7 replies to this topic

[Piyush Patel]

Dear all,
What is the basis for control valve sizing for a hydrocarbon containg vessel with split range pressure control? To elaborate, the vessel is under nitrogen padding with split range pressure control system. If the vessel pressure is less than set pressure, vent control valve will close and nitrogen inlet valve will open to put more nitrogen inside vessel. If vessel pressure is higher than set pressure, nitrogen inlet valve will close and vent valve will open to vent the excess vapor/gases and vessel will get depressured to set value.

Now my questen is, how to size the inlet and vent cotrol valve? Any guidelines available?

For more clarity refer attached sketch.

Attached Files

•  split_range_pressure_control.xls   20KB   207 downloads

[Qalander (Chem)]

Dear all,
What is the basis for control valve sizing for a hydrocarbon containg vessel with split range pressure control? To elaborate, the vessel is under nitrogen padding with split range pressure control system. If the vessel pressure is less than set pressure, vent control valve will close and nitrogen inlet valve will open to put more nitrogen inside vessel. If vessel pressure is higher than set pressure, nitrogen inlet valve will close and vent valve will open to vent the excess vapor/gases and vessel will get depressured to set value.

Now my questen is, how to size the inlet and vent cotrol valve? Any guidelines available?

For more clarity refer attached sketch.

Dear

I assume all the inflow and outflow streams balance and

Min., Max.,Normal Flow rates in/out and

rate of level rise and drop(again normal, Min. Max.)

all will have to be considered for this purpose

a hazop sort of exercise needs to be undertaken.

Hope this helps

Best regards
qalander

[ankur2061]

Dear all,
What is the basis for control valve sizing for a hydrocarbon containg vessel with split range pressure control? To elaborate, the vessel is under nitrogen padding with split range pressure control system. If the vessel pressure is less than set pressure, vent control valve will close and nitrogen inlet valve will open to put more nitrogen inside vessel. If vessel pressure is higher than set pressure, nitrogen inlet valve will close and vent valve will open to vent the excess vapor/gases and vessel will get depressured to set value.

Now my questen is, how to size the inlet and vent cotrol valve? Any guidelines available?

For more clarity refer attached sketch.

Piyush,

Sizing from process view-point will involve the following data to be generated:

1. Flow Rate:

a. The inert gas feed valve flow rate will be based on the inbreathing (liquid outflow + thermal inbreathing ).

b. The vent valve feed rate will be based on the outbreathing (liquid inflow + thermal outbreathing).

Inbreathing & outbreathing calculations can be done according to venting calculations as given API 2000 or alternatively I have made a post for tank venting calculations in the Relief Device sub-forum which can be utilized for doing the same calculations:

http://www.cheresour...?showtopic=7327

2. Pressure Drop:

a. Minimum pressure drop: This will correspond to the pressure difference betwen the min N2 operating pressure at the valve inlet and the maximum operating pressure of your vessel.

b. Maximum Pressure drop: This will correspond to the maximum N2 operating pressure at the valve inlet and the minimum operating pressure of your vessel.

Other data provided by process for sizing the valve is the fluid, it's physical properties (specific gravity, viscosity) and its critical properties.

All the above mentioned data when provided to the control valve vendor/manufacturer will enable the vendor/manufacturer to size the valve.

Regards,
Ankur.

[Qalander (Chem)]

Thanks ankur for focused explanation of issue.
Best regards
Qalander

[Piyush Patel]

Ankur,

Thank you for your detailed explanation. I also referred the link u mentioned. In my case, I need to transfer the liquid from one tank to other tank by pressure difference, not by pumping. In this case, first I have to pressurize first tank and depressurize second tank (receiving tank) such that liquid will transfer by delta P bet two tanks.

What will be the limiting case? liquid outflow rate or tank pressurization by nitrogen (inert gas)? I think inert gas control valve should be designed on this case. If you have sample calculation for such case, I would appreciate if you put it here.

thank you.
regards// piyush

[fallah]

Dear

I assume all the inflow and outflow streams balance and

Min., Max.,Normal Flow rates in/out and

rate of level rise and drop(again normal, Min. Max.)

all will have to be considered for this purpose

a hazop sort of exercise needs to be undertaken.

Hope this helps

Best regards
qalander

You are notified sizing (here sizing of inlet and outlet of control valve) is not included in HAZOP study intentions.

[Arvind Iyer]

Dear Piyush,

I was thinking of the normal operating case.

Is this the operating philosophy of transfering the liquid by pressure difference :-

a) on a continuous basis? This would mean that the first vessel is always set at a higher pressure and the second always at a lower pressure.

Or,

b) only an intermediate operation? This would mean that you would rise the pressure in vessel 1 and depressurise the second as an intermediate operation whenever transfer is needed.

I think that in both the cases, the deciding factor is the liquid flow rate of transfer. You do not want the transfer to be too fast. The velocity in the line should not be exceeded as per your project guidelines to prevent any flashing in the second vessel.

Experts to comment on this. i have put in my view for your kind scrutiny.

Best Regards,
Arvind

[Piyush Patel]

Dear Arvind,

It is the intermediate transfer as and when required. So one has to pressurize and depressurize the vessels as per requirment and how fast transfer is required. Press. diff bet two vessels also depends on the elevation difference between two vessels. I agree with your point that liquid flow will be deciding factor. However, if we size control valve with very small liquid flow (which is process requirement), it would take much time to pressurize the vessel. So I think one has to optimize between these two cases. But still I am in doubt whether we can consider gas inlet flow same as liquid flow? If so, should we take gas flow as AM3/hr or NM3/hr?