re starting NH3 two stage system.

[cool kiwi]

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.:)

[US Iceman]

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.

[PaulZ]

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

[Poodzy]

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.

[Poodzy]

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.

[US Iceman]

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.

[aawood1]

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.

[brian_chapin]

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.