VSD or throttle control strategy for RO High pressure pumps 1

[baasi]

We have two stage RO plant of capacity 75 m3/h (permeate)[/color] and two high pressure pumps(51 m3/h each) are installed to maintain the feed flow. The raw water flowrate is 94 m3/hr . The throttle valve flow control strategy is applied along with the bypass. The whole scenario is shown in attached picture.
Note : Membranes type is CPA3 Hydranautics
High pressure pumps are vertical multistage centrifugal pump with 37 KW rated capacity motor.

Some of the parameters are as follows:
1- Inlet pressure of the High pressure pumps is 2 bar
2- Outlet pressure of the High pressure pumps when both are running in parallel is 20 bar
3- Pressure after throttle valve is 15 bar which means requirement of the membranes is 15 bar
4- Pressure drop after first stage of RO is 12 bar
5- Booster pump boost the pressure to 15 bar again for 2nd stage RO.

I have some questions related to it and would appreciate the reply from experts.

1- What throttle valve actually do? Does it control pressure or flow?
2- As mentioned above the pressure requirement for the membrane is 15 Bar but the pumps are generating 20 bars. Why do we need pumps or higher pressure and flow?
3- Would it be feasible/worth it to install VFD on one pump and other pump will work on fixed speed.
4- Do you think pump are sized properly?

 

[1503-44]

I wouldn't do that, but each to his own.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[ashtree]

All the above answers are good advice but I am going to propose a slightly different approach because there are some gaps in your information.

First of all as someone has suggested above you need to start with the concentrate flow control valve and when the plant is running adjust it until the recommended concentrate flow is achieved. Too little flow may scale the membranes too much is a waste of water and energy. You will need to know what that concentrate flow should be. Membrane manufacturers publish minimum concentrate flows but this will also be dictated by the water chemistry and the permissible recovery rate.

You would then adjust the gate valve after the high pressure pumps until you get the desired permeate flow. Depending on the design of the system and the equipment used you might have to also readjust the concentrate control valve to get it back to where you need it but probably not much. I would suggest that you adjust the system by small amounts at a time and allow the system to settle for a while after each adjustment. The permeate flow will be dictated by feed water quality , permissible recovery rates, and membrane area.

Doing this will allow you to see what are the relevant flows and pressures in the system to get the desired flows. Forget about trying to maintain 15bar on the first stage and 15 bar on the second stage. This will cause a flux rate imbalance in favor of the first stage because of the higher second stage conductivity. What you are actually looking for is a 3 bar boost on the second stage inlet and a focus on achieving the recommended/required flow rates. The pressures will be what they will be and if the designers have it right then 3 bar will approximately balance the flux rate between the two stages. Without knowing everything about the system its impossible to know whether or not the 3 bar is enough

Once you know the actual operating pressures and flow rates you can decide whether or not VFDs are worthwhile.



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