Petroleum refineries act as a "nodal industry" because it is provides fuel for heat and lighting, lubricants for machinery and raw materials for a number of manufacturing industries like synthetic textile, fertilizer and numerous chemical industry. That's why petroleum is called as nodal industry.

Introduction:
Petroleum refineries are often referred to as "nodal industries" due to their crucial role in supporting and connecting various sectors of the economy. This term is used to highlight the central position and significance of petroleum refineries in the industrial network.

Explanation:
Petroleum refineries serve as nodal industries primarily because of the following reasons:

1. Raw Material Source:
- Petroleum refineries are the primary source of refined petroleum products such as gasoline, diesel, jet fuel, and various other chemicals.
- These products serve as raw materials for several downstream industries, including synthetic textile, fertilizer, and chemical industries.
- By providing the necessary raw materials, petroleum refineries act as essential nodes that facilitate the functioning of these industries.

2. Interlinking Various Sectors:
- Petroleum refineries establish strong interconnections between different economic sectors.
- They serve as crucial links by supplying petroleum products to diverse industries, including transportation, agriculture, manufacturing, and construction.
- By connecting these sectors, petroleum refineries play a pivotal role in sustaining the overall economic growth and development.

3. Value Addition:
- Petroleum refineries add value to the crude oil extracted from oil wells by converting it into various refined products.
- These refined products undergo further processing in downstream industries to produce goods such as textiles, fertilizers, plastics, and chemicals.
- Through this value addition process, petroleum refineries contribute to the growth and expansion of multiple industries, making them central to the economic landscape.

4. Employment Generation:
- Petroleum refineries are often large-scale industrial complexes that require a significant workforce to operate and maintain.
- They create employment opportunities for a diverse range of skilled and unskilled workers, contributing to local, regional, and national employment levels.
- The employment generated by petroleum refineries further extends to the associated industries, thus fostering economic stability.

Conclusion:
Petroleum refineries are rightly termed as "nodal industries" due to their pivotal role in supporting and connecting various sectors of the economy. They act as the primary source of raw materials, interlink different industries, add value to crude oil, and generate employment. By understanding and appreciating the significance of petroleum refineries, we can recognize the critical role they play in driving economic growth and development.