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Reciprocating Compressor Unloader Valves


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#1 Mayank Joshi

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Posted 06 May 2013 - 04:40 PM

Hi,

   Most recip compressors have options for increasing the Load by loader valves or through clearance pocket .

If a compressor has 2 set of loader valves for 50% load and 100 % load , and if the compressor is in no load condition , are the loader valves completely closed ie not allowing any gas or are they completely open ie they allow gas to come in and the gas is released through them on discharge stroke . Most compressors have a trip   
after some time in no load condition why is that given ???



#2 Bobby Strain

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Posted 06 May 2013 - 06:42 PM

You should visit one of the compressor manufacturer's site for a discussion on how reciprocating compressors and controls work. You should be able to find them with Google. The proper terminology is valve unloaders.



#3 Art Montemayor

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Posted 06 May 2013 - 06:44 PM

Mayank:

 

The proper term(s) is UNLOADER VALVE(S) – not “loader” valves.

 

There are basically two types of Unloaders used on reciprocating compressor suction valves – and not on discharge valves:

  1. Unloaders that use “fingers” to depress the suction valve plates and keep the valves open while this is done;
  2. Unloaders that use a “lifting” device to lift the entire suction valve assembly off of its seat and render the valve inoperative during this time.

Which type are you referring to?  If you are referring to the first type, then yes, the compressor is essentially in a “no load” condition while the valve plates are depressed – but only if all the valves being unloaded constitute ALL of the compressor suction valves for that compression stage.  Here, I have to assume you mean that the compressor is only a single stage reciprocating compressor (but you fail to mention that).

 

A reciprocating compressor cannot continue to be 100% unloaded 100% of the running time while using valve unloaders because of re-compression heat build up in the cylinder.  This is the reason that I personally prefer clearance pockets or speed control in order to control a reciprocating compressor’s capacity.

 

I have many years' experience designing, operating, and modifying reciprocating compressors in many gas services and can upload capacity control information on reciprocating compressors if you (and others) so desire it.   Just let me know.

 

 

 



#4 Mayank Joshi

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Posted 07 May 2013 - 03:40 AM

Okay , Thanks for the reply.

  so the valves near the suction VOLUME bottle which have instrument Air SUpply are known as UNloaders.??

Can u please provide something on how UNLOADERS work .?

 

In my above question I  wanted to ask in 0% load condition are the unloader valves closed ie in 0 load a compressor draws in no gas.???



#5 Art Montemayor

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Posted 07 May 2013 - 09:01 PM

Mayank:

 

I believe there is some mix-up occurring here.

 

You haven’t even supplied a sketch (or a clear, detailed photo if you are dealing with an actual compressor) of what you are describing as “the valves near the suction VOLUME bottle which have instrument Air Supply”.  So, therefore, I can’t understand what you mean.  What do you call a “volume bottle”?  Normally, the activation of the valve Unloaders is done by using pneumatic action and that explains the instrument air.

 

To make this thread a true engineering discussion, please furnish additional basic information, such as:

  • Are your queries academic in nature because you want to learn more on the subject?
  • Are your queries related to an actual compressor installation operating at present?
  • If you are dealing with an actual compressor, tell us all its features or simply furnish a detailed Data Sheet on the compressor.
  • What gas are you compressing and what are the suction and discharge conditions?

I am attaching a paper on Compressor valve unloading and I will post more material in a future post as I find it in my files.

 

Attached File  Compressor Valves and Unloaders.pdf   402.01KB   1089 downloads

 



#6 Art Montemayor

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Posted 07 May 2013 - 10:01 PM

Mayank:

 

Here, attached, is the detailed information on reciprocating compressor valves and how they can be designed to furnish compressor capacity unloading.

 

I hope this helps to answer your questions.

 

Attached File  Unloader Selection for Recip Compressors.pdf   361.21KB   842 downloads



#7 Mayank Joshi

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Posted 08 May 2013 - 06:38 AM

hi ,

I am attaching a pic in which i am stating what I mean by suction Volume bottle and what I mean by unloaders.
I am talking about a 2 stage reciprocating compressor , gas service hydrogen .

my question is that when a recip compressor is at 0 load it discharges no gas ,  is it because , all the unloaders are closed during zero load or is it because all unloaders are open ie gas coming in through unloaders goes out through the same unloaders???

Attached Files



#8 Art Montemayor

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Posted 08 May 2013 - 09:30 PM

When you have Suction Valve Unloader fingers, your reciprocating compressor's capacity is reduced to zero when the fingers are actuated in that part of the cylinder.  (You have failed to tell us if your cylinders are single or double-acting)

 

You are getting confused with Suction Bottles.  They are nothing more that surge chambers and have nothing to do with compressor capacity control.  Please study the literature that I have uploaded for you and you will understand how valve unloaders work and why.   Don't forget: there are other ways to unload a reciprocating compressor - more energy efficient and flexible ways than the valve unloaders.

 

All that valve unloaders do when they are activated is to maintain the suction valves (which are nothing but simple check valves) completely open so that any gas that enters the cylinder is also allowed to reverse direction when the piston displaces it.  Therefore, there is no compression taking place because the "check" valves are kept open.  It is that simple.   I have given many seminars on reciprocating compressors and the best way I know of to explain how a compressor cylinder's valves work is to open and close the door of the classroom,   This is the same action that a suction valve does every time the piston strokes.

 

I hope I have explained it such that you can understand its simplicity.

Why didn't you answer all of my questions?



#9 Mayank Joshi

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Posted 09 May 2013 - 04:07 AM

To make this thread a true engineering discussion, please furnish additional basic information, such as:

  • Are your queries academic in nature because you want to learn more on the subject?

Yes I was just curious , and wanted to learn to clear some doubts.

  • Are your queries related to an actual compressor installation operating at present?

Yes , It is neuvo pignone double stage double acting 4 cylinder compressor taking gas at 2.5 kg /cm2 at suction Mol. wt 4.5.
compression ratio approximately 3.3 in 1st stage , and the rest is achieved by 2nd stage.
Actually a doubt came while we were discussing compressors at work , about the functioning of unloaders , someone said that when unloader is not loaded it does not allow any gas to come into the compressor that is why a trip is given at no load , and I had this notion that they remian completely open when not in load .

I know what suction bottles are for , actually here the valve unloaders are referred to as loader , and the the discharge valves are called as unloaders , which I checked from the web is wrong nomenclature , so to tell about there position I wrote about the suction volume bottle .

i am sorry for taking your time and irritating you but thanks a lot for your help.



#10 Art Montemayor

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Posted 09 May 2013 - 06:37 AM

Mayank:

 

You have not irritated me.  Allow me to close this topic with some honest advice:

 

People on this Forum are here to help you - for free.  When we ask questions, it isn't to bother or molest you; it is meant to gather information to better help you.  It is common sense to know that if you don't supply information regarding your questions or concerns, no one can help - even if they want to.  I have much more information and experience with compressors that I would gladly share with you and others.  But I can't do it without being asked.

 

I hope you realize the value of the information I have furnished you and that you can use it to your advantage.  You may feel suspicious of me or this Forum and for that reason do not respond positively from the very outset.  If so, I wish you well and hope you get over it and go on with what knowledge you have received and profit from it.

 

Good Luck.



#11 Mayank Joshi

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Posted 09 May 2013 - 10:28 AM

hey!! you really got me wrong i said irritated bcoz i felt that my questions are too basic in nature and i felt indebted since you took out time to read and write the answers to my doubts , and i dont feel suspicious of you or the forum coz this is the best online forum available to clear the doubts .

 

There are a lot of questions that are in my mind if you could just give me your email id i would like to trouble you with my doubts or is it fine answering them here .??



#12 Art Montemayor

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Posted 09 May 2013 - 10:48 AM

Mayank:

 

I will do my best to answer any and all questions you present regarding reciprocating gas compressors - or any other topic that I might have some expertise on.   That is what our Forums are designed to do - help out other engineers who are eager and want to broaden their knowledge base.

 

But you have to help us help you out by furnishing as much basic data  and background that enables us to give you an accurate and useful response.   Often, people don't furnish sufficient basic data and we are forced to ask questions regarding their queries.  Just use one new Thread per topic; don't mix all your topics into one thread.   You can generate as many threads as you feel you need to.



#13 Narayan Singhania

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Posted 04 January 2014 - 12:33 PM

Hello Mr.Montemayor,

 

 

I would be very much interested to know about Hydrocom unloading system of Hoerbiger. Are these better than traditional unloaders considering the cost involved to install them.

 

Warm Regards

Narayan Singhania

narayan@compressor.in



#14 Adwtsai

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Posted 13 June 2015 - 10:33 AM

Hi Mr. Montemayor,

I'm preparing start-up procedure of reciprocating compressor. The type is double acting. I'd like to know when conducting no load function by using Unloader, should I open valve at suction vessel in order to intake air from outside?

Thanks, Andrew

#15 Art Montemayor

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Posted 13 June 2015 - 01:18 PM

Adwtsai:

 

Rather than continue to repeat myself over and over, I'll cut and paste what I've already stated numerous  times: you have to help us help you out by furnishing as much basic data  and background that enables us to give you an accurate and useful response.

  • What gas are you compressing?
  • Show a sketch of your flow diagram, showing all block, vent, and blowdown valves;
  • Are you compressing gas into an existing, pressurized process?
  • What is your 1st stage process suction pressure?
  • What is your final process discharge pressure?
  • How many stages of compression do you have?
  • How are you controlling the compressor delivery capacity?

If this is another topic different from this thread's, then start a new thread with this topic.



#16 Anshu18

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Posted 21 October 2015 - 07:51 AM

Hello Everyone,

Little diversion from the current topic.

In order to give the background of our previous discussions, it was regarding the Recycle valve which is of "Fail-open" type. With each trip, this valve is opened & the Recip Compressor train reaches settle-out condition.

As we are in bidding stage, there are few more learnings on this issue. I would like to share it with you all. For ref, I have attached the sketch & the tentative pressure levels.

It is to be noted that the Compressors are driven by gas engines.

The explanation vendors are giving for FO type Recycle valve & settle-out across the train is as follows:

  1. In order to overcome the starting torque, the Recycle valve has to be opened. With this valve open, there will be higher pressure at the first stage cylinder at suction than the discharge. This will open the suction valves & subsequently discharge valves.
  2. If the Recycle valve doesn't open after every trip, the compressor cannot be started because of starting torque. So other option is to blowdown the train & then start the compressor. This loss of inventory is not acceptable.

Are these correct reasons for this design?

The compressor has double acting cylinders with suction unloaders.

Attached Files



#17 Art Montemayor

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Posted 21 October 2015 - 09:46 AM

Anshu18:

 

Firstly, the title of this thread is “Reciprocating Compressor Unloader Valves” and has nothing to do with reciprocating compressor recycle valves.  The two are distinctly different animals.  You should not highjack a thread in our Forums, as explained in our Forum Guidelines.  You should have started a new thread on your specific topic.

 

Secondly, a succinct and proper response to your query could easily be the answer of our learned and experienced colleague, Bobby Strain, who in thread Fail Mode Of Recycle Valve In Reciprocating Compressor (found at http://www.cheresour...ng-compressor/) stated: “My philosophy is to always fail a valve closed unless there is good reason not to do so.”

 

Additionally, a complete and detailed response to this particular topic of employing a Fail Open design on a reciprocating compressor recycle valve was discussed and explained in detail in the thread found at: http://www.cheresour...ressor-recycle/.

 

A reciprocating compressor should always be started (and stopped) in the completely unloaded condition - unless advised otherwise by the compressor manufacturer.  This is so not because I say it, but rather because that is what the basic design of a positive displacement device requires.  The torque required of a natural gas engine is one thing; but there are other considerations that must be applied - such as process conditions, the limits of the compressor’s mechanical design, potential high pressure in the suction side, etc..

 

Without thorough knowledge of your process application the Forum is unable to state whether what your supplier states is the correct and proper way to start (and stop) your reciprocating compressor.  To further treat with this topic, please start a new, specific thread on this subject, supplying complete and detailed data and information.



#18 Malik Umar

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Posted 22 March 2016 - 01:53 AM

Thanks a lot Sir Montemayor for your valuable sharing. 




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