Blistering Hot Gas Solenoid Diaphragm Issue

[dewars_aus]

G'day,

I've got a problem with hot gas defrost valves blistering the diaphragms that I'm hoping someone may have some thoughts on.

We have machines state wide which are suffering the same problem, and machines Australia wide looked after by other contractors which are suffering a similiar fate.

The unit in question is a frozen beverage dispenser, which uses factory fitted Parker hot gas defrost valves for a ~8 minute defrost every 3 or 4 hours (depending on programming preference). After an average of 8-9 months of operation, the diaphragms blister to the point that they no longer seal off when the solenoid closes - which causes serious capacity problems.

The Parker diaphragms are rated at around 110°C so we tried *ahem* "modifying" Danfoss diaphragms from an EVR6 solenoid (rated 130°C) to fit. The first of these diaphragms failed recently, after just 2 months of operation.

Using temperature sensitive strips, I have proved that the hottest the discharge pipe gets is between 99°C-104°C. During a defrost period, the temperature is below 94°C at both the discharge and the hot gas valve itself (obviously). This suggests that the problem is not neccasarily temperature related.

The unit is on R404A and polyester oil. The distance between compressor discharge and hot gas valve is less than 1000mm and uninsulated.

I've emailed both Parker and Danfoss but havent got a response just yet, although from previous phone conversations I dont think we'll get anything useful.

We are really looking for a better diaphragm material as the company who manufacturers the machine has been fairly unhelpful in this matter and uses any excuse to deny a warranty payment. If we try changing valve bodies or modify the pipework too much, we'll void warranty on the machines we work on. Since some components have a 5 year warranty, we cant go voiding it after the 8 months of operation it takes for the defrost valve to fail. We are trying to help our customers out by replacing the (inadequate?) diaphragms with a better quality item, but unfortunately need to do so without changing to a direct acting valve.

I am not entirely sure why these valves would be blistering, although we are thinking prehaps the design of the valve and the blast they get from the close proximity to the discharge pipe is the problem. Since the valve allows the gas to shoot vertically up into the face of the diaphragm, could hot oil vapour be carried through at a temperature higher than discharge gas temperatures and "blast" the face of the diaphragm? Could the coating of the material be comprimised by a combination of heat and pressure? Is the sealant used not compatible with R404A (although the packaging / sales pitch says it is)?

I've attached a few pictures of the diaphragms. You can see that the blisters line up with the holes in the brass body.

Thanks in advance for any replies,

Regards,
Andrew

[US Iceman]

Hi Andrew,


Since the valve allows the gas to shoot vertically up into the face of the diaphragm, could hot oil vapour be carried through at a temperature higher than discharge gas temperatures and "blast" the face of the diaphragm?

The problem could also be due to high velocity impingement on the diaphragms causing it to delaminate (layers of the diaphragm separating). The diaphragm looks like it is a composite material, similar to carbon fiber.

Have you asked the equipment manufacturer to verify the valve sizing? All of those little holes in the valve body appear to be in the same vicinity of the "blisters".

My first suggestion is to work on the dispenser manufacturer since they built it. The valve may be simply misapplied for the system.

[Brian_UK]

May be a stupid question but what is the purpose of the plate with the holes in?

If the plate was removed would it reduce the jet velocity effect on the diaphram?

[Dan]

May be a stupid question but what is the purpose of the plate with the holes in?

If the plate was removed would it reduce the jet velocity effect on the diaphram?

I had the same question, Brian, although I didn't realize it was a plate. I have never seen solenoids that distribute the gas in such a manner. If it's a plate, I'd yank it and see what happens.

[dewars_aus]

G'day again...


Thank for the replies. The blisters do indeed line up with the holes in the plate. I am unsure of the purpose of this plate, but an older model machine I recently checked which utilises the same valve also has this plate, yet basically no problems with blistering. (Ive seen *1* in 8 years of doing thes machines, on a machine over 10 years old) The distance from compressor to valve is similiar, although the gas is different. In this case, the case *was* R502, which has now been changed to R408A.

R408A does not contain R134a (unlike R404A) so its tempting to follow up the idea of a reaction causing it to delaminate. Parker has responded asking for MSDS information for R404A to verify its compatibility, so thats the next office job for me today :) No word from Danfoss yet :S

The fact the same valve used on the same machine (with similiar operating pressures and temperatures, although different gas) has basically no problems makes me think its not a temperature thing, although plotting a ethalpy diagram shows gas temperature would be more like 130oC discharge.... Still not sure how the gas would be at 130oC with a pipe surface temp of only 90oC considering copper and oil is a fantastic temperature conductor :) After 8 minutes, regardless of velocity, I would have thought pipe temp would equal gas temp....

I'll see what Parker has to say about the MSDS and possible incompatibility - that would solve the problem quite nicely and we can approach the manufacturer with a known issue as opposed to a series of ideas they just poo-poo and use as an excuse to deny warranty claims...

[US Iceman]

The fact the same valve used on the same machine (with similar operating pressures and temperatures, although different gas) has basically no problems makes me think its not a temperature thing...


Hi Andrew,

As you say, it appears everything may be similar. But, I think you suspect something must be different to be causing this problem.

Have you calculated the mass and volume flow through the valve yet? I don't know what the difference in mass flow is between R-502 and R-404a since we don't have information on the compressor.

If the mass flow is higher on the R-404a system to produce the same refrigerating effect of R-502, the gas velocities will certainly be greater.

The solubility of the refrigerant and oil may also be one of the root causes of this.

This is a guess about the plate... Perhaps it is there to provide a uniform gas flow to support the diaphragm.

A similar thing is done on large reciprocating compressors for the valves.


Parker has responded asking for MSDS information for R404A to verify its compatibility...

From my experience the manufacturers test their materials with the oil and refrigerant mixtures to ensure compatibility and long life.

Does Parker suspect the refrigerant may be at fault? Purity, water content, etc???



although plotting a enthalpy diagram shows gas temperature would be more like 130oC discharge.... Still not sure how the gas would be at 130oC with a pipe surface temp of only 90oC...


If you plot the discharge temperature at constant entropy the discharge temperature will of course be much higher than the pipe.

The temperature you are plotting is inside of the compressor (pistons, valves, cylinders, etc.) In fact, the actual discharge temperature in the cylinder will probably be greater than the calculated value from the P-H diagram.


Once the gas pressure has increased to the desired level (the compression of the gas has been completed in the cylinder), the gas leaves the compressor and flows into the discharge line. At this point, the pipe may be substantially cooler due to radiation, convection, and conduction.



we can approach the manufacturer with a known issue as opposed to a series of ideas they just poo-poo and use as an excuse to deny warranty claims...


The manufacturer should give you some award for helping them with their product development cycle.:D

[mr cool]

would it be possible to use sat gas from top of reciver to defrost with instead of discharge vapour, i have also had soli diaphragms blister like this with 404a and 408a i think that it may have somthing to do with blended refrigerants as you never had any problems with 22 or 502, i dont think that the diaphragms are made to deal with the uses we need them for as per usual, one valve covers all gasses and states of refrierants in one handy box! and with anything thats one size fits all there is always someone left needing more. i have also known systems to be adapted to take r404a and the dischage pipe has needed to be larger to deal with the increase in expansion after compression this has only been discovered after a few costly failiures!

[donato]

Environmental Stress Cracking (ESC) in PVC caused by SW32 oil; there is a picture of a crack on page 15/16 but it's in dutch, sorry. maybe it has to do with the oil.
http://www.robklimaat.nl/docs/3750070130.pdf

can you tell us the temperature raise of the valve at defrost?