Mycom recip takes a dive

[danny_s_pike]

Hello everyone, this is my first post. I have a doozy of a problem that I can't figure out. I'm hoping someone here might offer me a little insight.

About 3 years ago, we installed a Mycom recip (P6WBHE), running single stage. This compressor replaced an old Sabroe recip, which ran fine for years, but suffered from wear and tear and old age. So, the Mycom was installed and ran fine up until last Spring. I was freezing product in a blast freezer when the compressor and attached motor (Belt driven) took a dive in RPM. The first time it did this ever. After the dive, it came back up to full RPM. After this happened, the compressor would take a dive more frequently, especially when it had the most load on it, like starting a new freeze period or just after defrost.

After this, the compressor started to make some top end noise, so we stripped it down and found that all 6 pistons were worn around the wrist pins. The holes in the pistons were enlongated. We installed new pistons, rings, wrist pins, inspected the crank and bearings for wear, checked unloader plates, etc. We started the compressor up and the diving stopped. We also upgraded the motor to 100HP, as the 75HP motor was in fact a little too weak to run the compressor, according to spec.

I have now ran the compressor and froze 5 loads of product. Last night, it took a dive again. I couldn't believe it! I inspected the oil filter in the base and the cuno filter as well. It seems that the new oil has now turned black again.

When the compressor takes a dive, I've observed the following values:
suction pressure=10 PSI (Normal for us)
discharge pressure=205 PSI
using R404a *****
oil pressure=60PSI
oil temp=101F

Oil separator is working fine, with no loss of oil. We have a big accumulator before the suction, and there is no evidence of liquid slugging at the suction. The values I mentioned do not change in any way while the compressor is bogging down. Any help is appreciated. Thanks.

[The Viking]

From your description my first guess is that there is liquid refrigerant entering the compressor.

Worth checking the super heat, solenoid valve (if fitted) and all lines/parts that could allow liquid in to the suction line.

When the compressor were changed did you also change the solenoid and expansion valves? Was the system fully recommissioned?

:cool:

[danny_s_pike]

Hi thanks for your reply. My first thoughts were that liquid could be entering the suction of the compressor, but the unloader plates are in excellent shape. The suction pressure is only at 10PSI and there's a big accumulator that would certainly hold back any liquid. The oil separator on the discharge side is working correctly. There is no frost on the line between the oil separator and the compressor. The crankcase is warm to the touch while running, leading me to believe that there is no liquid refrigerant in the base.

There is an oil cooler attached to the compressor that uses refrigerant for cooling. The return line from the cooler goes into the suction, but that is always warm to the touch. That line is a bare copper pipe, so it's easy to tell that no liquid is entering into that area.

All of the tx and solenoid valves are from the old system. They were working fine and I'm still using the same refrigerant as the old system used.

The premature wear of the pistons is telling me that the problem must be in the heads of the compressor, yet there isn't any liquid slugging at the compressor suction. It seems like the oil is not reaching the heads maybe?

[Brian_UK]

Were there any signs of the pistons dragging on the bores?

Are the oilways through the crankshaft and any feeding the small end bearings free of sludge or blockages of any sort?

[danny_s_pike]

I didn't see any marks, the cylinder sleeves looked very good. The rings didn't look bad either, only the holes in the pistons where the wrist pins fit through were damaged. Perhaps the oilways through the crankshaft may be blocked or partially blocked by deposits... I will also check the oil feed to the bearings. I will certainly give an update with my findings... Thank you for your suggestions!

[monkey spanners]

Wrist pin wear can be a sign of worn valve plates, basically there is always pressure pushing the pistons down so it can eventually push the oil out on the top of the wrist pin and lead to wear.

Whats the discharge superheat like? Paint discoloured on the heads?

Is it possible it is heat seizing?

(I mostly work on smaller stuff so take that into account when reading my ideas!)

[Magoo]

What viscosity grade of oil are you using.

[NH3LVR]

What kind of starter does this have on it?
Sounds like the HZ could be falling off and giving the appearance of bogging down, or something along those lines.
If you are taking liquid you should know that right away.

[Magoo]

Piston small end bearing wear would indicate lack of lubrication, those bearing surfaces are at the end of the lubrication circuit. The small end bearing in the conrod gets pressure lubed, the pin in the piston is generally stable and is plash fed lubricated. Your sump oil temp is too low, should be average +10' C above saturated discharge temp., the viscosity rating of oil should be/ I think 68. I believe the damage is happening during pull-down period, higher suction pressures, low SDT and low sump oil temps., will circulate liquid refrigerant in oil circuit without any obvious foaming etc.. The wear / oval would make compressor extremely noisy.

[NH3LVR]

I made an error.
I failed to read your post thoroughly and did not notice the part about the oil turning black.
My apologies.
I would strongly suggest you check the discharge temperature when this is happening.
If a relief valve between suction and discharge is leaking under load it would cause high temps.
I would think it would raise the suction pressure as well though.
Keep us informed.

[RANGER1]

Maybe only Mycom could answer this one but they would require more info as well. Oil type, all temps, speed of compressor at flywheel, Quality of oil by oil analysis. Discharge temp should be about 60deg C. Also what type of oil cooling water or refrigerant (guessing refrigerant cooled. Maximum oil temp 50deg C.

[danny_s_pike]

Wow, thank you all for the suggestions, there have been some excellent points made here. NH3LVR, the black oil mentioned is definitely coming from the wearing pistons. Magoo, you're right, the compressor uses RL68H oil. The oil is cooled by way of refrigerant. I know that the oil temp can't be too low. I have a base heater that sits at around 75F while the compressor is idle. I have a thermostat that controls a solenoid valve for the refrigerant feed to the oil cooler, that one is set to open at 80F. I have another thermostat installed in the safety interlock that will shut the compressor down if oil temps rise over 115F. My guess is definitely a lack of lubrication to the ends of the connecting rods. All 6 of those are wearing down equally, telling me that the restriction must be happening before oil gets distributed to any of the connecting rods. I've attached pics of the new pistons, showing their wear after about 20 hours of use.1052710528

[PaulZ]

Hi Danny
The temp setting for your oil is too low, I would have the heater set to about 35oC / 95oF. 115oF / 46oC shut down is way to low. I would have the hi oil temp set at about 65oC / 149oF.
The wear you are getting is caused by lack of oil to the wrist pins, I would check the oil galleries in the crankshaft and in the conrods. With the oil that cold it would be fairly thick and this could also be contributing to the lack of lubrication to the top of the conrod.
The oil pressure of 60 psi / 413 kpa is a bit high, should be no more then 350 kpa / 50 psi.
Hope this helps
Paul

[passandscore]

Paul you recommendations are harmful!

Mycom oil temps for WA,WB & WJ = min @ 85f and max @ 120f. Any higher and you are overheating the oil which leads to even more problems.

Normal net oil pressure for WA,WB & WJ is between 20-43psi. Excessive oil pressure causes malfunction to the unloader system.

This information is from Mycom. Here is the link. http://www.mycomcanada.com/pdf/TechForum.pdf

[passandscore]

You state that the compressor (took a dive) motor RPM dropped. If the RPM drops the oil pump RPM drops and oil lubrication suffers. Minimum compressor speed is 800 RPM for a WB (max is 1200 RPM).

http://www.mycomcanada.com/pdf/TechForum.pdf

[danny_s_pike]

UPDATE: Those pics, plus more, were sent to Mycom in New Mexico for analysis. They tell me that they have never seen anything like this before. They have requested two conrods, which I sent away to Ontario for analysis. At present, they said if they can source or have the same model in inventory, they will be sending me one right away and taking this one back. I thank everyone who helped me troubleshoot this. It isn't the answer to the problem, but at least it's a resolution.

[Magoo]

OK, dumb question. Does compressor have water cooled body and heads?

[PaulZ]

Hi passandscore
You say my recommendations are harmful, how so.
I said the oil pressure of 60 psi is high and it should be no higher then 50 psi / 350kpa, I didn't say set it at 50 psi, sometimes you have to run a slightly higher oil pressure then the max in the book. If you read my post re oil temps I said have the oil heater set to 35oC / 95oF and I said a cutout of 65oC my Mycom book says an operating range of 30 - 55oC the cutout is 10oC higher.
Regards
Paul

[RANGER1]

Wow, thank you all for the suggestions, there have been some excellent points made here. NH3LVR, the black oil mentioned is definitely coming from the wearing pistons. Magoo, you're right, the compressor uses RL68H oil. The oil is cooled by way of refrigerant. I know that the oil temp can't be too low. I have a base heater that sits at around 75F while the compressor is idle. I have a thermostat that controls a solenoid valve for the refrigerant feed to the oil cooler, that one is set to open at 80F. I have another thermostat installed in the safety interlock that will shut the compressor down if oil temps rise over 115F. My guess is definitely a lack of lubrication to the ends of the connecting rods. All 6 of those are wearing down equally, telling me that the restriction must be happening before oil gets distributed to any of the connecting rods. I've attached pics of the new pistons, showing their wear after about 20 hours of use.1052710528
Danny, from this post you have refrigerant cooled oil cooler on 80F (27C) which is way to low, I would set this at 115F (46C) ON & 104F (40c) OFF. High oil temp cut out 140F (60C) as maximum running oil temp is 122F (50C). Oil pressure 200-400 kpa OK so set to 250 kpa (36psi) oil trip 150kpa (22psi).ith oil temp I would even try to run with no refrigerant cooled oil cooler & see where it stabilizes (keep all high oil temp protection in place of course). I have seen R22 plant with this type of set up & it never turns on, as is cool enough (stabilizes at 40-45C). I would also suggest checking discharge temps as should be below 70C (158F) under normal operating conditions.
Only suggesting these thing for possible new compressor Mycom supplies. Have you noticed any abnormal oil foaming in crankcase? Suction superheat also critical to keep dry gas entering compressor. Oil return critical as well if it contains any liquid refrigerant in it.

[danny_s_pike]

UPDATE AND ANSWER! Mycom has examined the parts and finally, after looking through their own records, determined that they installed the wrong valves at the factory. Clearances were off and the heads were heating up. I've been dying to know what went wrong and here it is. RANGER1, I will definitely keep your suggestions and I will follow Mycom's settings to a tee. Thanks guys!

[danny_s_pike]

Oh, Magoo, the compressor doesn't have water jackets. The heads are air cooled and the oil cooler is refrigerant cooled.

[Brian_UK]

Thanks for the update and let's hope that the problem is finally resolved.

[RANGER1]

UPDATE AND ANSWER! Mycom has examined the parts and finally, after looking through their own records, determined that they installed the wrong valves at the factory. Clearances were off and the heads were heating up. I've been dying to know what went wrong and here it is. RANGER1, I will definitely keep your suggestions and I will follow Mycom's settings to a tee. Thanks guys!

Very interesting findings, great news. Unusual to have a machine that big on R404a. In Australia I have not seen the newer series WBH as we tend to use screws compressors for almost everything. Mycom Canada seem to have been very efficient in resolving problem & actually admitting fault.