The famous Swiss psychiatrist and psychotherapist Dr. Carl Jung is often quoted as saying, “What you resist persists.” I always took it to mean that because I had problems with calculus in college, I would always be in professional positions that would require me to be proficient in the subject. I was painfully correct.
If you do not set goals and face these situations, the things that you resist will persist by repeatedly popping up, typically at the most inopportune times. In this new year, I propose we work on items on this “resist” list of common damage-causing issues for pumps.
1. NPSHA Mystery
The subject that requires the most attention is understanding net positive suction head available (NPSHa). Some people resist learning about it because the name and subject can be difficult and confusing. Others believe they know all about the concept, but their calculations and applications prove otherwise. Their pumps consequently cavitate, incurring expensive damage and downtime.
The “resisting” category also includes anyone who thinks that NPSHa is purely a function of suction pressure, suction energy and/or static height (submergence), as in having a flooded suction. I propose that the subject is really not that difficult. There are numerous sources of information and instruction on this topic. Pumps & Systems has published many articles on the subject—several in the last couple of years. A good starting place is my Pumps & Systems column from August 2015 titled “Rethinking NPSH” (read it here).
It all comes back to knowing the basic formula in Equation 1.
2. Best Efficiency Point
Operating the pump away from the best efficiency point (BEP) is the second most common issue affecting pumps. I acknowledge that, in many applications, operators or owners are helpless to do anything about the situation because of circumstances beyond their control. Others can look to change something in the system that allows the pump to operate where it is designed to run.
Just as someone should not drive a car down the interstate in first gear, end users should not operate a pump near the shutoff point. Helpful options include variable speed operation, trimming the impeller, installing a different size pump or a different model pump, and/or changing something on the system that could be as easy as installing a three-way control valve or an orifice.
3. Pipe Strain: The Silent Pump Killer
A pump is the most expensive piping hangar you can purchase. It seems that piping systems are often not properly designed, installed and/or anchored. Provisions for thermal expansion and contraction are often not considered.
I observed a situation where pipe strain was the suspected root cause of bearing and seal issues. After we instructed the mechanics to remove the pump foundation bolts, the 3,200-pound pump was lifted several inches by the piping. For those who are not sure, that is an example of severe pipe strain.
Another way to check is to place dial indicators on the coupling in the horizontal and vertical planes, then unbolt either suction or discharge piping. If the dial indicators show more than 0.002 inches of movement, there is too much pipe strain. Repeat for the other flange. The coupling should have inserts/spacers removed.
4. Preparing for Startup
Pumps of any size, other than fractional horsepower close-coupled, rarely arrive at the final site ready to start up. Pumps are not “plug and play.” End users must add oil to the bearing housing, set the rotor and impeller clearances, set the mechanical seal, and perform a rotational check of the driver prior to installing the coupling. Read and understand the instruction book.
The alignment of the driver to the pump is critical. No matter how well the pump was aligned in the manufacturer’s facility, the alignment was lost the second the pump was transported. If you aligned the pump when it was set in place, it was likely lost when you connected the piping.
6. Oil Level & Cleanliness
More oil is usually not better. In ball bearings with splash lubrication systems, the optimal oil level is when the oil is touching the very bottom of the bottom ball. Adding more oil just increases friction and heat. Remember this important point: The biggest cause of bearing failure is contaminated oil.
7. Running the Pump Dry
Submergence (simple submergence) is defined as the distance (D) measured vertically from the liquid’s surface to the centerline of the inlet suction pipe. A more important descriptive term is required submergence, also known as minimum or critical submergence (SC).