Question about what type of flow meter application to use

[vaavas]

Hello,
I was wondering what method is more accurate to measure flow given the below application?

Im helping with a project that involves measuring flow for feedwater (steam cycle power plant). It’s to measure flow at the discharge of a pump and the flow meter will control a valve on the pump recirc line to maintain minimum flow. The design conditions for the demin water at min flow are: 307 deg F at 160 gpm, 400 psig. The conditions can reach 360 gpm.

There were 2 options I considered.

Option No. 1 - Install Differential Pressure Transmitter and Pneumatic Control Valve
Option No. 1 is to install the following:

● Differential pressure transmitter at the pump discharge and suction piping.
● Resized HDP recirculation line with pneumatic control valve.

The differential pressure transmitter will send a signal to the Distributed Control System (DCS). The DCS will control the pneumatic control valve to maintain pressure for minimum flow based on the unit’s HDP curve.

Option No. 2 - Install Flow Meter and Pneumatic Control Valve
Option No. 2 is to install the following (see attached sketch):

● Flow meter within the HDP discharge piping.
● Resized HDP recirculation line with a pneumatic control valve.

The flow meter will send a signal to the DCS. The DCS will control the pneumatic control valve to maintain minimum flow based on the unit’s HDP curve.
The flow meter is a Karman Vortex measurement principle. This is the option Im recommending (see attached sketch).

my train of thought is that installing a flowmeter will be more accurate because of its direct method to measure and control flow. The differential pressure transmitter is not the preferred option because of its indirect method to measure and control flow. It introduces a margin of error by using pressure as a control variable to correspond to the minimum flow from the HDP curve.

Let me know if this seems incorrect to any of you guys. Thanks in advanced for your help and time.

 

[bimr]

Yes, the vortex flow meter will be more accurate. However, the vortex meter is 3 times the cost of a Differential Pressure flow meter.

What is the reason that you desire the higher accuracy and what accuracy are you trying to achieve?

 

[vaavas]

Thanks bimr. Higher accuracy is desired (please correct me, if the diff press transmitter can perform good enough to get the same results):

To reduce inaccurate measurements that can cause the pump to still be repaired from operating below its minimum flow rating.
to avoid future problems/ modifications to the control logic as the pump efficiency decreases over time.

There was also concern about the same pressure corresponding to 2 flows on the pump curve (droop).

 

[vaavas]

Bimr, do you know of a website or sometype of resource that I can look up the level of accuracy for a diff press trans and a vortex flow meter?

 

[bimr]

https://automation.isa.org/2013/01/...e-ugly-of-differential-pressure-flow-devices/

Try Bela Liptak's Instrument Engineers Handbook:

ftp://ftp.unicauca.edu.co/Facultades/FIET/DEIC/Materias/Instrumentacion%20Industrial/Instrument_Engineers__Handbook_-_Process_Measurement_and_Analysis/Instrument%20Engineers'%20Handbook%20-%20Process%20Measurement%20and%20Analysis/1083ch2_1.pdf

 

[mk3223]

The Option 2 should be for your system since the "flow" is the main concern for the pump operation, which be controlled directly if possible. Depending on the pump characteristic, the pressure control can be an issue for the pump with a flat curve.
Regarding the type of the flow meter, verify it with the Control Engineer for the accuracy of the different meter types as needed. And then, you may decide the one based on the design requirement, cost and lead time.

 

[LittleInch]

That was going to be my response. The "accuracy" of a PD type measurement is wholly dependant on the shape of the pump curve at that point. Fairly steep and the flowrate can be worked out quite aaccurately, fairly flat and the uncertainty could be very high.

Or you could look at ARV valves which do this without external input to a good level of accuracy (10% or better).

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

 

[georgeverghese]

Either vortex meter or the dp cell would do just as well, but agree that the vortex meter's extra cost here may not be justified since its higher turndown of 10:1 is not required in this application. But make sure the feed to this pump is from a tank or vessel where the water is adequately degassed; your diagram doesnt show this detail on pump suction source configuration; else neither will work.

By the way, why do you have this thermocouple signal going to the min flow recycle controller in DCS? Water density variation with temp is not much, so in the case of a dp cell application, it wouldnt be necessary to compensate for density variation for flow computation. To my memory, vortex meters dont need density compensation in the case for water either.

 

[vaavas]

Thanks everyone. your help/input is much appreciated.

georgeverghese: Sorry, I left the thermocouple information out. The thermocouple is to alarm in the DCS incase the pump's maximum operating temperature is exceeded.

 

[LittleInch]

That's good.

So what are you going to do?

If you need some time be sure and come back and tell us to close the loop and give us all some feedback.

Thing I forgot to say was that the min flow level from the vendor is not a precise number to be followed / required to 2% accuracy. 10% is Ok and 20% probably fine as well. Min flow is all run on guidelines and with some allowance for variations so that the pump vendor can't be blamed if someone runs their pump at that flow for long periods / continuously.

So flow meter level accuracy may not be needed.

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

 

[vaavas]

Thanks LittleInch. Im going to look more into how accurate the Diff PT compared to the Vortex flow meter accuracy, to see if the Diff PT can do the job. I'll post my findings. Probably in 2-4 weeks from now. thanks again.

 

[bimr]

The flow meter accuracies are listed in the attachment.

 

[georgeverghese]

Depending on what the backpressure at the exit of the min flow FCV is, presume you're aware that flashing in the CV throat can severely restrict flowrate.

 

[lilliput1]

Why bother with flowmeter control? Why not just put in a combination flow measuring and balancing valve on the recirc line and set it for the minimum flow when the level control valve downstream of the pump is closed?

 

[danw2]

>To reduce inaccurate measurements that can cause the pump to still be repaired from operating below its minimum flow rating.

That says to me that accuracy in the low flow region is needed for what you need.

Vortex is "more accurate" than DP, but it has its own weakness - a low flow cutoff (flow meter's output drops to zero) inherent in the meter's output when the rate of vortice production drops below some manufacturer determined minimum at some minimum flow rate.

Every vortex flow meter vendor can size the meter for the anticipated flow rate and tell you what the low flow cutoff limit is, and you need to be sure that the low flow rate the pump needs is somewhat higher than the vortex meter's cutoff, otherwise you'll have no flow rate value when you need it.

Notice on the graph below that the flow "bottoms out" (that's 4.0mA from a 4-20mA output) at 15.2 gpm, not a zero gpm.

That same characteristic is evident in the flow chart:

Notice that Qmin values are not zero, but some value that is about 7 to 9 % of Qmax.

 

[georgeverghese]

@danw2, Thanks for this confirmation; A turndown to 7-9% of Qmax is about the same as the 10:1 ratio stated earlier for vortex meters.
Think the bigger concern here is really not with the selection between dp cells or vortex meters, but whether the FCV has been sized to account for internal flashing due to low backpressure, if the corresponding throat pressure at the CV is less than Psat for steam at the max operating temp of 307degF, which is approx 77psia - no response from the OP on this.

 

[vaavas]

First, thanks everyone for your continued input and time with this issue. please forgive me for my delayed responses, I've been having to work on other projects so haven't had the required time to really dig into these issues that are being brought up. I should free up within the next 2 weeks. You guys really help me so I don't want to burn any bridges or upset anyone on this site from my delayed responses.

Georgeverghese:
The max operating conditions of the new control valve:
Inlet (pump discharge piping): 400 psig, 307 deg F.
Outlet (low pressure feed water heater): 80 psig, 550 deg F. I don't think the area that this is discharged into at the shell is actually at this temperature.

There is an existing pipe already there (no control valve, 1" diameter) that is from the discharge pipe to the low pressure feed water heater shell drain. the feed water heater has a baffle to block the discharged water as it drains into the FWH shell. Im not sure if the water flashes when it enters the FWH and have not heard of any issues from our operations or field engineers on the FWH tubes being damaged from this.


Thanks again everyone and will be providing more feed back within the next 2 weeks.

 

[georgeverghese]

Assuming LP feedwater preheater shellside min normal operating pressure is 80psig = 95psia.

Overall dp across FCV = 415-95 = 325psi

So, dp at throat of FCV = 325/0.7 = 464psi, assuming recovery pressure characteristic is similar to that for a restriction orifice. This doesnt make sense, since this means internal pressure at the FCV = 415-464 = -50psia.

Still, this rough calc indicates a smoke trail for flashing inside the FCV - have you checked with the CV vendor what the throat pressure in the FCV is ? It shouldnt be less than 77psia as indicated earlier, else the valve should be sized for two phase flashing.