Determine pressure drop of RO in minimum recirculation line

[Orsiz]

Dear all,

I'm a new member of this forum but I've read some interesting topics in the past and I feel I can learn a lot from experienced members here as I'm a young graduate.

I have a question regarding the design of a restriction orifice in centrifugal pump operation. The function of the RO is to have a continuous minimum flow during operation. In plant designs I've seen, they typically place the orifice in a line going back directly from the pump discharge to the suction vessel. In our system, the line going back to the suction vessel is located much more downstream. The branch is located at the end of the main process line and this main process line consists of piping, fittings (bends, manual valves, check valves) a heat exchanger and filters.

To determine the bore diameter, I need the minimum pump flow (from vendor) and pressure drop. I know the minimum pump flow (16 gpm, the flow through the main process line is 100 gpm) but I'm not sure of how to determine the pressure drop. My way of working is to look at the total differential head at minimum flow (from pump curve) and calculate the suction head. The sum of these two is equal to the discharge head. If I calculate the frictional losses (due to fittings, heat exchanger and filter) and static head, I know which pressure drop the RO needs because the sum of these is the discharge head.

My questions:
- Which pressure drop do I take for the filters? The clean pressure drop or the maximum pressure drop (dirty filters)? If I take the max pressure drop of the filters, my flow through the orifice will be greater when I have a clean filterelement so then my flow through the main process line (100 gpm) will be lower as more flow goes through the orifice. The other way around, If I take the pressure drop of a clean filterelement, then I think there is the risk of going below the minimum pump flow due to saturation of the filter.
- Do I calculate the losses due to fittings, heat exchanger and filters at the flow rate of 116 gpm (sum of both flows)?
- Is it a good idea to have this line so much downstream of pump discharge?

Thanks!

 

[ashtree]

Can you sketch out a flow diagram for us so it is clear in our minds what you are trying to do.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[georgeverghese]

Agree with the concern regarding the uncertainty of the dp across the filters. The branch tee (leading to this RO) should be located upstream of this filter and any other devices that may show variable dp at a fixed flow of 16gpm.

 

[Orsiz]

@ashtree: I drew a simple PFD of the process (RO marked in green), I hope this helps.

 

[Latexman]

#3 No, it is a bad idea. Dirty filters, mis-valving, etc. will dead-head the pump. Too many opportunities for error, especially if not fully automated.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

I'm not a fan of continuous flow bypasses. They are a constant power loss and to me a rather brutal way of maintaining flow. Have you thought about an ARV. That way you don't have to worry about pressure drop as it varies on flow. If you add up the lost energy it might make commercial sense to use it.

If you want to use an RO here then you will need to look at worst case pressure drop. The min flow figure is quite low so I wouldn't want it to be much lower.

It's in the wrong place though.

You only need to calculate pressure drop at 16 gpm.

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

 

[LittleInch]

Forgot to say welcome. That was a well laid out post. Just add the diagram to the OP next time. Pictures and diagrams are worth 1000 words.

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

 

[Orsiz]

Thank you for the replies. the use of an ARV is probably too late as we're in a late project phase but I'll definitely address it. Based on your replies, I think the best way for now is too use the pressure drop of a dirty filterelement (an alarm is generated at a certain filter pressure drop so I can use this value) to calculate the pressure drop needed by the RO. The flow through the RO will be greater initially but this is less of an issue than dead-heading the pump.

 

[mk3223]

Is this a dirty operation which is designed with four filters?
IMHO, at the first opportunity as the system is in service, the RO bypass line can be relocated close to the pump discharge, i.e. the upstream of the heat exchanger. So, it can safely protect the pump.

 

[Orsiz]

@mk3223: The operation is not dirty, the fluid is water in which any type of contamination is not allowed. I totally agree that upstream of the heat exchanger is a better location but the intention of the design is to have some kind of recirculation through the whole system instead of just downstream of pump discharge.

 

[LittleInch]

I'm curious as to why you show an actuated valve around the RO? Why?

Can you add an actuated valve on the RO line and open & close it based on flow?

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

 

[Orsiz]

@LittleInch: I made a mistake here, thanks for mentioning it. The actuator shouldn't be there as it is just a manual globe valve to bypass the RO.

 

[mk3223]

The system can work safely once the operation procedure and training are provided properly.
But, the pump could be vulnerable if the flow is blocked in the heat exchanger or filters. The long circulation piping route can protect the pump in expense of the energy and extra piping material.

 

[georgeverghese]

An RO will work in this application only when the Q-h curve for this pump at around the 16gpm mark is reasonably sloped. If it looks flat (or it is more or less flat), the RO wont work.

 

[LittleInch]

I thought it would be the other way around? The RO is sized for a pressure drop and a flow. So long as the pressure drop stays fixed so will the flow?

As the pressure reduces as you increase flow and pressure drop in the HX and the filters increase and the pump head falls then the flow through the RO will reduce a bit but that's no bad thing.

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

 

[Orsiz]

@georgeverghese: Can you explain this in more detail please? Te Q-h curve for this pump is indeed more or less flat around 16 gpm but this minimum flow was given by the vendor so normally the RO should work if the system pressure drop is fixed (I follow LittleInch here).

 

[georgeverghese]

Oops, got this muddled up with power savings considerations. Yes, the RO would still work even if the curve is flat at around 16gpm, it is that you'd be expending more driver power when flow is in excess of 16gpm (in comparison to a min FIC - FCV control loop).

 

[dcasto]

The problem I see with your diagram is that the exchanger in the line will either heat up the inlet tank or cool it, not sure the duty direction. Unless the fluid is terribly full of junk to be filtered, put the min flow RO just around the pump(s).