1. Field of the Invention
This invention relates to a main relief valve for a safety relief system, and more particularly to means to control the fluid discharge rate for the main relief valve in a safety relief system.
2. Description of Prior Art
Heretofore, safety relief systems for a pressure vessel, such as a tank, have utilized pilot operated relief valves. The main safety relief valve normally has a piston type valve member with a dome chamber over the valve member. A separate pilot valve continuously senses the tank pressure, and when the tank pressure increases to set pressure, the pilot valve is opened to vent the dome chamber and reduce the dome pressure thereby providing opening of the main valve member. When the tank pressure decreases, the pilot valve closes to repressurize the dome chamber from the tank pressure for closing of the main valve member.
Flow restrictors or restrictions for controlling fluid flow upon opening of a valve member have also been utilized heretofore with the selection of a predetermined flow restrictor positioned within and connected to the inlet for the valve member. The prior restrictors limit the maximum discharge rate through the flow passage but are not connected to and movable with the valve member. For example, U.S. Pat. No. 4,960,260 shows interchangeable flow restrictors removably connected to an inlet restriction for a control valve. However, the flow restrictor is not removably mounted on the valve member for movement with the valve member, and particularly on a piston type valve member for a relief valve.
The main safety relief valve is required by various regulations, including the ASME Boiler and Pressure Vessel Code, Unfired Pressure Vessels, Section VIII, to have a certain coefficient of discharge (K.sub.d) for various sizes and pressure ranges of main relief valves. The requirements for obtaining a consistency in the coefficient of discharge (K.sub.d) is set forth in the aforementioned ASME Code, Section VIII, paragraph VG 131.
It is desirable that a single main relief valve, particularly the valve body, be provided for a plurality of various pilot valves so that it is only necessary to stock one main relief valve thereby reducing the overall inventory of valves. Further, it is desirable that various interchangeable piston type valve members be provided for the common main relief valve with the valve members providing a consistency in the coefficient of discharge (K.sub.d). A supply of separate piston type valve members having interchangeable flow restrictors thereon of different sizes and different orifice restrictions is desirable in order to obtain the flow control desired for the main relief valve, thereby minimizing the inventory of valve parts and reducing customer stocked valve parts to result in improved service to customers.