Ammonia plant running conditions

[seanf]

Just to see if there's something to learn from you guys.

Single Sabroe screw ammonia plant with HP receiver level control, pumped liquid to a tunnel, two evaporative condensers of different sizes, thermosiphon oil cooler.


Links to a few pictures, not very good but might help.
Original layout https://www.flickr.com/photos/s34n/5...ream/lightbox/
--New condenser layout https://www.flickr.com/photos/s34n/5...ream/lightbox/
--Liquid line pipework https://www.flickr.com/photos/s34n/5...ream/lightbox/
--Smaller cond https://www.flickr.com/photos/s34n/5...ream/lightbox/
--Larger cond trap https://www.flickr.com/photos/s34n/5...ream/lightbox/
--Larger cond https://www.flickr.com/photos/s34n/5...ream/lightbox/
--Level Control https://www.flickr.com/photos/s34n/5...ream/lightbox/


Reported that the suction pressure dropped low and tunnel temps went high. Been told sites way to fix it was to switch the compressor off and keep the two liquid pumps running in hand. Once the suction pressure had built back up they switched the compressor back on and its back to normal. I didn't get to see how it was in fault. In the past others have added gas as the fix.


How it was found running after site had restarted the comp,


-- Compressor seemed to be at full capacity. Discharge pressure between 9.5-10.5 bar.


-- Receiver level just under half full, marked up as the normal level being about 5/8 full or what looks to be slightly above the NH3 line on the Danfoss level controller. The liquid feed to the surge drum seems to be piped halfway into the receiver going by the drawing and the text in the manual.


-- Surge drum level, bottom sight glass full, second glass sometimes a bit of splashing at the bottom. Dont know their normal operating level.


-- Two liquid pumps running, think originally designed for one, pressure on the outlet gauge read 2.5bar. The gauge up by the compressor showed about -0.5bar suction and 0.5bar pumped liquid diff.


-- Both condensers running with fixed speed fans and water pumps running.
The smaller condenser if I'm remembering correctly has air leaks from large holes in the fan housings, didnt get to check the coil or spray nozzles. Its inlet pipe temp was at saturation temperature or lower, outlet about the same. Air on temp 18.5 DegC.
The larger condenser inlet temp was up at around 48 DegC, and outlet at saturation temp or lower.


-- Air purger, the foul gas line was open to the smaller condenser's liquid outlet purge point. No bubbles coming from the foul gas blow off. Got a little bit of air out manually purging from the purgers gauge connection, swapped over to the larger condenser purge point and got more from there but again not much. No real change in discharge pressure or condenser pipe temps.


-- The surge drum's level controller controlled fill valve was frosty on the outlet, the manual bypass was closed but sweating at the strainer, and the feed from the oil cooler to the surge drum was frosty after the hand valve.


--The temp display on the control panel showed -29 to -31 DegC, meant to be the tunnel air temp. Said to normally be -32 to -35 DegC.


Switched off the smaller condensers fan and the discharge went up by about 0.5bar and receiver level stayed the same. Switched off its water pump as well and the pressure went up to 12bar, the receiver level dropped to about a 1/4, and then built back up after switching the fan and pump back on. I don't know if the level changed due to the surge fill valve opening or lack of liquid draining from the condensers.


My questions are,


Is that liquid feed from the bottom of the oil cooler to the surge drum just there to stop oil from logging in the refrigerant side of the oil cooler, or is there another reason for it?


Is the low inlet and outlet pipe temp of the smaller condenser due to lack of gas flow through it, and is that lack of flow due to a low rate of condensation of the ammonia vapour?


With the different sized condensers, are both of the traps at appropriate size, height and locations from the outlets? The larger condensers trap is about 300mm below the gantry.


Does the bottom entry of the liquid line to the receiver now cause a problem with this two condenser setup, with each condenser having its own trap?


I thought the discharge pressure seemed a little high, suction a little low (if the gauge was reading correctly), maybe these running condition were just giving the plant low cooling capacity. If not, would you have expected the condenser liquid outlet temps to have been warmer, considering the smaller condenser seems to be operating poorly?


When the site engineers switched the compressor off, is it likely liquid had been building in the condensers and was then able to drain to the receiver?


Cheers