Whether the contract labor about whose promotion is demanded by the union have gained the experience of 20-25 years in your establishment alone?
If you are a PE, it is not your obligation to consider such a demand for promotion of the contract labor as they are not your employees.
If all such contract labor have gained so much experience in your establishment alone, the bonafides of the very contract would be questionable. Promotion of an employee implies elevation to a higher position coupled with hike in compensation. When no change in the existing nature of work merely by the efflux of time is possible, no use in promotion of such an incumbent despite his gaining long experience. The motive behind such an attempt of the union may be to gain more salary/wages for such workers with long experience. As far as I know there is no rule obligating an employer to give promotion to employees simply by virtue of their long service in the establishment.