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  1. #1
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    Smile re starting NH3 two stage system.



    Hi ,Im still a bit of a novice at the industrial ammonia plant that I'm required to operate single handed as one of two shift engineers at a meat freezing works and still getting to grips with the theory of our two stage/pressure system .My main concern is in the event of an electrical failure [happens sometimes here ] after power is restored . Which stage compressors should be put online first and why??
    We have combinations of screw and reciprocating type on both high and low pressure stages? we have two "pots' for each stage and operate at 200kpa and 10kpa respectively supplying liquid refrig to both higher and lower temp chillers blast freezers and cold stores. This place is a bit of a museum with lots of "additions" to the plant over about 100 yrs[yes true!] of continuos service . The newest piece of equipment is 1970 howden screw compressor all the way down to 1917 Haslam horizontal recip compressor . So anyone out there that can put up with a green horn ex marine engineer asking basic questions on refrigeration ,Id be most grateful for the help.



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    Re: re starting NH3 two stage system.

    Typically you will want to start a small high stage compressor first to pull down the suction pressure gradually. As the high stage suction pressure (also could be called the intermediate pressure) begins to decrease you can begin to start more high stage compressors. This first step should be done slowly because if you drop the intermediate pressure too fast you will cause a lot of boiling in the vessels connected to the intermediate pressure.

    Once the intermediate pressure is within a reasonable range of the normal operating condition; you want to start the low-stage compressors in the same manner as the process described above. (the comment on vessels also applies here as well).

    You may also have to turn off some evaporator fans initially to allow the respective compressors to pull down the suction pressure of the respective suction pressure.

    The key to this is... progress slowly with the re-start and don't get in a hurry. I'm sure some others will have some helpful comments also.
    If all else fails, ask for help.


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    Re: re starting NH3 two stage system.

    Hi cool kiwi
    Agree with US Iceman, it may be better to shut some of the rooms off altogether not just the fans if the plant has been off for a while. If everything comes on at the same time you could end up with vessel level problems, high or low levels.
    It might also be necessary to turn the liquid pumps off until you get the suction under control, reduce pump cavitation.
    The reason you start the high stage first is the low stage compressors discharge into the high stage.
    There's plenty of guys on this site that can assist you with any problems or questions, just remember to give as much info as possible.
    Good luck
    Paul

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    Re: re starting NH3 two stage system.

    We have the same problem here Cool Kiwwi. But like Iceman and Paulz said starting the high side compressors is the way to go. I am going to assume your plant is like mine. We have to turn off the liquid (nh3) pumps until we get some equipment on.

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    Lightbulb Re: re starting NH3 two stage system.

    Quote Originally Posted by Poodzy View Post
    We have the same problem here Cool Kiwwi. But like Iceman and Paulz said starting the high side compressors is the way to go. I am going to assume your plant is like mine. We have to turn off the liquid (nh3) pumps until we get some equipment on.
    sorry cant edit. The reason we turn off the liquids is mainly so we dont get spikes in are head pressure.

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    Re: re starting NH3 two stage system.

    I did not mention the refrigerant pumps because I assumed this was a momentary power outage. if the power is off for some time, then what PaulZ suggested is absolutely correct.

    If you have some type of central control system, simply turn off the liquid feed solenoids, fan motors for the evaporators, and refrigerant pumps before you re-start a small compressor. This will allow you to pull down the intermediate pressure much quicker. This also allows the refrigerant pumps to cool down again before you start them.

    After the intermediate pressure is down to within less than say 1/2 bar of the normal intermediate pressure then you could start a low stage compressor.

    The low stage start-up is the same process you use for the high stage. However, you should NOT start the low stage first under any circumstance. if you do, there would not be any high stage compressor to compress that gas.

    This is the same procedure you would also use for a cascade system (CO2 & NH3), if and when you might ever have the privilege of working on one of those.
    If all else fails, ask for help.


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    Re: re starting NH3 two stage system.

    Hi Cool Kiwi. We have the same thing with electrical failure's at time's on my plant. And as USICEMAN said the best and only way is to start the high side of the plant and lower the pressure on the low side side dleivery. As for the ammonia pump's the ones we have are wired so they will not come back in on untill you switch them on, as they will only load the plant pressure when you do not want it to.
    Arthur.

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    Re: re starting NH3 two stage system.

    Some very good advice in here that I wish I hadn't had to learn the hard way.

    Definitely slow is the key - especially if the shutdown has been for a few hours, or at least long enough to let the liquid in your vessels warm significantly from their normal temp.

    I had an engineer start a system a good 40 times in a row because he wouldn't let the liquid in the -55f vessel cool back down slowly. Let's just say that when you have a system down for eight hours and the liquid is at -20f, you do not want to pull 16" of vacuum on that vessel.

    Well, that is unless you want to start a system 40 times in a row...

    Our procedure now is to:

    1) Shut down the load - fans and pumps.
    2) Start high side and wait until the intermediate vessel equalizes and head pressure is normal.
    3) Start low side at intermediate pressure setting and start up the load slowly.
    4) Drop 2# on the low side every 5 minutes or so keeping an eye on the vessel level. Wild fluctuations mean rapid boiling.

    Eventually everything stabilizes and your low side is set back to normal.

    Just fyi the system in question is 16,000# -55f (16")on low side suction temp, 10f(24#) on high side suction temp.

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    Re: re starting NH3 two stage system.

    Hi Iceman , thanks for the tip ,Ive got the old museum under some control now and finding my way nicely.One thing bothering me is my intermediate stage vessel [HP pot in kiwiland] is the level control .Its a pilot operated philips valve via a sporlan Tx style pilot with immersed bulbs/electric heaters in a liquid leg on side of the vessel. The problem I'm experiencing is when the bulbs are immersed in liquid Nh3 they fail to open the pilots . Ive replaced these sporlan bulbs/ tubes /element units. with tested new units [at atmospheric pressure in liquid Nh3] but to no avail . Could it be that the liquid leg on the side of the vessel has a "slug" of oil much like the sight glass that is insulating the bulbs but giving a level that the float swith can operate with??That is how the levels are controlling now. I hope that makes Q makes sense. thanks in advance
    cool kiwi

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    Re: re starting NH3 two stage system.

    I work at a ice cream plant, and when our ice cream roller bed type hardener goes into energy management we get a lot of liquid to the intercooler and sometimes shut down on high level because head pressure drops at night and on cold days and we can't transfer from the accumulator via dump tank to the cpr. to sum up the process of this system intercooler at intermediate transfers to accumulator at low stage pressure accumulator fills the dump tank by gravity during vent, then dump tank transfers liquid to the cpr with head pressure.

    How would i deal with the liquid in the intercooler when the system is down for a couple hours? i could try to transfer to the accumulator but if i have no head pressure there is no way of dumping to the cpr, i think the intercooler also feeds the recirculator on set level in the intercooler or via manual valve then i could pump it to loads in the plant. I'm not sure here never been in this situation but one day I'm sure it will happen to me, just wanted to be ready and post something that might help others out too.

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    Re: re starting NH3 two stage system.

    A pressure regulating valve with wide open function placed in the discharge piping after the gas supply to the dump tank should do the trick. When the dump is activated, this regulator will raise gas pressure to drive liquid to the CPR. Otherwise, it is wide open.
    Phillips calls this a system and they call the valve a PMR--BL, http://www.haphillips.com/products.html

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    Re: re starting NH3 two stage system.

    Quote Originally Posted by NH3ISFORME View Post
    I work at a ice cream plant, and when our ice cream roller bed type hardener goes into energy management we get a lot of liquid to the intercooler and sometimes shut down on high level because head pressure drops at night and on cold days and we can't transfer from the accumulator via dump tank to the cpr. to sum up the process of this system intercooler at intermediate transfers to accumulator at low stage pressure accumulator fills the dump tank by gravity during vent, then dump tank transfers liquid to the cpr with head pressure.

    How would i deal with the liquid in the intercooler when the system is down for a couple hours? i could try to transfer to the accumulator but if i have no head pressure there is no way of dumping to the cpr, i think the intercooler also feeds the recirculator on set level in the intercooler or via manual valve then i could pump it to loads in the plant. I'm not sure here never been in this situation but one day I'm sure it will happen to me, just wanted to be ready and post something that might help others out too.
    What? soory dont get your lingo? Without a doubt Start your LP with out your HP machines Period ! its the perfect recipie for disaster. As the other fridgees explained get that Ip in control before even thinking about running the low stage. Wheres your PLC and scada.what age do we live in ? Get this place automated and concentrate your hard work in keeping this place running another 100 years. As for your make up system it sounds simular to a danfoss RT 280 A seems oddball but unsure of sporlan, before my time! be interested to have a look at this museum? I work in auckland, sounds like your down the line splashing ammonia around the place.

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    Re: re starting NH3 two stage system.

    Quote Originally Posted by nh34life View Post
    what? Soory dont get your lingo? Without a doubt start your lp with out your hp machines period ! Its the perfect recipie for disaster. As the other fridgees explained get that ip in control before even thinking about running the low stage. Wheres your plc and scada.what age do we live in ? Get this place automated and concentrate your hard work in keeping this place running another 100 years. As for your make up system it sounds simular to a danfoss rt 280 a seems oddball but unsure of sporlan, before my time! Be interested to have a look at this museum? I work in auckland, sounds like your down the line splashing ammonia around the place.
    dont start lp b4 hp!

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    Re: re starting NH3 two stage system.

    cool kiwi ,
    When sporlan bulb is immersed in liquid it should close pilot line as well as main phillips valve .

    When level in HP pot is low , heater will warm up bulb and open sporlan valve venting liquid off top of phillips valve .
    This will then open to increase level .

    As you say oil may be in column , but you describe it opposite to what I would have thought .
    If theres a lot of oil in column would suggest not much ice forming around it , but level may be higher than required in HP pot .

    Have you tried draining any oil out ?

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    Re: re starting NH3 two stage system.

    Hi,

    Quote Originally Posted by Nh34life View Post
    dont start lp b4 hp!
    why not? system is very cold i.e. low pressures ... no production and no load ... so it is possible to start LP comps (if there is permission for start LP without HP) to get some pressure to pump/transfer liquid from one vessel to another ...


    ... this is only a theory... grown up on practical experience ..

    ... of course to do all in a proper way .... we have to know much more about that plant ...

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

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    Re: re starting NH3 two stage system.

    Josip If I started the lp comps. before the hp comps the lp comps. would shut down as soon as the intermediate pressure reached about 25 psi. Thinking about it in my engine room, if it has been off for several hours the pressure usually has equalized at around 40 to 60 psi. the low side comps would not start against that pressure. C.D.
    Last edited by CHIEF DELPAC; 15-02-2010 at 11:50 PM.
    NH3 for me

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    Re: re starting NH3 two stage system.

    NH3ISFORME If your pressures are equal and you have a high level. Close in on the suction valve to the high side comp. The valve will act as an expansion valve. As the head pressure increases and the levels in the vessels drop you can open the valve more.I have found that when an engine room has been off for a long time it takes at least 2 hours to start the engine room and bring all refrigeration equipment back on line. C.D.
    NH3 for me

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    Re: re starting NH3 two stage system.

    Hi, CHIEF DELPAC

    Quote Originally Posted by CHIEF DELPAC View Post
    Josip If I started the lp comps. before the hp comps the lp comps. would shut down as soon as the intermediate pressure reached about 25 psi. Thinking about it in my engine room, if it has been off for several hours the pressure usually has equalized at around 40 to 60 psi. the low side comps would not start against that pressure. C.D.
    partially, agree with you, but in auto mode LP should start up HP comp ... also LP comp must be able to run up to 5 bar-70psi without trip due to high discharge pressure (now-days maybe less due to cheaper el. motors with less power) ... this must be designed due to delays to reach a proper capacity both for LP comps and HP comps ... at least this is my experience with plant designed by Sabroe with screws on LP and HP side back to 1984.

    that plant was a low temp cold store with two freezing tunnels .... of course to run freezing tunnels it is much better to start HP comp and then LP comp to achieve low temp before starting with freezing tunnel/s ... i.e. we can have situation where suction pressure in LP separator rise thus giving a starting signal to a LP comp and then discharge press from LP comp will rise a suction pressure in HP separator and HP compressor should start ... all this will happen within few minutes ...

    anyhow .... different design of refrigeration plants give us more or less possibilities for playing with plant and in your case maybe that is not possible ... thus must be restarted in some other sequence ... HP comp-LP comp...

    definitely, we can find a lot of poor designs due to different reasons .... but the worst is ignorance ... very often in this modern era ... thanks to computers which must be of help and not the main source of designs ... but that is another story

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

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    Re: re starting NH3 two stage system.

    Josip The engineroom I operate is completely manual, none of that modern automatic stuff. The engineroom I run Has had 5 or 6 different owners. There has been either no money or no desire to upgrade the engineroom. My experience has been in this and other places that the operating and maintaining crews have little input on the design and building of refrigeration systems. Cost determines the equipment that is purchased. The lowest bidder gets the job. So its here it is run it. C.D.
    NH3 for me

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    Re: re starting NH3 two stage system.

    Hi, CHIEF DELPAC

    Quote Originally Posted by CHIEF DELPAC View Post
    Josip The engineroom I operate is completely manual, none of that modern automatic stuff. The engineroom I run Has had 5 or 6 different owners. There has been either no money or no desire to upgrade the engineroom. My experience has been in this and other places that the operating and maintaining crews have little input on the design and building of refrigeration systems. Cost determines the equipment that is purchased. The lowest bidder gets the job. So its here it is run it. C.D.
    unfortunately that is true.... owners does not like to invest some money in automation to make your life easier .... they are thinking only about profit and struggle is what remain to you ...

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
    No job is as important as to jeopardize the safety of you or those that you work with.

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    Re: re starting NH3 two stage system.

    Quote Originally Posted by Josip View Post
    Hi,



    why not? system is very cold i.e. low pressures ... no production and no load ... so it is possible to start LP comps (if there is permission for start LP without HP) to get some pressure to pump/transfer liquid from one vessel to another ...



    ... this is only a theory... grown up on practical experience ..

    ... of course to do all in a proper way .... we have to know much more about that plant ...

    Best regards, Josip
    Because this is like launching a boat without starting the engines, What if the ip discharge doesnt trip and blows the reliefs,what about your excessive load on the ip comp start. this would create a miriad of issues. The only time i would run lp first is for maintenance purposes

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    Re: re starting NH3 two stage system.

    Hi, Nh34life

    Quote Originally Posted by Nh34life View Post
    Because this is like launching a boat without starting the engines, What if the ip discharge doesnt trip and blows the reliefs,what about your excessive load on the ip comp start. this would create a miriad of issues. The only time i would run lp first is for maintenance purposes
    Agree with you ... partially

    I'm not trying to teach anyone to be stupid ... see my sigs

    but it is possible to launch the boat with sails or by rowing ... this is almost the same .... if you know what are you doing ... without need to step over the safety or some other limits ... kind of answer you gave in your last sentence ...


    this was theoretical discussion with some advices/notes from my theoretical and practical background ... it is normal to start HP system first ... but, there is always one "but" ... hope you agree with me too , at least partially...

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
    No job is as important as to jeopardize the safety of you or those that you work with.

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