5 replies to this topic

[Mystified]

Hi,

 

I am trying to calculate vent losses from the liquid oxygen tank. Tank has an equal percentage globe valve in the vent line. I am thinking of creatng a virtual flow meter wherein instantaneous flow through valve will be deduced from the opening. Calculated flow will be totalized for say 24hr period.

 

I have valve data sheet with three operating conditions and calculated CV/opening. rated Cv is also available.

 

What i don't have is valve range-ability. You may ask why shouldn't i ask valve supplier. This is old set up and we don't have details of purchase from which vendor can take out information.

 

I am using below formula to calculate flow from opening.

 

V = (e^x/R)*Vmax

 

x = LnR*H

 

Where H is % opening. (H = 0 ~ 1)

R is range-ability

V is flow rate at any opening

Vmax is maximum flow rate through the valve.

 

Is there any other method to find out range-ability from valve data sheet values.

 

I tried to approximate flow otherway.

 

As it is a equal % valve, equal increment of valve opening should result in equal % increment in flow.

 

I calculated % increment in flow  from data sheet, and used same % to calcualte flow for other openings.

 

Rated CV comes at ~85% opening, instead at100%. See the attached excel sheet.

 

Please tell me where i am going wrong.

Attached Files

•  Tanks vent valves.xlsx   13.82KB   93 downloads

[proinwv]

When the manufacturer designs an equal percentage trim, they must select a rangeability in determining the contour of the plug. In reality, testing often shows that the dimensions, especially at the low flow end must be extremely closely held in order to produce a true =% trim.

 

So, most trims are in fact, close to but not true =%. Depending upon the manufacturer, a particular trim may be "tweaked" to match the theoretical characteristic. The reality is that for the overwhelming number of applications, that accuracy is not needed because the controller will adjust the valve position to match the process.

 

If the valve were to meet the requirements of Cv rating by ISA standards, you should see a Cv that is about +/- 5% of catalog.  Your 85% is a calculated value and may well not resemble the valve capacity.

 

If you have the ability to test the valve and establish a flow curve for it, then you would see what you have. I have seen =% trim that is actually a series of of straight lines, approximating a smooth curve. This is due to machining constraints, and defining the machined shape by a series of straight lines. The more segments, the smoother the curve. If you were to take the plug out and examined it closely at a magnification you would likely see this. This is the reality.

[Mystified]

Thanks for the insight and info in detail.

[BOUKHLAZ]

Dear All,

 

I try a weeks ago to design an open gooseneck vent (diameter) of water storage tanks, i didn't find answer for my question even si I read a lot of topic here in this forum and other forum, I would like to ask you some questions if possible:

 

• What are the criteria in term of velocity and pressure drop we have to follow when doing the line sizing of the vent?
• What will be the pressure drop ? do we consider only 2 elbows, pipe entrance and exit ? is there any assumption for the straight pipe length and for the insect/dust screen pressure drop?
• The total pressure drop is the design pressure of the tank (overpressure) is it correct ? how do we calculate the negative design pressure (vacuum)?
• In case of outbreathing of water storage tank, do we use the air properties to do the calculation? (there are no properties of water vapor at standard conditions)

 

Your help would be greatly appreciated.

 

Thanks in advance.

 

Best regards.

 

Zakaria.

[fallah]

Zakaria,

 

Try using what included in following link and come back for asking any required clarification:

 

http://www.cheresour...ank-vent-sizing

[femolyn1]

Hi, I am trying to estimate the total vented hydrocarbon from tanks ( Wash tank, settling tank, water tank, slop tank, pop tank, desand tank, sales tank) in an oil battery. I would like to use the empirical correlations but confused as to how to do the calculation for all the tanks. The sample calculation I saw for the EUB Rule of thumb , standing correlation and Vasquez&beggs only considered an emulsion  treater and a separator and stressed the need to consider the condition upstream