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3

Pump Sizing Considerations

bernoulli theorem centrifugal pump head highest point

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#1 panagiotis

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Posted 07 April 2024 - 12:10 PM

Hello everyone,

 

Let's assume that we have a discharge routing which goes up to 10 m and then down to the inlet nozzle of the tank, which is 5 m from the ground. 

 

The pump is sized for a height difference 5 m instead of the highest point, which is 10 m.

 

When it is necessary to size the pump for the highest elevation? (For example when the flowrate of the pump is small and the head is large?? To be sure that during the start of the pump the liquid will arrive at the highest point??) 

 

Let me know if something is not clear in my question.



#2 fallah

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Posted 07 April 2024 - 01:07 PM

Hello everyone,

 

Let's assume that we have a discharge routing which goes up to 10 m and then down to the inlet nozzle of the tank, which is 5 m from the ground. 

 

The pump is sized for a height difference 5 m instead of the highest point, which is 10 m.

 

When it is necessary to size the pump for the highest elevation? (For example when the flowrate of the pump is small and the head is large?? To be sure that during the start of the pump the liquid will arrive at the highest point??) 

 

Let me know if something is not clear in my question.

 

Hi,

 

The system you described is almost clear but to completely clear the matter you can upload a simple sketch of the system...

 

At a glance to the matter, appears the required pump's head at start up is almost equal to 10 m plus the equivalent head loss due to friction and fittings but after start up it will be decreased by around 5 m due to head compensation through the liquid falling from the high point to the tank inlet. The pump can be sized for height difference of 5 m but at the start up the working point moves to the left of the pump curve at required head with lower flow rate then moves back to the normal working point with higher flow rate.



#3 panagiotis

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Posted 07 April 2024 - 01:35 PM

Hi, you understood perfectly. There is not a real sketch. It is a random example. It is not 100% clear to me, when we need to size the pump for the highest elevation point? 

Indeed when the liquid is falling to the inlet nozzle, then gravity helps. That's why we size the pump for 5 m instead of 10 m according to Bernoulli. But, my concern is when it would be wise to size the pump for the highest elevation point?



#4 fallah

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Posted 07 April 2024 - 06:47 PM

But, my concern is when it would be wise to size the pump for the highest elevation point?

 

When after the highest point the line not to fall to compensate part of the head to the high point!



#5 breizh

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Posted 07 April 2024 - 08:03 PM

Hi,
Naser has been very diplomatic and soft. Your design must take into account all the conditions, including start up.

 

Being in operation for more than 30 years I cannot imagine a design where start up is not taken into consideration. Those guys have enough on their plate, can you imagine the consequences? Safety, Quality,Etc.

 

Breizh



#6 katmar

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Posted 08 April 2024 - 04:21 AM

I agree with fallah and breizh - you must provide a pump that can deliver 10m of head or the plant will never start up. This can be quite a difficult thing to do because you want the pump to operate stably over a 2:1 head range. Most centrifugal pumps have fairly flat curves near the Best Operating Point and this 2:1 head range may cause flow problems (see below).

The second consideration is that in the 5m downflow leg there will only be pressure recovery if the line runs full. If the line is larger than necessary it will not run full and there will be no pressure recovery. This links back to the first point regarding the pressure range. If the pump can deliver 10m of head it might be at a very low flow and this might not be sufficient to fill the downleg and get the pressure recovery you want. The pump would continue delivering liquid but at the lower rate and you would not achieve the required flow rate for normal operation.

The solution to this problem will depend on the capacity of the plant. A common solution in small plants is to design the pump for the full flow at a head of 10m and then design the downleg large enough to prevent it ever running full and therefore preventing any pressure recovery. This allows the pump to run stably at constant conditions. Depending on the fluid being pumped you might choose to even add a vent at the top of the downleg to ensure that it does not run full.

An alternative would be to note that the downleg has to be quite seriously over-sized to prevent it ever running full. If the downleg can achieve a Froude Number of 1.0 with the pump running at the 10m point it will quickly become full and the pump head will drop to 5m. If you go for this solution discuss it with the pump supplier to ensure that the pump can happily switch back and forth between the 10m and 5m operating points.

This over-designing of the pump is only feasible in small plants where the capital and running costs of the pump do not destroy the economics of the plant.

In larger plants you would need a more sophisticated solution, perhaps even installing two pumps where one is specifically to overcome the start up requirement and it can then be shut off once the pressure recovery is established and the main pump can be designed for the full flow at only 5m head. But such a solution requires careful design and a degree of automation (or highly trained and skilled operators).


 



#7 latexman

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Posted 08 April 2024 - 05:45 AM

In the old days I would have said, see if a 2 speed pump (motor) might help you in this situation Nowadays, a VFD is your friend.

#8 Pilesar

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Posted 08 April 2024 - 07:56 AM

A variation of 5 meters of head in a centrifugal pump discharge line is not of much consequence in a real world application. The flow control valve in a real pump system will likely have at least this much differential pressure drop range. The pump should be sized for the normal operating conditions and the system checked for operability at all expected operating conditions. The difference in height of 5 meters to the highest point does not matter in itself... it is the variation in total system differential pressure that matters. This will no longer be a 2 to 1 pressure range as in the simplified assumptions. The system pressure drop calculations should take into account the line, check valves, control valves, and the other piping devices. When the flow control valve and the entire pump system are sized properly, a single speed centrifugal pump should be a fine choice. Pump systems typically see many more operating conditions than a single design operating point.

  Theoretical questions with sparse description are an okay diversion for discussion but I learn more from the real world problems that appear in these forums. I consider possible 'what if' objections to my first paragraph. What if the pump and system are very small? Then efficiency won't matter. Just size the pump as needed. What if the pump and system are extremely large? Just size the pump as needed. If 5 meters of extra differential head on startup make the system inoperable, then adjust the system so that it can operate. A VFD drive might make sense for a very large pump, but there would need to be more justification for the cost than an extra 5 meters head on startup. 



#9 katmar

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Posted 08 April 2024 - 09:47 AM

The way I suggested that sizing the pump for full flow at 10m of head for a small installation, and then backed up by Pilesar's suggestion that a full sized pump can be considered in almost all installations, was the way it was done when we were young engineers but it shouldn't be done that way anymore.

Latexman is correct in recommending a VFD. Even for small installations this does not need to be a more expensive option because it will quite likely eliminate an expensive control valve and positioner. Simply let the pump run at a higher speed for a few seconds at start up and then it will automatically slow down and save the power that would be lost in a control valve.

Even if the cost factor of the wasted electricity is not large, it is just the right thing not to waste energy. And in my world it is unusual to overrate the pump head by 5m.

The downleg still needs to be sized to ensure that pressure recovery occurs, but this is a minor consideration compared with the pump and control system selection.



#10 fallah

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Posted 08 April 2024 - 10:15 AM

Suppose the piping route and pipe size are such that we are sure for 5 m pressure recovery at falling section...

 

In general, all we need to operate the pump is creating the conditions leading to two system curves intersect the pump curve, one with 10 m head for pump start up and another with 5 m head for pump normal operation. If we would start the pump with initially closed discharge valve and gradual opening of it, the system curve with 10 m head will easily be created. Then we could completely open the discharge valve to create the system curve for normal operation.



#11 panagiotis

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Posted 08 April 2024 - 11:22 AM

Hello everyone,

Give me time to read all your answers!

And I will definitely come up with more questions!!

#12 breizh

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Posted 08 April 2024 - 07:20 PM

Hi,

This is not the way I understand the post. To me all the equipment is in place and there is an issue at start up due to failure in design.

other comments for similar situation.

https://www.eng-tips....cfm?qid=240286

 

Of course, there are options to consider but $ are needed, the best one being a good design.

My view.

Breizh 






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