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# Compressor Blocked Discharge

7 replies to this topic
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### #1 Asgar

Asgar

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Posted 24 September 2020 - 07:35 AM

Hi All

How can I calculate the compressor relief temperature and relief flow rate in the event of blocked discharge,I have compressor curves,how should I determine relief temperature through hysys,I have to do for the following cases
1.single compressor driven by single Motor
2.2compressors driven by single Motor

In the event of checking adequacy should I consider the air cooler duty to my relief temperature?

Kindly Answer as iam new to this blocked discharge of compressor

### #2 latexman

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Posted 24 September 2020 - 08:05 AM

In the event of checking adequacy should I consider the air cooler duty to my relief temperature?

I recommend reading API 521  4.2.3 on Double Jeopardy and making a decision.

The temperature can be calculated using most thermodynamic textbooks and simulators.  I suspect the equations are in API 520 and/or 521, compressor references, compressor vendors literature/websites, etc.  Back test or fine tune your efficiency assumption with whatever you find using actual normal operations, if possible.

Edited by latexman, 24 September 2020 - 08:08 AM.

### #3 Asgar

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Posted 26 September 2020 - 07:10 AM

hi , thanks for replying, can u answer my query regarding design

### #4 latexman

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Posted 26 September 2020 - 07:56 AM

### #5 Jiten_process

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Posted 27 September 2020 - 03:52 AM

@Asgar,

I think you are talking about centrifugal compressor. Blocked outlet scenario for centrifugal compressor must be evaluated very carefully as there are certain consideration behind the deriving the worst case blocked outlet relief case.

Usually, you have compressor discharge pressure VS flow curve at normal operating conditions. Blocked outlet can occure when you are off your normal operating coditions. Hence, blocked outlet scenario should consider below cases

1) compressor being operated at maximum suction pressure (higher than normal suction pressure)

2) compressor being operated with fluid with different molecular weight. specially higher than normal case

3) compressor being operated at its maximum speed.

Usually, you should ask vendor about the curve with above consideration and confirm with vendor for the accurate blocked outlet relief flow. otherwise, based on the efficiency data by compressor compressor vendor, you can simulate it for governing case considering power constraints. Note that, in absence of vendor supplied curve, if you are using HYSYS to evaluate the relief flow, it must be carefully done given that the procedure is different for fixed speed compressor vs variable speed compressor. Also, it it two stage compressor, 2nd stage discharge relief flow depends on 1st stage compresor operating condition. I can give you quick method for fixed speed and single stage compressor using simulator as below

- take the maximum suction pressure at suction

- model compressor curve in the hysys given by the vendor

- keep discharge pressure same as relief valve set pressure or relieving pressure

- keep increasing the flowrate till the time you see power calculated by simulator is equivalant to maximum power constraints by vendor.

- add 5% margin on calculated flowrate to abosrb any uncertainly/error.

- repeate the above steps for lowest molecular weight and highest molecular weight and use PSV sizing for governing case

Above step, can give you the relieving temperature also readily from simulator.

If you are at proposal stage and dont have vendor info, you can use guidline as - for variable speed compressor relief flow rate can be in the range of 1.5-1.8 times the design flow rate and for fixed speed machines, this can be in the range of 1.3 to 1.5 times design flow rate.

Trust this helps.

Good luck.

### #6 Asgar

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Posted 01 October 2020 - 11:22 PM

@jiten process , Thanks for Your help , In the phase of adequacy check, do we consider Air cooler duty , because it greatly varies my relieving temperature if i use air cooler duty , Should we use or not? previously in one ADNOC project , they have considered the Air cooler duty at the compressor discharge , the relief stream is taken as discharge of air cooler , can u please answer this?

### #7 latexman

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Posted 03 October 2020 - 09:39 AM

Size the relief with and without air cooler duty. Does it make any difference to the selected relief?

### #8 Jiten_process

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Posted 04 October 2020 - 04:05 AM

@Asgar,

The relief valve usually installed at the discharge of compressor nearest to its discharge flange. Mostly, before the discharge check valve (non slam type). Hence, during blocked outlet, its compressor discharge stream which is going to be released directly. Check valve will prevent discharge system volume flowing back towards the relief valve. The procedure i have outlined is for this kind of installation.

I need to look at your system before commenting. Having relief valve downstream of air cooler, that too for compressor blocked outlet scenario is not a common. you should not have any obstacles in between the discharge flange to downstream of air cooler which i dont think is usually the case.

A general guidline in any relief system design is "worst case scenario". you need to review the sequence of events carefully during the scenario and confirm whether is there any credible case in which you cant take credit of air cooler operation. Let me give you example, before you reach relief valve set pressure in compressor blocked case, you would hit the PAHH trip at discharge which will initiate PSD to trip compressor as well as discharge air cooler along with other series of trips. why do we still size the relief valve is because considering the failure frequency (PFD) of trip function. Imagine, the failure is partial, wherein air cooler is trip but not a compressor. Point is, you cant take favorable credit of controls or rotating equipment unless it is proven that they are available absolutely in all cases. Having said that, you can take credit of natural cooling (at max. ambient temp). But you need to run your air cooler thermal design with relief stream flowing inside the tubes with natural cooling mechanism. I think, there is a provision in HTRI for that. I doubt this will give you substantial benefit though. As I said, without details of your system, difficult to comment a direct answer.

As @latexman said, do relief valve sizing in both the cases, take governing one.

Good luck.

Edited by Jiten_process, 04 October 2020 - 04:09 AM.