21 replies to this topic

[ulka_19]

Hello Everyone,

 

I am involved in a project of tank farm. Tanks in different enclosures are having different class products stored in it. For class A products we have provided PVRV and for other classes Goose neck Vent. As per API 2000 i have calculated venting requirement and thus specified the size for vents. My question is whether I should follow the same procudure for PVRV sizing as well. Like tentatively we have specified two nos. of PVRV of 10". However when I calculate venting requirement the area required comes out to be 225 cm2 and this one single 10" PVRV gives an area of 508 cm2. So I would like to know if I have done the right calculation. And the difference in Vent sizing calculation and PVRV calculation.

 

 

Thanks in advance.

 

I am attaching my calculation.

Attached Files

•  PVRV Sizing.pdf   67.13KB   1019 downloads

Edited by ulka_19, 17 March 2014 - 10:49 PM.

[ankur2061]

ulka_19,

 

PVRV is sized based on normal venting requirements (inbreathing / outbreathing). The size depends on the higher of the flow rates (Nm3/h) calculated for inbreathing and outbreathing. API 2000 6th edition introduced a new method for normal venting calculations. Refer to the link below on one of the most popular old threads on "Cheresources" related to tank venting based on API STD 2000. A spreadsheet link for doing normal venting calculations as per API STD 2000 6th Edition is also provided in the same thread.

 

http://www.cheresour...g-requirements/

 

Based on the calculated normal venting rate, vendors such as "Groth" and "Protectoseal" will provide you the size for the PVRV. Refer the link below for the size of PVRVs based on the Series 1200 PVRVs manufactured by Groth.

 

http://www.grothcorp...rialCatalog.pdf

 

Pages 15-18 provide tables for various PVRV sizes depending on vent flow rate and pressure / vacuum settings.

 

Similarly "Protectoseal" provides data for their valves at the following link:

 

http://www.protectos...series8540H.cfm

 

Regards,

Ankur

[ulka_19]

Thanks Sir.

 

As per the Groth Catalogue and my venting requirement (Based on API 2000, 6th Edition only) 6" PVRV is sufficient. But we have already specified two nos. of 10" PVRV and our main concern was to confirm whether it is sufficient and not underdesigned. So my calculations say that it is not underdesigned but highly overdesigned.

 

I was wondering if this much overdesign is permissible or should we go for a size change?

[ankur2061]

ulka_19,

 

The obvious disadvantage of having an oversized equipment is higher first cost. Other than that, handling, installation and maintenance becomes more difficult with bigger sized equipment.

 

For oversized relief devices there is one typical phenomena called "valve chatter". At best it increases wear and tear of the relief device leading to frequent maintenance and at worst it may not do it's job of overpressure or vacuum protection leading to catastrophic equipment failure.

 

Refer the link below which has an attachment as powerpoint presentation for relief devices and has a brief section that discusses relief device "chatter". It is an excellent read and highly recommended.

 

http://www.cheresour...-relief-valves/

 

Hope this helps.

 

Regards,

Ankur

[ulka_19]

Relief Devices PPT is very informative.

 

One more thing I wanted to confirm is that Venting Capacity is calculated based on inbreathing & outbreathing capacity inclusive of thermal & product filling or withdrawal inbreathing & outbreathing. Do we also consider the emergency venting capacity in case of fire in this calculation? 

 

Because if that is the case then my wetted area is greater than 260 m2 and that will give me a venting capacity required of 19910 Nm3/hr and for this we will require a bigger size of PVRV.

 

Thanks.

[ankur2061]

ulka_19,

 

The size of the venting device is based on the "higher" of the Inbreathing (Pump-out + thermal) or Outbreathing (Pump-in + thermal). The venting flow rate thus determined is the normal venting rate.

 

Emergency venting is due to external fire and if such a case is found to be credible for a storage tank, a separate emergency venting device is provided on the tank, for which a separate nozzle or opening is provided on the storage tank. The first step in the requirement of an emergency vent is to establish whether the tank can face an external fire.

 

Regards,

Ankur.

[ulka_19]

Thank You Sir.

[easrikrishna]

hi, this is krishna,

 

i am planning to install PVRV  for 6000kl capacity of storage tanks where nitrogen blanketing is to be provided. and when i compared with ur file, i was unable to find the std values from API 2000 5th edition. may i know how u calculated venting capacity for outbreathing and inbreathing..?

[ulka_19]

Hello Krishna,

 

Well I have done my calculations as per API 2000 6th Edition and I found it self explanatory so please let me know the specific portion which you are not able to understand then it will become easy for me to explain.

 

Thanks.

[ulka_19]

Hello,

 

I am again stuck. I am referring API 2000 6th Edition. I had calculated Themal Inbreathing and Outbreathing from Table A.3, which says "Normal Venting Requirements for Thermal Effects". And from this table I found thermal outbreathing 726.8 m3/hr and thermal inbreathing 726.8 m3/ hr, as flash point is less than 37.8^oC. But now as vendor has given his calculation, values are totally different. He has calculated Thermal Outbreathing with formula V_OT=Y.V_TK^0.9.R_i and Thermal Inbreathing with V_IT=C.V_TK^0.7.R_i

 

However Outbreathing is same as 726.8m3/hr but Thermal Inbreathing mentioned by vendor is 2656 m3/hr, he has considered C=6.5 as given in API 2000 Table-2. Because of this additional venting requirement, venting area required is more and thus the size.

 

So I wanted to know if the inbreathing and outbreathing capacity given in Table A-3 "Normal Venting Requirements for Thermal Effects" is not same as Thermal Inbreathing and Thermal Outbreathing.

[ankur2061]

Akanksha,

 

I suggest you read this old but very popular thread on "Tank Venting Requirements" very carefully to understand the difference between Table A-3 and the calculation equations in the main body of API 2000. It will be a difficult choice, but on a personal note I believe the Table A-3 is good enough. Here is the link:

 

http://www.cheresour...g-requirements/

 

Regards,

Ankur.

[ulka_19]

Thanks for your prompt reply.

 

I went through the full thread and found it really interesting. So as it is an open ended choice between the formula and the table I will go by the table.

 

But one thing I was wondering is that You have earlier mentioned that there could be chattering issue in oversize PVRV so as we see the difference in formula and table is so large that if one goes for the formula calculated inbreathing, would have to go for the bigger size and in practice we see that Table A-3 gives suficient capacity so will it be a good design if we follow the emperical formula?

 

And since that thread is posted in 2010 and now its 2014 so I want to know what is the trend in Process Industries after that. Do people go for the overarted thermal Inreathing ?

Edited by ulka_19, 26 March 2014 - 12:32 AM.

[ankur2061]

Akanksha,

 

As an engineering consultant the choice is entirely yours from a good engineering practice viewpoint and as per what the standard says. I do not know where the tank is located in your case and what it contains.

 

The thermal inbreathing calculation equations in API 2000 are based on European standard EN 14015:2005. So if you are dealing with a European client it might make more sense to use the equations. Using the equations might also make more sense if ambient conditions dictate very low temperatures specifically after a sudden rain shower or snow storm. 

 

If the above is not true then you can probably safely use the thermal inbreathing table given in API 2000.

 

Hope this helps.

 

Regards,

Ankur.

[easrikrishna]

Inbreathing:
If tank capacity <= 3500 m³
Total Inbreathing = Pumpout + 0.178*tank capacity (m³)
If tank capacity >3500 m³
Total Inbreathing = Pumpout + 3.2*(tank capacity (m³))^0.651
Outbreathing:
Liquids with flashpoint >37.8 deg C or NBP above 149 deg C
If tank capacity <= 3500 m³
Total Outbreathing = 1.069*Pump-in + 0.107*Tank Capacity(m³)
If tank capacity > 3500 m³
Total Outbreathing = 1.069*Pump-in + 1.92*(Tank Capacity(m³))^0.651
Liquids with flashpoint <37.8 deg C or NBP below 149 deg C
If tank capacity <= 3500 m³
Total Outbreathing = 2.138*Pump-in + 0.178*Tank Capacity(m³)
If tank capacity > 3500 m³
Total Outbreathing = 2.138*Pump-in + 3.2*(Tank Capacity(m³))^0.651
Note: The Pumpout and Pump-in are given in m³/h.

 

 

I have used the above formula for Inbreathing and outbreathing flow rates, i got 1222m3/hr for Inbreathing and 575m3/hr for outbreathing. but the vendor has given me that inbreathing is 3261 and out is 858. and according to him, He has calculated Thermal Outbreathing with formula V_OT=Y.V_TK^0.9.R_i and Thermal Inbreathing with V_IT=C.V_TK^0.7.R_{i. and C is 6.5}

 

As per my calc i get 80NB size of valve, but he got 250NB of size. With these huge variations in size, i strucked here to choose better option.

 

Location of tank, and service of the tank, and all other requirements provided to vendor, but he came with above values.

 

please help me in this regard.

 

Thanking you

 

Krishna

[ankur2061]

Krishna,

 

What help do you want? You have the option of utilizing Table A.3 in Annex. A for thermal inbreathing / outbreathing of API 2000 which give lower rates. 

 

Previous discussions on this contradiction in thermal inbreathing / outbreathing rates have discussed that the tank vents which were designed earlier based on Table A.3 rates are working fine and there has been no finding to suggest that the tanks have failed (buckled or ruptured) due to the calculations and Table provided in Annex A. Remember that, what is now given as Annex A in API 2000, 6th edition use to be there in the main body in the earlier revision, i.e. 5th edition.

 

Having said that, the client or owner's representative engineer has to be convinced with your approach in sizing the PVRV.

 

Regards,

Ankur

[ulka_19]

Krishna,

 

I think you have not read the whole thread, same issue has been discussed above and resolved as well.

 

Please follow the thread carefully and one more thing if you are calculating venting requirements using equation then for both Pumping and thermal Inbreathing / outbreathing use equations given in 4.3.2.2 & 4.3.2.3 (API 2000 6th Edition) and if you are calculating from Annex A then for both venting Requirements - Pumping & thermal use table A.2 & A.3 respectively. This should not be done that for pumping you use the equation and thermal venting you calculate from table or vice versa.

 

Hope this helps.

[ulka_19]

Eh. There I am again with my doubt

 

For Pressure and vacuum valves calculation Eqn No. 21 is given in API 2000 as,

 

q_th= 125.15*p_i*A_min sqrt[ (1/M.Z.Ti)(k/k-1) {(p_o/p_i)^(2/k) - (p_o/p_i)^ (k+1/k)}]

 

Where p_o= Abs. Pressure at device outlet

            p_i= Abs. Pressure at device inlet

 

However for overpressure case this makes sense but for vacuum case when the pressure at inlet is less than the pressure at outlet, the ratio p_o/p_i becomes more than 1 and as value of (k+1/k) is greater than 2/k so the term inside the square root becomes negative.

 

Have I done anything wrong or assumed it the wrong way?

 

I checked by reversing the value as well like instead of p_o/p_i, in vacuum case I took p_i/p_o but the calculated venting capacity by reversing is quite different from the value provided by the vendor so please guide me how to calculate size in vacuum case.

Edited by ulka_19, 27 March 2014 - 03:55 AM.

[ulka_19]

Please guide me how to size the vent valve in case of vacuum.

[Hhussain]

Hi Ulka,

 

what is Amin in your calculation

 

Huss

[ulka_19]

A_min is the minimum flow area of the device, expressed in square centimeters.

[chemsac2]

ulka,

 

For vacuum calculation, Pi is atmospheric pressure and Po is vacuum set point of PVRV i.e. pressure inside the tank which has reached making vacuum port open. In such a case, Po/Pi is always going to be less than 1. This would make value of square root always +ve.

 

Amin in equation is vacuum port area and that equation is just orifice equation with Pi as inlet pressure to orifice (atmospheric for vacuum port) and Po is pressure at outlet of orifice (tank vacuum setting for vacuum port). 

 

As for calculated value of qth from equation 21 being different from vendor offer, are you taking into account of discharge coefficient of PVRV to calculate actual flowrate from valve?

 

You can refer to page 28 from attached catalogue as to how PVRV model and size is decided by vendors. 

 

Regards,

 

Sachin

 

 

Attached Files

•  Marvac_Catalogue.pdf   873.64KB   238 downloads

[ulka_19]

Thanks for your response chemsac2.

 

Yes I am considering the coefficient of discharge for getting the actual flowrate.