Suction Pressure: Maximum vs Rated

[furio2]

Dear all,

I would be grateful if you could help me with a basic question about centrifugal pumps: which is the difference between the maximum and the rated suction pressure?

Thank you for your time.

 

[BigInch]

Rated suction pressure would be the suction pressure needed when the pump is discharging its rated flow.

Maximum suction pressure is the rating of the pump's suction flange, or possibly some other factor which limits suction pressure to the given maximum value.



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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[TenPenny]

Rated suction pressure would be the suction pressure at normal (rated) operating conditions, as specified by the plant/engineer/user.

Maximum suction pressure could be two different things: it could be the maximum suction pressure that the pump could see, as specified by the plant/engineer/user, for an upset condition.
OR
It could be the maximum suction pressure that the pump is capable of withstanding, based on the operating speed, so that the pump does not exceed its own pressure rating. This would be the maximum suction pressure as specified by the pump manufacturer (not as commonly seen as previous meaning).

Most commonly, the engineer will specify the maximum suction pressure that the pump is expected to see, so that the pump manufacturer can select a pump that is capable of withstanding that pressure.

 

[JJPellin]

I have (almost) never seen a maximum suction pressure rating as a design limit for a pump. As already noted, this is more commonly a value provided by the end user to define the expected range of suction conditions (min, max and rated). The one exception that comes to mind involves Sundyne pumps. The OEM specifies a maximum suction pressure based on the configuration of the upper output shaft thrust bearing. The standard bearing is only capable of operating up to 200 psi suction. For higher suction pressures, a tilt-pad thrust bearing can be provided.

Johnny Pellin

 

[BigInch]

Right. I would imagine its not something you see often in a refinery. Suction pressure limits and suction flange ratings are more important in pipeline work. When flows are low, or the pipeline is shutdown, you have full static head (could be thousands of meters) at the suction flange of the pump station in the next valley. A lot of flatlanders get it wrong there.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[furio2]

Thank you very much for your help.

I will refer to the TenPenny explaination in which he defines the "maximum suction pressure" as the maximum pressure that the pump could see for an upset condition.

 

[BigInch]

Why include the "for an upset condition"?

Max pressure is max pressure. Doesn't matter if it is an upset condition or not.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[furio2]

Right, but I have understood that the rated suction pressure is the maximum suction pressure in "normal" conditions, i.e., for a pump that takes suction from the bottom of a tank or vessel the rated suction pressure could be calculated as the sum of the operating pressure + the head @ maximum level - pressure drops.
The maximum suction pressure could be calculated as the sum of the set pressure of the psv (the design pressure of the tank/vessel in most cases) + the head @very high level - pressure drops (pressure drops have to be taken into consideration in this case?). I have considered this last condition as an upset condition because it is different from the "normal" condition. Isn't a correct interpretation?

 

[BigInch]

furio2. I think you're over complicating this. If the specifying engineer simply said maximum suction pressure, a pump manufacturer would automatically take it as an "in the process" condition and wouldn't care if it was upset or not. If the pump manufacturer simply said maximum suction pressure, the process engineer would set his relief valves accordingly. Any other definition could easily confuse both the process engineer and the pump manufacturer. If you are intent on making up some "conditional" upset or non-upset" term, I'd suggest you add the word "Operating", such as maximum suction operating pressure, if you simply must.

AND

I will refer to the TenPenny explaination in which he defines the "maximum suction pressure" as the maximum pressure that the pump could see for an upset condition.

That's fine, as long as you include OFF as an upset condition.

For example the way you are calculating maximum suction pressure would not give the maximum suction pressure in one of the two most very normal conditions ON, or OFF. In the OFF condition, you would not have suction line losses and, if that was a butane tank, the sum now might easily be enough to put you over the limit of a lot of maximum flange pressure ratings. If you speced the pump max suction pressure in that manner, I can assure you that it is very possible you would get a flange that didn't work when the pump was OFF. Not nice.

I don't see the real use of the term, maximum suction pressure, for anything other than a flange rating anyway. If suction pressure's head equivalent is above NPSHR and below flange ratings, who cares what it is. Change product and a previous suction pressure or discharge pressure doesn't mean any thing at all. Pressures change with specific gravity. That's why specing a pressure really only means something to pumps when applied to FLANGE RATINGS and other components.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

This standard also uses my definition of max suction pressure.

http://www.red-bag.com/jcms/index.p...ules-to-establish-mechanical-design-pressures

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[TenPenny]

BigInch, you're correct, I used 'upset condition' when I meant 'other than normal operating condition', which would include 'off'.

 

[BigInch]

OK, then the only problem I have with your "normal (rated)" definition is that it implies you couldn't operate the pump normally at anything other than its rated flow & head ???



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[TenPenny]

Isn't that what 'normal' means, the normal operating point? Which is generally what you have the pump rated for?

I will defer to your preference. You can take the topic and run with it.

 

[BigInch]

Look at my first response.

I thought it was run out back then. What more do you really need to say.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[dabluffrat]

JJ, That's not just for Sundyne pumps.

In vertical pumps, or vert in-line pumps, max suction pressure can also be the max pressure to prevent an up thrust condition and maintain 2 yr L10 bearing life.

Did you know that 76.4% of all statistics are made up...