Application of Vacuum Dewatering Process in Transformer Oil Production in Summer

Wang Yongzhong Wang Jiatao

(Petrochemical Plant of PetroChina Liaohe Oilfield Company, Panjin, Liaoning)

Abstract: Aiming at the problem of water absorption of naphthenic transformer oil, this paper adopts vacuum oil filter to carry out vacuum dehydration treatment. Through the investigation of dehydration effect under different conditions, it is proved that the application of vacuum dehydration technology in the summer production of transformer oil in our factory is successful.

Keywords: vacuum dehydrated transformer oil

1 Preface

The transformer oil of our plant is produced from Liaohe low pour point cycloalkyl crude oil. Because it contains high naphthenic hydrocarbons and aromatic hydrocarbons, it has excellent electrical performance, aging resistance, anti sludge generation performance and low temperature flow performance. Since its industrial production in 1997, it has been widely used in the electrical sector. But it is precisely because of the high content of naphthenic hydrocarbons and aromatic hydrocarbons that the naphthenic transformer oil has a higher moisture absorption tendency than the paraffin based transformer oil [1]. In addition, the geographical location of our factory (when the relative humidity of the air in summer is the highest, it exceeds 85%) causes the water content of our transformer oil to increase in summer production due to the invasion of moist air, thus affecting the electrical performance of the transformer oil, The voltage withstand performance of transformer oil is greatly reduced, which affects the production of transformer oil in our factory.

The water content is limited in foreign electrical insulating oil standards. For example, IEC 296 and BS 148 standards stipulate that the water content of the oil shall not be higher than 30mg/kg when delivered in batches, 40mg/kg when delivered in barrels, and 35mg/kg in ASTM D3487 standards. China's transformer oil standard GB2536 does not give specific indicators of water content, and it is clear in the notes that qualitative filter paper is allowed to filter when determining the breakdown voltage. However, in recent years, some domestic oil users, especially the power sector, have put forward higher requirements on the quality of the transformer oil they will use. For example, a transformer factory requires that the transformer oil purchased should meet the requirements of the breakdown voltage in the quality indicators without any treatment, This puts forward higher requirements for transformer oil production plants, especially for naphthenic transformer oil production plants.

2. The way of water intrusion and the method of removing water in the process of transformer oil production

2.1 Ways of water intrusion during the production of transformer oil

The factors affecting the breakdown voltage of transformer oil include water, impurities and temperature, among which water is the main factor affecting the breakdown voltage. The invasion ways are as follows:

① In the clay refining process, due to the low temperature at the furnace outlet, the water cannot be completely gasified, the oil and clay paste stay in the clay evaporation tower for a short time, the water evaporation is incomplete, the stripping steam carries water, the system vacuum is low, the water content of the air used for cleaning the filter is high, the water content of the raw oil or clay is high, and the internal leakage of the cooler and other reasons, the base oil will contain water;

② During the storage of base oil or transformer oil in the tank, respiration occurs due to the rise and fall of oil temperature. At this time, if the air humidity is high, the water in the air will dissolve into the oil;

③ When transformer oil is mixed with additives, the water content in the additives is relatively large, which will also increase the water content of transformer oil.

The practice in recent years has proved that the unqualified breakdown voltage index of transformer oil in our factory occurred from June to August, which also indicates that the index is mainly due to the influence of moisture.

2.2 Method of removing water from transformer oil

Some domestic transformer oil production plants, such as Kelian, use plate and frame filter for filtering and dehydration, some small blending plants use the method of filtering after clay treatment, and power departments generally use vacuum degassing and dehydration to treat transformer oil.

Due to the high air humidity in summer in the geographical location where our factory is located, if the plate and frame filter is used for filtering and dewatering, the water absorption of the filter paper is limited, and at the same time of filtering, the moisture in the air invades again, so the dewatering effect is not good. If the unqualified transformer oil is filtered by adding clay, the loss of oil products will be increased and the production cost will be increased, as well as the load of the clay plant for continuous production. However, the power industry generally adopts vacuum degassing dehydration method, which is not limited by climate and geographical conditions, and can be used to solve the water content problem of transformer oil in our plant. The ZYA large pumping speed two-stage vacuum oil filter produced by Chongqing Submersible Optical Electromechanical Equipment Research Institute has been widely used in the power industry for transformer oil injection and drying treatment of damp transformer oil. Therefore, our factory has selected ZYA500 vacuum oil filter for dehydration treatment of transformer oil.

Application of Model ZYA-500 High speed Double stage Vacuum Oil Filter in Transformer Oil Production in Summer

3.1 Technical transformation of transformer oil dehydration system

After the transformer oil to be treated is heated to a certain temperature in the tank, it is pumped into the vacuum oil filter. After further heating by the heating system in the oil filter, it enters the vacuum tank for vacuum dehydration. The dehydrated oil enters another tank after being cooled by a cooler with a heat exchange area of 85m2. The principle flow chart is as follows:

Raw material tank to be dehydrated → vacuum oil filter → cooler → qualified transformer oil tank or ex factory

After the above transformation, the transformer oil system has the ability to vacuum dewater the water bearing transformer oil at any time.

3.2 Industrial test

3.2.1 Investigation on vacuum dehydration effect of transformer oil with different water content

We selected two tanks of 25 # transformer oil with different quality for vacuum dehydration at 50 ℃ and 57 ℃ and vacuum degree of -0.098MPa respectively. The quality before and after treatment is shown in Table 1:

Table 1 Vacuum Dewatering Effect of Transformer Oil with Different Water Content

Serial No

Dewatering temperature

（℃）

vacuum degree

（Mpa）

Before processing

After processing

breakdown voltage 

water content

breakdown voltage 

water content

one

fifty

-0.098

33.7kv

37.0mg/kg

43.6kv

15.1mg/kg

two

fifty-seven

-0.098

45.5kv

14.7mg/kg

49.7kv

12.4mg/kg

It can be seen from Table - 1 that the oil filter has a certain dehydration effect on transformer oil with different water content. After dehydration, the breakdown voltage of the oil has been increased to varying degrees, and the water content has been further reduced.

3.2.2 Influence of different dehydration temperature on dehydration effect of transformer oil

We carried out vacuum dehydration for several tanks of transformer oil with unqualified breakdown voltage, and investigated the influence of different dehydration temperatures on dehydration effect (see Table - 2).

Table - 2 Dehydration Effect at Different Temperatures

Raw material tank No

105#

106#

109#

103#

Vacuum dehydration temperature/℃

fifty-two

fifty-six

fifty-eight

sixty

Breakdown voltage of dehydrated oil products/kv

forty-six point six

forty-eight point one

forty-nine point two

fifty-two point four

It can be seen that with the increase of dehydration temperature, the breakdown voltage value of the dehydrated oil products increases. The several tanks of oil we are currently handling are basically dehydrated below 60 ℃. The quality of the dehydrated oil products (breakdown voltage) fully meets the quality requirements of the transformer oil products produced by our factory at present (more than 35kv).

3.2.3 Investigation on dehydration effect of vacuum oil filter during continuous operation

In order to further investigate the dehydration effect of the oil filter during continuous dehydration treatment, we continuously treated 1180 tons of transformer oil with unqualified breakdown voltage for 77 hours. During the treatment period, the laboratory regularly sampled and analyzed the oil filter outlet to track the dehydration effect of the oil filter. See Table - 3 for the tracking and analysis results. It can be seen from Table - 3 that the analysis of distillate outlet is above 40kv. This data shows that it is feasible to realize synchronous operation with clay plant if the dehydration facility is connected in series to the base oil supply line.

Table 3 Breakdown Voltage Analysis Results of Dehydrated Oil Products during Continuous Operation of Oil Filter

Working time of oil filter

Breakdown voltage/kv

First sampling analysis

Second sampling analysis

Day 1

forty-eight point one

forty point two

Day 2

forty-eight point two

forty-seven point six

Day 3

forty-six point six

forty-eight point nine

Day 4

forty-one point five

fifty-two point four

3.2.4 Investigation on cooling effect of cooler

Lowering the storage temperature of transformer oil can reduce the water intake [2]. The cooling effect of cooler at different dehydration temperatures is listed in Table - 4:

Table 4 Cooling conditions of cooler at different dehydration temperatures

Dehydration temperature/℃

fifty

forty-seven

fifty-six

fifty-seven

fifty-seven

fifty-seven

fifty-six

sixty

Oil temperature after cooling/℃

thirty-two point five

thirty-eight

forty

thirty-nine

forty

forty

thirty-nine point five

forty point five

Processing capacity/m3 · h-1

fifteen

twenty-five

twenty-five

twenty-five

twenty-five

twenty-five

twenty-five

twenty-five

It can be seen that under the maximum flow rate, when the dehydration temperature reaches 60 ℃, the maximum temperature of the oil after cooling is 40.5 ℃, which can meet the storage requirements of our transformer oil.

3.2.5 Due to the limitation of climatic conditions, if the dehydrated transformer oil cannot be delivered in time or effective protection is implemented, the breakdown voltage of the oil will also be reduced again. Therefore, we investigated the quality of the dehydrated transformer oil immediately sealed and stored for 50 days. According to the analysis of a transformer factory, the breakdown voltage of the tank of transformer oil is 58kv, and the water content is 12.6mg/kg.

3.3 Use ZYA-500 large pumping speed two-stage vacuum oil filter for vacuum dehydration of transformer oil

Since the rainy season, the 6 tanks of transformer oil produced by our factory have continuously failed to meet the breakdown voltage. Therefore, we used vacuum oil filters to vacuum dewater the unqualified transformer oil in turn, and adjusted the dehydration temperature according to the temperature of each tank of oil. The breakdown voltage of the dehydrated oil has significantly increased. See Table 5 for the quality comparison data of oil products before and after dehydration:

Table 5 Quality Comparison of ZYA-500 High speed Double stage Vacuum Oil Filter before and after Transformer Oil Dehydration Treatment

Before stripping

Tank No

108#

106#

105#

103#

107#

109#

Breakdown voltage/kv

twenty-two point four

twenty-nine point five

twenty-nine

twenty-nine point four

twenty-one point two

twenty-two point three

Dehydration temperature/℃

fifty-one

fifty-five

fifty-three

fifty-eight

fifty-six

fifty-nine

Vacuum degree/MPa

-0.098

-0.098

-0.098

-0.098

-0.098

-0.098

Oil temperature after cooling/℃

thirty-five

thirty-seven

thirty-six

thirty-eight

thirty-seven point five

thirty-nine

After stripping

Tank No

one hundred and four

102#

109#

109#

103#

106#

Breakdown voltage/kv

forty-one point eight

fifty-four point eight

forty-seven

fifty-four point five

forty-seven point eight

fifty-two point five

4 Conclusions and Suggestions

After the transformer oil containing trace water was vacuum treated with ZYA-500 large pumping speed two-stage vacuum oil filter, the problem of low breakdown voltage in summer production of transformer oil in our factory was completely solved.

① When the vacuum degree is -0.098MPa and the dehydration temperature is 51 ℃, the breakdown voltage of the dehydrated transformer oil can reach more than 40kv. Further increasing the dehydration temperature to more than 55 ℃ will further increase the breakdown voltage of the dehydrated transformer oil to more than 50kv.

② After the transformer oil after vacuum dehydration is cooled by a cooler with a heat exchange area of 85m2, its temperature can be reduced to below 40 ℃, preventing the qualified oil from absorbing water again.

After a period of operation, it is found that the system still has some problems, which need to be further improved, namely:

① The processing capacity of the oil filter is relatively small, which is only 25m3/h in fact. Under the current process flow, the dehydration processing time is long (about 20t/h), which cannot meet the need of direct loading and transportation of transformer oil after dehydration. Therefore, if considering the need of direct transportation of transformer oil out of the factory after treatment, a vacuum oil filter with large processing capacity (about 50 m3/h) should be added.

② The transformer oil to be dehydrated must first be put into the mixing tank and heated to the predetermined temperature before vacuum dehydration. The process is long and it takes at least 3 days to process a tank of oil. If the clay refined oil can be cooled to 65~70 ℃ before it leaves the unit, then it will enter the buffer tank in the blending tank farm and be pumped to the dehydration system for vacuum dehydration, which will not only save the energy consumed by reheating before dehydration, but also improve the dehydration efficiency.

③ If the dehydrated transformer oil cannot be transported out of the factory in time, it can be stored in barrels.

reference

[1] Application Technology of Petroleum Products compiled by Oufeng

[2] The special series of lubricating oil starts from Book 4: Blending of lubricating oil and its application and additives