SAB 202S cap and Vi slide headache

I was on a site today with a sab202 that didn't pull as much amps as it should due to a bad calibration of the cap slide.



Someone had tampered with the calibration in the unisabII and adjusted the min and max points.

so, I calibrated the cap slide at the mechanical 0% with the screws on the potentiometer in front, removed the increase solenoid on the Vi and started the compressor.
Checked that the Vi remained in 0% (1.3) and then ran the capacity slide up to max by using a permanent solenoid (with care mind you ;))
With the suction valve I got full movement and had 270 amps for a minute (280 max) before I set the 100% point.

I took the capacity back down to 0% and did the same steps on the Vi slide.

During the test below I was at a suction pressure of approx -38 - -42C (0- -0,3 bar) and discharge of 25-29C (9-10 bar) (NH3)

Now I started the compressor in auto as they would normally do, and the slide moved from mech 0% to around 20-25% and set that as regulator 0% as it should.

the Vi slide moved to 35% (2.5)

The cap increased to around 70% where it should lock with the Vi slide, and then the cap slide went down to 50% as the Vi slide followed up to 100% (4.5).
When the Vi hit 100%, the increase solenoid is permanently on to keep it in position.

However, I never got the amps up to more than 210-220 amps, while according to the site machinist (who I believe), at this suction pressure, the compressor should be working on the amp limit and increase decrease on the motor protection in the unisab because the motor is a bit to small.

I went back to manual and checked my calibrations and both slides hit 0 and 100 % when the should.

so I started it up in auto again and let it go up to 100% (50% on the cap slide and 100% on the Vi slide), then I put my permanent solenoid on the cap slide increase solenoid, and since the cap piston has a larger surface than the Vi piston, It slowly pushed the cap slide towards the Vi slide, moving them both, and when the indicator closed on 2.5 Vi (75% of the total movement on the cap slide) the amps started increasing, and at around 1.9 Vi (85% of the total movement on the cap slide) we were stable at 270 amps because now the suction pressure was down to -42C.

I have never opened a SAB202 and haven't seen the slides with my own eyes, but according to the drawings they are similar to those on a howden, so in my mind, there is no reason why the amps should increase when I moved the slide manually, unless they weren't fully locked in the first place, and when I moved the cap slide it hit a point where the pressure behind the cap piston and Vi piston won over the spring pressure between them.

As I see it now, we are looking at a mechanical problem rather than a control problem, I was thinking maybe the cap piston seal was leaking, and the spring was pushing the slides apart, but if the piston is open to the rotors on the backside, it should increase capacity.


Any thoughts? :)

Tycho, Vi SAB 202 is ever really a headache.
To change the sealing ring? I'm not sure that many will end your problem, i can not, there are many variations, cylinder capacity, discharge port when the control line, how long the compressor sevice?

Tyco,
With the 202 its discharge pressure on back side of piston & oil pressure on indicator side.
I have had a few where cylinder is scored, as oil for unloaders is directly of the oil pump through a strainer.

Could you set both slides in a set position by taking connections off solenoid coils.
Maybe try in a few positions to see what happens.
This could mean scored cylinder or damaged seal, also leaking solenoid valve or even dedris or contaminents under ball position 207E in manual (this could also cause creeping of unloader piston).
You might want to check maximum amps & discharge pressure settings in Unisab as well.

Another thought could be a worn out spindle for capacity indication, especially if it hunt a lot at full load area when backing of on amps etc.

Had one recently where amp calibration was out as well as discharge pressure transducer out of whack also.

Tyco,
I've thought a bit more after reading your post a few times.

From what I understand on the last start Vi went to 100% when main slide was at 50%.This does'nt sound right does it?
I thought machine would load to 100% then the Vi slide would adjust with main slide to correct Vi as per Unisab.
When it unloads Vi slide stays in same position.
The Vi slide should also be slow to adjust (2 minutes from 0 to 100% from the manual).

Quote:

---

Originally Posted by josef

Tycho, Vi SAB 202 is ever really a headache.
To change the sealing ring? I'm not sure that many will end your problem, i can not, there are many variations, cylinder capacity, discharge port when the control line, how long the compressor sevice?

---

the compressor has 15.000 hours

What I meant was that the slide valve went to 100%, then moved back down to 50% along with the Vi slide just as you said, and yes, it takes a while to reach 100% on the Vi slide.


I was back on the site today, and one tip I got was to adjust the calibration of the cap slide in the unisab to show 90% when it was really in 100% so it would keep pushing on the Vi slide.

I tried this with no result.

Then I figured I would check that the cap slide was REALLY in 100% and that there were no mechanical hangups that came into play while the compressor was running.

So with the compressor stopped, I closed the valved to the cap slide solenoids and disconnected the oil supply pipe so I could attach a oil pump (yellow jacket hand pump), and pump the slide to a certain 100% with oil.
I do this on howdens with the LPI indicator because then you won't have to deal with the regulator unloading the compressor while you are running to push the button for 100% calibration :)

Back to the point, I started pumping and kept going til I got the indicator to showing 15-20%, by then I needed a break, so I stopped and the indicator promptly went back to 0%
To check that it wasnt my pump leaking, I did it again with a small ball valve fitted and closed it as I was pushing the last stroke on the pump, same result, it went back to 0% in a second.

My conclusion is that the cap seal is leaking, but I find it strange, since the back of the slide is open to the rotors, I would have thought it would increase capacity with a broken seal, also it seemed to hold the capacity when the compressor was running.

Any more thoughts before I dismantle the cap slide on friday will be appreciated

I had so much corrupted regulating cilinder capacity that had to change when manchette long bad, metal on metal.

Hi, Tycho :)

Quote:

---

Originally Posted by Tycho

What I meant was that the slide valve went to 100%, then moved back down to 50% along with the Vi slide just as you said, and yes, it takes a while to reach 100% on the Vi slide.


I was back on the site today, and one tip I got was to adjust the calibration of the cap slide in the unisab to show 90% when it was really in 100% so it would keep pushing on the Vi slide.

I tried this with no result.

Then I figured I would check that the cap slide was REALLY in 100% and that there were no mechanical hangups that came into play while the compressor was running.

So with the compressor stopped, I closed the valved to the cap slide solenoids and disconnected the oil supply pipe so I could attach a oil pump (yellow jacket hand pump), and pump the slide to a certain 100% with oil.
I do this on howdens with the LPI indicator because then you won't have to deal with the regulator unloading the compressor while you are running to push the button for 100% calibration :)

Back to the point, I started pumping and kept going til I got the indicator to showing 15-20%, by then I needed a break, so I stopped and the indicator promptly went back to 0%
To check that it wasnt my pump leaking, I did it again with a small ball valve fitted and closed it as I was pushing the last stroke on the pump, same result, it went back to 0% in a second.

My conclusion is that the cap seal is leaking, but I find it strange, since the back of the slide is open to the rotors, I would have thought it would increase capacity with a broken seal, also it seemed to hold the capacity when the compressor was running.

Any more thoughts before I dismantle the cap slide on friday will be appreciated

---

Our experience with SAB 202 capacity and Vi regulation is not the same as your because we have another type of transmitter (capacity rod type and not turning type) ... regulation is a little bit different, but generally first you must adjust zero capacity with zero screw (we do it with stopped compressor) ... then start compressor and increase capacity up to 100% manually (check your solenoids 70NO and 71NC) controlling ampere meter at 100% capacity solenoid 73 is open and oil pressure is pressing Vi slide pos 190 to capacity slide pos 200. solenoid 74 remains closed ... to get the proper Vi we need some time because both solenoids are going to open/close to get the right Vi but no changes in compressor capacity ... at lower capacity Vi slide move away from capacity slide now controlled via Unisab II and Vi slide position is controlled by pressure ratio ...

One more suggestion ... please check the strainer within solenoid block ... dismantle solenoids and check the seats .... and position of solenoids ... one is NO and another NC it is possible, if someone tried to clean solenoids and reinstall them in wrong places ... about exchanged solenoids I hope I'm wrong, but I saw too many unbelievable things and now I start to believe in everything;)

Regarding V-shape spring teflon seal ... we found that around 10000 working hours seal is usually worn and need to be replaced to protect cylinder and piston .... in case of big damage of that seal, when you start compressor capacity slide is moving to 100% within 20-30 seconds from start and oil return pipe to suction became very warm/hot due to discharge gasses passing (when you dismantle capacity cilynder cover there is no oil (only behind capacity slide) .... your compressor has 15000 working hours ... so ...

Another possibility ... this seal is V-shape turned inside (to not allow passing of discharge gasses) and need counter pressure of discharge gas to be tight with cylinder wall ... when your compressor is standstill there is no discharge gas counter pressure and spring force is very big seal is very soft and you have oil leaking into discharge chamber even if seal is not damaged ... you can see how soft is seal during replacement;)

with Howden and Stal is little bit different ... there we have cylindrical teflon seal reinforced with underneath O-ring because we use oil pressure on both sides of capacity cylinder therefor we can move the piston with oil pump ...

My warm suggestion is to check and replace V-seal ...

Hope this will help a little ... good luck

Best regards, Josip :)

Quote:

---

Originally Posted by Josip

Hi, Tycho :)



Our experience with SAB 202 capacity and Vi regulation is not the same as your because we have another type of transmitter (capacity rod type and not turning type) ... regulation is a little bit different, but generally first you must adjust zero capacity with zero screw (we do it with stopped compressor) ... then start compressor and increase capacity up to 100% manually (check your solenoids 70NO and 71NC) controlling ampere meter at 100% capacity solenoid 73 is open and oil pressure is pressing Vi slide pos 190 to capacity slide pos 200. solenoid 74 remains closed ... to get the proper Vi we need some time because both solenoids are going to open/close to get the right Vi but no changes in compressor capacity ... at lower capacity Vi slide move away from capacity slide now controlled via Unisab II and Vi slide position is controlled by pressure ratio ...

One more suggestion ... please check the strainer within solenoid block ... dismantle solenoids and check the seats .... and position of solenoids ... one is NO and another NC it is possible, if someone tried to clean solenoids and reinstall them in wrong places ... about exchanged solenoids I hope I'm wrong, but I saw too many unbelievable things and now I start to believe in everything;)

Regarding V-shape spring teflon seal ... we found that around 10000 working hours seal is usually worn and need to be replaced to protect cylinder and piston .... in case of big damage of that seal, when you start compressor capacity slide is moving to 100% within 20-30 seconds from start and oil return pipe to suction became very warm/hot due to discharge gasses passing (when you dismantle capacity cilynder cover there is no oil (only behind capacity slide) .... your compressor has 15000 working hours ... so ...

Another possibility ... this seal is V-shape turned inside (to not allow passing of discharge gasses) and need counter pressure of discharge gas to be tight with cylinder wall ... when your compressor is standstill there is no discharge gas counter pressure and spring force is very big seal is very soft and you have oil leaking into discharge chamber even if seal is not damaged ... you can see how soft is seal during replacement;)

with Howden and Stal is little bit different ... there we have cylindrical teflon seal reinforced with underneath O-ring because we use oil pressure on both sides of capacity cylinder therefor we can move the piston with oil pump ...

My warm suggestion is to check and replace V-seal ...

Hope this will help a little ... good luck

Best regards, Josip :)

---

good points, but howden xrv range also have the V seal (U sleeve) and on those it is no problem to use a hand oilpump to move the piston ;)

the teflon ring with the o-ring is on the wrv range, and they also have a seal around the slide valve rod, separating it from the discharge end of the rotors :)

*edit*

also I have checked the solenoids :)

the problem with the apparent loss in effect is when the compressor goes to 4.5 Vi other than that everything seems fine

Quote:

---

Originally Posted by Tycho

What I meant was that the slide valve went to 100%, then moved back down to 50% along with the Vi slide just as you said, and yes, it takes a while to reach 100% on the Vi slide.


I was back on the site today, and one tip I got was to adjust the calibration of the cap slide in the unisab to show 90% when it was really in 100% so it would keep pushing on the Vi slide.

I tried this with no result.

Then I figured I would check that the cap slide was REALLY in 100% and that there were no mechanical hangups that came into play while the compressor was running.

So with the compressor stopped, I closed the valved to the cap slide solenoids and disconnected the oil supply pipe so I could attach a oil pump (yellow jacket hand pump), and pump the slide to a certain 100% with oil.
I do this on howdens with the LPI indicator because then you won't have to deal with the regulator unloading the compressor while you are running to push the button for 100% calibration :)

Back to the point, I started pumping and kept going til I got the indicator to showing 15-20%, by then I needed a break, so I stopped and the indicator promptly went back to 0%
To check that it wasnt my pump leaking, I did it again with a small ball valve fitted and closed it as I was pushing the last stroke on the pump, same result, it went back to 0% in a second.

My conclusion is that the cap seal is leaking, but I find it strange, since the back of the slide is open to the rotors, I would have thought it would increase capacity with a broken seal, also it seemed to hold the capacity when the compressor was running.

Any more thoughts before I dismantle the cap slide on friday will be appreciated

---

With the 202 you cannot test like a Howden or any other machine.

The way these compressors unload when machine is off makes this test impossible.

The slide valve unloader push rod has a ball & seat which is allows oil to vent into discharge area from front of unloader piston when machine is turned off & pressures equalize.

During operation the ball is held on its seat by discharge pressure.

Without discharge pressure on ball, oil you pump in will basically vent straight into the discharge port (which I mentioned in a previous reply, BALL POSITION 207E).

The manual also suggest as you said, to set Unisab above & below 0 & 100% limits to calibrate both Vi & slide.

Tyco do you think you have all the instuctions you need, as not sure what information you do have?
If not maybe I or someone can email them to you.

Quote:

---

Originally Posted by RANGER1

With the 202 you cannot test like a Howden or any other machine.

The way these compressors unload when machine is off makes this test impossible.

The slide valve unloader push rod has a ball & seat which is allows oil to vent into discharge area from front of unloader piston when machine is turned off & pressures equalize.

During operation the ball is held on its seat by discharge pressure.

Without discharge pressure on ball, oil you pump in will basically vent straight into the discharge port (which I mentioned in a previous reply, BALL POSITION 207E).

The manual also suggest as you said, to set Unisab above & below 0 & 100% limits to calibrate both Vi & slide.

Tyco do you think you have all the instuctions you need, as not sure what information you do have?
If not maybe I or someone can email them to you.

---

Seems I missed the part about the unloader, THANK YOU! more to check :)

I thought it was the NO solenoid that de energized to decrease the slide.

I don't have any papers on the compressor other than a side view cutaway drawing, for the unisabII I have all the instructions

Allrighty!
First, a huge thanks to Josip for sending me the manual for the SAB202! (however I didn't check in here to see it before I went on site)


Ok, so I went back to the site today, repair kit for cap and Vi slide in hand. that way, I could at least open it up and look at it even if I might not need to overhaul it.

First thing, I pulled a hose up to the roof, 3 or 4 stories up and tied it down, connected it to the compressor discharge service valve and let the pressure out.

While I was relieving the pressure, I dismantled the cap indicator until I was down do the slide cover, had a smoke and did my preparations, oil absorbing pads under the cover, made ready a cut in half 5 liter container and pulled an oil drum over to act as my working table after I covered it with an oil absorbing pad and some rags. :)

had another smoke.

Removed two of the bolts holding the slide cover and put in two M12 threaded bars with extra large discs and nuts, to hold the cover in place (large discs to not chip more paint than necessary and make it look like a sloppy job (some use only a nut, but it chips the paint and scores the metal around the bolt holes and it looks like **** afterwards))

Pressure is down to 0.0 bar, but of course there will be fumes, so I don my mask, remove the last two bolts and start releasing the tension on the spring via the threaded bars.

Tension is of and I see the cap seal (U-sleave, C-ring, V-seal, or what have you, i will call it U-sleave), it looks fairly decent, no marks on it, but there are some tiny tiny metal shavings stuck in it all around as is not all that unusual, if I had not had a new one, I would have re-used it.

I pulled the complete slide valve out, which is nice to be able to do, on this, sabroe is better than howden.

Replaced the u-sleave, and accompanying o-ring, and the two "paper" strips on the end where the Vi slide interlocks with the cap slide.

Moved aside the ring holding the ball, mentioned earlier in the thread, in place and inspected the ball and the hole, seemed fine and dandy, but polished the hole with a 3m pad and cleaned it with brake cleaner then replaced the ball.

started to dismantle the Vi slide, but the ring holding the coupling cover in place made it impossible to remove the Vi cover no matter how you moved tilted it or tried to find a good angle (poor planning by Sabroe :)), and the only way to take it out would be to cut of a piece, of to take of the entire coupling on the compressor side.

So I didn't get the Vi cover off, due to time restraints and customer requests, I didn't take the Vi slide out.
Halfway consoling myself that the Cap slide seal was in good shape and that the Vi shouldn't be any worse.

So, I assembled the cap slide again and put everything in place, feeling down at heart because I didn't really find anything that was apparent out of order.

Vacuuming the compressor unit gave me time for some smokes and doing some office work on the computer before the site machinist arrived and we chatted for a while before we had 4 mBar.

started the compressor and used my permanent solenoids to calibrate the slides again.

Vi slide was off by 10%, showing 110% after having been left with the inc. solenoid on for 2 minutes after reaching 110%, corrected.

Cap slide was showing -17%, corrected and was spot on at 100%.

Switched to auto and let it run...

0.3 bar suction pressure throughout, regulated by hand on the suction valve

Vi slide starts moving from 0% to 100%, reaching 100% at the same time, the cap slide is moving.

They meet at Vi (read from the front indicator) of 2.5 and then move to Vi of 4.5 and the compressor is pulling 220 amps.

Again, I apply my dirty trick of the permanent solenoid on the inc valve of the cap slide and the amps rise and now I take it all the way to full travel of the slide, and it pulls 275 amps, the Vi slide shows 0%

So I am 5 amps below the limit at 100% slide and 0% Vi (Vi of 1.5), if I let it go down to 4.5 Vi it only pulls 210-220 amps.

I am a bit focused on the amps, since the site engineer told me that when the compressor runs like this (210-220 amps) they extend the freezing time in the freezing tunnel by 3-4 hours, and normally it would butt against the amp limit of 280 amps and work between 270-280 amps

So in my despair I go into settings and set Vi control to manual instead of auto, just to see what happens.

What happens is this, the cap slide increases to 100%, full travel (Vi of 1.5), and when we reach the setpoint, the cap slide decreases and the Vi slide follows. Noticed that both solenoids on the Vi slide are energized when it is set to manual, however, the larger surface area of the cap piston makes it push the Vi piston to 0%

A downside is that they don't get auto Vi, and during startup the Vi slide will go to 4.5, making it so that they have to be more careful during start up to not overload the compressor.


I'm completely at my wits end here...

I have THOROUGHLY read both the unisab II manual and the manual Josip sent me, and according to those, I have done everything right, but the unisab is somehow deciding that it should be running at 210-220 amps.

there have been no power outages, all the settings are still there for other things, so the unisab has not lost it's memory (don't remember if it has RAM memory that is reliant of the battery backup).

as I said, I am at my wits end...the unisab want's to be there, It didn't want to be there before, so what has changed...

Apart from normal is that the compressor has not been running for two months

"%!%%&!&%/%/(##!!!!!!!

Hi, Tycho :)

Quote:

---

Originally Posted by Tycho

Allrighty!
First, a huge thanks to Josip for sending me the manual for the SAB202! (however I didn't check in here to see it before I went on site)


Ok, so I went back to the site today, repair kit for cap and Vi slide in hand. that way, I could at least open it up and look at it even if I might not need to overhaul it.

First thing, I pulled a hose up to the roof, 3 or 4 stories up and tied it down, connected it to the compressor discharge service valve and let the pressure out.

While I was relieving the pressure, I dismantled the cap indicator until I was down do the slide cover, had a smoke and did my preparations, oil absorbing pads under the cover, made ready a cut in half 5 liter container and pulled an oil drum over to act as my working table after I covered it with an oil absorbing pad and some rags. :)

had another smoke.

Removed two of the bolts holding the slide cover and put in two M12 threaded bars with extra large discs and nuts, to hold the cover in place (large discs to not chip more paint than necessary and make it look like a sloppy job (some use only a nut, but it chips the paint and scores the metal around the bolt holes and it looks like **** afterwards))

Pressure is down to 0.0 bar, but of course there will be fumes, so I don my mask, remove the last two bolts and start releasing the tension on the spring via the threaded bars.

Tension is of and I see the cap seal (U-sleave, C-ring, V-seal, or what have you, i will call it U-sleave), it looks fairly decent, no marks on it, but there are some tiny tiny metal shavings stuck in it all around as is not all that unusual, if I had not had a new one, I would have re-used it.

I pulled the complete slide valve out, which is nice to be able to do, on this, sabroe is better than howden.

Replaced the u-sleave, and accompanying o-ring, and the two "paper" strips on the end where the Vi slide interlocks with the cap slide.

Moved aside the ring holding the ball, mentioned earlier in the thread, in place and inspected the ball and the hole, seemed fine and dandy, but polished the hole with a 3m pad and cleaned it with brake cleaner then replaced the ball.

started to dismantle the Vi slide, but the ring holding the coupling cover in place made it impossible to remove the Vi cover no matter how you moved tilted it or tried to find a good angle (poor planning by Sabroe :)), and the only way to take it out would be to cut of a piece, of to take of the entire coupling on the compressor side.

So I didn't get the Vi cover off, due to time restraints and customer requests, I didn't take the Vi slide out.
Halfway consoling myself that the Cap slide seal was in good shape and that the Vi shouldn't be any worse.

So, I assembled the cap slide again and put everything in place, feeling down at heart because I didn't really find anything that was apparent out of order.

Vacuuming the compressor unit gave me time for some smokes and doing some office work on the computer before the site machinist arrived and we chatted for a while before we had 4 mBar.

started the compressor and used my permanent solenoids to calibrate the slides again.

Vi slide was off by 10%, showing 110% after having been left with the inc. solenoid on for 2 minutes after reaching 110%, corrected.

Cap slide was showing -17%, corrected and was spot on at 100%.

Switched to auto and let it run...

0.3 bar suction pressure throughout, regulated by hand on the suction valve

Vi slide starts moving from 0% to 100%, reaching 100% at the same time, the cap slide is moving.

They meet at Vi (read from the front indicator) of 2.5 and then move to Vi of 4.5 and the compressor is pulling 220 amps.

Again, I apply my dirty trick of the permanent solenoid on the inc valve of the cap slide and the amps rise and now I take it all the way to full travel of the slide, and it pulls 275 amps, the Vi slide shows 0%

So I am 5 amps below the limit at 100% slide and 0% Vi (Vi of 1.5), if I let it go down to 4.5 Vi it only pulls 210-220 amps.

I am a bit focused on the amps, since the site engineer told me that when the compressor runs like this (210-220 amps) they extend the freezing time in the freezing tunnel by 3-4 hours, and normally it would butt against the amp limit of 280 amps and work between 270-280 amps

So in my despair I go into settings and set Vi control to manual instead of auto, just to see what happens.

What happens is this, the cap slide increases to 100%, full travel (Vi of 1.5), and when we reach the setpoint, the cap slide decreases and the Vi slide follows. Noticed that both solenoids on the Vi slide are energized when it is set to manual, however, the larger surface area of the cap piston makes it push the Vi piston to 0%

A downside is that they don't get auto Vi, and during startup the Vi slide will go to 4.5, making it so that they have to be more careful during start up to not overload the compressor.


I'm completely at my wits end here...

I have THOROUGHLY read both the unisab II manual and the manual Josip sent me, and according to those, I have done everything right, but the unisab is somehow deciding that it should be running at 210-220 amps.

there have been no power outages, all the settings are still there for other things, so the unisab has not lost it's memory (don't remember if it has RAM memory that is reliant of the battery backup).

as I said, I am at my wits end...the unisab want's to be there, It didn't want to be there before, so what has changed...

Apart from normal is that the compressor has not been running for two months

"%!%%&!&%/%/(##!!!!!!!

---

sorry, you put a lot of effort and result is not good ... I have received manual from RANGER_1 asking me to forward it to you, because he was absent and you were short in time;) ...

regarding Vi cover, to take it out, to replace V-seal we need to remove coupling flange from compressor and the plate for coupling protection, but in general this compressor has a good design .... also we need to disconnect small pipes for Vi hydraulic control, then remove the cap pos. 24 to be able to remove cylinder screw pos. 192 used like stop screw for Vi slide ... I'll suggest you to take it out for throughout check and replace V-seal ...

as per your post, seems you done everything correct, but there is always easy to make some mistake, pls don't take it wrong .. . I was there and sometimes we have to repeat complete procedure 2 or 3 times and still today we do not know why what we did wrong ...

first I'll suggest you to check all configuration settings within Unisab, if someone tampered some settings then a lot of things can be wrong ... when you are sure that both mechanical and software part are correct then try again to set-up Cap slide and Vi slide ... please read your Engineering manual pages 37-39.

One very important thing is ... when you open compressor and have no pressure within, please, check calibration of pressure transmitters ... this can be done wrong and can cause wrong calculation of Vi...


Good luck ...

Best regards, Josip :)

Tyco,
Its a difficult one by the sounds of it.
Can you describe what the capacity control solenoids are doing ie enegised/de-energised etc when system is in auto.
It describes the Vi one pemanently energized at 100% capacity to keep slides together.
Is this the case etc.
You also describe the Vi slide moving before capacity slide reaches 100%.
I bought this up before, but thought I'd mention again.
This is also a single stage blast freezer plant, is there a pulldown, or is it always around 0 to -3 bar suction.
Would there be any reason for the Vi to be anything but 4.5 to 5.0 Vi?

As Josip mentions tranducer calibration, Unisab etc




Attachment 9395Attachment 9396

Which control valve you have manual valve or EVRB block, hand (old model) many unreliable, but I think that as you type rotor grinding and regulating valve does not work, bad discharge port.

Hi Guys

Based on the operating conditions you listed,w ouldn't you expect to see higher amperages at 100% cap slide and 0% vi due to under compressing the gas at this high compression ratio? When I calibrate capacity slides and vi slides on low temp compressors, I have had to force the suction higher and the discharge lower in order to keep the compressor motor from tripping on high amperage, because calibrations need to be done at lowest vi position. When the vi increases after the calibration is complete, the amperage falls.

C Ice

final tally...

As I said earlier, I set the Vi control to manual and it was working fine, however, a few weeks back when they started production, and had problems again with the capacity of the compressor. *sigh*
With dark clouds over my head I went back.

What I haven't mentioned before is that it is running in parallel with a Howden wrv 255 (never any problem with that btw :)) and that compressor was also running this time.

during me working there we didnt start it because they had no production and they didn't want any power spikes (if they have a spike in consumption, they have to pay the average of that for the whole month)

anyways, I came, looked at it, set the Vi back to auto, and the compressor started working like a charm.

During my working and testing we had ran the SAB alone, but we pulled down the LP side slowly to allow it to work around the setpoint.

so in conclusion, even though we were just pulling down the LP drum (5 metric tons on nh3 I think)), and after having pulled it down, it was still to much for the compressor to handle alone and the unisab did it's wicked magic and probably it was regulating like it should under those conditions, even though for me looking at the running conditions and reading the manual it should have acted otherwise. *sigh*

This is why I like working on systems of our own design, because then I know the innermost secrets of the PLC :D

I would like to than everyone for their input here, I have learned new things, so it was not all bad :)

Some advice if I may A very good starting point when working on a UNISAB controlled compressor. Spend 20 minutes and record all of the setpoints manually on the prescribed form. Often there are many different fiddling fingers in a plant and often they have no idea what they are doing. I think in this particular instance that possibly the fiddling fingers have been at it and then when you were away they corrected the problem. Sometimes its the over zealous plant engineer trying to save some Krone. And as you said energy is a problem here. The other problem could quite well be the capacity slide and/or VI slide position indicator. They are both 4-20mA and often on the older UNISAB Boards the return signal is corrupted possibly by a loose joint. This causes the 4-20 to fluctuate. But usually it is so quick that you cannot see it but it effects the calculation on the algorithim causing the slides to do some strange things. I don't think your problem is cured yet and you might have a few more hours of work there. Be strong!

Quote:

---

Originally Posted by piewie

Some advice if I may A very good starting point when working on a UNISAB controlled compressor. Spend 20 minutes and record all of the setpoints manually on the prescribed form. Often there are many different fiddling fingers in a plant and often they have no idea what they are doing. I think in this particular instance that possibly the fiddling fingers have been at it and then when you were away they corrected the problem. Sometimes its the over zealous plant engineer trying to save some Krone. And as you said energy is a problem here. The other problem could quite well be the capacity slide and/or VI slide position indicator. They are both 4-20mA and often on the older UNISAB Boards the return signal is corrupted possibly by a loose joint. This causes the 4-20 to fluctuate. But usually it is so quick that you cannot see it but it effects the calculation on the algorithim causing the slides to do some strange things. I don't think your problem is cured yet and you might have a few more hours of work there. Be strong!

---

I know all about the fiddling fingers :)
I once was on a ship, and was actually waiting for them when they pulled in, so I jumped on board when they got the first mooring on and went down to the compressor room, where I found a solenoid with an extension cord to a wall socket on the increase capacity valve of a compressor :)
about a minute after I walked in, the ship's engineer walked in and said "Oh ****, you weren't supposed to see that" :)

Anyway, this compressor has been running like an absolute beauty since September last year (Knock on wood).

I like the Unisab in it's apparent simplicity, and how it "just works", but some times there are so many variables in play that it can make you go cross eyed trying to figure out what is causing what, and there is no feedback whatsoever as to what is causing the regulator to do this or that, and it may make you think something is mechanical wrong, when in fact the regulator has noticed some discrepancy that needs to be corrected when it's not even noticeable to the guy who is watching the display, and he is left there scratching his head.

That is why, on our in house PLC program, that is probably nowhere near the unisab in sophistication, but does the job just as well. I have demanded, and gotten my will through, that there should be a status message on screen for 15 seconds for every action the PLC makes.

Take for example om a screw compressor system, everybody knows that when the slidevalve get's the final pulse and closes, you will experience a spike in amps at some pressures.

So on our system, the slide valve closes, the motor exceeds max amps, so the plc sends a signal to the decrease solenoid, and on our display you will get the message "Motor current to high, capacity decreased"
This message will stay there for 15 seconds after the fact.

on a Unisab II wich I am most familiar with, it would have shown a down arrow, and you would have been non the wiser as to why :)

I really like the unisab, and have sold my share of conversions from the old danfoss controller with the temperature transmitter and dials for setpoint and deadband (don't remember what they were called), because it was an easy conversion.

I like the new Unisab III, but again, it has to many submenus and to much going on, and while it has a lot of information for a service engineer, trying to get a site engineer to go through the menus while on the phone is just too much to intuitively use for them... I can't count the hours I have spent on the phone with operators, just to make them tell me what was the alarm that made the system stop and get the answer that they have an angry face on their display.

That is why we have made our systems wireless, we have a coded box on all of our systems, since the beginning of this year, that connects to a box at our main office, where we can log into each system and watch the system running along with the operator and see what is happening, instead of asking them "can you start the system and write down this and that and tell me/email me"

so be it a shore system or a fishing vessel, as long as you have an internet connection, we can monitor your system in realtime.

of course they have the option to disconnect it.

We wouldn't want them to believe that at any moment the system would shut down, and a messsage on the screen says "You have 1 unpaid bill! to make your system operational again please pay now" :D