Hot Alignment 5

[steamdog]

I have been searching for information about the temperature at which an additional hot alignment of base mounted pumps is required. Most of the pumps I work with are in the HVAC industry where the temperature is 180-190 deg F maximum, and we don't normally hot align the pumps. I recently was part of a job with boiler feed water pumps and the temperature is about 212 degrees, and they are asking for a hot alignment (we had these cold aligned already), and I am not sure it is neccessary. Any input will be appreciated.

 

[JJPellin]

We require that hot alignment compensation is included on any pump with a product temperature above 200 °F. However, almost all of our pumps are API pumps and are split line mounted. I would not feel comfortable setting a single temperature value for hot alignment of any and all foot mounted ANSI pumps. I would prefer to come at it another way.

If you have a pump that you think might be a candidate for hot alignment or cold alignment with a thermal offset, test it. Measure the distance from the shaft centerline down to the base of the feet. Take a surface temperature of the foot/case every inch or two along that distance and do the math. A known temperature distribution over a known distance of a known material provides a known growth.

If you know the expected growth, there are two things you can do with it. First, determine if this amount of growth would exceed your alignment tolerance (if you have one). For example, for a pump running at 3600 rpm, we require that the final alignment is within 0.002’ offset with an angularity of less than 0.003”/1”. So, if the calculated growth was more than 0.002”, we would know that the thermal growth would necessarily grow the alignment beyond my allowable tolerance. I need to do a hot alignment or cold align with a thermal offset.

We no longer allow for hot alignment because of energy control considerations. It is not practical to lock out a pump, establish energy control, and issue a work permit fast enough to get a good hot alignment. And, we are not allowed to align a pump with the valves open so that it stays hot. So, we align the pump cold with a pre-determined thermal growth offset. Once you calculate the growth of a pump as I describe above, you already have a good starting point for a cold alignment target. If you calculated that the pump is going to grow 0.005”, then you cold align the pump so be 0.005” lower than the motor so that it will grow into alignment when it runs. Even if you are allowed to do a “quick” not alignment, I would still suggest doing a cold alignment with a thermal growth offset. I think it is likely to be more accurate unless you have pipe strain or base problems that are changing the actual growth very much as compared to the calculated growth. .

Depending on the type and size of driver, you may also need to allow for vertical growth of the driver. But, most motors do not grow as much as a hot pump. It would not be too difficult to estimate motor growth in the vertical based on motor frame and speed. A simple table might be all you need to compare the calculated growth of the pump to the expected growth of the motor to get the net difference which determined if you need to allow a hot alignment offset and what offset you should use.

Lastly, if you are aligning pumps cold and the vibration data tells you that you still have an alignment problem, then you need to consider hot aligning or cold aligning with an offset. If the vibration and reliability are good, then cold alignment may be good enough.

Johnny Pellin

 

[steamdog]

Johnny,
The above is one of the best answers to a question (mine or others) that I have seen. Thanks for your expertise AND for the method for me to make the required steps in the field.

 

[electricpete]

That is a very good complete discussion. We encounter the same difficulties in not being able tag out a pump and get to it while it's still hot.

If you're forced into a non-ideal situation where hot alignment is not possible and there is not any or accurate thermal-adjusted cold targets, then checking vibration is one valuable datapoint to assess whether there is an alignment problem.... BUT I would suggest not to get too much sense of security from that because misalignment can cause large static forces and may not show up with correspondingly large dynamic forces that cause vibration.

Beyond estimating targets from thermal profile, the off-line-to-running change in position can be measured directly with various instruments; Ludecda Permalign is one. Acculign is another. This info can be used to adjust future cold alignment targets.



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(2B)+(2B)' ?

 

[steamdog]

I was talking to my alignment guy, they have electric blankets that they sometimes use to do a hot alignment (with some leisure time) and not worry about the pump cooling down, or forcing some flow etc....

 

[JJPellin]

Electricpete provided a valuable caution about the use of vibration in this regard. I need to be careful to be specific when I speak of vibration analysis. Overall vibration levels can actually be reduced by excessive misalignment in some situations. I rely on my experienced vibration technicians to recognize the characteristic frequencies and phase angle relationships that are indicative of misalignment. These relationships may be highly dependant on the type of coupling used and other variables. Just because vibration levels are low does not mean that the alignment is good. I didn't intend to suggest that. Thanks Pete.

Johnny Pellin

 

[JJPellin]

I had one obvious error in my first reply. It should be 0.0003"/1" angularity tolerance, not 0.003"/1".

Johnny Pellin

 

[Artisi]

Your question has been well and truly answered, however, my 2 cents worth is, if hot alignment is going to be a problem look at pump /motor coupling that is capable of handling some misalignment. Not sure what the coupling I have in mind is called in your part of the world, maybe Fenner, Fenner Dodge or just Dodge.