Pump Suction and Discharge Velocities - Original Pipe or Nozzle Diameter? 1

[sweetliver]

Hi All,

We are going to replace an existing pump with one of a higher capacity (maximum expected operating flowrate is 340 GPM). The suction line size is 6 inch, while the discharge line size is 4". These corresponds to liquid velocities of 3.8 ft/s for suction, and 8.6 ft/s for discharge. However, the pump we are planning to select has flanges of 3" in the suction (6" x 3" reduction nozzle) and 2" in the discharge (4" x 2" increase nozzle). According to the calculated liquid velocities in these reduced sections, the suction nozzle will have a velocity of 14.7 ft/s, while the discharge nozzle will have a velocity speed of 32.4 ft/s.

My role of thumb is to not to exceed 8 ft/s in the suction and not to exceed 15 ft/s in the discharge.

Does the liquid velocity in the nozzle matter, and should I only stick to the size and the liquid velocities of the main lines?

Thank you in advance.

 

[LittleInch]

It's really about the pump manufacturer trying to save money by having a smaller casting and hence sizing is essentially up to them. You just need to have reducers on bother nozzles to increase diameter.

But that is a big decrease in size and substantial velocity. Are you at the far end of that pumps envelope? I would be happier with the next pump casing size up myself.

Talk to the supplier and see what they say.

 

[Artisi]

Ignore the inlet and outlet nozzle diameters, the pump has been designed for the maximum flow rate achievable, with all testing and results based on the flow thru' the pump measured at the pump inlet flange and the pump discharge flange.
Just fit an appropriate flange adapters to the pump.
However, without checking, a 3x2 pump sounds marginal for a 320 gpm flowrate.

 

[goutam_freelance]

NPSHA should be calculated after considering the pressure drop across 6"x3" inlet nozzle.

Also with such high velocities, you can expect more erosion in inlet/outlet nozzles.

 

[sweetliver]

It's really about the pump manufacturer trying to save money by having a smaller casting and hence sizing is essentially up to them. You just need to have reducers on bother nozzles to increase diameter.

But that is a big decrease in size and substantial velocity. Are you at the far end of that pumps envelope? I would be happier with the next pump casing size up myself.

Talk to the supplier and see what they say.

Thank you for your response. I have a made a mistake in the information I have provided in the post.

The suction pump nozzle size is 4" (6" x 4" reducer, 8.6 fps @ the 4" cross-section)
The discharge pump nozzle size is 2" (2" x 4" reducer, 32.4 fps @ the 2" cross-section)

 

[sweetliver]

NPSHA should be calculated after considering the pressure drop across 6"x3" inlet nozzle.

Also with such high velocities, you can expect more erosion in inlet/outlet nozzles.

Thank you for your response. There was an error in the information I provided. It is a 6" x 4" inlet nozzle.

The suction pump nozzle size is 4" (6" x 4" reducer, 8.6 fps @ the 4" cross-section)
The discharge pump nozzle size is 2" (2" x 4" reducer, 32.4 fps @ the 2" cross-section)

 

[pierreick]

Hi,
Why do you change the manufacturer of the pump?
Yes, NPSH to be calculated, 6fts is a good value for suction line, 10 to 12 fts for discharge .
Pierre

 

[sweetliver]

Hi,
Why do you change the manufacturer of the pump?
Yes, NPSH to be calculated, 6fts is a good value for suction line, 10 to 12 fts for discharge .
Pierre

Thanks Pierreick,

The system already exists but we are going to replace the pump only to meet a higher capacity demand in the process unit. The existing pump will not be able to meet the demanded flow rate.

We are relying on the NPSHA stated in the existing pump datasheet. However, I am going to recalculate the pressure drop and see the source and destination pressure for validation.

 

[pierreick]

Hi,
Hold on ! you need to recalculate the entire system including the NPSH available, you cannot rely on existing data sheet.
You did not answer my question about pump selection, why did you change the brand? $$$?
With the existing supplier an increase on flowrate would have led to bigger or equivalent pipe diameters. Not the opposite!
Another point to consider in your design is erosion velocity. Did you check?
Good luck
Pierre

 

[sweetliver]

Hi,
Hold on ! you need to recalculate the entire system including the NPSH available, you cannot rely on existing data sheet.
You did not answer my question about pump selection, why did you change the brand? $$$?
With the existing supplier an increase on flowrate would have led to bigger or equivalent pipe diameters. Not the opposite!
Another point to consider in your design is erosion velocity. Did you check?
Good luck
Pierre

Hi Pierreick,

The system has not changed. The vessel (column) pressure is the same, the suction line will remain with the same diameter (6"), length, fittings, and elevation. The specs of the pumped fluid will remain the same. NPSHA is influenced by these systems.

We have not changed anything yet! We are considering the change in the future. We are in a project phase.

We are replacing the existing pump because it is not fulfilling the new capacity requirement. This pump was installed in the 1970s. The new capacity requirements will change due to process change.

The whole post is about the velocity in reducers of the suction and the discharge. The liquid velocities in the main lines (6" for suction and 4" in discharge) are acceptable.

 

[Artisi]

Why are you so hung up on the velocity in the reducers, all it will do is add a minute amount of headloss to the overall total.

Just calculate the head loss for each reducer and add it into the friction loss calc. for the system.

 

[pierreick]

Hi,
You are going to install a reducer; this means you are going to increase the head loss, same when you increase the flow rate thus the NPSHa will decrease.
Similarly, the discharge line will see more head loss due to velocity increase (bigger flow rate) and enlargement thus TDH will decrease.
Now it's up to you.
Make sure the new pump curve matches your need.
Pierre

 

[SB1976]

Hi,
No doubt the velocity will increase impacting head losses in the system thereof (V^2/2g). However, this increment is marginal and can be ignored safely. Absolutely no issues with that in terms of pump performance.

 

[sweetliver]

Hi,
You are going to install a reducer; this means you are going to increase the head loss, same when you increase the flow rate thus the NPSHa will decrease.
Similarly, the discharge line will see more head loss due to velocity increase (bigger flow rate) and enlargement thus TDH will decrease.
Now it's up to you.
Make sure the new pump curve matches your need.
Pierre

Hello Pierreick,

Thank you for your response. I did the calculation today. I got a 3ft drop in NPSHA (existing NPSHA vs New flow rate NPSHA). I could not find a reducer friction loss coefficient. However, I used ones for expansion and contracting diameters found in fluid mechanics books (or Cameron Hydraulic Data).

But honestly, this matter is not addressing my original question. I don't think the high velocity of the new flow rate affects only the head loss. "Pump Users Handbook" mentions that:

1- "the suction piping should be at least as large as the pump suction nozzle and be sized to ensure that the maximum liquid velocity at any point in the inlet piping does not exceed 8 ft/sec"

2- "System piping is also influenced by process considerations, with the maximum recommended velocity at any point in the pump discharge piping being 15 ft/sec".

Why is the book emphasizing that velocities should be within range at all points?

 

[sweetliver]

Hi,
No doubt the velocity will increase impacting head losses in the system thereof (V^2/2g). However, this increment is marginal and can be ignored safely. Absolutely no issues with that in terms of pump performance.

Thank you for your response. Can you give me your point of view about this statement in Bloch's Pump User's Handbook: Life Extension:

1- "the suction piping should be at least as large as the pump suction nozzle and be sized to ensure that the maximum liquid velocity at any point in the inlet piping does not exceed 8 ft/sec"

2- "System piping is also influenced by process considerations, with the maximum recommended velocity at any point in the pump discharge piping being 15 ft/sec".

Why is the book emphasizing that velocities should be within range at all points?

 

[sweetliver]

Why are you so hung up on the velocity in the reducers, all it will do is add a minute amount of headloss to the overall total.

Just calculate the head loss for each reducer and add it into the friction loss calc. for the system.

Thank you for the response. Do you recommend any friction loss coefficient for a 6"x3" reducer?

Also, Can you give me your point of view about this statement in Bloch's Pump User's Handbook: Life Extension:

1- "the suction piping should be at least as large as the pump suction nozzle and be sized to ensure that the maximum liquid velocity at any point in the inlet piping does not exceed 8 ft/sec"

2- "System piping is also influenced by process considerations, with the maximum recommended velocity at any point in the pump discharge piping being 15 ft/sec".

Why is the book emphasizing that velocities should be within range at all points?

 

[LittleInch]

They are good guidelines to use for the design of the PIPING. Nothing to do with the size of the pump nozzle itself.

There is nothing you can do about the size of the flange on the pump. As long as the vendor says it will do the duty and the efficency isn't lowered by this then just add reducers on both ends to increase the size of the piping.

So "does the liquid velocity matter...". To me no.

And don't confuse guidelines with rules. I've seen >7m/sec in some short lines before now which isn't great, but size was limited and the overall impact was low.

 

[Snickster]

The attached old edition of API 14E provides pipe sizing criteria.

 

[Snickster]

This is copy of Norsok P-001 which also provides sizing criteria.

 

[Snickster]

Here is page from Hydraulic Institute concerning suction piping.