deciding wether one or two pump is needed
deciding wether one or two pump is needed
(OP)
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
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





RE: deciding wether one or two pump is needed
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.
RE: deciding wether one or two pump is needed
"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
RE: deciding wether one or two pump is needed
- 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.
RE: deciding wether one or two pump is needed
Naresuan University
Phitsanulok
Thailand
RE: deciding wether one or two pump is needed
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.
RE: deciding wether one or two pump is needed
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
RE: deciding wether one or two pump is needed
Still think 2x50% or 3x50% is the way to go.
Naresuan University
Phitsanulok
Thailand
RE: deciding wether one or two pump is needed
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.
RE: deciding wether one or two pump is needed
Naresuan University
Phitsanulok
Thailand
RE: deciding wether one or two pump is needed
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
RE: deciding wether one or two pump is needed
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,
RE: deciding wether one or two pump is needed
Regards checman
RE: deciding wether one or two pump is needed
RE: deciding wether one or two pump is needed
Naresuan University
Phitsanulok
Thailand
RE: deciding wether one or two pump is needed
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:
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.
RE: deciding wether one or two pump is needed
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)x0.92 = 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)1/2~93%. 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,
RE: deciding wether one or two pump is needed
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 ΔPf of the system even narrower than the one estimated by quark, as if the reduction weren't 0.92, but, say, 0.91.8.
RE: deciding wether one or two pump is needed
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%).
RE: deciding wether one or two pump is needed
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.
RE: deciding wether one or two pump is needed
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."
I too was a litle puzzled by this posting, but getting back to 2 x50% versus 3x50%, it all comes down to economics, and your faith in the installed equipment etc.
2x50% units is fine as it always gives at least 50% capacity should one unit ever go-down or has to be shut for unscheduled maintenace, however 3x50% will (usually) give 100% capacity even allowing for the failure of 1 unit or unscheduled maintenance on 1 unit, it also give you the benefit of having one unit out of service for routine maintenance.
Naresuan University
Phitsanulok
Thailand
RE: deciding wether one or two pump is needed
I for one always feel that it would be nice to hear that our collective help overcame a problem - but guess that's in a perfect world, anyway, maybe we have helped someone else along the way to make it all worthwhile.
Naresuan University
Phitsanulok
Thailand