A petition for leave to intervene may be filed in any proceeding before the Administration. The petition will be granted by the presiding officer if the proposed intervenor establishes that it has a substantial interest in the proceeding and will not unduly broaden the issues therein or unduly delay the proceeding. All such petitions shall be filed prior to the opening of the prehearing conference, or if none is held, before the commencement of hearing, unless petitioner shows good cause for allowing the petition at a later time. Intervention petitions shall be served in the same manner as other petitions, and shall be subject to answer. Intervention petitions will be granted where necessary to protect substantial interests of the petitioner and where intervention will not materially broaden the issues. A person granted permission to intervene becomes a party to the proceeding.