Pump down control problem

[frank]

I'm in the process of servicing / re-building 3 Carrier air cooled condensing units with Carlyle 06E 5 99, 6 cylinder recip (no unloading) compressors. http://www.carlylecompressor.com/corp/details/0,2938,CLI1_DIV24_ETI9588,00.html#112158h

One of them is failing to hold pump down despite having new valve plates / gaskets fitted. No problems from the other 2. The machine is working well and sounds sweet but even with the suction stop valve shut I'm getting equalisation taking place and the machine cycles on LP.

Any ideas as to why?

[US Iceman]

Frank, does the compressor have an internal relief valve or device?

How fast does the LP recycle?

[Brian_UK]

Is the suction stop valve faulty ? bad seat or valve stem perhaps.

New valve plate not seated correctly or also faulty?

Head gasket damaged.

[slingblade]

Pump it over with the liquid line valve closed, leave the suction valve open. once pumped over close the discharge service valve. if the pressure does not rise on the suction very much (as i suspect it wont) then your valve plate/s are in trouble.

[Argus]

Frank, Carrier 06Es had a problem with breaking cylinder gaskets ? or they did when I was on the tools that gave the problem you describe.

I think that it was a problem with small slugs of liquid collecting under the dome on the off cycle. The result was poor performance on the affected pair of cylinders and rapid pressure equalisation when it was turned off.

Valve it all off and take off the head casting of the affected cylinder. The valves are arranged in rows and the head casting consists of two chambers side by side, suction and discharge ? there is a wavy shaped web in the dome plate that separates the two parts. This has a gasket, usually metal if I recall, that contacts the dome plate with the valve plate below. It will sometimes break in the centre and the cylinders unload at this point permanently.

Hope this helps. It wasn?t an epidemic fault, but it used to crop up from time to time and prompted me to look for the possibility of liquid being able to collect or to condense at this point on the off-cycle.

It important to get the torque settings right on these heads.

Hope it helps.
________
TT600 (http://www.cyclechaos.com/wiki/Yamaha_TT600)

[NoNickName]

solenoid liquid valve leaking, been there seen that. I'm with Brian

[monkey spanners]

Has it got an oil separator? Have seen leaky float once causing comp to cycle on LP.

[frank]

Monkey Spanners

No oil separator.

Nonick

With the sol valve closed and the suction stop valve closed - no increase in pressure in the suction line so it appears that the sol valve is holding.

Sling

Done that. The problem is definately on the high to low side.

Brian

The suction stop valve is OK as I've been able to isolate the compressor and remove the heads. The valve plates are all brand new as are the gaskets (valve plate & head)

Iceman

I'm not sure if there is an internal relief valve - I'm going to contact carrier (as it's their condensing unit)

Argus

Spot on with the description.

I've considered that the gasket was faulty or blown but this wasn't the case. I've had all 3 heads off 3 times now and replaced all gaskets but the problem persists.

Since rebuilding the compressor I've tested pump down during a run cycle (15/20 minutes) to see if it holds. Even with the correct torque settings, new gaskets, new valve plates, stop valve shut, it will pump down but then start to equalise and cycle on LP.

Even when I've had the comp isolated with the valves shut and evacuated, when opening the discharge valve to break the vacuum the suction starts to increase.

It appears that these machines have suffered damage from liquid starts in the past, no sump heater on one, no pump down control on the other and this one.

Thanks for the replies :)

[NoNickName]

With the sol valve closed and the suction stop valve closed - no increase in pressure in the suction line so it appears that the sol valve is holding.




Is it holding vacuum? It is, as you say no increase in pressure. Then, what is the problem?

[frank]

Is it holding vacuum? It is, as you say no increase in pressure. Then, what is the problem?

The problem is that the compressor is not holding pump down. The high side is leaking to the low side. The sol valve is holding so the increase in pressure is not coming via the low side i.e. through the sol valve.

[US Iceman]

Frank,

This sounds like one of those jobs that makes you go, hummm.:o



Even when I've had the comp isolated with the valves shut and evacuated, when opening the discharge valve to break the vacuum the suction starts to increase.


If the equalization starts immediately when the discharge valve is cracked open, the problem has to be related to discharge gas leaking, right? So, what could be the potential leak paths?

Leaking discharge valves (not likely if new valve plates, but maybe possible) Don't those valves have roll pins holding them in place and seat in the compressor deck?

internal relief valve that re-seated, but still leaking slightly.

Or, a warped deck on the compressor. Can you check this with a machinst's rule for flatness? If the deck or head cover is warped, the gasket could still leak, even when torqued to the prescribed values.

Or, if the compressor has had liquid slugging damage, something in the head cover casting could be cracked.

How much fun can a guy have on one system Frank?:rolleyes:

[chillin out]

Fit a one way valve on the discharge.... job done.

How badly is it leaking back?

What controll system does it use, is it for a/c?

Are you sure it's meant to 'pump down'?

Some of these packs are designed to always run at least 1 comp.

Chillin:) :)

[US Iceman]

Frank,

I looked at the link you posted for the compressor.

In the service guide book (020-611):

On page 58 the exploded view shows item 22 as the internal relief valve (but it's not visible in the picture)

Page 76 provides a descripition of the internal relief valves on 06E compressors.




Pressure Relief Valve

All 06E compressors and all 06CC 50-99cfm compressors are equipped with a pressure relief valve that relieves from discharge pressure to crankcase pressure. It is located under the center cylinder head on all 6 cylinder 06E and 06CC compressors, and under the discharge service valve on all 4 cylinder 06E compressors. The relief valve is sealed with a metal gasket:

Part Number Description

EB51FN272 400psi Relief Valve (differential pressure)
6G65-1251 Relief Valve, Gasket
An 11/16" drag link socket can be useful for removing/installing the relief valve.


I have seen this occur on other recip compressors. If the valve relieves it may reseat improperly and does the same thing you are describing.

I have also seen the valves stick open and boy Oh boy does the compressor get hot in a hurry!

[slingblade]

Sounds viable to me, but internal relief valves and bursting discs dont just pop for the fun of it.
You could be looking at blocked condesers and linked out H.P. switches to boot.
Best of British to you Frank.

[US Iceman]

... but internal relief valves and bursting discs dont just pop for the fun of it.


Absolutely right. The relief valve may try to reseat (sometimes they will, most of the time they don't)

A rupture disc is different though. Once it ruptures, it's all over.

400 psig is pretty darn high, but I've seen this on air-cooled condensers all too often. As mentioned previously, when the high-pressure safety switch keeps tripping someone might bypass the switch.

This problem also occurs when someone closes the discharge service valve first, while the compressor is running.

People do the strangest things...:rolleyes:

[Below 0]

Hi
I m with the iceman on this, check the internal relief valve.After all its just a spring that may have lost enough of its tension to cause you sleepless nights.I worked on carrier containers for ten years and had a variety of relief valves and rupture discs that leaked when they should not have.It seems you have tried everything else what have you got to lose. At the very least you can then eliminate it.

Cheers

[frank]

Thanks guys. Sorry for the late response but the wife won't let me on the 'puter for too long till my poorly eye is healed up :(

Chillin

The Carrier unit is an air cooled condensing unit that has not worked correctly for some time and the client has asked me to have a look see if I can fix it.
It is 1 of 3 on A/C for a bingo club and you could see where liquid slugging had damaged the discharge reeds, hence the new valve plates.

These units don't come with any suction accumulator and this one didn't even have a sump heater installed, so without pumpdown holding it appears to have been collecting liquid in the crankcase.

The BMS system controls the unit via the sol valve so when it isn't being called it simply equalizes and bangs in and out on LP switch, about every 3 minutes.

Iceman

Thanks for the heads up on the internal valve, I'll have a look next week - if the rain stays away :)

[US Iceman]

My pleasure Frank. Take care of yourself and heal quickly.