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Alternating Pumps

September 26, 2010
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Larry Bachus
(a.k.a. "Pump Guy")

Critical pumps are frequently installed in pairs, side by side, so one pump can operate while the other is at rest or in the shop. Usually, one of the two pumps is called the “primary” pump, and the other pump is considered the “stand-by” or “back-up” pump.

It is best to abandon the concepts of primary pump and stand-by pump. Alternate or switch these pumps once or twice per month, or about every 1,000 hours of operation.

This practice exercises both pumps to extract the most production from the investment in the equipment. There are a number of benefits to alternating or switching pumps in a process.

Consider the Pump Bearings
As lubricated bearings rotate, the oil or grease coats and cushions the rotating parts with a protective film. The protective film is constantly replenished as the bearing spins.

When the equipment sits idle, the weight of the rotating assembly (impeller, shaft, sleeve, wear bands, and mechanical seal) rests on the bearings. With time, the lubricating film is displaced by the weight of the equipment. This leaves metal-to-metal contact between the stationary balls and races of the bearing assembly.

Vibrations travel through the ground and the piping from nearby pumps, motors and compressors. This causes Brinell damage inside the bearing as the stationary balls and races vibrate against each other without the protective lube film.

Consider Lubricant Contamination
Many process pumps are shipped from the factory with a breather cap on the bearing housing. When idle, the sun heats the bearings at mid-day. The breather cap allows hot air to escape.

At night, colder, moisture-laden air is drawn into the bearing housing through the breather cap. The next day, the sun causes the moisture to condense on the internal ceiling of the bearing housing. Water droplets fall into the oil. This fouls the lubricant.

Consider Secondary Elastomer Components
Most process pumps have gaskets, rubber lip seals, and mechanical seals with o-rings and other sealing elements. Rubber has a natural desire to flex and stretch.

When o-rings are compressed and contained in a fixed enclosure for extended periods with heat, the o-rings re-cure and redefine the interference fit, assuming the shape of the containing groove or slot. The industry calls this “compression setting.” The heat causes the o-rings to harden and vulcanize to the enclosure.
All storeroom techs and shop mechanics know that rubber sheet gaskets will dry rot with time on the shelf. These gaskets will also dry rot inside idle equipment. Oil lip seals work by deformation and compression on the pump shaft. When the shaft is idle for long periods, the rubber lip dries, hardens, loses its elasticity and vulcanizes to the shaft. The sealing lip will rip when the pump starts. Oil leakage results.

The Pump Guy column has appeared monthly in Flow Control magazine since 2006. Numerous times in this column, I’ve stated that pumps go into the shop because of mysterious bearing and/or seal failure. Notice how idle “stand-by” pumps lead to unnecessary failure and maintenance.

Here is another reason to alternate “A” and “B” pumps – vibration analysis. Vibration analysis is the key element of many reliability programs for rotating equipment. The equipment must operate to collect vibration data. You cannot collect vibration data on an idle pump.

Imagine your recently rebuilt stand-by pump sitting idle in-line for eight months with a bent shaft or unbalanced impeller … and no vibration data? What’s gonna’ happen at the critical moment when you need that pump?

So, there are numerous reasons to swap your “A” and “B” pumps in a process and abandon the concepts of the primary and stand-by pump.

Let me offer some specific guidelines on alternating your pumps:
Designate and identify your pumps as “A” and “B”. I suggest you label, mark and paint your pumps. Let’s say that pump A is operating. You will switch to run pump B. With pump A still operating, follow your start-up procedure and add pump B to the process. Observe pump “B” for 10 to 20 seconds as it comes up to speed. Verify that pump B is healthy and contributing to the process with an opened check valve.

Pumps A and B were installed into the system to operate alone. They are not parallel pumps. They are not designed to run together. Both pumps will be stressed as they try to pump twice the flow through a discharge pipe meant for the flow of one pump. So, don’t waste time and move briskly through this procedure. When you are satisfied that pump B is healthy and contributing, shut down pump A.

The check valve on the discharge pipe of pump A should close. Verify no reverse or re-circulated flow through pump A. If the check valve holds on pump A, then restrict (throttle) the control valve on the discharge of pump A about 80 percent (20 percent open). This prepares pump A for start-up.

If the check valve on pump A leaks and passes fluid, then close the control valve and isolation valve on pump A to ensure the flow to the process (with pump B). This also protects pump A from damage. With pump B operating, schedule repair on the check valve to pump A.

The fruits of good operation are reliable pumps, extended seal life, long bearing life, cooler running temperatures, less vibration and noise, energy conservation, and scheduled (preventive) maintenance.

We’ll discuss these and many other issues at upcoming Pump Guy Seminars. For a full course outline and to register for this event, visit
www.flowcontrolnet work.com/events.asp.

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 larry@bachusinc.com or 615 361-7295.

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