Interpreting the evidence

Let's interpret the physical evidence that you might see at these close tolerances and their source. To begin:

1. You might see gouge or wear marks all around the circumferences of close tolerances on the rotary elements, and a corresponding wear spot at approximately 60° from the cutwater on the stationary elements.

■ This would be induced or caused by operation, if the plant operators strangle valves to control production flow. Any other discharge flow restriction (clogged filter, pipe obstruction, or un-calibrated automatic valve) would produce the same evidence. Talk with the plant engineer about this situation and show him the evidence on the pump.

■ This could also be induced by design, if the pumps are oversiz.ed, or by high velocity and friction head in discharge piping of inferior diameter.

■ This could be induced or caused by maintenance in cases where the mechanic installs a check valve in reverse, or uses inadequate practices when rebuilding valves, cutting and placing flange ring gaskets at pipe joints, or exchanging and replacing incorrect valves. Our recommendation is to always use good maintenance practices.

AUTHOR'S NOTE

Of the three sources of problems, design, operation and maintenance, the mechanic is really responsible for a small part. The truth is that the majority of pump problems begin with changes to design, and plant operations after the system was commissioned.

■ If the condition should be occasional, the solution could be to install a variable speed motor. If the condition is permanent, the solution could be to reduce the impeller diameter, replace the pump, or increase the diameter of the pipe. If normal operations require living with the condition, then increase the diameter of the pump shaft to improve the L/D factor.

2. You may see the same evidence all around the circumference of the close tolerance rotary elements, with gouge or wear spots on the stationary elements at about 240° from the pump cutwater.

■ These marks are caused or induced by operations or by design. This evidence is revealed when operating the pump too far to the right of the BEP on its curve. Perhaps the pump is inadequate and doesn't meet the flow and head requirements of the system. It could also be that there is a loss of resistance in the discharge piping. A big hole in the discharge piping could present the same evidence.

■ If you must live with this condition, you need to increase the diameter of the shaft to improve the L/D factor and deflection resistance.

3. If you see the same evidence, gouge and wear marks around the circumference of close tolerance rotary elements, and spots or arcs on the close tolerance stationary elements at about 180° from the cutwater, or straight down:

■ This would be a problem induced by inadequate design, caused by pipe strain probably in a high temperature (thermal expansion) application. The volute of the pump and the stationary elements are growing up from the floor due to thermal expansion, against the rotary elements. You need to speak with the plant engineer and show him the evidence. A possible solution is to change your ANSI standard pump for a 'High Temperature' or API design in this application.

See the following graphs, Figure 9-13, depicting thermal expansion. The picture on the left shows an ANSI pump where thermal growth is straight up from the base. On the right we see a high temperature pump where thermal growth occurs 360 degrees around the volute.

The coefficient of thermal expansion of 316 stainless steel is 9.7 x 10 6 in/in per degree Fahrenheit. The metric equivalent is 17.5 x l(h(> mm/mm. per degree Centigrade. See the next Table and note the expansion on a pump whose centerline is 10 inch above its base.

ATEMPERATURE ATEMPERATURE THERMAL EXPANSION

F° C° Inches Millimeters

100 °F

55 °C

0.0097

0.245

200 °F

110 °C

0.0190

0.490

300 °F

165 °C

0.0291

0.735

400° F

220 °C

0.0388

0.900

500 °F

275 °C

0.0485

1.230

600 °F

330 °C

0.0582

1.470

As you can see, the pump casing will grow against its shaft almost 0.030 inches with an increase of 300°F. There are many tolerances in a pump that are tighter than 0.030 inches. This means a rotary element will scrape and rub a stationary element.

You may even see the same evidence of gouges and wear around the circumference of strict tolerance rotary elements, leaving a corresponding spot on the stationary elements at any other point around the volute circle of the pump.

This condition is probably misalignment, indicating a maintenance problem. The mechanic should be trained to correct this. Follow correct alignment procedures, as well as correct bolt torque procedures.

Inspect gasket surfaces for knots and irregularities. Look for bent dowel pins and misaligned jack bolts, dirt and any other factor that might lead to misalignment.

Next we'll discuss evidence marks and prints that are different, but to the untrained eye, they may appear the same. You may see a spot or arc of wear and gouging on the rotary elements, and a circumferential wear circle on the bore of the close tolerance stationary elements. This is a maintenance-induced problem. This is the sign of a physically bent shaft, or a shaft that is not round, or a dynamic imbalance in the shaft-sleeve-impeller assembly. The solution is to put the shaft on a lathe or dynamic balancer, verify its condition, and correct before the next installation.

The next condition and physical evidence we'll mention is rare, but we need to cover it in case you should ever see it. You might see scratch and gouge marks all around the circumference of strict tolerance rotary element ODs, and stationary element bores alike. This condition and marks is evidence of a 'Lack of Control'. It could be from any of the aforementioned reasons up to this moment, and even including vibration, damaged and misapplied bearings.

The problem could be maintenance, operation, or design, or a combination of any or all these factors. In all honesty, you should never sec this set of evidence marks because it indicates a lack of control. Now because the mechanic cannot control operational problems or design problems, the first phase to correct this situation is to control the mechanical maintenance factors, like alignment, proper bolting and torque sequences, be sure shafts are straight and round, and dynamically balance all rotary components. Reinstall the pump and wait for the next failure. Once the maintenance factors are under control, there should appear a clear vision and path to resolve any operational and/or design weaknesses.

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