11 replies to this topic

[Devandra Godara]

Hello Everyone,

 

I am currently working on Relief load calculation for our refinery. We have 2 columns. ( Say Column 1 and COlumn 2). Column 2 is fed from Bottom product of Column 1. Column 1 feed pressure is controlled by PIC controller (PV Valve). The set point pressure at PIC is 4.5 Kg/cm2-g. Normal Column 1 bottom pressure is 9.5 & normal Column 2 feed pressure is 1.6 kg/cm2-g. I have also attached a hand drawing of the system.

 

Now Due to some power failure both column goes to relief. Column 1 bottom pressure increases to 15 kg/cm2-g & column 2 pressure increases to its relief which is 4.5 kg/cm2-g. Detailed hydraulics show that pressure at PIC is 9.8 kg/cm2-g.

 

As per the controlling action of the valve, the valve will open 100% to decreases it's upstream pressure. But as soon as I am opening the valve to 100 %, I am not getting enough pressure drop in the system to get downstream pressure of Column 2 (4.5 Kg/cm2-g). 

 

To match the hydraulics of the system, I have to decreases the valve opening from 100%. Both the situation are contradicting each other. What would happen in reality ?

 

I have attached a diagram for better understanding of the system. 

Attached Files

•  IMG20220908124927.jpg   70.49KB   0 downloads

Edited by Devandra Godara, 08 September 2022 - 05:08 AM.

[breizh]

Hi,

Did you perform any Hazop on this process? Power failure should have been taken account and position of control valve in case of power outage, same for utilities failure (compressed air, cooling water).

Edit

In other words, a backup power supply should be in place for strategic motors, let say a diesel generator.

 

Why is the pressure increasing at the bottom? 

 

Breizh

[latexman]

What is the MAWP of Column 2?

[Devandra Godara]

MAWP of Column 2 is 4.5 Kg/cm2-g (Relief pressure of Column 2) - Latexman. 

 

Column 1 condenser stops during the partial power failure resulting in increase of Column 1 pressure - Breizh

[latexman]

The allowed accumulation for Column 2 is 4.95 Kg/cm2-g (4.5x1.1).  Depends on Code.

 

Is the flow choked at the inlet nozzle expansion to Column 2, AND Column 2's relief maintains pressure <= 4.95 Kg/cm2-g?  Or, are you saying Column 2's relief does not have enough capacity for this scenario?

[Devandra Godara]

My Question is about the opening of Control valve. It is designed to maintain a pressure of 4.5 kg/cm2-g at Battery Limit conditions. Now Since the pressure at PIC has increased to 9.8 kg/cm2-g (As show in attachment), the PV will allow it self to open 100%.

 

But while doing the hydraulics I am getting higher pressure at the Column 2 inlet which is not possible hydraulically.

[Bobby Strain]

This is all confusing without a drawing.

 

Bobby

[latexman]

My Question is about the opening of Control valve. It is designed to maintain a pressure of 4.5 kg/cm2-g at Battery Limit conditions. Now Since the pressure at PIC has increased to 9.8 kg/cm2-g (As show in attachment), the PV will allow it self to open 100%.

 

But while doing the hydraulics I am getting higher pressure at the Column 2 inlet which is not possible hydraulically.

 

 

Bobby is right.  A P&ID would show us some details, including the failure position of PV.  What is the failure position of PV during power failure?  Fail open, fail closed, fail last, what is it?  This is critical to know.

 

In the OP it sounds like you want the PIC to close the PV, but power has failed!  The PV will go to it's failed postion, and stay there during the power failure.  In it's failed position during a power failure, the PV cannot control anything.

 

In reality, during a power failure, equipment goes to their failed state/position, and one has to estimate/calculate the resulting flows, then size PRDs to relieve those scenarios, if needed.  I think that is the reality.  Others may have input on this.

[Nikolay_]

Could you please explain with what you are trying to match the hydraulic calculation? If your control valve is placed between columns and has reached 100 % opening so feed pressure of Column 1 to be increased, didn't it?

Edited by Nikolai T, 08 September 2022 - 10:24 PM.

[Devandra Godara]

Hwy Nikolai,

I am trying to match downstream pressure of Column 2 during relief. As Column 2 maximum pressure is 4.3 Kg/cm2-g(Relieving pressure of PSV) with varying the opening of PV. 

 

Now 2 actions are possible. (i.e. either PV will open 100% or PV will open say X%)

 

Action 1:- As the Pressure at PV is 9.8 kg/cm2-g during relief (Set point = 4.5 Kg/cm2-g), as per controlling action the valve will open 100% to allow more flow (i.e. more pressure drop) to achieve its set point of 4.5 kg/cm2-g. but in this case, I am getting 5.4 kg/cm2-g pressure (Column 2 pressure at relief - 4.3 kg/cm2-g)

 

Action 2: Now to achieve column 2 pressure, PV has to close itself, which is not line with its controlling philosophy. 

 

which action will take place in reality ?

 

For others, the PV valve is pneumatically controlled.

[latexman]

Action 1:- As the Pressure at PV is 9.8 kg/cm2-g during relief (Set point = 4.5 Kg/cm2-g), as per controlling action the valve will open 100% to allow more flow (i.e. more pressure drop) to achieve its set point of 4.5 kg/cm2-g. but in this case, I am getting 5.4 kg/cm2-g pressure (Column 2 pressure at relief - 4.3 kg/cm2-g)

 

Action 2: Now to achieve column 2 pressure, PV has to close itself, which is not line with its controlling philosophy. 

 

which action will take place in reality ?

 

How many times are you going to ask this question and have it answered?  Column 1 is above set point and PIC will open PV to 100% eventually.  Plus, you said it's during a power failure, so PV will go to it's pre-determined failure mode, and stay there until power is restored and the PV is reset.  Again, action 1 is the reality.  The PV at 100% open during high column 1 pressure is simply a fixed restriction in the line, and the pressures and pipe and fittings determine the flow rate, until column 1 drops below set pressure.  The PV at 100% open during power failure is simply a fixed restriction in the line, and the pressures and pipe and fittings determine the flow rate.

[Nikolay_]

Ok.

 

PSV is not control valve and relief pressure of PSV shouldn't be equal set point of control valve. The calculation should account flow through PSV at relief pressure.

 

Hope this helps.