24 replies to this topic

[zulu]

Hello Guys

can any one plz explain ' Settle out pressure of a compressor' . i know its the equilibrium pressure b/w comp suction and discharge while compressor is trip and no vent or flaring is taking place( shutdown without blowdown). but during blowdown and consequently flaring what will be this settle out pressure and do one needs to account this pressure for sizing of blowdown valve , RO and flare header???
looking forward for a reply. thanks in advance

[fallah]

Compressor shut down ----> SDVs closed ----> Anti surge valve opened ----> Compressor settles out ----> Depressurizing,if fire case is applicable

Sizing of relevant BDV and RO, in connection to flare, would be done based on output of proper software and considering that the time of depressurizing should be such that compressor sealing system not being subjected to any damage.

[herl_m]

Lets say I'm using hysys for blowdown simulation. All i need to do now is specify the volume for streams which are involved in blowdown process. Without calculating firsthand the settle-out pressure, the hysys will give me the settle-out pressure. Of course, the settle-out pressure calculated will be lower than PZAHH. For sizing the RO size of BDV, the initial pressure IS the settle-out pressure and not the PZAHH. Correct? What about low temp study? Is it true thst this settle-out pressure is the initial pressure or the PZAHH?

Pls help.

[fallah]

I think the initial pressure for sizing of RO (installed after BDV) in compressor system,should be settle out pressure.

[herl_m]

Thanx fallah!

[zulu]

thanks a lot for ur answer.

the problem is the sequence u hav mentioned is theoretical one. whats happening is at the same time all these actions are taking place ( SDV closed, anti surge valve open and blowdown valve opening). so i guess settle-out pressure will b achieved after some time ( few seconds). then it means the initial impact on RO( when blowdown valve will be open) will be the operating pressure of that system which will definitely be higher than the settle out pressure. so the sizing of RO will be based on initial impact pressure or the settle out pressure ??? i will appreciate ur reply.

[fallah]

Depressuring in the compressor, if intended, would be done manually after compressor shut down and settle out (closing of SDVs followed by pressure equalization of suction and discharge parts of compressor).

Therefore, initial pressure at the time of BDV opening should be settle out pressure.

[zulu]

I am afraid this is not the case with me; at the same time all these valves opening/ closing so initial impact will be the operating pressure of the system.

can u plz tell me how do u relate RO with 15 min of de pressuring??? i mean u need to de-pressure the system in 15 min, u calculate the peak blow down rate( kg/hr) from Hysys and then u give this mass flow while sizing blowdown vlave and RO, how this RO will let that mass flow to pass in 15 mins????

[fallah]

You should do a dynamic simulation!!

-With initial pressure,time duration (15 min),...,the peak flowrate would be accessible by proper software (HYSYS,...),

-Using peak flowrate the depressuring line size,hence BDV size,would be calculated,

-Using line size,peak flowrate,max Delta P,the size of the RO could be calculated.

[chemsac2]

Raheel,

Analyse the situation dynamically before jumping to a dynamic simulation software. I think your system has an interlock which activates on compressor trip and closes suction and discharge ON-OFF valves and depressurizes compressor inventory.

If this is the case, system would behave like this:

1. Time zero ==> compressor trips
2. Interlock activates and anti-surge valve opens instantly
3. Suction and discharge ON-OFF valves start closing at around 1"/second rate (typical rate for ON-OFF valve closure is considered to be 1 second per 1". If your line size is 6", closing time would typically be 6 s)
4. Although compressor has tripped, it does not mean compressor would come to stand-still instantly. Typical coast down time would be 1-4 seconds depending on system. Specific cases may require as large as 10-15 minutes for compressor to reach stand-still. For this analysis consider stand-still time is 2 minutes.
5. At the end of 6 seconds, situation would be like this:
- Suction ON-OFF valve closed
- Discharge ON-OFF valve closed
- depressurization valve opened
- Compressor not yet stand-still but running at slightly below operating rpm
6. As depressurization proceeds, discharge pressure decreases. Compressor sees this as a low system resistance and hence compressor discharge pressure would decrease and flow increase. But, anti-surge system sensing low suction flow would open more and more to avoid reaching a situation where compressor sends out more gas than is coming in.
7. This would go on till compressor reaches stand-still.

To answer your question on RO sizing:

Why is restriction orifice provided on depressurization lines?

RO is to ensure that depressurization is not so fast that material undergoes brittle fracture. API suggests that RO be sized to reach 7 kg/cm2 or half the design pressure within 15 minutes.

Since depressurization is a dynamic process (pressure in system decreases, flow across RO decreases), no single value of pressure can be used for RO sizing be it settle-out pressure or normal discharge pressure. RO inlet pressure would vary from normal discharge pressure to settle-out pressure to even less. Exercise is necessarily dynamic especially if case is critical involving fire.

Using settle-out pressure as RO inlet pressure leads to bigger orifice than normal discharge pressure and hence is used conservatively.

Hope it helps.

Sachin

[fallah]

Anti surge valve be opened after suction/discharge SDVs to be closed, due to sensing low (even no) flow condition.

BDV to open (if depressuring is required) manually after compressor settle out,therefore settle out pressure should be considered as RO upstream pressure in its sizing.

[Shahine]

Is it right that during BDV caculation and RO sizing to consider the initial pressure = the design pressure because we need to depressurize the suction line & compressor and discharge line from design pressure (to get minimum temperature also more safe) to 7 barg or 50% from design pressure, so settle out pressure not affect in sizing calculation for RO?

but i think settle out pressure must be consider to determine design pressure for suction line which must be higher than settle out pressure.

PLZ, confirm that ?

[fallah]

Your questions aren't so clear. I think if you read the above discussions you can meet your requirements.

Anyway, don't hesitate to submit any new concerns other than submitted in this thread.

[Shahine]

Thank you for replying. But I mean that initial pressure that we use in hysys simulator (depressurization tool) to determine peak flow rate (also, RO area) equal to the design pressure of compressor not settle out pressure!

And design pressure for suction line must be higher than settle out pressure.

[fallah]

If you accept the sequence I submitted in my post of 17 June 2010 - 05:52 AM , then you should accept that the initial pressure of compressor depressuring is the settle out pressure; otherwise, please submit your own sequence!

Indeed, the design pressure of the suction line is equal to settle out pressure.

[Shahine]

I Accept your sequence, I prefer to select initial pressure = design pressure of compressor due to :

1- usually, there is PSV at compressor discharge line and set pressure for this PSV < or = design pressure of compressor , So, may be during reliefing we need to shutdown the compressor and depressurized it ,so pressure at this moment at BDV will equal to design pressure.

2-As you said "Using settle-out pressure as RO inlet pressure leads to bigger orifice than normal discharge pressure and hence is used conservatively", so, I think considering initial pressure = the design pressure of compressor will give bigger area,


3- Min. design temperature, usually, according to depressurization calculation, So,to be more safe selecting Initial pressure = design pressure give Min. temperature.

[fallah]

Philosophy of depressuring of compressor is based on sequence submitted in this thread and you accepted it.

To be much more safe, everybody can consider initial pressure even more than design pressure!

[Shahine]

Sure your consequence may be right in some cases as you said "Analyse the situation dynamically before jumping to a dynamic simulation software. I think your system has an interlock which activates on compressor trip and closes suction and discharge ON-OFF valves and depressurizes compressor inventory.",these sequence may be occur during shutdown. Not occur during BDV openning due to failure, so what will happen if BDV open due to failure action [open alone] .

1- pressure in this case may be equal to discharge pressure or may be equal to design pressure [i mean this failure action for BDV may be occur at the same time when PSV is reliefed due to any resone that cause overpressur].

So, during depressurization considering initial pressure = design pressure not only more safe to get lower temperature or higher area but also it's real sequence may be occur , and it's wrong to neglect it.

[fallah]

You are right to consider BDV malfunction leading to full open during normal operation of the compressor,as a credible scenario,specially due to BDV would be FO (fail open position).

But as you know,normally dedicated BDV of compressor depressuring has been installed on the suction vessel,therefore senses suction pressure (not discharge pressure).Of course,there is PSLL at the suction line as a safeguard triggered the compressor to be tripped.

[Shahine]

May be you are right ,but realy I involved in Gas processing plant has two compressor ,Export Gas compressor & refrigerant compressor both of them have BDV on suction drum (as you said), and also have BDV on discharge line to depressurized the compressor and air cooler.

I do not know definite reason for this BDV on discharge line may be to protect compressor seal from high depressurization rate .


I'm sorry for delaying the reply, but if you can , PLZ , Help me to Know the reason for this BDV on discharge compressor line.

[Propacket]

Sorry to jump into the thread.

Shahine,

In my opinion, BDV on compressor dischrage is dedicated for compressor blowdown. BDV on the inlet of compressor is for the blowdown of plant located upstream of compressor.

[fallah]

As far as i know, in order to seal leak protection, MOV'S on suction/discharge lines might be replaced by SDV'S as per compressor vendor advise and at this condition a BDV needed for depressuring the trapped gas between two replaced SDV'S.

[Shahine]

P.Engr you are well come, I think you are right but why we cannot use only BDV on suction system to depressurize suction drum and compressor.?

Thanks you fallah, but I can not understand your meaning?

[fallah]

Thanks you fallah, but I can not understand your meaning?

You are right,let me explain about my statement:

Compressor vendor may require a shutdown with depressurization in the case of seal leak or high vibration.The system to be depressurized includes the suction drum and the after cooler.This depressurization would be done through a dedicated BDV located at the compressor discharge.

[Shahine]

Thank you fallah for this clarification