Heat transfer oil expansion tank level
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The liquid level in the expansion tank of our unit’s heat transfer oil system has been experiencing irregular fluctuations upward and downward recently. I would like to ask fellow experts: what are the factors that can cause such fluctuations in the liquid level? Can fluctuations in production load or changes in ambient temperature cause fluctuations in liquid level?
Has any medium leaked into the heat transfer oil system, and has the temperature of the expansion tank changed?
The methanol cracking hydrogen production heat transfer oil system, right? Was it a leak in the heat exchange equipment that caused the process medium to enter the heat transfer oil system? Check whether the combustible gas alarm at the low-level tank discharge outlet is triggering an alarm; if no combustible gas alarm is installed, use a portable combustible gas alarm for detection. It is also possible that the heat transfer oil has oxidized, deteriorated, and decomposed; in such a case, a sample needs to be taken for testing.
Our unit’s heat transfer oil system supplies two production lines; the heat transfer oil pumps are used in pairs with one as a spare. No other fluids have been detected entering the heat transfer oil system at present. Recently, there have been slight fluctuations in user load, while changes in ambient temperature have not been significant. The liquid level has remained relatively stable; it’s just that one production line was shut down recently. However, the heat transfer oil system is still in operation at low temperatures. Later, it was found that the oil level on the production line in use was fluctuating.
Yes, the hydrogen production system in my facility uses methanol cracking, and high-pressure steam is employed for heating. In the past, there have been instances where sand holes in the heat exchanger allowed oil and gas to enter the system. We have encountered and investigated all of the problems you mentioned, and so far none of those issues have arisen again. The current situation is simply that the liquid level in the expansion tank fluctuates slightly up and down in an irregular manner; we have not encountered such a situation before.
The level of the sump is closely related to temperature fluctuations under operating conditions: when the temperature of the heat transfer oil is high, the level is high; when the temperature is low, the level is low. Under normal operating conditions, the level is not greatly affected by temperature changes.
This scenario was also considered, and some additional new heat transfer oil was added, but there was still no significant change in the situation. (After we added oil, the level gauge showed an increase of about five centimeters.)
If factors such as oil degradation, water entering the oil, and process gas intrusion are ruled out, it should be related to the oil temperature. Heat transfer oil experiences certain thermal expansion due to temperature changes. As you mentioned, your heat transfer oil is heated by steam heaters; therefore, when one production line is shut down, it may become more difficult to control the oil temperature. I believe it is related to the way you control the oil temperature.
If your heat exchanger has developed sand holes, it should be re-processed with oil; during this process, it is important to ensure that air is vented from the converter and the vapor superheater as well. Another point is that once the oil has been heated, the auxiliary exhaust valve must be closed (the actual requirement is to close this valve once the oil temperature exceeds 170 degrees Celsius); otherwise, if there is no nitrogen seal in the high-level tank, the heat transfer oil is prone to oxidation and deterioration. If the auxiliary exhaust valve is not turned off, the temperature of the expansion tank will be relatively high, and there may also be fluctuations in the liquid level. After shutting off the auxiliary exhaust valve, local convection will decrease, the temperature of the sump will drop, and the liquid level will stabilize.
We are still analyzing and investigating the cause of this issue!
This was indeed not taken into account before; it’s necessary to check the opening and closing condition of the valve as well.