Ron Carlson

The affinity laws below are well known and used daily by pump people.

Per standard nomenclature, Q is flow rate, H is head, P is power, and NPSH3 is Net Positive Suction Head for a 3 percent head drop. 

For single stage pumps, these equations work very well. Some corrections need to be made when modifying multistage pumps, particularly when changing the diameter. When trimming impellers, the increased vane tip clearance will lower the efficiency and performance a couple of percentage points. Usually about 1.5 percent per stage. If you are trimming impellers on a 10-stage pump, you will need to reduce the amount of trim by roughly 15 percent to avoid reducing the capacity and head below what you would originally expect from the affinity laws. The power saved would also be lowered because of the reduced efficiency.

NPSH3 is not affected by changing the diameter because the impeller eye does not change. However, real world fluids do not behave exactly according to the affinity laws. For hydrocarbons, an exponent of 1.8 will give better predictions than the theoretical 2. Some chemicals, such as caustic, can use and exponent of 2.1 to 2.2 for better predictions. User experience with each liquid will refine your technique. Unfortunately, industry does not run tests for these affects, and the few that do, do not share or publish the results.

From the equations above, you would not understand why the various methods of achieving high head vary so greatly in efficiency and power requirements. Why is a large diameter impeller turning at motor speed so much less efficient than the smaller impeller turning much faster and a multistage pump more efficient still? The answer is the friction developed by the impeller disc as it moves through the liquid. The friction power modifies the power equation by the following:

The 5 for an exponent means the efficiency decreases very rapidly as the diameter increases so a large diameter impeller requires much more power than a smaller impeller. From the original power equation we learn that speed carries a cubic exponent. So a multistage pump running at a lower RPM will be more efficient than a pump with a small impeller turning at high RPM which will be more efficient than a low speed pump with a larger diameter.

This information does not have much affect on low power applications. However, when a multistage pump may be greater than 75 percent efficient and a high speed pump is 25 percent efficient and a large diameter pump would run below 10 percent efficiency the cost of power over the life of the application can make a significant difference in the life cycle cost of the pump system.

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Pumps: The Affinity Laws and Beyond

After determining how long you want the equipment to last and identifying the process and fluid properties, it is time to select the pump.  Factors for minimum life cycle costs include installation, maintenance, energy, safety and environmental and possibly other considerations.

A conventional pump will require a foundation which will cost in the neighborhood of $10,000 to $15,000 +.  Most canned motor pump (CMP) should be mounted on stilts for a few hundred dollars (this can also help reduce pipe stress).  From the pumps I had quoted over the last decade, a conventional API pump costs roughly twice that of a canned motor pump that meets API 685 and includes a bearing wear indicator.  You will need to communicate with your vendor about the specifics for this.  The purchase differential becomes much greater when you add seals and support systems to meet current safety and environmental regulations.

The facility where I worked was very careful to insure a CMP was selected to meet the process conditions and system needs.  The result was a cumulative mean time between repairs (MTBR) of over 15 years for a population of over 100.  22 CMPs were installed in our trial unit and in the next 11 years, one bearing replacement had to be performed for an MTBR of over 200 years.  Unless the liner of a CMP is breached, most repairs consist of replacing the bearings which can be accomplished in a few hours for under $5,000 for parts and labor.

CMPs are less efficient than conventional pumps when you consider just the pumps.  When the power to drive the seal(s), coupling and motor efficiencies are factored in, there are few percentage points’ difference between a conventional and CMP (sometimes the CPM uses less power).  Depending on the cost of electricity, this may be a deciding factor in your decision.

Canned motor pumps provide true secondary containment and, assuming proper materials were selected, will not leak to the environment.  This eliminates safety and environmental risk factors.  The pump may stop moving liquid, but there will not be a release to the environment.  Conventional pumps with mechanical seals will always have the possibility of leaking so mitigation measures may need to be installed.

Selection of CMPs requires complete and honest communication with the vendor as they are not appropieate for all applications (no one pump technology is). 

 In conclusion, canned motor pumps are much less expensive to install, maintain, and are competitive with conventional pumps for operating costs.  They also include the prevention of safety and environmental incidents.

Pump selection: Canned motor vs. conventional

For rolling element bearings that are not lubricated by a forced lubrication system, there are a few methods available to the user. Oil mist has been presented as the best method of lubricating bearings. It is the best and has some advantages.

As an automatic system, there is little operator action required other than to insure the resevoir has oil in it. By maintaining positive pressure in the bearing housing, contaminants from the atmosphere, moisture and dust, cannot enter the bearing housing. This assumes that high quality bearing isolators have been installed, as they should be for all installations. 

Oil mist can also save some energy by reducing the drag of the bearings through the oil. The downside is that oil mist systems are expensive to install, especially as retrofits to existing units. Oil mist can be installed as a supplement to a wet sump.

The most cost effective method of rolling element bearing lubrication is the wet sump method. Install high quality bearing isolators to minimize contamination. Also install a drain sight glass that will give a visual indication of contamination so the operator can drain water from the bearing housing or detect particulates that indicate more serious issues with the bearings. Utilizing a synthetic oil appropriate for the application will enable extending oil changes to 6 months, a year, or even longer. Consult your oil provider for recommendations.

Operators do have to check oil levels on their rounds to ensure that sufficient oil is available for the oil rings or flingers. Keeping the oil level below the bearings saves significant power by not forcing the bearings to plow through the oil and generate heat. Flingers are becoming the preferred method of oil distribution due to being positively driven by the shaft, but oil rings also work well if properly set up, including making sure the pump shaft is level. Following this guideline will provide bearing reliability that is comparable to oil mist at considerably less initial investment.

Vertical shafts present special difficulties as oil is slippery and gravity works to move it along the shaft and out of the bearing housing. There are magnetic bearing isolators that provide a more positive seal and minimize the leakage. Oil mist is better for vertical applications as there is no oil level to be maintained for bearing lubrication. Wet sump designs must have an oil pump to move the liquid from the sump to the bearings. The oil pump designs I have experienced work well from the factory, but are very difficult to repair while maintaining the tolerances necessary for efficient operation.

For more information on lubrication, PIP REIE686/API RP686, PIP REIE686A and API 614 are excellent sources for lubrication information. PIP stands for Process Industry Practices and is a consortium (non-profit) the strives to develop best practices for a variety of industries. Visit pip.org and take a tour. API is the American Petroleum Institute and is well known.