deciding wether one or two pump is needed

[martino6000]

HALLOW EVERYONE!
I AM CONSTRUCTING A PIPELINE FOR COOLING TOWER .I NEEDED PUMP FOR PUMPING COOLLED WATER BACK TO HEAT EXCHANGER.BASED ON MY PIPELINE DESIGN AND VOLUME FLOW RATE;I HAVE CALCULATED OVERHEAD PRESSURE DROP.I HAVE BEEN SEACHING FOR A SUITABLE PUMP THAT COULD MACTH MY CALCULATED VALUES, I DID GET SOME BUT THEY ARE ALL SPECIFIED TO BE IN USE FOR OTHER PURPOSES e.g IN CHEMICAL INDUSTRIES:IS IT ADVISABLE TO USE THESE PUMPS FOR MY COOLING WATER SYSTEM:HERE ARE MY VALUES.
IS IT BETTER TO USE TWO PUMPS IN PARALLEL CONECTION SO THAT I CAN EASLY GET BETTER PUMP/BEST OPERATING POINT FOR MY SYSTEM.YOUR HELP WILL BE HIGHLY APPRECIATED.
Volume flow:900 cu meter/hr
pump head:98m
Static head:27m
kinetic viscosity:0,68cst
temperature:40°C
PumP power:299KW
Pumpen type:Coolling water pump

 

[cryotechnic]

Well... My opinion is that, if you can find a pump (and I think you can) with design vallues who match your calculated vallues, it wouldn't be a problem to instal just 1 pump.
Ofcourse the choose of installing 1 or 2 pumps depends also on how critical the cooling system is for your whole proces.

For more advice ask your local sales rep.

 

[sprintcar]

Contact the pump vendors - they have the data to fit your application and can recommend the best approach.

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

 

[Bjegovic]

number of factors is relevant, to mention some of them:
- how much your flow / head conditions vary? ploting pump vs system curves for different layouts is the best way to figure out how the system will behave

- you will install a stand-by pump, right? in that case, purchasing 3 (2+1) small pumps instead of 2 (1+1)large pumps may be more feasible

- compare the cost of the two designs,taking into account all valves and fittings necessary.

 

[Artisi]

A pump of this size is nothing special, any of the major "big" pump manufacturers could accommodate this duty with one pump. However, in some case you might be better with 2 x 50% units to give at least 50% standby capacity or even 3 x 50% pumps which would ensure 100% under all but extreme upset conditions.

Naresuan University
Phitsanulok
Thailand

 

[Montemayor]

What Artisi is describing is the logic that I have always been exposed to and have applied in an operating, industrial plant. I was taught to never "put all your eggs in one basket" when operating an expensive and needed industrial process. We were will to go down 50% in plant production for a brief time, but never 100%.

Capacity turn-down can also be handled much easier with multiple pumps than with one, solitary model. No industrial plant can operate for long without debating how to reduce its capcity due to market demand changes and equipment repairs or maintenance needs. All plant utility services have to adjust accordingly.

 

[checman]

Martino6000

One pump on a VFD will work best. I crunched the numbers and ran some curves. You apparently have a fair share of system losses at max flow and assuming you have sized the pump at max demand and with a little cushion the VFD will give you a payback. Because of the system losses It may be hard to get two pumps to run at the same time and both be efficient at less than max flow. It is nice not to have all your eggs in one basket but in this case it would not be in your best interest.

Regards Checman

 

[Artisi]

I cannot agree with chemman for this application, it seems the duty is fairly well defined so I cannot see the point of a very expensive VFD system as there was no mention of a variable duty and the question was is 1 or 2 units the best arrangement.
Still think 2x50% or 3x50% is the way to go.


Naresuan University
Phitsanulok
Thailand

 

[25362]

Artisi, something in your philosophy is debatable. If the flows are well-defined, and no variable duty is expected, as you say, it seems 2x100% would be the best selection (one pump as a stand-by), the supply of cooling water being a critical item.
However, since I agree with the expectance of reduced duties, for climatic or process reasons; thus it seems to me your approach (3x50%) would be the right one.

 

[Artisi]

I don't have any argument with 2 x 100% pumps - but a cost analysis on 2x100% verses 3 x 50% should be undertaken.

Naresuan University
Phitsanulok
Thailand

 

[checman]

Martino6000
The point I was making is that a single pump can efficiently handle this application. If the load varries a FVD has a pay back when you are talking the HP you will need. Also with a VFD you can go with a max impeller (normally the most efficient)and run the pump a little slower and gain efficiency. When you are talking this large of pump and motor your energy use will be much larger than the cost of equipment in most cases. If the pump is critical by all means have a back up.

Regards checman

 

[quark]

If the system requires a constant flowrate then I am along the lines of what Artisi said. 27meters of static head(with 98meters total head at full flow) limits the flowrate not to go below 50%(if we employ a pump with VFD) and a 3x50 arrangement can also be good, in this case.

I am against the idea of going for a bigger size pump and then slowing it down to the required duty point along the BEP. As the head delivering capacity is proportional to the square of speed and flowrate is linear with speed, the pump will always run at higher flowrates than actually required and thus increasing energy cost. This may offset whatever benefit we get from improving hydraulic efficiency.

Regards,

 

[checman]

I am not sure what pump or program quark ran to get his information. Large fan type pumps designed for services like this have very flat curves and the flow rate changes dramatically with small changes in head. Quark must have not run curves for a pump like that. The pump I selected and ran went from 84.7 over all efficiency to 85.1. with a max vs. cut impeller. Not that large of a difference in this case but with 379 hp you will find the saving will add up. The rpm went from 1775 to 1720 rpm. Couple this with limiting the loss through a valve for changing demands to a minimum you could save big. Proof is in the pudding pick a pump and run the curves.

Regards checman

 

[macmil]

I've been watching this one with interest. There are two aspects here. The first is the solution to the pump requirements. The second is the required degree of redundancy, or back up. Yes, I agree the two aspects are intertwined, but I believe it's important to address the pumping problem first, and then consider the degree of redundancy required.

 

[Artisi]

One other aspect that should be addressed is the head loss through the pipe line - a careful analysis of pipe size /friction loss / capital cost should undertaken - at the moment friction loss is 70 metres - a lot of friction and a lot of power being consumed. Remember friction loss for a given flow is a function of pipe diameter to the 5th power.

Naresuan University
Phitsanulok
Thailand

 

[25362]

When system head derives mainly from friction losses, speed control is best, because pump efficiency remains practically constant, it is gentle to pump and valves so that their life span is increased. Operating costs are reduced, as well as power input due to the higher pump efficiency all along.

A given percentage of the cooling water flow rate should be filtered back to the cooling tower basin.

When speed, n, is reduced the law of similarity tells us that flow rate, Q, head, H, and NPSH are proportionally changed as follows:

Q [ε] n; H [ε] n[sup]2[/sup]; NPSH [ε] n[sup]2[/sup]​

Besides, one should remember that the power required is proportional to flow rate multiplied by head, and head is about proportional to flow rate squared. Artisi's and checman's advices should both be seriously pondered.

 

[quark]

checman,

25362 briefed about what I am saying. I have no issues, absolutely, with the pump and its increased efficiency. My point is about the system characteristic. For example, we consider a reduction of 10% in speed to match our duty point from a bigger size impeller. As I have no data about the bigger impeller we are talking about, I would just consider the actual point.

With 10% reduction in speed, flow will reduce by 10% and the head reduces by 19%. For the 98m head pump the new head at 90% speed will be 79.38m. The static component out of this is 27m, so the pump can take care of 52.38m frictional losses. But frictional losses with 10% decreased flowrate will be (98-27)x0.9[sup]2[/sup] = 57.51m. Now our pump is unable to deliver a head of 5.13m.

If we have to provide 84.51m(57.51+27static)of head the pump should run at 1-(84.51/98)[sup]1/2[/sup]~93%. So we are providing 3% more flowrate and the increase in power consumption will be 3% approx.

If the system losses are purely dynamic then the reduction in head capacity is exactly offset by the reduction in losses.

Regards,

 

[25362]

To strenghten quark's point let's look at the well-known Moody diagram.
When the Re number is below the fully rough flow line, a reduction in Re by 10% may increase the friction factor making the reduction in [Δ]P[sub]f[/sub] of the system even narrower than the one estimated by quark, as if the reduction weren't 0.9[sup]2[/sup], but, say, 0.9[sup]1.8[/sup].

 

[stephenweinstein]

Regarding 2 x 100% pumps versus 3 x 50%:

With 3 x 50%, you are more likely to have two simulatenously out of service and therefore fail to achieve a total of 100% from what is in service.

With 2 x 50%, you are more likely to have everything simulataneously out of service and therefore have 0% in service.

For example, if you are using pumps to put air in car tires, you may decide that sometimes having it take twice as long as normal (50% of normal speed) is acceptable, but ever not being able to do it (0% of normal speed) is unacceptable.

On the other hand, if you are running the cooling system for a nuclear power plant and are required to shut down the reactor anytime that pumping capacity gets below a certain threshold, then you may feel that sometimes being at 0% is acceptable (you can afford some downtime) but frequently being at 50% is not (because you cannot run at 50%).

 

[Montemayor]

stephenweinstein:

I don't understand your belief that pumps are used to fill vehicle tires. Pumps pump liquids; compressors fill tires. The two handle totally different fluids.

Additionally, please explain your reasoning for asserting that "With 3 x 50%, you are more likely to have two simulatenously out of service". Is this your belief or someone's theorem or finding? While this could be true, what is the engineering logic behind it? The same comment applies to your statement: "With 2 x 50%, you are more likely to have everything simulataneously out of service".

I have operated cooling towers with 2 x 50% capacity pumps in many sites and never had any failures - much less everything simultaneously out of service. Perhaps I'm just lucky, but I'm willing to learn about this probability.