15 replies to this topic

[pasquale]

I HAVE A SURGE DRUM FOR HVGO BLANKETING WITH LP NATURAL GAS.

THE TANK HAS ALSO A RELIEF VALVE.

I DON'T UNDERSTAND HOW THE VAPOR FLOW OF LP NATURAL GAS MUST BE CALCULATED.

CAN ANYONE HELP ME?

I HAVE ATTACHED A SIMPLE TANK SCHEME.

Attached Files

•  TANK.xls   16.5KB   246 downloads

[proinwv]

Pasquale

We need more information on this process.

What are you trying to accomplish? Information on the process, the sizes, flowrates etc would be helpful.

There is information on blanketing on my site, but I don't know if this would be helpful at this stage. Maybe it would help you ask a more detailed question.

PAUL

[Art Montemayor]

Pasquale:

Maybe the attached version of your Workbook can be of help to you.

Attached Files

•  Pasquale__s_Surge_Drum.xls   52KB   305 downloads

[proinwv]

Well with Art's drawing I see two things to comment on.

1-The PRV vents to atmosphere and this seems not advisable with the gas involved.

2- Will the gas blanket the ullage space in that will it mix? Is not some purge necessary? What about a gas inlet and outlet to flow thru and improve mixing?

What do you think?

PAUL

[npp]

The liquid level in the surge drum is varying dependent on the rate of supplying and withdrawing. The blanketing gas volume must be changed accordingly to keep the pressure constant. This is done by a system with a fuel gas supply line and a purge line. A pressure controller with split range control is used. If the pressure is lower than setpoint fuel gas valve is opened. If the pressure is high, the purge valve is opened to release gas to flare.
The gas flow is thus dependent on the change of the level. The maximal flow rate can be estimated as a case with no supplying and maximal withdrawing.

[proinwv]

You stated "The gas flow is thus dependent on the change of the level. The maximal flow rate can be estimated as a case with no supplying and maximal withdrawing."

Is not this the answer to your question? Simply the gas flow will equal the withdrawl rate from the vessel. One cu.ft/min out will be replaced by the same in gas flow.

The system would be more responsive and accurate in pressure control if the blanket gas was controlled by a PRV between the gas supply and the tank. PRV's are very fast in their response time.

The addition of a vacuum vent would offer backup protection to the tank in case of blanket gas failure.

I hope that this is of use to you. If not, please respond.

Best regards.

[pasquale]

You stated "The gas flow is thus dependent on the change of the level. The maximal flow rate can be estimated as a case with no supplying and maximal withdrawing."

Is not this the answer to your question? Simply the gas flow will equal the withdrawl rate from the vessel. One cu.ft/min out will be replaced by the same in gas flow.

The system would be more responsive and accurate in pressure control if the blanket gas was controlled by a PRV between the gas supply and the tank. PRV's are very fast in their response time.

The addition of a vacuum vent would offer backup protection to the tank in case of blanket gas failure.

I hope that this is of use to you. If not, please respond.

Best regards.

Hy guys!

I have the P&ID of this surge drum and obviously the PRV is connected to the flare header !

The answer of proinwv to my question is what i need, in fact for the sizing of the LP natural gas inlet valve

i need to known the maximum flow rate of gas.

I am fully agree with you!

Best regards

PS sorry for my english.... I'm italian

[proinwv]

No apology is necessary for your English. It is far better than my Italian. so I defer to you as the language expert.

[pasquale]

Hi proinwv !

What do you means with the statement " The addition of a vacuum vent would offer backup protection to the tank in case of gas failure" .

What is a vacuum vent ?

Moreover the pressure control of this drum is a classical split range; in fact for low pressure signal the control system opens the inlet gas valve and for high pressure signal opens the other valve for the exit of the sour gas.

[proinwv]

Hi Pasquale,

A vacuum vent is to provide protection in case of the tank pressure going below atmospheric. This can happen if the withdrawl volumetric rate is less than the incoming. Also the pressure can drop due to a change in temperature .

I am not sure of the factors that can effect your particular operation so I mention this for your information.

You might find some of the articles on my website helpful on this subject.

PAUL

[pasquale]

Hi Paul

In my tank i have a liquid stream of gas oil that enters and the same stream is sent with a pump (under the vessel) at the reaction section of my plant.

The gas in the tank is used for mantaing the suction pressure to the set value 345 kPa (g).
so in normal operating condition the same amount of liquid that enters in the vessel exits .

In this condition the liquid level is costant and the pressure also, so the two pressure contro valve (gas lines)
are, for me, closed.

a change in liquid level can cause pressure variations and then the opening of the pressure control valve.

What do you think ?
In nornal operating conditions the two gas valves are closed?

Moreover I don't think that this drum can go under vacuum

[proinwv]

Pasquale,

You know and understand your system better than I but as you describe it, NORMALLY there will not be a vacuum.

Can a vacuum occur in an abnormal situation?...an equipment failure, or operator error for example. And if this is possible, then what would result from it? Damage to the system, injury of people, damage to the environment? Should this be possible then use a vacuum valve to prevent the damage. Vacuum valves, are also known as vacuum vent valves and vacuum breakers.

PAUL

[djack77494]

Pasquale,
A scenario that could result in vacuum conditions developing would go something like this.
1) The vessel nearly full and at its normal operating pressure.
2) Undetected by plant personnel, the blanket gas (inlet) valve fails closed.
[This is not detected because the valve is normally closed.]
3) A loss of feed to the tank occurs, but withdrawal continues.
4) The result is a drop in the liquid level, with a corresponding expansion of the trapped gas above the liquid.
The expansion causes the gas pressure to drop proportional to the "old" gas volume/"new" gas volume.
5) If the expansion has been sufficient, vacuum conditions will develop.
Hence the need to consider a vacuum relief valve, essentially a PSV operating in reverse.

However, if your blanket gas is not compatable with air, you have an additional complication and cannot just allow for air entry into the vessel.

HTH,
Doug

[pasquale]

Hy guys

The scenario described by djack77494 is very interesting but i have a level control connected to a safety
integrity system that can block with a valve the liquid withdrawal from the tank.

Best regards

[Leticia]

Hi,

I have a 3000 cubic meters tank in which we storage a product that needs blanketing with nitrogen. We have design the nitrogen supply system following API2000. The tank is keep at 10 mbar of pressure and there is a blanketing valve, the problem is that I don´t understand the nitrogen flow, when we pump the product at 45m3/h the flow of nitrogen is more or less 70m3/h, but when we pump at 90m3/h the flow of nitrogen rises until 250m3/h, what I mean is that It follows and exponential curve. Is it normal? Is my valve working well? Where can I look for information about it? Thanks!

Best regards,

Leticia

Attached Files

•  TANK_CONDITIONs.xls   76KB   158 downloads
•  TANK_CONDITIONs.xls   76KB   108 downloads

[proinwv]

There are a number of explanations possible. I am not suggesting that these are the answers but let me list what comes to mind.

Metering accuracy of the flow rates, both liquid and gas. You need to be reading scfh for the gas or if acfh, then measured at tank vapor space pressure, not gas supply pressures.

Venting of gas during pump off due to a combination of blanketing gas pressure control problems and pressure vent operation. (Some low pressure regulators have a pressure rise characteristic during flowing conditions to counteract droop.)

By the way Leticia, you should have started a new post rather than tagging onto this existing one.