The city shall not sell, exchange, lease, or in any way alien or dispose of the property, easements, income or other equipment, privilege or asset belonging to and appertaining to any utility which it may acquire, unless and except the proposition for such purpose shall first have been submitted, at a special election held for the purpose in the manner provided in this charter, to the qualified voters of the city and approved by them by a majority vote of the electors voting thereon. All contracts, negotiations, grants, leases or other forms of transfer in violation of this provision, shall be void and of no effect as against the city. The provisions of this section shall not, however, apply to the sale or exchange of any articles or equipment of any city-owned utility as are worn out or useless, or which could, with advantage to the service, be replaced by new and improved machinery or equipment.