss 11,000 as a magic number. It is nonsense to accept a pump with Nss 10,750, and reject a similar pump with Nss 11,001. Regarding pump operation in a dynamic (constantly changing) system, there is no discernable difference between these two numbers.
If the process engineer insists on operating pumps with no gauges, no curves, and no operator training to deal with system upsets, then he/she should specify his/her pumps with Nss 6,000 or less, and abandon the 11,000 number.
I admit Nss 6,000 is as unrealistic as Ns 11,000. What is realistic? How can a specification engineer incorporate suction-specific speed into a pump selection?
Have a meeting (in person or conference call) with the process engineer, the production engineer, and all involved in purchasing the pump. Ask some questions. I can’t cover all of them here, but I’ll offer two examples.
Example #1: Imagine a system where the pump will fill a standing tank from the bottom at 2,000 GPM with a four-pole motor. You want to know the resistance in feet against the pump when the liquid level in the tank is at its lowest level for production. You also want to know the resistance in feet against the pump when the liquid level in the tank is at its highest production level.
Let’s say the pump operates at 100 percent of design when the tank is at lowest production level. And the pump operates at 60 percent of design when the tank is at its highest production level (high resistance against the pump).
Now you know how the pump will migrate on its curve as the liquid level varies in the tank. This is the performance window for this pump on its curve.
Next, you want to know the frequency at which the tank is at its maximum level. For example, does this occur once per week for less than five minutes? If so, then don’t worry about it.
However, you must consider the implications if the high tank level offers maximum resistance against the pump four times per day for two hours each time. You want a pump that can operate safely for eight hours per day at 60 percent of design and as much as 16 hours per day somewhere between 60 percent and 100 percent of design. This will suggest a more realistic Nss 9,700 or less, rather than an arbitrary 11,000. (See graph.)
Example #2: Imagine the bottling operation at a “bottled water” plant. A bottling carousel can fill five-gallon jugs of water at 100 jugs per minute, plus or minus one jug, in an eight-hour day. The pump is designed at 500 gpm to the carousel. If the carousel jams, or ceases to receive empty jugs at the carousel loading point, a sensor shuts off the pump.
This pump runs eight hours per day at 99 percent to 101 percent of design. Forget Nss. If you used the magic number of 11,000 as your sole criterion, you’d reject a perfectly good pump based on Nss.
The Pump Guy advises: Design the piping system with included artificial resistance to resolve system upsets in both directions. Most design engineers don’t include artificial resistance in their designs. This is a major flaw resulting in runaway pump maintenance. Train the operators to interpret curves and gauges.
Install and key the instrumentation so the pump operates conservatively at 90 percent to 110 percent of design as much as possible. (I show how to do this in my pump lectures.) Assure adequate suction energy (NPSHa margin over NPSHr), at all potential duty points. Then, the suction-specific speed issue becomes a moot point.
Maybe you are wondering how to design a piping system to resolve upsets in both directions? Maybe you want to contain runaway pump failure? I’ll show you how to break through your pump reliability barriers at the Pump Guy Seminars in 2010. We have upcoming dates in Houston & LA.
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 firstname.lastname@example.org or 615 361-7295.