2 replies to this topic

[Bill B]

I have an interesting thermal relief problem and would like some input. I am evaluating a cooling water line that protects 8000 HP electric motor windings. The scenario would be blocked in cooling water. Has anyone run into this? If so, what % of the HP could I assume is transferred to heat energy?

The set point of the RV is 50 psig, so the saturation temperature would be ~300°F. Would you assume that I would reach that point, and perform the relief calcs for steam? Thanks.

[Art Montemayor]

Bill:

This is an interesting scenario. It raises additional questions:

1) How does the cooling water "protect" the motor windings? Is it in direct contact with the windings?

2) Presumably, the protection to the motor is to prevent an overheating of the windings due to a mechanical overload. If not, is the cooling of the windings required during normal operation?

3) If one is trying to protect the windings from getting "too" hot, what is the maximum temperature that the winding are allowed to reach – and beyond which a hazard can occur (i.e., windings' insulation breakdown, short-circuit, electrical fire, etc., etc.)?

4) How is the windings' operating temperature monitored continuously?

You state that you have a thermal relief valve installed in the cooling water circuit going to/ coming from the motor's electrical windings and that the set point on this relief device is 50 psig. If the relief device is truly a "thermal relief valve" and not a conventional PSV, then it protects the system by reacting to a buildup of hydraulic pressure in the system caused by an increase in the system's liquid expansion that, in turn, is caused by a temperature increase in the liquid. What all this means is that the system being protected MUST BE 100% LIQUID-FULL. The system cannot have any vapor or gas pockets within it because that would constitute a compressible section that doesn't follow the rules of hydraulic expansion.

Also, the relief valve in this case does not "protect" the motor windings. Even if the relief device activates and releases excessive hydraulic pressure (which would be about one or two cubic centimeters of liquid), the motor windings still continue to overheat. Nothing about the operation of the relief valve protects the motor and the windings. The relief valve only protects the cooling water line from rupturing due to excessive hydraulic pressure. The hazard involving the motor windings continues and must be mitigated by some other means.

Having stated all of the above, we can now restrict ourselves to what happens when the relief device activates. The initial discharge, as I stated, will be about one or two cubic centimeters of hot water. If the heating continues and the block valves remains closed, the water will continue to increase in temperature until reaching a condition where it starts to convert itself to saturated steam – approximately 297 oF at the 50 psig set point. The resulting discharge of fluid will be a mixture of saturated steam and water – a 2-phase flow. This is a very special condition that should always be taken into consideration because it puts a sizing constraint on the relief device. You need a larger relief valve to discharge a 2-phase flow than you would to discharge either a gas or a liquid. I would not apply a thermal relief valve on this application. I believe you require a special-designed PSV to relieve what I just described.

Correct me if I have assumed anything wrongly.

[djack77494]

Art,as usual has provided an excellent analysis of the problem. I think the possibility of ever generating a steam mix is virtually nil however, as a motor of this size should have a thermal overload which would trip the motor before that happened.