13 replies to this topic

[DanV]

Hi.
I have 10'' EIV ball valve and 6" FCV in serial in a 10" line upstream from light crude oil Desalter vessel protected by PSV.
I have FCV (is downstream from EIV) max flow rate but don't have EIV's.
My doubt: is FCV's max flow rate sufficient for PSV sizing?
If know than how can I get EIV's max flow rate? It is not in the data sheet.
Appreciate very much anybody's comments.

[fallah]

DanV,

 

FCV size is lower than ball valve size, then FCV's maximum flowrate is sufficient for downstream PSV sizing...

 

But be careful, the passing flowrate through FCV for PSV sizing should be calculated at relieving conditions as follows:

 

Q_relief=Q_max (deltaP_relief/deltaP_max)^1/2

[DanV]

Thanks Naser, very much appreciated.

 

Just to verify my understanding of delta P-s in formula and if you could point out the reference on how it was derived:

 

deltaPrelief = PFCVinlet - PPSVoutlet/relief

 

deltaPmax = PFCVinlet - PFCVoutlet

 

 

Best Regards,

 

Dan

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[fallah]

DanV,

 

 

Both deltaP_relief and deltaP_max are deltaP across the control valve but the former is in max flow conditions and the latter in PSV relieving conditions...

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[DanV]

Naser,

 

These are my numbers as in CV and PSV datasheets:

 

CV:

 

Flow rate at max flow: 475 Am³/h (?! why Am³/hr I am not sure, it is liquid, blended crude not gas)

 

Inlet pressure:              2280 kPa-a (?! why kPa-a in datasheet not sure; Gage P would be 2280 - 93 = 2187 kPa-g)

Pressure Drop:            472 kPa

 

PSV:

 

Required Capacity:     560 m³/hr

 

Set Pressure:              1100 kPa-g

 

Barometric pressure:   93 kPa-a  

 

% Allowable Overpressure: 10% means P_relief = 1210 kPa-g

 

When I calculate Q_{relief as per your formula and my understanding of it I get 683 m3/hr. If that is correct than PSV is undersized. Please if you would be willing to comment would help a lot.}

 

_{Thanks for your time and effort.}

 

_Dan

[fallah]

DanV,

 

Please consider these points and do a recalculation:

 

1) Specify exact relief scenario, is it control valve failure to be wide open?

 

2) Fluid sate? Liquid or gas?

 

3) The control valve outlet pressure in relieving conditions should be evaluated just at the control valve outlet that would be higher than relieving pressure.

 

4) You are calculating required relief load but you mentioned it is 560 m3/hr before relief load calculation. It might this value being PSV rated capacity, please recheck.

 

5) You should calculate the relief load in mass flowrate.

Edited by fallah, 12 September 2013 - 11:16 AM.

[DanV]

Thanks Naser for all comments above.

 

1) Exact relief scenario is control valve failure to be wide open.

 

2) It's Liquid crude oil

 

3) 'The control valve outlet pressure in relieving conditions should be evaluated just at the control valve outlet that would be higher than relieving pressure.' - How do I evaluate/determine this outlet pressure? I have numerous heat exchangers in between desalter and control valve. 

 

4) 560 m3/hr is required relief load in PSV datasheet based on original blocked flow overpressure scenario. You are right, PSV rated  capacity is higher and I can find out that. Means my statement about PSV might be undersized is premature. 

 

5) Ok. Thanks for correction.

 

Appreciate very much your time. Am very grateful.

[fallah]

 

 

3) 'The control valve outlet pressure in relieving conditions should be evaluated just at the control valve outlet that would be higher than relieving pressure.' - How do I evaluate/determine this outlet pressure? I have numerous heat exchangers in between desalter and control valve. 

 

 

 

DanV,

 

You should estimate the pressure drop between control valve outlet and PSV inlet at relieving condition to obtain the control valve outlet pressure...

[DanV]

Thanks Naser for your comment. Much appreciated.

[DanV]

Hi Naser.

 

The relation you gave me earlier Q_relief=Q_max (deltaP_relief/deltaP_max)^1/2 is derived with an assumption that Cv_max = Cv_relief.  Could you please explain why we can do that? Cv depends on deltaP as far as I know.

 

Thanks for your time.

 

DanV

[fallah]

DanV,

 

From where you realized that there is such assumption?

Edited by fallah, 07 October 2013 - 04:39 AM.

[TechExpert]

DanV 

 

Are you calculating like this:

 

Qrelief=Cvrelief X f(x) X (deltaP_relief / sp. grav)^1/2

 

Qmax=Cvmax X f(x) X (deltaPmax / sp. grav)^1/2v

 

^{Taking ratio:}

Q_relief=Q_max (deltaP_relief/deltaP_max)^1/2 

 

Assuming Cvmax=Cvrelief 

 

where f(x) is control valve opening (assumed 1 means CV 100% open), sp. grav=specific gravity (assumed same for both cases)

 

Falah! Please put some light over this.

 

Regadrs

PKS

[fallah]

PKS,

 

The assumption: Cv_max = Cv_relief in the case of PSV relief due to control valve's full open failure could be correct...

[DanV]

Hi PKS,

 

Sorry for late reply.

 

Correct, that is the way how I came up to Fallah's formula provided earlier.

 

DanV