Built up back-pressure criteria for PSVs on API 650 tanks 1

[Sawsan311]

Dear All,

considering the requirement of API 2000 section 3.6.2 to have the PSVs for the API 650 tanks being set at a lower pressure than the tank design pressure to allow sufficient flow relieving capacity for these tanks as they have low safety design factors...

Considering overpressure is 0% and the set pressure is less than the tank's design pressure.. Do you all agree that the tail pipes of vent lines shall be checked such that the built up backpressure is always less than the PSV set point?

Appreciate your views..

Thanks

Regards,

 

[don1980]

No, that statement isn't sufficiently correct.

It's correct to say that API 650 tanks are not allowed any pressure accumulation (the tank's design pressure is the peak allowable pressure), but that isn't automatically accomplished by limiting the outlet pipe's friction loss to the relief device set pressure.

That's because there is no prescribed set pressure for these devices. i think you're probably assuming that the set pressure can't exceed 50% of the tank's design pressure, but that isn't true. The set pressure can be any value as long as it results in preventing the relief pressure from exceeding the tank design pressure. For example, say the relief device is set at 80% of the tank's design pressure, which is perfectly OK as long as the relief device reaches the necessary relief flow at 20% overpressure. In this example the relieving pressure is going to exceed the allowable limit (tank design P) if the friction loss in the outlet pipe is equal to the set pressure (80% of the tank's design P).

So, the correct statement for such applications (API 650 tank which has a tailpipe on the relief device) is to say that the tailpipe friction loss must be accounted for. That is, the set pressure must be sufficiently adjusted to ensure that the relieving pressure isn't greater than the tank's design presure.

 

[Sawsan311]

Thanks Don,

Actually I am in total agreement with your detailed statement, The only thing that I would like to highlight that API 2000 section 3.6.2 generally stated that the set pressure shall be set at a value that will not allow the tank's design pressure to be exceeded. I would not prefer the concept of overpressure to API 650 except for emergency venting devices for fire relief i.e.20% overpressure.

For the selection of the set pressure of the tank'sPSV, tank vendor shall take into account the tank's maximum liquid level.

I think in your below statement, you meant (80% of the the tank's design ) instead of 75% of tank's design P):
Quote:
'' In this example the relieving pressure is going to exceed the allowable limit (tank design P) if the friction loss in the outlet pipe is equal to the set pressure (75% of the tank's design P)''

We both agree that the estimation of the built up backpressure driving the sizing of the PSV tail pipe shall be taken into consideration while adjusting the set pressure of the PSV during relieving.

I have another question.. Consider a storage tank having a split range controller acting on an inlet PCV on the blanketing line (0-50%)and (50-100%) controller output acting on a PCV on tank's pressure control vent line. This loop serves in maintaining the tank's pressure and optimizing the consumption of blanketing gas.
Is it a general design practice to combine the outlet pressure control valve tail pipe with the tank's PSV outlet tail pipe.. or you agree that is recommended to keep each separately in order to avoid any effects on PSV relieving pressure during relieving since a failure of the pressure controller can cause both PCV and PSV to open..

thanks

regards,

 

[Latexman]

I would not prefer the concept of overpressure to API 650 except for emergency venting devices for fire relief.i.e.20% overpressure.

That is not correct. The fire exposure emergency vent relieving pressure cannot exceed the design rating for API-650.


Good Luck,
Latexman

 

[Sawsan311]

Thanks Latexman,
Actually I was also seeking for a clarification with regards to the setting of the API 650 tanks emergency fire vents i.e. blow out hatch since I have come across one of the design engineering guidelines stating that they can be set at 110% of the tank's design pressure with an overpressure up to 20% ..Do emergency vents also shall be set at or below tank's design pressure??

Regards,

 

[Latexman]

Since API-650 stipulates there is no accumulation above the design rating, that design engineering guideline is wrong. Normal vents and emergency vents must be set below the design pressure on API-650 tanks. It's Code.

A set of rough rules of thumb I use for API-650 is:
Emergency vent set pressure = 1/2 design pressure.
Normal vent set pressure = 1/2 emergency vent set pressure.
Pad gas set pressure = 1/2 normal vent set pressure.

Good Luck,
Latexman

 

[MJCronin]

This is a very important and informative discussion .....

But, IMHO, the next question that it prompts is:

For the API-650 tank specifier/purchaser, what is the most appropriate design pressure that should be specified???, .... given that there are minimum actuation pressures for relief devices.

My guess is that about 14 inches of water should be the minimum design pressure ...

What do you all think ???




MJCronin
Sr. Process Engineer

 

[Sawsan311]

Thanks Latexman for your inputs..just a small question, the Pad gas set pressure.. you mean the blanketing gas pressure control valve on the tank??

Mr.MJCronin,considering API is limited for tank's design pressure up to 0.172 barg,above this value and up to 1.03 barg,tank is classified by API 620. subject discussion applies mainly to API 650 usually fixed roof tanks since floating roof tanks are non-blanketed and have their vapor space controlled and suppressed.

The selection of the tank's design pressure is associated with the thickness of the tank's plates considering liquid full conditions. Therefore,only tank vendor can confirm the recommended settings of the PVRV and emergency vents.

Our role in piping design is to ensure that that PVRV flowing capacity is not jeopardized by the increase in backpressure above its setpoint during relieving. As per API 2000 7th edition section C2.2,due to the nature of the weight loaded PRVs,they require to be set at pressure less than the tank's design pressure to provide the sufficient relieving capacity. Simultaneously,their setting of the normal PSV set pressure shall be compared to the setting of the emergency vent valve (75% to 90%) of EV set point (which is defined in API as the leakage pressure).

Appreciate your views..

 

[don1980]

This is a good topic to discuss because it's one that engineers generally don't sufficiently understand - it's like an engineering blind-spot. Engineers tend to decide these details arbitrarily, or based on some rule-of-thumb, and they overlook the the obvious need to optimize each case based on that tank's particular needs. And this oversight results in excessive costs (initial and long-term) and unnecessarily high environmental consequences.

So, what should we be doing? First, don't apply a one-size-fits-all rule. Assess the volatility of the liquid. Specifically, determine the vapor pressure at the tank's operating temperature, and remember that the vapor composition is proportional to the vapor pressure. Unless the liquid is something like mineral oil (essentially non-volatile) then minimize the vapor losses by setting the relief devices as high as possible. That is, maximize the operating pressure range - the range over which the tank vapor can expanded and compress without any external venting. Maximize the set pressure of the relief device(s) because this maximizes the operating pressure range of the tank. This may require a larger body size for the relief device (higher device cost), but that's generally an acceptable trade-off because these devices are relatively inexpensive. Over the life of the tank, the lost product vapor, lost pad gas, and reduced environmental offense will easily offset the higher cost for the relief device.

If the tank is padded, then minimize the set pressure as low as possible. That minimized the frequency of padding and the amount of pad gas used. The higher the pad gas setting, the more external breathing.

 

[MJCronin]

Good general discussion here :

https://www.petrolplaza.com/knowledge/1992

This has been discussed before on these fora...

https://www.eng-tips.com/viewthread.cfm?qid=225293

In my opinion, for the large API-650 tank purchaser in the budgetary phase of the project a reasonable tank design pressure would be 8 to 14 IWC (inches of water column).

I believe that this number range would not put excessive demands on the tank fabricator (and thereby get a budgetary tank price quote that is too expensive and out of the norm) while, at the same time be high enough to allow for back pressure, accumulation and tail pipe friction.

Maximum operating pressure would be 7 to 10 IWC, in accordance with the suggestions of Latexman, above.

With this range, a reasonable set of pressure relief devices could be purchased ..

Now what do you all think a reasonable design vacuum pressure for an API-650 style would be ?

Vacuum design pressure that I have used in the past is -2" IWC

MJCronin
Sr. Process Engineer