The waterworks system so acquired by any city under sections 91.090 to 91.300 shall be entirely exempt from seizure, levy or sale under any judgment, execution, decree of court, or other proceedings in invitum, except as expressly provided in said sections, for the payment of the bonds issued in payment for the purchase price of said waterworks system, or the contract price for the construction of the same, and the city or board of waterworks commissioners shall not have power to sell, convey, mortgage or pledge the same, except as expressly provided in said sections.