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Pump discharge line size calculation
4

Pump discharge line size calculation

Pump discharge line size calculation

(OP)
Hello professionals
Thank you for going to help me.
We have pump with capacity of
180 m3/hr , head = 45 meters.
Pump nozzle size is 100mm x 80mm
But in our plant we already installed piping for suction with 150mm inlet piping and discharge with 250mm outlet piping to the plant
(This is cooling water supply and return line both are the same size as 250mm)
So I calculated the velocity method (Q=A X V) with the known pipe size and flow rate I get 1m/s.

Few more details for your reference:

1.Pipe material is Mild steel "C" class pipe
2.Pipe length from cooling tower sump to plant supply (250mm) EOL is 80 meters.
3.Pipe length from Main header to sub-header supply (200mm) EOL is 30 meters.

My doubt is all about discharge nozzle size with discharge piping size.
Did we take wrong piping size for discharge line?
Is there any rule of thumb or design considerations for pump piping as applicable to liquid flow.

I have attached my hand sketch for more clarity.

RE: Pump discharge line size calculation

Hi,
1 m/s velocity is too low for water!
Pierre

RE: Pump discharge line size calculation

(OP)
pierreick hm this is really too low velocity 😔
Thank you sir.

RE: Pump discharge line size calculation

"Did we take wrong piping size for discharge line?
Is there any rule of thumb or design considerations for pump piping as applicable to liquid flow."

Piping size is always a compromise between lower cost of the pipe material /welding / fittings vs increased pumping costs over time.

For pipelines, say >10km long, a start point for velocity is about 2m/sec which has proved over time to be the correct balance between low pipe cost and low pump cost / energy usage. Many people prioritise CAPEX over OPEX, but if you add in lifetime energy costs, you often find you go up a pipe size and lower velocity.

Piping on the other hand a higher velocity - prob around 3m/sec, is usually most economic due to the high cost of fittings (bends, elbows, tees, flanges and most importantly valves of which there are a lot more on piping than pipelines.

You normally want to flow at 1m/sec minimum as this velocity will blow your bubbles / air pockets out. Less than 0.5 you could get air building up at high points and dirt at low points.

At 180 m3/hr my initial guess would have been a 6" / 150mm pipe, possibly 8" / 200 NB if the run was quite long. 130m isn't long, so yes the 250mm looks a bit too big, but your energy bills for the pump will be quite low. If this works 24/7/52, then you might get payback for the bigger pipe in 5 -7 years. You normally want to go up at least one if not two pipe sizes compared to the discharge flange size of the pump and at least one pipe size higher for the inlet / suction line, especially if its at low pressure / gravity fed.

however I guess now your pump was sized based on a low frictional loss in the bigger pipe, so you can't change it now without changing the pump for a more powerful one.

but this is why you do design concepts to find the most appropriate size for your particular circumstances. Any numbers are just an initial guide. There are no fixed rules saying it must be this velocity or not more than say 3m/sec. You sometimes see lines running at 5-6m/sec if it makes sense. Surge can be an issue over 2.5 to 3m/sec, but just look at it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump discharge line size calculation

I agree with LittleInch's comments.

In the world of fluid dynamics, the general saying is the pressure is cheap and flow is expensive. That is certainly true where moving large volumes of fluid comes into focus.

With 45m of head available, the primary objective is to minimise the head loss between the pump and the cooling tower and if you have a need for pressure to supply sufficient flow to the equipment, then head loss prevention is critical.

It is assumed that you have done the calculation to check for head loss from friction losses and bend losses etc. With all of the losses due to friction, fittings, bends and drop legs etc, if the total head loss is too high, then you naturally have to consider larger diameters, with the data available to justify the decision. After all, larger diameter means heavier supports and brackets and the CAPEX gets high. If you go with marginal flow performance and have turbulent flow and higher pressure at the pump outlet, the OPPEX is going to be higher and the higher input energy will likely shorten the working life.

RE: Pump discharge line size calculation

(OP)
LittleInch sir I hope it will not affect our process requirement now due to low friction loss. We have less piping distance than you mentioned >10km. As you told "There are no fixed rules saying it must be this velocity or not more than say 3m/sec". We should consider our velocity need (PIPING COST). This is really sense so I can reduce my velocity even more little upto 0.5m/s so I can get the same flow rate but the problem is the piping cost and other fittings will increase. Finally I can get the same flow rate ay EOL with this 1m/s velocity but with high cost in CAPEX.

RE: Pump discharge line size calculation

(OP)
FluidPowerUser sir In the world of fluid dynamics, the general saying is the pressure is cheap and flow is expensive. As you told we have achieved our desired pressure to omit head loss but with high energy and piping cost. Yes it is really lot of materials and energy cost involved in this. I still have to understand some deep concepts in this. I will e-mail this thread. I hope it will definitely give clarification. Thank you sir ☺️.

RE: Pump discharge line size calculation

Have you calculated the head loss in the system so that you can make a justified selection of the bore size?

If there is no requirement for heat exchange, then go as slow as you can to keep the head losses to a minimum. How deep do you want to go in terms of the concepts?

RE: Pump discharge line size calculation

(OP)
FluidPowerUser sir we didn't calculate head loss and this head 45mtr is taken by our managing director (he is also not sure about this design because I did conversation with him so he is not clear about this design and he took it based on his experience)

We selected bore size based on cross sectional area calculation because we would have to take few branches from the main header so the 250mm pipe meets our requirements so we proceed with this size that's it apart from the factors there is no calculation is involved in this selection.

If there is no requirement for heat exchange, then go as slow as you can to keep the head losses to a minimum.

Yes this is for heat transferring purpose for 33kl fermenters. But please clarify me here in this scenario I have 1m/s velocity but same flow rate

1.)so flow rate doesn't depend on velocity?
(In my understanding)
2.) If I want to reduce head loss should I need to select bigger pipe size with higher head ?
(Due to large area of pipes the fluid can flow easily without so much accumulation in small pipe area )
3.) So there is no rule for selecting one size bigger or two size bigger than pump discharge nozzle for piping ? (Like rule of thumb like that it is purely based one our needs like pipe cost and material cost , valves , fitting , space , pipe racks etc)

Can you please explain me sir 🙏



RE: Pump discharge line size calculation

Have you selected the pump performance - (180m^3/hr at 45m head) based on the heat transfer requirements?

Can you change the pump if your calculations show insufficient flow or head?

What is the water return temperature at the pump inlet expected to be? This is for the NPSH requirements and the vapour pressure of the water. Your spec sheet for the pump is showing you need to have 4.2m at the inlet. If the water is cool, then temperature won't come into the equation so much, but if you are expecting hot water at the inlet, then the inlet pressure requirements may change. I know there is a cooling tower, but it's all relative and it's important to be specific about the temperatures in the system.

Are you planning to use the by-pass valve to create a pressure drop in the ring and effectively force flow into the 8" line and through the equipment to provide cooling?

To answer your questions

1) Flow rate and Velocity

Your pump is rated to 180m^3/h at 45m head. You need to design your system around this specification so that your total system restriction, as plotted on a chart, is below the curve of the pump. DO you have the performance curve for the pump?

In the pump, it is the diameter of the impeller that dictates the max pressure and it's the height of the impeller vanes that dictate the flow rate.

The volute of the pump is then designed around the impeller to give the ideal output geometry. In your case, the inlet is 100mm DIA and the outlet is 80mm.

The average velocity at the discharge of the pump will be Q/A

Assuming that pump is operating on its performance curve, the velocity of the fluid will be 0.05m^3/sec (Q)

The area of the 80mm outlet is 5.027x10^-3 m (A)

Therefore the average velocity of the fluid at the put of the tube is 0.05/5.027x10^-3 = 9.95 m/s

You can keep this velocity if you want and stick with the same diameter of tube, or you can increase the diameter to reduce the velocity.

2) Head loss

Research 3 things - Head loss equation, bend loss coefficient and Moody Chart.

The head loss equation will help you to identify the losses and the Moody chart will help you to understand your flow regime (Laminar - mixed flow - fully turbulent). The bend loss coefficient table will help you understand the losses in fittings, elbows and valves etc.

Everything in the line will cause a pressure loss.

For every 10m of height your pipework goes up, the load on the pump will go up by 10m or 1 BAR.

3) There are no "rules" as such, just the laws of physics. As stated, it's a balance between installation costs and running costs. As long as your total system restriction is not more than the 45m head that the pump can produce, you will be OK. If your system losses exceed the 45m, the pump will deliver less flow.

Pump drive power is the flow x head with a bit added for the 80% efficiency of the pump.

RE: Pump discharge line size calculation

(OP)
FluidPowerUser
Have you selected the pump performance - (180m^3/hr at 45m head) based on the heat transfer requirements?

My managing director calculated heat transfer requirements because they have equivalent design sheet and when they calculate they were not involving me at that conversation.( Still I don't know how to do them 😔)

Can you change the pump if your calculations show insufficient flow or head?

Sir we have already installed one standby pump with the same capacity (180m3/hr with 45 MTR head) parallel configuration.
And one more we have exact space for the pump base frame so no more space even 2" gap also impossible for new pump.

What is the water return temperature at the pump inlet expected to be?

The pump is below the sump configuration even though I'll calculate the NPSHA soon.

Are you planning to use the by-pass valve to create a pressure drop in the ring and effectively force flow into the 8" line and through the equipment to provide cooling?

Yes sir it is for future consideration but not for routine operation. ( Will affect cooling performance?)

DO you have the performance curve for the pump?

No sir we don't have pump performance curve and this is really new to me because I have never seen this kind of information in our design but I can get them with our pump supplier. (Does it available on internet like common chart for all centrifugal pumps?)

Research 3 things - Head loss equation, bend loss coefficient and Moody Chart.

Of course sir here lot of new concepts I'm have to go through because this gives me more and more deep knowledge about pumps and their design.

As long as your total system restriction is not more than the 45m head that the pump can produce, you will be OK. If your system losses exceed the 45m, the pump will deliver less flow.

Finally I get the concept of less flow from your above sentence.

Pump drive power is the flow x power with a bit added for the 80% efficiency of the pump.

This one I couldn't understand please explain me little bit.

RE: Pump discharge line size calculation

The pump performance details can be found in spxflow.com.

The data you provided for the pump shows the efficiency is 80%. That means for every 1000 watts of power you put into the motor, you’ll get 800 watts of output. Efficiency is measure of the output vs the input in terms of energy or power.

RE: Pump discharge line size calculation

(OP)
Thank you so much sir. I have learned so much of knowledge form this thread especially from your conversation. Again thank you so much sir 🙏❤️.

RE: Pump discharge line size calculation

You are welcome and good luck!!!

RE: Pump discharge line size calculation

Hi,
To add to this discussion a document about cooling water, I don't know whether you have it in place.
Last remark is about velocity in pipe, if too low it can promote the deposit of salt on the wall and favor corrosion.
Higher velocity will help for self-cleaning of the pipes.
Good luck.
Pierre

RE: Pump discharge line size calculation

Yes, Pierreick is correct. Laminar flow is ideal for pressure reduction, turbulent flow is good for cleaning the walls of the tubes and valves. The flow pattern in tubes and pipes is to have almost zero velocity near the walls, so turbulent flow helps to prevent deposits and therefore higher flow rates are required.

RE: Pump discharge line size calculation

Cement line carbon steel will enable a longer life span in comparison to plain pipe. Pipe ID will be some what less.

RE: Pump discharge line size calculation

(OP)
pierreick sir thank you for sharing the document. It is really very helpful for me😊

RE: Pump discharge line size calculation

(OP)
Sorry to interrupt you all again. Today I checked my pump name plate it has Q (rated) = 180 m3/hr
Max 284 m3/hr.
Same as head (rated) 45 Mtr but max 65 MTR.

What does it mean ? If anyone have seen this picture please explain me a Little bit.

I attached the nameplate image.

RE: Pump discharge line size calculation

The data on the plate shows that the rated flow and head are 180 cubic metres per hour and 45m, respectively.

The max figures are the flow and head that the pump can reach, but not at the same time.

If the head goes up, the flow comes down and likewise, if the flow goes up, the head comes down. The performance details will also tell you the BEP, best efficiency point for the pump. The BEP should be at the rated point for the pump. So 80% efficiency at Q=180 and H=45. If you run the pump with flow and pressure away from the BEP, the efficiency will be less than 80%.

RE: Pump discharge line size calculation

(OP)
FluidPowerUser sir
The max figures are the flow and head that the pump can reach, but not at the same time.

So there is some possibility to achieve that max figures? If so what will affect them to achieve that max figures like head loss or any other parameters like improper piping size.

This is really interesting and new for me sir.
 

RE: Pump discharge line size calculation

Those numbers could be two things.

1, the max flow and head available with the biggest impellor that will fit in that pump.

Or

2 The max flow at the end of the curve. So the head would be lower than 45.
Maxhead is probably no flow / shut in head.

If the resistance to flow in your system is lower than 45m at 180 m3/ hr then the flow will increase until the frictional l and other losses equa, the head out out by the pump.

This could be because of bigger pipe, shorter ,length, less valves a d other fittings than calculated, opening up control valves, less resistance to flow in any item

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump discharge line size calculation

As mentioned before a centrifugal pump will change its output depending on the conditions placed in the discharge line.

The pump rating for H and Q are at the best efficiency point.

If you add more restrictions in the line, the pressure will go up, but the flow will be lower. Likewise, if you reduce the restriction in the line, the pressure will go down and flow will increase.

The max values on the pump plate are telling you the max that the pump will do, irrespective of how low or high the system’s restriction is.

RE: Pump discharge line size calculation

The chart is very useful, but max head will be at zero flow in most instances.

The point marked pump max head is just higher than the rated head. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump discharge line size calculation

(OP)
LittleInch sir If the resistance to flow in your system is lower than 45m at 180 m3/ hr then the flow will increase until the frictional l and other losses equa, the head out out by the pump.

So this is just a guess work because our BEP
denotes 180 m3/hr as rated. If so the capacity of the pump is more than denoted value like a straight line without any restriction like bends, valves, change of pipe section, elevation etc.

Yes if I don't have any restriction to the flow absolutely I can achieve that MAX figure.

RE: Pump discharge line size calculation

(OP)
FluidPowerUser sir If you add more restrictions in the line, the pressure will go up, but the flow will be lower. Likewise, if you reduce the restriction in the line, the pressure will go down and flow will increase.

The max values on the pump plate are telling you the max that the pump will do, irrespective of how low or high the system’s restriction is.

So I must have reduce the restrictions in my piping (straight pipe without any bends or fitting etc) to get more flow even if I want to get that Max flow rate, yes it is achievable.

RE: Pump discharge line size calculation

I can’t agree with that point, but I’m not being argumentative.

We don’t know if the impeller is unshrouded, single shrouded or double shrouded. Also, the volute is designed around the impeller to match the head and flow performance. If you change the impeller in the volute, the performance of the pump will change dramatically. It’s the OD of the impeller that determines the pressure that the pump can produce as the flow is generated by the vanes of the impeller and the fluid is flung outwards and collected in the volute.

Zero flow on a centrifugal pump will result in zero pressure as pressure is simply the resistance of flow. It will also not cool the pump seal, so what would be the point of having an unachievable performance point on the plate?

That’s just my opinion based on experience of pump design.

RE: Pump discharge line size calculation

I did say most pumps but shut in head/ no flow is not zero head. There is internal re circulation. It is normally the highest pressure the pump will generate.

Shut in head is quoted to make sure the piping etc can withstand the pressure if the flow is stopped/ valve closed somewhere downstream.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump discharge line size calculation

OK then….

Just semantics then.

In response to the last question from the OP.

Normally, you’d design the system to get what you need in terms of heat transfer and operating performance, then calculate the flow and head you need, then select a pump that can achieve the desired output.

In this case, you effectively have 45m of head or 4.5 BAR to play with and you need to make sure there is enough left at the end of the line to keep the pump out of cavitation. Just having a flooded suction might not be enough. Your pump needs 4.5m of head at the inlet.

RE: Pump discharge line size calculation

Errrr NPSH is absolute head.

And no one has stated what this is.

Pumps deliver differential head so add that to the inlet pressure.

But I've got lost where this thread is going...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump discharge line size calculation

The thread is jumping all over the place from pumps to pressure drop to inlet pressure.

NPSH required is stated as 4.2m

That’s 4.2 metres of absolute pressure or 420 millibars minus the vapour pressure of the water and we don’t know that. So I was just making reference to it as it needs to be calculated by the OP.

RE: Pump discharge line size calculation

Think the OP needs to read this

https://www.pumpfundamentals.com/

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump discharge line size calculation

(OP)
FluidPowerUser sir Just having a flooded suction might not be enough. Your pump needs 4.5m of head at the inlet.

But I have calculated our NPSHA it is just 3.12 MTR.

I referred this site for my calculations: https://pumpsdesign.com/npsha/


I have attached the image please review it and if any error I my calculations guide me to correct them. Thank you sir.

RE: Pump discharge line size calculation

That pipe entrance loss looks looks very high.

How was it calculated?

But then you haven't allowed much for your strainer. Is this a very big mesh?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump discharge line size calculation

Read it properly.

It says 0.63 milli bar, not bar,

So basically 0.00063 bar or negligible.

I would allow 2m for the strainer basket.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump discharge line size calculation

(OP)
LittleInch sir
I have corrected my calculations.

NPSHA= (Absolute pressure head - vapour pressure
+ static head - pump suction line losses)

NPSHA = (10.33-0.238+1.8-2.51)
= 9.3 meters

But our pump needs 4.2 NPSHR

If so I have enough head to this system.

Can you please review this sir.

RE: Pump discharge line size calculation

That looks better.

You do need to make sure that suction line strainer doesn't get blocked or create a large pressure drop.

And don't forget NPSH is not cavitation limit. Always allow 1 to 2 m above NPSHR to make sure your pump doesn't start cavitating.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump discharge line size calculation

And remember to include a min flow recirculation line. See the pump datasheet for min flow value, and also for NPSHr.
If this pump is on auto start with existing parallel pump, then the pump starting up may go out to end of curve ( EOC) during startup, since system frictional resistance will decay to zero since the active pump would be coasting down to stop - check the pump motor power is sufficient for EOC operation.

RE: Pump discharge line size calculation

(OP)
LittleInch sir
We plan to install strainer after 5 meter run of the pipe but due to come space concern we installed near to suction manifold.
Thank you sir 😊



RE: Pump discharge line size calculation

(OP)
georgeverghese sir
The pump will run in manual mode and recirculation we should have planned.

check the pump motor power is sufficient for EOC operation.

Can u help me how to calculate power consumption at EOC operation?

RE: Pump discharge line size calculation

(OP)
I have attached the pump suction piping.

We don't have enough space to position strainer at "y" axis so we decided to install them on "x" axis.

Does it make any increase in pressure drop ?

I know yoy are all busy professionals anyway consider my post when you have free time. Thank you all 🙏

RE: Pump discharge line size calculation

One way to find EOC motor power is to look up the Q-h curve and see what power is at 284m3/hr. However, if these pump are not configured to autostart, you can avoid EOC operation by throttling the discharge on startup, if motor power is not adequate for EOC operation.

RE: Pump discharge line size calculation

(OP)
pierreick
thank you so much sir for sharing the booklet and your valuable time for me.🙏❤️.

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