A Standby Pump Can Be Damaged While It Stands Still

What changes when the shaft stops

In a turning bearing, each rolling element keeps moving onto fresh raceway and lubricant is carried back into every contact. When the shaft stops, that renewal stops. A few rolling elements in the lower part of the bearing, the load zone, now carry the weight of the shaft at the same spots, hour after hour (SKF, p. 26; NTN-SNR, Causes of bearing failures, PDF p. 22).

How lubricant reaches those spots changes too. In an oil-bath housing the oil settles to its static level. SKF asks for that level to reach the middle of the lowest rolling element at standstill, with the sight glass marked for both the static and the running level (SKF, p. 14).

The margin is small: Flowserve's Mark 3 manual, for a housing without the optional oil slinger, keeps the level within about 3 mm of the sight-glass centre (Flowserve 71569102, section 5.3.1.1). So a standby's oil has to be read against the static mark, and a small loss matters.

Where an oil ring feeds the bearing, it lifts oil only because the turning shaft drives it, so on a stopped standby it lifts none (SKF Evolution, oil pick-up ring; Sulzer, US10274015B2, background). A grease-lubricated bearing keeps its grease in the housing, but grease pushed out of a loaded contact may not find its way back while the shaft is still.

How a running neighbour marks a still bearing

Pump A shakes the plinth and the shared suction and discharge pipework, and through them pump B. In B's bearings the loaded rolling elements rock back and forth over a tiny distance without ever rolling away. NTN-SNR's failure guide, PDF p. 22, describes the chain: lubricant is forced out of the contact, metal touches metal, microscopic welds form and tear, and the loose particles oxidise and act as an abrasive that speeds the wear.

The result is a row of shallow depressions in the raceway at the same spacing as the rolling elements. This is false brinelling. SKF's handbook, following the damage classification in ISO 15243, files it under frictional corrosion (SKF, pp. 40 and 41; SKF Evolution on ISO 15243:2017). In a grease-lubricated bearing the marks are typically reddish-brown; with oil they can look mirror-shiny (SKF, p. 52).

The marks are a history of the stops, not just the starts. SKF shows several sets at roller spacing in a bearing from auxiliary equipment with long idle periods. Vibration level, frequency and time at rest all affect how much damage forms (SKF, p. 52).

Pumps are not the only machines exposed. NTN-SNR gives an emergency generator on the same platform as a running one, and an emergency fan connected with an operating fan (NTN-SNR, PDF p. 22). In each case the bearing can be damaged while the machine stands by, and the first sign may be noise when it is finally needed.

Water collects while the pump waits

Vibration is not the only standstill risk. A housing that warms while running and cools after it stops can draw in humid air, and the moisture condenses on the bearing. NTN-SNR describes this cycle for a bearing in a very damp atmosphere that is started and stopped often: water builds up a little with each stop and corrosion spreads through the bearing (NTN-SNR, PDF p. 28). A standby that is run long enough to warm up and then stopped in damp weather can go through the same cycle.

Free water that does get in settles to the bottom of a stopped bearing and can etch the raceway, again at rolling-element spacing (SKF, pp. 28 and 50).

Before relying on a standby that has sat through a humid season such as the monsoon, check its drain or low point for water. Baldor's storage procedure for large motors asks for the same check at the drain before a stored motor that has been exposed to humid weather is operated (Baldor MN439, p. 1). Our article on water in oil covers the laboratory side of a water result.

Why no single rotation interval fits

The two bearing makers agree on the broad remedy. NTN-SNR says periodic rotation of a stationary machine in a vibration area, even slowly, lets the lubricant spread and keeps the load off the same spot. SKF says to rotate the shaft of standby machinery periodically, and adds that isolating the machine from the vibration is a real solution, though not always a practical one (NTN-SNR, PDF p. 22; SKF, p. 26).

Neither says how often. That has to come from the manual for your machine, and manuals give different answers because they describe different machines in different states.

Read the state column before comparing intervals. KSB gives three answers in one manual: storage, prolonged shutdown and installed standby (sections 3.3, 6.3.1 and 7.2.1). The motor needs its own instruction too.

Conditions can change that instruction. Baldor's monthly turn becomes weekly above 60 percent relative humidity in storage (MN439, p. 3). Even two WEG documents give different intervals for their respective motors (guide 50031071; T Line manual, section 3.3.2.6). Those differences are reasons to find the applicable document, not to average the intervals.

Most of these intervals are written for storage. Among the motor documents, only the WEG T Line manual also applies its routine to a motor that is installed but stopped for two months or more (WEG 12364818, section 3.3.2). For an installed standby, Flowserve's maintenance checklist asks daily whether the pump is at the applicable temperature and available to start, and gives no rotation figure at all (Flowserve 71569102, section 8.2).

The stated reasons differ as well: deposits in the pump, sticking seal faces, brinelling, rust, grease distribution. Each instruction can be right for the document that gives it. None of them is an instruction for your pump.

Find the instruction for your machine as it stands today

Start by naming the state. A pump in the store, a pump installed but not yet commissioned, a standby on auto-start and a unit in a long shutdown are four separate situations, and the instruction you need belongs to only one of them.

Collect every document for the pump set: the pump manual, the motor manual and any separate gearbox, coupling or seal instructions. Search them for storage, shutdown, stand-by, standby, prolonged, rotate and turn. For the state you named, write down:

• the maker, document number, revision and section;
• the task: turn by hand or run, how many turns or minutes, and how often;
• the conditions attached, such as humidity, oil level or the process conditions a run needs;
• the reason the document gives, if it gives one.

If the pump and motor documents disagree, or none covers the state your machine is in, ask the manufacturers and file the reply with the record. Do not borrow the interval used on the pump next door.

Two safety points belong in the written task. Turning a shaft by hand means working on a machine that may be set to start automatically. KSB's manual tells the operator to make sure the pump set cannot be started unintentionally before working on it, so the turn needs the site's isolation procedure (KSB 1969.85/03, section 7.1).

A run is a real start, so it needs the conditions the manual sets for one, such as enough liquid supplied for the check run and a primed pump (KSB, section 6.3.1; Flowserve, section 6).

What a turn shows, and what a run shows

Turning the shaft moves the loaded rolling elements to new spots and lets lubricant spread over the old ones. It also shows that the shaft is free, which is why Flowserve's pre-commissioning checks end with a hand turn (Flowserve, section 6). It cannot show that the pump will start, deliver its flow or run quietly.

When you turn by hand, finish in a different position each time, as WEG asks for its motors in storage (WEG guide 50031071, section 2). A paint mark on the coupling lets the next person see where the shaft was left.

A planned run turns the bearings at speed and sets any oil ring working again. It also gives you operating readings, provided the required start conditions are met. Whether to turn, run, or do both still belongs to the applicable manual and the machine's owner.

Keep the comparison fair. Record speed, duty or load, the pumped liquid and its temperature, ambient conditions, measurement points and method, and the time elapsed since starting. Compare with B's earlier runs under similar conditions. A short check run is not directly comparable with a duty pump that has been running for hours. If conditions differ, record the difference before interpreting the readings.

The Flowserve Mark 3 manual, Table 6, explains that bearing temperature depends on operating conditions and takes time to settle. Use the requirements for the actual machine, not a borrowed settling time.

The automatic handover is a separate test for the controls and operations owners. A planned run does not establish how quickly B takes over after an unexpected loss of A. For readers looking after data-centre cooling pumps, Reliability Engine's Redundant on Paper works through that gap for a coolant distribution unit's spare pump.

Give the standby its own record

A standby does not age like its twin. It has fewer running hours, long spells at rest, and its neighbour's vibration arriving through the plinth and pipework. It needs its own history, not a line in the duty pump's file.

A record is worth keeping only if some entries change what happens next:

• Water at the drain or low point. Follow the manual's response before relying on the pump, and trace where the water came from.
• Oil off its static mark. Correct the level and find out why it moved.
• A shaft that is hard to turn. Do not start the pump until the cause is known.
• Changed running condition. If B is noisier, hotter or rougher under comparable conditions, record the difference and follow the site's abnormal-operation procedure. Refer an unexplained change for condition assessment. Compare with A only when its operating conditions and measurement method are comparable too.
• A missed turn or run. Record it as missed, with the reason and the next action. A completed task and a healthy machine are two different things; the record needs evidence of both.

If B's oil is sampled, label it as B's and say whether it was taken after a run or at standstill. Our article on where you take an oil sample explains why the operating state belongs on the label.

When a standby bearing comes out, keep the evidence

Keep the removed bearing unwashed. NTN-SNR's guide is direct about this: never clean a bearing before examination, because the lubricant and any particles in it are part of the evidence (NTN-SNR, PDF p. 10). Note which bearing it was, which way round it was fitted and where the load zone sat.

Marks at rolling-element spacing show that something happened while the bearing was not turning, but more than one cause leaves that pattern. Vibration at standstill gives shallow, worn false brinelling marks. Water at standstill gives corrosion or etching. A static overload, such as a mounting force pushed through the rolling elements, leaves dents at the same spacing (SKF, pp. 50, 52 and 56).

Each points to a different fix: isolation and rotation, sealing and drainage, or handling and fitting practice. So have the bearing examined by someone qualified in damage analysis before deciding which fix the standby needs.

Start with one pair

Choose one pair and check three things: B's present state, the pump and motor instructions that apply to it, and the evidence from its last comparable run. If one is missing, make it a named action rather than a blank nobody owns.

Write the document numbers on B's record and mark the coupling position after each required hand turn. On the next planned run, keep the operating conditions, elapsed run time and measurement method with the readings. That gives the next check a usable baseline.

The next unexpected trip is a poor time to discover that nobody knows how B sounded on its last planned run.

Related training

Machinery Lubrication I (MLT I / MLA I) covers lubricant application, storage and handling, contamination control and field inspections, the skills a standby routine draws on. Practical Industrial Lubrication Orientation Training (PILOT) is an in-house, plant-floor course for the people who grease, top up, inspect and handle lubricants.