Rule 318. Designation of agent for service. A person may be designated by a natural person, corporation or partnership as an agent for service in a writing, executed and acknowledged in the same manner as a deed, with the consent of the agent endorsed thereon. The writing shall be filed in the office of the clerk of the county in which the principal to be served resides or has its principal office. The designation shall remain in effect for three years from such filing unless it has been revoked by the filing of a revocation, or by the death, judicial declaration of incompetency or legal termination of the agent or principal.