16 replies to this topic

[robbiejustice]

Hi,

 

I have a PSV on the discharge of a pressure reducing valve (PRV) in case of PRV failure. When checking the inlet losses meet the 3% of set pressure rule I have found that it cannot be met by increasing the inlet line size because the determining factor is the pressure drop across the inlet reducer to the relief valve. This is my system:

 

Fluid: Saturated steam

Set pressure: 3 barg

PSV inlet size: 2"

PSV rated flow at set pressure: 3002 kg/h

 

The length of line between the main line and relief valve reducer is 190mm. The incremental pressure drop through the main line is negligible.

 

Has anyone had this problem before and can suggest a solution?

 

Any help/advise very much appreciated.

[fallah]

Hi,

 

Please submit the detail of your calculation to be able to evaluate...

[robbiejustice]

Hi Fallah,

 

I am using a company in-house standard spreadsheet so I can't post the calculation on here. I am pretty confident that the calculation is correct but in terms of inputs:

 

Fluid: Saturated steam

Set pressure: 3 barg

PSV inlet size: 2"

PSV rated flow at set pressure: 3002 kg/h

Density = 2.2 kg/m3

viscosity = 0.01 cP

Pipe is schd 40

 

Fittings:

Straight length = 190mm

1 off tee through branch

1 off reducer X x 50mm ( X being the line size)

[shan]

Use a pilot PSV.

[Bobby Strain]

If you give us complete information we might give you some useful feedback.

 

Bobby

[latexman]

A dimensioned and labeled hand sketch would be great.  What's the line size and schedule of the straight length and branch tee?  What's the overpressure?  I don't think the reducer by itself should be over 3%.  How is it characterized in the software?  For a commercial reducer I use K = 0.1 based on the velocity on the small side (2" Sch. 40).  That should be < 3% for a reasonable installation.

Edited by latexman, 09 January 2016 - 08:13 AM.

[CMA010]

3002 kg/hr saturated steam passing thru a PSV set at 3.0 barg with a 2" inlet? Are you sure?

[robbiejustice]

Hi thanks for the responses.

 

I've attached a sketch and the capacity chart for the PSV.

 

Away from work now so I'll respond again when I am back on Monday and can access the DP spreadsheet.

Attached Files

•  PSV.jpg   74.19KB   6 downloads
•  Screen Shot 2016-01-09 at 13.15.36.png   78.96KB   5 downloads

Edited by robbiejustice, 09 January 2016 - 12:11 PM.

[latexman]

If you are pretty confident the in-house standard spreadsheet is correct, what do you want us to do?  I recommend you consult with the person responsible for the in-house standard spreadsheet and become 100% confident it is correct or incorrect.  If it is correct, you have to make a change.  The 3% rule is . . . a rule, right?  Do not exceed 3% for a conventional PSV.  That's what it means.  Switch to a pilot operated, or switch to a PSV with a larger inlet and same orifice size.  That specific PSV has a very large orifice size for a 50 mm inlet.  It is not typical in my experience.  There are lots of 80 mm inlet PSVs on the market that will work.

Edited by latexman, 09 January 2016 - 11:34 AM.

[robbiejustice]

Ok, well thanks for having a look I just thought I might be missing something. I have had a word with the vendor as I can't have been the first to have this issue with this valve but I didn't get anywhere. I'll have a look into options for changing the PSV.

Edited by robbiejustice, 09 January 2016 - 12:20 PM.

[latexman]

I just ran this on my company's software.  What's the flow requirement for the worst case scenario?  Not the rated capacity of the PSV.

[robbiejustice]

2500 kg/h...

[latexman]

Our software says 1.1% drop for the reducer and 2.5% drop for the branch tee and straight pipe.  3.6% > 3%, so you are on the right track changing the PSV.

[robbiejustice]

Thanks Latexman.

[Bobby Strain]

Maybe you don't need to do anything other than set the blowdown on the valve. There is nothing mandatory about 3% inlet loss that I am aware of. That just covers most situations where the valve blowdown is set low. I presume your operating pressure is sufficiently low to allow you to set a greater blowdown. Check with the valve manufacturer.

 

Bobby

[latexman]

When I looked I didn't see a blowdown ring/adjustment on the Spirax Sarco SV60, but checking with the vendor may prove fruitful.

[robbiejustice]

I am trying to find this out from the vendor at the moment, the datasheet states 10%.