Avoid repeat pump failures

Repeat failures are an indication that the underlying failure causes either haven't been discovered or have been ignored.

By Heinz P. Bloch, P.E., Process Machinery Consulting

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In brief:

  • Most pump problems develop gradually. Others manifest themselves sporadically.
  • The onset of pump problems is not the same for different pumps or different services.
  • The vulnerability of operating process pumps in parallel is not always appreciated by pump purchasers.

Equipment failures cost money. Repeat failures are an indication that the underlying failure causes either haven’t been discovered or have been ignored.

But if your facility has 600 pumps and they survive an average of three years, there will be 200 repair events per year. Even at a mere $6,000 per repair, that’s $1.2 million each year. Suppose your competitor knows how to avoid repeat failures and sends his pumps to the shop only every six years. His yearly outlay will be $600,000, and his repair and maintenance crews can be deployed for failure prevention tasks instead of fix-up tasks.

So, what exactly is it that best-of-class users do to set themselves apart from the rest?

Pumps have a defined operating range

We start with the obvious. All machines have a defined operating range. A Boeing 787 cannot land on a 300 ft (100 m) landing strip. It cannot fly at 50 knots. A sailplane can do all of that; it’s designed for different performance and seats two people. It’s no different with process pumps; they, too, have operating ranges and performance limitations.

Pump & Turbo

Heinz P. Bloch, owner of Process Machinery Consulting, will be leading a tutorial, “Breaking the Cycle of Pump Repairs,” on Oct. 1 at 2:00 PM and again on Oct. 2 at 10:30 AM  at the 29th Pump and 42nd Turbomachinery Symposia in Houston.

Most pump problems develop gradually. Others manifest themselves sporadically. They could be operations-related vibration excursions due to fluid vaporization, but hydraulics influence pump life. Perhaps we can draw parallels with human illnesses. Most serious health issues develop gradually. A few can be easily cured and go away after a while if we make wise decisions. Some illnesses manifest themselves with extreme suddenness and can have devastating consequences. All of these problems could have been diagnosed earlier if we had used more time, money, and well-targeted efforts.

The onset of pump problems is not the same for different pumps or different services. Attempts to identify best practices led Paul Barringer and Ed Nelson to explain the effects of deviations (Figure 1). While focusing on the best efficiency point (BEP), Barringer and Nelson plotted eight traditional non-BEP problem areas on a representative H/Q curve. The plot supports the notion that pump reliability can approach zero as one operates farther away from the BEP. At some combination of age, load, speed, temperature, or whatever, reliability goes to zero with every conceivable creation of man.

Figure 1. The Barringer-Nelson curve shows reliability impact of operation away from BEP.
Figure 1. The Barringer-Nelson curve shows reliability impact of operation away from BEP. (Source: Paul Barringer)

The implications of the Barringer-Nelson curve are easy to visualize. Just because pumps are able to run at lower-than-BEP flows doesn’t mean it’s good to operate there. Compare it to a vehicle able to go 12 mph in sixth gear, or 57 mph in first gear. It can be done, but will likely prove costly if done for very long. Pioneering efforts to define minimum allowable flows can be traced back many decades and attention is drawn to the sketch by Irving Taylor (Figure 2). His work is worth mentioning because Taylor approximated in a single illustration what others have tried to convey in complex words and elaborate mathematical expressions. Although Taylor’s relationships are typical at best, he deserves much credit because he kept the average user in mind. More scientific approaches were documented by Taylor’s very famous contemporary, Igor Karassik in “Pump Handbook.”

Figure 2. Pump manufacturers usually plot only the NPSHR (subscript R) trend associated with the lowermost curve. At that time a head drop or pressure fluctuation of 3% exists at BEP flow.
Figure 2. Pump manufacturers usually plot only the NPSHR (subscript R) trend associated with the lowermost curve. At that time a head drop or pressure fluctuation of 3% exists at BEP flow. (Source: “The Most Persistent Pump-Application Problems for Petroleum and Power Engineers,” by Irving Taylor)

Taylor suggested a demarcation line between low and high suction specific speeds (Nss numbers) at somewhere between 8,000 and 12,000. His data are supported by surveys taken after 1977 at Amoco in Texas City, Texas, by Nelson and Jerry Hallam; there also were other plant locations which pointed to suction-specific speeds of 9,000 or 9,500 as Nss numbers that deserve attention. Many pumps with Nss numbers higher than approximately 9,500 will degrade when being operated at flow rates much lower or higher than BEP. By how much the life expectancy or repair-free operating time of these pumps will be reduced is speculative, at best. Whether these life reductions will amount to 10% off normal or 60% off normal is the subject of much debate and requires reviews on a pump-specific basis.

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