Subj. system uses two stage "grasso" type reciprocating compressors but requires following manual operations during start-up to avoid liquid stroke :
a) start compressor
b) operator checks by sound for automatic loading of one
cylinder
b) immediately upon loading of the said cylinder the 2nd stage
suction valve is gradually opened taking care liquid does not
enter compressor
c) next the 1st stage suction valve is gradually opened again
taking care liquid does not enter compressor
Any ideas ?
Sounds like a recipe for a wrecked compressor. By trottling in the two suctions you are causing the liquid entrapped in the suction lines to expand into vapour(mostly) During this start up stage you risk allowing liquid into the compressor cylinders wrecking all before it, also by trottling the suction you reduce pressure in the crankcase causing refrigerant in the oil to boil off rapidly leading to marginal lubrication.
Something needs fixed. You probably have a shell and tube evaporator with a surge vessel on top. The liquid level is too high in the evaporator (chiller) leading to pull over on start up. Fix the feed valves or partial pump out the chiller before shutting the plant down (tempory fix) and keep the shut off valve before the feed valves shut whilst the plant is off and while running the plant up.
If you are still having problems post again giving more info on the plant and its operation.
Regards. Andy.
Thank you very much for taking the time to reply. I found your post very interesting. Here's are few more system details.
Hope you will have the time to read on.
System consists of three nos. two stage compressors with two running & one standby. Yes we have a shell and tube evaporator with a surge vessel on top. Besides there's also a direct contact interstage cooler. So the output of the 1st stage compression goes to the inter-cooler where it bubbles through liquid ammonia that comes in from the ammonia reciever for condensor. The liquid levels in both the inter-cooler & evaporator (chiller) are automatically maintained so long as the system is running. Every time we need to start the system we take the steps mentioned in my post. You have advised :
"Fix the feed valves or partial pump out the chiller before shutting the plant down (tempory fix) and keep the shut off valve before the feed valves shut whilst the plant is off and while running the plant up."
The feed valves are directly on the two suctions of each compressor (usually one running at a time). As for the shut-off valves before the feed valves, we have only one located at gas outlet of surge drum. Gas outlet from inter-cooler does not have any shut-off valve. I fully agree that :
"During the start up stage you risk allowing liquid into the compressor cylinders wrecking all before it, also by trottling the suction you reduce pressure in the crankcase causing refrigerant in the oil to boil off rapidly leading to marginal lubrication."
I request your help to I rectify this situation & while doing so I would like to automate the start-up sequence of operations.
"But rereading the original post I now have the impression the
is about what the heading suggests, configuring a plant
controlls to automate the described starting procedure as
protection against those rare occasions, it's more like a
question about why are we having to start the system in this
fashion, what's wrong that we have to start with this manual
type procedure?"
I can't understand why you have gained this impression. I meant exactly what I said about automating the described starting procedure. I would like some guidance as to how I should go about automating the system start-up without having a liquid stroke. For e.g. does one monitor the suction superheat while opening the suction feed valves or the suction pressure etc. ?
Hi,
I'm not sure I exactly know what I'm talking about. I'm an ammonia tech, but all I know are screw compressors. What we depend on is having a large enough suction trap/accumulator to keep liquid out of the compressor. Liquid level in the accumulator is watched by alarm system and there's usually enough time between alarm and shut-down to keep ahead of the process. Ammonia solenoids seem very prone to leaking
Jim Heffernan
Tillamook, Oregon
:)
Hi, Luciom. I tend to agree that ther is a fundimental problem with your system, leading from a design flaw or a maintenance issue. Your start up problem may be related to your shut down proceedure. The liquid level in the chiller may be correct for the operating parameters at shutdown time. Either the level increases during the shutdown period (faulty liquid feed solinoides or floats) or the level is quite high during the last operation surging over during a rapid startup sequence. It would be possible to partialy pump out or lower the level in the chiller before shut down, ensuring a lower level on startup and decreasing the possibility of liquid carry over.(ie shut the liquid line shut off valve before the chiller feed solinoides/floats and don't open this until the plant is up and running at a near to normal suction pressure). It is also possilbe to fit a crankcase pressure regulation valve on the first start compressor to trottle the suction to lower the pressure/ loads on this compressor crankcase. This will also decrease the rapid boiling action in the chiller and leave you witha plant which is semi-automatic during start-up.
Please check your liquid feed solinoides/ floats for proper operation good seat seals as NH3 needs very little in the lines of a leaking seat to fill up a vessel over the space of a day or so.
I hope this is of some help to you. I feel there is more to this problem than start up proceedures, hopefully we can hewlp find you a solution to your problem.
Regards. Andy.
Dear Andy, Your posts are most interesting. It indicates you have had the patience to read the details I filled-in in reply to your post. Once again thanks for your time.
Since the shut-down procedure cannot be ensured (frequent unannounced power outages) your second option of having a crankcase pressure regulation valve sounds promising. I suppose I will need two of these as my compressors are two stage. Can you give me more details of how these work ? I was thinking the start-up sequence would look like this. Compressor starts with liquid feed solenoids shut & the crankcase pressure regulation valve in control. Once the suction pressure is near to normal the solenoids can come into action. How does this sound ?
By the way did you recieve the email I sent directly to your email address ?
Rgds., lucio
Hi, Lucio. Yes I did receive your email. In reply to your previous post. You could fit two crankcase regulators, one per stage, I think you will get away with one. Is your compressor a RC4-2/11 or the Kilosker electric company copy. From memory they start with a higher ratio of low to high stage heads than at full load. This means you are drawing a smaller amount of the intercooler than the low stage accordingly, this leads to the low stage discharge suppressing the boiling in the intercooler, leaving the only rapid boiling coming from the low stage. I you fit a crankcase regulator to the low stage it works by restricting the pressure into the compressor like trottling the suction service valve. This is acheived by purely using system pressure and the pressure drop accross the valve. You can then open a pilot solinoide valve fitted to the regulator to allow almost full flow through the valve(just the pressure drop required to keep the valve open) but I would keep the valve in circuit as the pressure drop would not be any higher. Also I would recomend lowering the level in chiller during normal operation anything which is just covering the chiller tubes is sufficent, but you may require an even lower level to ensure a safe startup after the system faults.
Try one crankcase valve, with the liquid feed valve closed until you reach the lowest solinoide cut in point available to you. Then operate the system as normal. If you feel you need another on the high stage fit if after evaluating the performance of the low stage crankcase regulator.
Regards. Andy.
Dear Andy, Our compressor is the equivalent of a RC6-3 & it is of Kirloskar Compressors make (I wonder how you are familiar with those) however at start-up it automatically first loads only one high stage head probably for reasons mention by Fridgetech below so there's no supression of boiling in the intercooler right ?
But I want to know more about the CPR. What exactly are these & how are they constructed ? How is the pressure into the compressor restricted ? Is it some control valve you speak of ? Where does the pilot solenoid fit in ? Can I learn more about the CPR some place on the internet ? How is the valve kept open by means of the pressure drop across it ? Sorry for the million questions but now I sense a solution to my problem is close at hand so please bear with me.
Rgds.,
lucio
Hi,
I'm without a clue about recip. compressors. But I think maybe level control problems are universal to ammonia systems.
If the float/solenoid circuit remains active while the system is shut-down, problems at start-up are inevitable. As the ammonia changes from liquid to boiling liquid the level rises a lot.
:)
Jim Heffernan
Hi, Me again. I have worked on kirloskars imported into N. Ireland by a consultant who works in India. The valve I have in my head is a Danfoss PM3 fitted with a cvpp lp and an evm n/c closed try their site danfoss.com. I have had a quick look around try under solinoides/pressure regulators in their literature section. What loads with the one high stage head two low? what about the next stage, three low one high. This would be more the ratio I would be thinking of, raising the intercooler/changing the pump ratio from 2:1 normal to 3:1. Nine cylinder compressor, who else but grasso!
Go easy on Marc he's a decent bloke really and game to be working at code. Someday Iam going to take a proper Open Uni course on computers, someday when I get time(wish full thinking).
Also I will build myself a basic web site with plants I have come across on it.
Regards. Andy.
Hi Andy, You have kindly given the list of valves & accessories required for my system. I checked-out the Danfoss site & downloaded the details of each component. Before I venture to incorporate your suggestions I will recapitulate. Please correct me if I am wrong.
Compressor starts with liquid feed valves closed & the crankcase pressure regulator in control. Once the required suction pressure is achieved the pilot solenoid will allow full flow through the valve. The liquid feed solenoids will cut-in only after the lowest possible level is achieved (we can adjust this as we have level transmitters installed as redundancy for the float switch).
Rgds.,
lucio
Dear Mr. O'Brien, your advice is noted by the poster or if you prefer imposter. I realise from your input how shallow is my knowledge in the field but I'm here to learn & I hope if you mind the other don't.
rgds.,
lucio
Dear Mr. O'Brien, i never asked for your assistance nor did i want to know what you think is for my good. i find your respect for my enthsiasm to learn is as shallow as my knowledge of this biz of which u think you know much more than i suspect you actually know (leaving aside a few acronyms).
Dear young Brien, kick this habit of giving away unasked-for advice or luck then go learn some code of ethics before coding anything else or shedding uncalled for tears
dear young O Brien, check-back to see where the excessive arrogance originated. now i see the vitriol has a deeper "colour-ful" origin. now you've shown your true colour. you seem to forget it's only skin deep & has no bearing on the grey matter
o.k. that's it for me; don't bother to post for i won't waste my time on it now that i've got to the bottom of this
dear young O Brien, check-back to see where the excessive arrogance originated. now i see the vitriol has a deeper "colour-ful" origin. now you've shown your true colour. you seem to forget it's only skin deep & has no bearing on the grey matter
o.k. that's it for me; don't bother to post for i won't waste my time on it now that i've got to the bottom of this
Whoa,
Lets's chill this a little. Shouldn't we be talking about refrigeration?
Luciom, I've been following this thread and I don't see the insult. It may be my ignorance, it sure looked to me like normal communications.
Jim (not Jeff) Heffernan
Tillamook, Oregon, USA
Having followed this thread, in my opinion you have gotten to the bottom of nothing. The "colour" you perceive has been provided by the glasses you are wearing, filtering everything in accordance with the obvious chip on your shoulder.
Marc, whoever you are you obviously think your **** doesn't stink! Wake up to yourself, this is an open discussion board for refrigeration, air conditioning and associated industries, it is not a councelling service, I know I don't come to be analyzed by ego tripping losers that cant admit their mistakes- we all make them mate! Or aren't you human? What is wrong with telling Luciom what a CPR does and where to bet one as Andy was so nice to do. I can see your point but it personally offends me that you "give advice" in the way you do, don't let distance fool you, speak to me like that and expect to see me in person, then we'll see whose still got the big mouth. And yes I am a member, just like you and my opinion counts just as much or as little as yours does. Also, you're putting a good image of yourself across to the rest of the industry...NOT! Where I come from word of mouth travels quick around the industry... hope your business goes well.
Well, it has got rather silly hasn't it! I've just read most of this thread for the first time......and quite honestly, I was enjoying it - I'm in the industrial NH3 game myself....for my sins!
Well, what went wrong? Why the slanging matches..lets get it back on track.
Nobody has really asked about the system fundamentals..ok, yes its a two stage, NH3 system with interstage cooling - open or closed flash? and what about the actual liquid control on the vessels? Is it low side control or is it a critically charged high side controlled system..perhaps with a notorious WITT float! If the system is slightly overcharged you may not notice it during normal run....but you could possibly notice systematic liquid carryover on start-up!
If you still need further info get back to me!
Frosty
just checked out your profile, certainly know how to live up to your interests! Mr 750's coming over to see you whinging poms in the oz winter (still ****s all over your summer!) and get some business done. Might have to make sure I look you up Fridgewreck and if you wake up one day in your work van with 4 flat tyres, a compound guage imprinted on your forhead and a 10" dial thermometer shoved up your ass... then you'll know 750 has said "G'day". Its an open discussion forum about refrigeration, don't let cultural history come into it because your country (south africa- doesn't deserve a capital s or a) have commited some of the most disgusting wrong doings in world history - who hasn't heard of apartheid! And they can't play Rugby or Cricket to well either, and to top it all off... you don't even live there, can't be that good! You must be very insecure, either that or you just have a small **** and can't handle it. Wake up to yourself, stop judging and start giving advice like everybody else here.
Sounds to me Frosty that you know what youre talking about!! unlike me as I've never touched an NH3 system.
:D
ooh! struck a nerve did I? face facts mate, your country is **** (as you admitted) and YOU are a product of it. How does it feel to be the faeces of the world? You make me laugh! Please write back, I'm enjoying this.
also fridgewreck you're right when you say Oz mechanics are better than sith ivrica (think of your accents) mechs. In fact, check out www.worldskills.com and click "english" then competitions and you may be suprised. Let me just say Montreal was beautiful and I loved kicking your fat pompous asses!
I don't know what this has got to do with the compression or expansion of refrigerant vapours but, - bloody heck - you can't buy this in the shops!!!!:)
You've got to laugh haven't you.....otherwise we might just burst into tears.........LOL - I love all this 'banter'
frosty
i was told my post was too silly & wasting others time & that led to my unneccessay outburst. It's nice to know others didn't think so.
Hi, Luciom, did you ever find a resolution to the initial problem with your fridge plant? If you did tell us about it.
Regards. Andy.
dear / Andy
iwas working in cfc cooling plants , now working in ammonia plant for freezing vegetables , using 3 sab 202,feeding 2 freezing tunnels & receiving from unified accumelator tank, my problem that i havnt diagram for the return system, can you please help me where can i find a similar diagram on the net, many thanks , my email mohsen_yehia@hotmail.com