5 replies to this topic

[Austro]

Hello, I am buying a reverse osmosis system in order to provide non-potable low TDS water for chemical process tanks. Currently the system is fed from the main water line which provides potable water feed at 40 psi. The system throughput requirements are approximately 250-300 gpm. This varies however, so how would I design my distribution pump for the RO system in order to always remain at the best efficiency point? Would a VFD be sufficient to maintain my operating condition at >90% of the BEP? Should I use a postive displacement pump instead of a centrifugal pump?

Also, is there any way to empirically develop a system curve for a piping network? Some of the piping is inaccessible and im not sure I can rely on the CAD drawings of the piping.

[DRS]

Your objective is not very clear. Generally, you have three pumps. The first one is feed pump from a storage tank or pipe header. Second one will be a multistage pump feeding RO membranes. Third can be RO water distribution pump.

You can control RO distribution pump by pressure sensing at the end point and will not be any issue. Membrane feed pump should better be constant flow type and you can acheive savings by using On/Off logic based upon RO water level control (presuming you have a storage tank). If you are manipulating speed of feed pump (to membrane feed pump), this will effect the performance of membrane feed pump. The head developed by the pump, at a different speed, varies by the square of ratio of speed multiplied by initial head.

If you don't provide enough feed pressure to the membranes, you will have more reject.

[Austro]

Feed pressure is at 40 psi from main water line. I think i just need a pump to pressurize the water going into the RO and a pump for distribution from the storage tank (plus small auxillary pumps like for CIP and circulating water in storage tank through UV). So, what you are saying is to use a centrifugal pump for distribution with a VFD and use a an on/off type system for the membrane feed pump and use a 300 GPM rated positive displacement pump?

My real question is... if I use a centrifugal pump with VFD for distribution and I design my pump to be at its BEP during normal operation... will I still be at the BEP of the pump when the GPM needed by the processes downstream is reduced. For example lets say I have four processes that use 75 GPM each. If I close two valves downstream I will increase my backpressure in the line and no longer be operating at the BEP. So then the VFD can adjust the speed of the motor to provide fewer GPM at a lower head and still maintain operation at the BEP point of the pump?

[DRS]

Few more questions.

Is it a recirculating system (the user side)?
What is the capacity of RO system ?
What is the capacity of storage tank?

You shouldn't really worry about the membrane feed pump even if it is a multistage centrifugal. All the systems I operated use centrifugals. They are cheaper and easy to operate than positive displacement pumps (ofcourse, other members may have different experience and idea)

If your requirement is variable type, then you can speed control the pump by using a pressure transmitter at the end of the supply header. If your system is recirculatory type then a pressure transmitter placed before the tank entry will control the system.

You can maintain the system at BEP at all flow conditions if your system curve lies in the isoefficiency region of the full load design duty point. In simple terms, various duty points should lie within the parabolic efficiency curve.

Developing a system curve is very easy. One of the simplest yet powerful method is to use parabola technique. Since, I am attaching here the procedure.

Please note that this method has some limitations as it assumes that system resistance is proportional to square of flowrate. There are systems which have system resistance proportional to the flowrate or cube of flowrate or the combination of the two.

The best method is to individually calculate system resistance for various flowrates and then plot it on the pump curve. This become very easy if you set this up in a spreadsheet.

Attached Files

•  System_Curve.xls   18KB   71 downloads

[Austro]

Few more questions.

Is it a recirculating system (the user side)?
What is the capacity of RO system ?
What is the capacity of storage tank?

You shouldn't really worry about the membrane feed pump even if it is a multistage centrifugal. All the systems I operated use centrifugals. They are cheaper and easy to operate than positive displacement pumps (ofcourse, other members may have different experience and idea)

If your requirement is variable type, then you can speed control the pump by using a pressure transmitter at the end of the supply header. If your system is recirculatory type then a pressure transmitter placed before the tank entry will control the system.

You can maintain the system at BEP at all flow conditions if your system curve lies in the isoefficiency region of the full load design duty point. In simple terms, various duty points should lie within the parabolic efficiency curve.

Developing a system curve is very easy. One of the simplest yet powerful method is to use parabola technique. Since, I am attaching here the procedure.

Please note that this method has some limitations as it assumes that system resistance is proportional to square of flowrate. There are systems which have system resistance proportional to the flowrate or cube of flowrate or the combination of the two.

The best method is to individually calculate system resistance for various flowrates and then plot it on the pump curve. This become very easy if you set this up in a spreadsheet.

First of all I want to thank you for your detailed and helpful response.

"Is it a recirculating system (the user side)?"

If you mean will any of the retentate be fed back to the feed then yes... I am shooting for at least 70% recovery of the feed.

What is the capacity of RO system ?

300 GPM total permeate @ 70% recovery.

What is the capacity of storage tank?

Designed storage tank for 5k gallons... this is another issue... I dont want the RO system going on and off to save on wear on tear so VFD on both the distribution and feed pump.. so then I guess there will also need to be a VFD on the recycle pump as well? Should I have a back for the recycle pump? I plan on having a back up feed and distribution pump with a selector switch.

What is an example of a circulatory system? Maybe using water to rinse parts and then treating the waste water and then feeding it back to rinse water tanks.... or maybe cooling water that needs to be very pure to avoid fouling?

[DRS]

By recirculation system, I mean, a loop where the water is drawn from a vessel and put back at the end of the loop, tap off and draw fluid from it partially. This is a common arrangement in Pharma Companies so that there is no stagnation in the entire pipe line.

If you maintain your tank volume from 500 gallons (Low Level) to 5000 gallons (High Level), you have about 15 minutes storage capacity at peak load and you have 4 cold starts of the system in an hour. This is not bad at all.

The most important factor for any RO system is to provide feed at sufficient pressure to overcome the osmotic pressure, which is proportional to the feed TDS. Now, when you reduce the pump speed to manipulate the flow requirement, the head developed reduces significantly (refer affinity laws). Since, you are unable to provide enough feed pressure, there will be less permeate and more reject.

Hope I tried to clarify the issue.