Oil in the TEV equilizer port

[herefishy]

It ocurred again today. It seems that mostly having condemned a TEV, that upon removal of the valve from the system, that often the external equalizer port of the valve is oil logged.

Is this observation a CAUSE of malfunction, or a SYMPTOM of the malfunctioning or misapplied component? .

I've conditioned myself to the point that when I open the equalizer connection and oil squirts all over me, that I am confident that my diagnosis is on the mark. And vice versa, the absence of oil in the TEV eq. port (might) cause me to further evaluate a system, aside from any concerns regarding TEV operation

[Prof Sporlan]

Is this observation a CAUSE of malfunction, or a SYMPTOM

Could be either....

Say we have a situation where the TEV is starving, and oil can trap in the equalizer line and get underneath the power head, either by poor equalizer piping or reverse cycle hot gas defrost. The oil will be essentially at evaporating temperature, and it can cool the head down causing the valve to starve. This is particularly a problem with a TEV having an MOP type charge, e.g., Sporlan 'ZP', as these charges will migrate to the head under these conditions. But even if the TEV doesn't have an MOP charge, the oil underneath the power head still can noticeably affect valve operation. Equalizer lines should always be piped in such a manner that they will not collect oil. Keeping the TEV power head on top (above the inlet an outlet connections) is also a good idea, particularly with internally equalized valves.

Internally equalized TEVs have a internal passageway from the outlet side of the valve to the underside of the power head for equalization purposes. As one can imagine, oil can collect at the outlet of the valve, particularly if it is feeding a bottom fed evaporator, and this oil can make its way to the power head if the TEV is upside down.

Of course, if the TEV is starving due to some other cause, refrigerant velocities in the evaporator are going to be lower, which will promote oil trapping in the evaporator and suction line. But then one needs to figure how the oil is getting into the equalizer line. If it is not due to poor equalizer piping or hot gas defrosts, a bad TEV pushrod seal might be a possibility. A bad seal could be the result of the TEV being overheated during installation.

[terrygoodrich]

The last time I checked... oil was a liquid. Doesn't it transmit pressure OK? It is more viscous than vapor but that would only be a problem if there was a flow. There should be no significant flow in the equalizer line.

Prof., Are you saying that oil under the power head can contain cold refrigerant that will condense the power element charge, causing a situation similar to a cross ambient condition where the valve will not open? Wouldn't that be only a temporary situation until the refrigerant is gone? What seals the pushrods ?

[Prof Sporlan]

Prof., Are you saying that oil under the power head can contain cold refrigerant that will condense the power element charge, causing a situation similar to a cross ambient condition where the valve will not open?

That is correct.

Wouldn't that be only a temporary situation until the refrigerant is gone?

Perhaps not. Having the refrigerant evaporate from the oil, which can take a long time, would obviously help the situation. But then the oil would still have to warm up sufficiently, preferably to some temperature above that of the sensing bulb. There is not much in the way of heat exchange opportunity for oil trapped underneath the diaphragm. Hot gas defrost could help here, though.

What seals the pushrods ?

For most balanced port TEVs, a neoprene type seal is typically used. For conventionally designed small capacity TEVs, close tolerance pushrod holes are used in lieu of a seal. Hole tolerances can be held close enough to prevent any significant leakage and allow the TEV to stroke open and closed freely.