Objective vs Subjective Salesmanship
This is in regards to the article "Objective vs. Subjective Salesmanship", from the February issue of WWJ. I agree that a "Subjective" or Sales Engineer can be so focused on his or her own product, that they try to make the entire system fit their own products specs. However, I also believe that an "Objective" or Systems Engineer is so focused on all aspects of the design, that he or she usually doesn't understand the intricacies of the individual components.
On the subject of Valves vs. VFD's, I also find it distressing that many Objective and Subjective engineers still so myth-guided, which leads to myth-applications in the system.
Myth #1 is that VFD's save energy. This is absolutely false. While maintaining constant pressure, even though the amps are reduced as the flow is reduced, a VFD always causes more energy to be used per gallon produced. This has become less of an issue as more engineers are finally beginning to understand.
Myth #2 is that Valves make pumps work harder. This is also absolutely not true. Restricting the flow with a valve will cause the amp draw to decrease as the flow rate decreases, almost exactly the same as when slowing the RPM with a VFD.
Myth #3 is that back pressure from a valve will destroy a pump. Not true! Extra back pressure only makes a pump think it is in a deeper well. Any pump is capable of a wide range of flow and pressure, without varying the speed, and without harming the pump. In a shallow well a pump will simply produce more flow than if in a deeper well. The extra back pressure from a control valve reduces the flow rate by making the pump think it is in a deeper well.
As for the example in the article, the first pump curve I looked up shows a 100 HP pump, producing 1,000 GPM, at 320' of head, using only 2 stages. This particular pump deadheads at 385', which is only 65' or 28 PSI more pressure than the design head. There is no way a 2 stage pump, that only produces and extra 28 PSI of back pressure, is going to overload a thrust bearing. It is almost impossible for a pump with a 200' setting to be able to build more back pressure than the thrust bearing can withstand. The lame excuse of excessive back pressure being the "single design factor" that eliminates the Control Valve as the best option, means the system engineer doesn't know, and is listening to myth-information from a VFD sales engineer.
Myth #4 is that a VFD delivers a wider range of flow than a Control Valve. Again, not true! Derating the motor with a Control Valve means that 5 GPM is the minimum safe flow rate for most large pumps. While creating a smaller motor from a larger one (not derating) using a VFD, requires a minimum flow of 55 GPM (8" motor in 10" casing) to be safe for the motor. Again, it sounds like someone is letting a VFD engineer blow wind up their skirt.
If we are looking for a "single design factor" that can override all of the advantages of a certain method, then we have plenty to choose from with VFD control. Harmonics or stray voltage, high voltage spikes to the motor, resonance frequency vibration at critical speeds, motor cooling at minimum flow, technical assistance required, system dependability and longevity, destructive bearing currents, environmental concerns for the placement of the VFD device itself, and many other negative side effects are present with VFD control. None of these side effects exist when using a Control Valve.
The lesson to be learned here is: Ultimately the installer or contractor is responsible for the final installation, not the "sales engineer" or the "systems engineer". The "systems engineer" will always take full credit for anything that goes right, and contractually passes the blame for anything that goes wrong to the installer. As the person with the responsibility, the installer must make sure that all of the component parameters are included and weighed before selecting and implementing any product. This includes the selection of an engineer. Selecting an engineer who understands the myths listed above, can be the best way to separate good engineers from those who can cause disasters by myth-applying certain products.
It could be said that "Subjective" or Sales Engineers learn more and more about less and less, until they know everything about nothing. While "Objective" or System Engineers learn less and less about more and more, until they know nothing about everything.
Cary Austin
Cycle Stop Valves, Inc.