Can FCV replace Frequency drive

[Watro]

Helow there,

<sup>Some one please advise the consequences if the SWRO high pressure pumps is controlled by a Flow control valve instead of VFD.
A quick comparison with pros/cons would help complete my ongoing study

Thanks'
Niss</sup>

 

[bimr]

Would not the answer depend on the process? Are this high pressure RO pumps? SWRO stand for sea water RO?

 

[Watro]

I apologize for giving meager info

Yes, This is Seawater RO plant where the feed RO high pressure pump pressure ranges 70 - 78 bar-g at discharge. The energy is recovered with Isobaric PX-ERD devices. I have never seen a SWRO desal plant with an auto FCV on pump discharge side, however would like to know WHAT IF and the pros and cons

Thanks
Niss'

 

[GameJudge]

Power consumption is the first thing that jumps to mind; running with a VFD means you are trying to target the output by modulating how you run your pump. Using an FCV means you're running your pump at speed and braking the output to push it along its curve.

 

[bimr]

The RO systems are typically designed to operate at a more or less constant flow rate and pressure. Having said that, the RO systems will experience some loss of productivity over time as the membranes foul.

For a feed pump sized for the maximum expected membrane pressure requirement, feed pressure control can be achieved by two methods:

Use a feed throttle valve to reduce pump discharge pressure as required by the membrane.

Or use a VFD to vary the frequency of the power to the feed pump motor. Varying pump speed allows the pump to vary discharge pressure with minimal energy loss. The VFD also allows a gradual increase in pressure (slower ramp up) which minimizes electrical starting load and reduces stress on the instrumentation and membranes. However, if the expected feed pressure variation is less than 4-5 bar then a solid-state soft-start motor starter may be a more economical solution. Typical VFDs have a 3% loss of electrical energy. Significant energy savings on RO systems is also not going to happen if the pump is properly sized.

VFDs are suitable to 600 kW. For larger RO systems, at higher power levels, medium voltage equipment is generally used, making VFDs extremely expensive. Other means of feed pressure control are available such as the high pressure hydraulic energy management integration (HP-HEMI) energy recovery device (ERD) that centralizes brine hydraulic energy recovery, feed pressure and flow control and brine pressure and flow control into one compact and fully integrated unit.

If PD pumps are used to provide feed pressure control, VFD or a feed throttle valve are not necessary.

It is more cost effective to properly size the feed pump than to try to optimize operations with a VFD or FCV. The size of the RO system also has an impact on the method of pressure control that is selected.

 

[BigInch]

The requirements do not appear to be VFD compatible. There is little point in using VFDs when flow rate and pressure are relatively constant. VFDs work best when flow is between 50-90% of design flow rate at least 50% of the time, or pressures at design flow rate vary by more than 25%. Still a number of potential problems associated with VFDs should cause one to think long and hard before installing them in lieu of a simple control valve.

 

[stanier]

VFDs have been over sold to industry. Lazy or overworked engineers routinely add a VSD to a drive when it is entirely unnecessary. An FCV can in most circumstances be used far more efficiently and less costly than a VFD.

The file I have attached has a presentation given to ASME, Engineers Australia and the Institution of Mechanical Engineers.

ôThe beautiful thing about learning is that no one can take it away from you.ö
---B.B. King

http://waterhammer.hopout.com.au/

 

[Watro]

Thanks for your all of your valuable inputs

Flow will be varying up to 20% from design, to meet some unusual cases

Further, the RO HP pump is rated approx. 2 MW which is 4.16 kV medium voltage connection

Also advise, is soft starters adequate to ramp up/ramp down without VFD and with FCV

Thanks'
Niss

 

[bimr]

For the size system that you are considering, the use of soft start controllers will most likely not be economical. Another "soft" start approach is to slowly open the feed valve.

 

[davefitz]

Refer to the "fan laws" or "Pump laws" to determine if a VFD is applicable to your system. The flowrate is proportional to pump rpm, the developed head is proportional to the rpm ^2, and the consumed power to the rpm ^3. Ideally the VFD is applicable to systems whose pressure profile also is proportional to flow ^2. However, if the required pump discharge pressure is independent of flow, then a VFD is not a useful addition; a soft start, constant speed pump may be a better solution.

"Whom the gods would destroy, they first make mad "