13 replies to this topic

Hello Dear All

This is my first post in Cheresourcess, I am a process engineer working in a refinery technical servicess dept. I have reviewed a P&IDs of a gas processing plant. I saw a very strange depressurezing logic in the process scheme of that gas plant. As per the depressurizing scheme of the gas plant, a centrifugal comrpessor has a isolation valve upstream of the inlet suction scrubber and down stream of the contrifugal compressor air cooled heat exchanger. The blowdonw valve is provided downstream of the air cooeld heat exhanger. There is no surge or recycle valve present in the system. I want to clarify, will this scheme works or do centrifugal give free farword flow, during static condtion.

Thanks in advance for your time.

Regards

[logic]

Hi

I have a similar situation, however the compressor type is a centrifugal compressor and in my case an anti surge valve ha been provided

I need to design the blow down valve downstream of the air cooler which will depressurise the entire compressor loop which includes suction scrubber, suction piping, compressor, discharge piping, air cooler and finally the discharge line of the air cooler upto the isolation valve

This i have to do using Hysys

Can someone please tell me the procedure of performing this calculation

your help will be much appreciated

Thanks all

[Erwin APRIANDI]

Hi Process Optimizer and Logic,

As I know there are two scenarios of depressuring

1. In case of fire; where the consider initial pressure is at normal operating condition or maximum operating condition to depress it to achieve 100 psi or 50 percent of the system design pressure, whichever is lower, in 15 minutes. As Per API STD 521

2. Adiabatic depressuring, or consideration of maintenance depressuring where the system pressure and temperature have settled to its stable condition and the pressure is depressurize atmospheric pressure

Most engineering company have it's own way on doing this calculation, however in most of the company process simulation depressuring dynamics such as what HYSYS have usually are used

The general steps are:
a. First is to determine the volume of the system for input to HYSYS depressuring dynamics
b. Input all data, and size the Cv of blowdown valve or Restrictive Orifice Size by using HYSYS depressuring dynamics (the sizing of the blowdown valve or RO are based on fire case)
c. the adiabatic depressuring is done for the prediction of MDMT (Minimum Design Metal Temperature) for this calculation the size of the blowdown valve or RO size is inputed based on the sizing using fire case scenario (but, to be honest MDMT calculation by HYSYS depressuring will lead to very low temperature)

For the compressor depessuring configuration, I can say yes mostly we will put the blowdown valve at the end of the system (downstream of after cooler, before the isolation valve). This is because mostly we will see a check valve installed downstream of centrifugal compressor therefore the only possible place to locate the blowdown valve is at the discharge end. No matter surge line is installed or not and we will also put the isolation valve installed upstream of the inlet suction scrubber and downstream of the centrifugal compressor after cooler, this is to enlarge the system volume so you will get less settle out pressure, where it can save you from requirement of special material due to low MDMT. However for the location of isolation valve further consideration such as plat layout and other aspect has also to be considered, so it is not a must that the configuration will be as what has previously mentioned

Edited by erwin.apriandi, 13 January 2011 - 08:13 PM.

[fallah]

nileshkmakwana,

It could be assumed that the PSV will lift before antisurge valves, and in this situation,the flare system will receive full discharge flow.Hence, the credit of quick opening of antisurge valve can not be used to reduce the safety relief load. The volumetric flowrate can be calculated based on given mass flowrate at the relieving pressure/temperature.

Fallah

[paulhorth]

There are two separate questions in this thread.
Nilesh has jumped in on top of the original question from Process Optimiser, with a totally different question.

I will try to respond to both of the questions.

First - Process Optimiser's question about blowdown. I agree with Erwin's reply, but I would add that if the antisurge valve is opened on shutdown, then it is possible to blow down from the suction side. I do not understand how you can have a centrifugal compressor without an antisurge valve.
I would also add that the suction and discharge ESD valves have to be where they are, which is outside the antisurge loop - if they were closer to the compressor, then a spurious closure of one of these valves could wreck the machine.

Next, the question from Nilesh k makwana, which is about blocked outlet relief for a compressor. It is vital to understand that the sizing case for the relief valve is ALWAYS MORE than the normal mass flow through the machine. The sizing case should take into account the flow at maximum suction pressure and maximum speed or max driver power, with no credit for the antisurge valve. On a recent gas plant project that I was involved in, this was not understood by the designer. When the operator discovered the error, the startup of the plant was delayed for six months while temporary fixes were put in place including a high suction pressure trip and a speed limiter. In the long term, larger relief valves are being fitted.
This maximum mass flow has to be derived from the performance curves, usually with assistance of the compressor supplier.

Paul

[Root]

Paul,

The refinery, i'm working have no antisurg valve and compressor is runing with out any problem since 11 years, the blowdown valve is given on discharge of the compressor.

Toor

[loupurdie]

Most of the compressor have at least one of the following operating limits:

* Minimum Operating Speed - the minimum speed for acceptable operation, below this value the compressor may be controlled to stop or go into an "Idle" condition.

* Maximum Allowable Speed - the maximum operating speed for the compressor. Beyond this value stresses may rise above prescribed limits and rotor vibrations may increase rapidly.

I think to have best Compressors, having heavy duty & durability to sustain long hours .

volvo parts

[paulhorth]

Toor,

Can you send in a P&ID of this compressor so that we can understand how it is protected? Because, if you are talking about a centrifugal compressor, then the absence of an antisurge valve is contrary to practice on every compressor I have ever dealt with on many North Sea projects and onshore gas plant projects over a 40 year career.

The Compressor Handbook by McGraw-Hill incudes a lot of information on antisurge systems, why they are needed and how to design them. It states " a trip-out of the driver should be interlocked to open the antisurge valve within three seconds........otherwise, the machine will surge causing mechanical damage........" and includes a photo of surge damage.

I note however, that you recently wrote in to this forum to ask how to calculate the velocity in a gas pipeline. This question suggests you do not have extensive process engineering experience, so perhaps you have not correctly grasped the functioning of the compressor in your refinery.

Paul

Edited by Art Montemayor, 16 June 2011 - 01:03 PM.

[Root]

Hi Paul,

Attached is our centrifugal compressor, please come out with detail coments on design philosphy of this control system. Compressor RPM is 9K and more is four pole and RPM is 1K. Integeration gear box is there. Gas flow is 60000+ m³ / hr and MW is 9 kg/m³.
Thanks
Toor

Attached Files

•  RGC and Net Gas Compressors.xlsx   411.27KB   265 downloads

[paulhorth]

Toor,

Your message is not clear. What does "More is four pole" mean? What does "MW is 9 kg/m3 " mean?

The diagram you have sent in is not a P&ID, but appears to be a screen display from a control room. This does not show the complete control system, nor does it show ESD valves, so it's difficult to make comments on the control philosophy as you ask.
Don't you have a P&ID?
The diagram shows three compressors, the Recycle compressor 6305J and the the Net Gas compressors 6307J and 6307JA. The symbols for 6307A/AJ look like recip compressors. It is not clear which of these you are talking about, but probably the first one??

However, I will make some comments.
You have said that your compressor (which one? ) does not have an antisurge recycle. But this diagram clearly shows recycle streams for all of the compressors.The recycle compressor has a stream going back from the discharge to the Combined Feed Exchanger through a control valve (no control signal is shown). The Net Gas Compressors (probably recips) have recycle streams coming from somewhere on the discharge side, feeding to both the second stage suction and the first stage suction under pressure control. This looks like a spillback capacity control whick would be typical for a recip.

To comment we would need to see how the recycle stream shown for the first compressor (the Recycle Compressor) is controlled. If there are any ESDVs between the return of this stream and the compressor suction, then i would suggest that the machine is not properly protected against surge, but it's hard to say with such incomplete information. My earlier comment about industry practice for centrifugal compressors still stands.

If you are interested in help and advice from this forum, why don't you take a little time writing your messages clearly and providing more complete information?

I am going to be off-line for a month from Tuesday next week, so I will not be able to follow up this discussion.

Paul

[Root]

Paul,

As you have lot of experience and I think you no need more explanation, but however, I will explain you what type of compressor we have.

• This compressor have 6 Pole and 4 pole motor option , 6 pole using during dry out with N2 and 4 pole during normal operation.
• No ESDV on this compressor is installed, Pressure is controlled through PCV, which is I marked.
• Compressor is getting tripped on Hi amps if there is any upset but not on surging or vibration.(off course compressor will trip on hi vibration, if Major surging will be there.)
• Net gas compressors are reciprocating and there is spill back for suction pressure control.
• Recycle gas compressor discharge is going two place one to CFE and other to net gas 1st stage suction drum where PCV installed for pressure control of entire loop.
• There is no control valve on discharge side, the valve you are looking , HCV, and we are manually controlling the recycle gas flow.
• Compressor motor RPM is 1.5K and Compressor RPM is 9K.
• MW stand for molecular weight.
• P&ID and screen shot is same there is no difference therefore I attached screen shot.
• After two days, I will remove this attachment.

I give you lot of information and hope you will come with positive out come.
Thanks
Toor

[paulhorth]

Paul,
As you have lot of experience and I think you no need more explanation, yes I do and so do all the other readers but however, I will explain you what type of compressor we have.

• This compressor have 6 Pole and 4 pole motor option , 6 pole using during dry out with N2 and 4 pole during normal operation.The number of poles is irrelevant to the compressor protection
• No ESDV on this compressor is installed, Bad practice,this would never be allowed offshore Pressure is controlled through PCV, which is I marked.Which is I marked? where? what?
• Compressor is getting tripped on Hi amps if there is any upset but not on surging or vibration.(off course compressor will trip on hi vibration, if Major surging will be there.) You say both it will trip and it won't trip in the same sentence - do you really understand? What about high pressure? High temperature?
• Net gas compressors are reciprocating and there is spill back for suction pressure control.As I thought
• Recycle gas compressor discharge is going two place one to CFE and other to net gas 1st stage suction drum where PCV installed doesn't say PCV on the screen shot. for pressure control of entire loop.pressure control where - suction or discharge? can't control both if flow is a variable
• There is no control valve on discharge side, the valve you are looking , HCV, and we are manually controlling the recycle gas flow.On what basis?
• Compressor motor RPM is 1.5K and Compressor RPM is 9K. Irrelevant
• MW stand for molecular weight. then why do you report a value of 9 kg/m3? that is not the unit for molecular weight. Do you understand?
• P&ID and screen shot is same there is no difference therefore I attached screen shot.There must be differences since the screen shot does not show control loops. Send the P&ID please
• After two days, I will remove this attachment.

I give you lot of information and hope you will come with positive out come.
I am going away tomorrow so this is all from me. My comments on the need for antisurge protection still stand . No oil company that I have worked for would allow a centrifugal compressor to be installed without one. If the design is indeed limited to what you have shown, these companies would shut down the plant tomorrow.
Thanks
Toor

[Root]

Paul,
As you have lot of experience and I think you no need more explanation, yes I do and so do all the other readers but however, I will explain you what type of compressor we have.

• This compressor have 6 Pole and 4 pole motor option , 6 pole using during dry out with N2 and 4 pole during normal operation.The number of poles is irrelevant to the compressor protection, why you said irrelevant, while this machine is exiting and working, its mean you don't have belief on tohers?
• No ESDV on this compressor is installed, Bad practice,this would never be allowed offshore Pressure is controlled through PCV, which is I marked.Which is I marked? where? what? PCV controll suction pressure through cascade contoll to discharge pressure.
• Compressor is getting tripped on Hi amps if there is any upset but not on surging or vibration.(off course compressor will trip on hi vibration, if Major surging will be there.) You say both it will trip and it won't trip in the same sentence - do you really understand? What about high pressure? High temperature?, Yes I will understand it, Hi temp trip is there but we never face problem as this compressor never come to this point.
• Net gas compressors are reciprocating and there is spill back for suction pressure control.As I thought
• Recycle gas compressor discharge is going two place one to CFE and other to net gas 1st stage suction drum where PCV installed doesn't say PCV on the screen shot.This is PCV no other name. for pressure control of entire loop.pressure control where - suction or discharge? can't control both if flow is a variable, Have you worked on Platformar ever, if yes then you will not comment like this. This is PCV and controlling the discharge pressure and suction as well because PT on compressor suction out put going to this PCV, to understand it once more study attachement.
• There is no control valve on discharge side, the valve you are looking , HCV, and we are manually controlling the recycle gas flow.On what basis?
• Compressor motor RPM is 1.5K and Compressor RPM is 9K. Irrelevant, why this irrrelevant, tell me indetail?
• MW stand for molecular weight. then why do you report a value of 9 kg/m3? that is not the unit for molecular weight. Do you understand? Sorry for this.
• P&ID and screen shot is same there is no difference therefore I attached screen shot.There must be differences since the screen shot does not show control loops. Send the P&ID please, PFD not getting uploaded, reason , I don't know!!!!
• After two days, I will remove this attachment.

I give you lot of information and hope you will come with positive out come.
I am going away tomorrow so this is all from me. My comments on the need for antisurge protection still stand . No oil company that I have worked for would allow a centrifugal compressor to be installed without one. If the design is indeed limited to what you have shown, these companies would shut down the plant tomorrow.
Nothing is end and you need to study more?
Thanks
Toor

[chemsac2]

Toor,

Few months back, even I thought that no centrifugal compressor was ever installed without anti-surge line. However, recently I have seen a closed-loop system that does not have anti-surge line. It was a small centrifugal compressor handling nitrogen and hydrogen in a batch mode.

My another firm belief was that anti-surge line should go to a cooler before returning to suction. I had to even change that belief when I read about cold recycle and hot recycle. Few applications do not need anti-surge line all the time (especially recycle compressors working in closed loop). In such a case, hot recycle is sufficient.

As for question of location of depressurization on downstream side of compressor, that makes sense to me. This would avoid reverse flow of gas from discharge to suction in the event of blowdown. Reverse flow of gas is not acceptable to avoid compressor impeller reverse rotation and surging.

Regards,

Sachin