Guest Post by Ricky Smith, CRL, CMRP, CMRT
Pipe stress is caused by misalignment of the mating surfaces of two pipe flanges creating abnormal internal stress of pump bearings, seals, motor bearings, couplings, and can possibly change the displacement of a pump.
General Rules which must be followed by maintenance personnel and contractors: (if you truly want to stop a long term pump problem)
- Pipe flanges attached to pumps must be aligned where the gap does not exceed the thickness of two gaskets or tolerance established by your company’s engineering standards.
- Pipe flange bolts must drop in without assistance.
- Cable pullers, come-a-longs, or long bars should not be used when aligning a flange which is connected to a pump.
- Validate the elimination of pipe stress by following the guidelines listed below.
Failure Modes experienced from Pipe Stress on Bearings:
- Wear caused by seals leaking
- Wear caused by static vibration
- Indentations caused by overloading while static
- Corrosion caused by inadequate lubrication caused by abnormal loading (seal leaking)
- Flaking caused by misalignment and excessive loading
WARNING: Ensure your contractors follow the same process to eliminate pipe stress. Pipe stress elimination should be validated during commissioning of new pump.
Follow this process if you want to inspect your pumps which may have pipe stress:
- Align the two shafts between your pump and driver (typically an electric motor) to tolerance recommended by the equipment vendor or your company’s engineering standards.
- Validate misalignment to insure motor and pumps shafts are aligned to specification.
- Disconnect the outlet flange on the pump.
- Revalidate laser alignment of shafts.
- If alignment has moved then you have pipe stress. Do the same for the inlet flange.
- Make corrections as stated in the following procedures to eliminate pipe stress.
Elimination of Pipe Stress – “The Ricky Smith Method for Pipe Stress” as learned from Dan Turner (his maintenance and engineering manager at Exxon during his career in the 1970s)
- Bolt flanges to pump and insert blind flange gasket along with two regular flanges between pump and mating flanges (cover the hole between welding area and inside the pump).
- Attached welding ground to flange. (do not attach ground lead to pump; welding group must always be attached to flange) WARNING: Failure to accomplish this one task properly will cause bearing failure by “electric arcing” which is a failure mode of bearings.
- Tack weld flange into place reverse welding each tack.
- Allow to cool for 10 minutes.
- Reverse stitch weld on opposite sides on the flange similarly used for cast iron welding.
- After initial reverse stitch weld then weld normally using electrode recommended by the American Welding Society (typically E-6010 5P or GTAW)
- After root pass; weld in any direction you wish.
- Allow to cool and then disconnect flange, replace gaskets and;
Validate bolts will drop into holes without pry bar.
Validate gap between flanges is no more than two gaskets thick.
Learn more about the effects of running equipment with pipe stress, watch LUDECA Shaft Alignment Know-How Pipe Stress video.
by Yolanda Lopez
Reposted from EASY-LASER® blog
There are basically two types of pipe strain: static and dynamic pipe strain. Static pipe strain occurs both when the machines are operating and when they’re not running. This is most likely the type you’ve been trying to eliminate. Dynamic pipe strain, on the other hand, is a condition that only takes place when the machines are operating.
PIPE STRAIN AND ALIGNMENT
Strain, which is the deflection and positional change resulting from pipe stress, comes from the suction and discharge piping, and creates stresses on the machine frame or casing that that in turn spread to the equipment body. A result of this is often a change in the alignment of the shaft. It also results in distortion of the machine casing which misaligns the bearings within the machine, resulting in very harmful vibration and increased radial loads on the bearings. So, before attempting shaft alignment, you should ensure that suction and discharge piping are not causing strain on your machines.
HOW CAN YOU CONTROL PIPE STRAIN?
Static pipe strain and its effects are fairly simple to control, and you can use any of the Easy-Laser® shaft alignment systems to measure the strain and help eliminate it. Here’s how: simply mount the system in the normal manner as when you perform the shaft alignment. Then:
- Position the measuring units at twelve o’clock. Use the Values program and set both units to zero.
- Rotate the shafts to the three o’clock position, and note if the values are not zero. Rotate the shafts back to twelve and confirm the zero setting.
- Now connect or disconnect the piping. Any changes to the values for the twelve and three o’clock positions is the result of pipe strain and should be corrected, which will require careful pipe fitting. What you are seeing is movement transmitted though the casing to the bearings and shaft.
Dynamic pipe strain is more difficult, because it only occurs when the machines and piping are at operating conditions. And a good proportion of this type of pipe strain may be a consequence of thermal expansion of the piping and the weight of system fluid. This explains why the dynamic pipe strain is not a fixed state and why it might be difficult to handle.
We understand that this can be quite a challenge, however, we believe you have a lot to gain from minimizing dynamic pipe strain. And there are effective tools for this purpose: Our Easy-Laser measuring program has a great feature that registers measurement values automatically for a specified amount of time and frequency of measurement. This is very useful for looking at differences between a machine that’s up and running and one that’s down. The data can also be transferred to the EasyLink™ software, where you can see the results more clearly, in the shape of a graph to scale.
The existence of pipe strain indicates that more than just alignment is needed for optimal machine performance; there are other important factors to take into consideration. This requires flexible measurement systems that will support you and help you reach your goal. You need more than a shaft alignment system; you need a total alignment solution.
We invite you to watch our Pipe Stress Know-How video to learn more about the effects of running equipment with pipe stress.
by Yolanda Lopez