Popping Pressure The pressure at which the valve disc rapidly moves from a slightly open (simmer) position to a practically full open position. In this pressure zone the valve is only slightly open and therefore discharging a small percentage of its rated capacity. Simmer The pressure zone between the valve set pressure and the popping pressure. Blow-Down The difference between the set pressure and the re-seating pressure expressed as a percentage of the set pressure or as a pressure difference. Re-Seat Pressure The pressure measured at the valve inlet at which the safety relief valve closes. Accumulation is the same as over-pressure when the valve is set at the design pressure of the vessel. The term refers to the vessel or system to be protected and not to the valve. Accumulation The pressure increase over a maximum safe working pressure of the vessel or system when the safety relief valve is discharging at its rated capacity is called accumulation. Usually expressed as a percentage of set pressure. Overpressure The pressure increase above set pressure at the valve inlet at which the discharge capacity is attained. DEFINITIONS Set Pressure The pressure measured at the valve inlet at which a safety relief valve should commence to lift under service conditions. It is characterised by a rapid opening action. Safety Relief Valve A valve which will automatically discharge gases, vapours or liquids, to prevent a predetermined safe pressure being exceeded. The term is commonly used for pressure relieving valves in which the lift is proportional to the increase in pressure above the set pressure. Relief Valve A valve which automatically discharges fluid, usually liquid, when a predetermined upstream pressure is exceeded. It is characterised by a rapid full opening action and is used for steam, gases or vapour service. Safety Valve A valve which automatically discharges gases and vapours so as to prevent a predetermined safe pressure being exceeded. A safety relief valve is designed to automatically discharge gas, vapour or liquid from any pressure containing system, preventing a predetermined safe pressure being exceeded, and protecting plant and personnel. Safety relief valves should be used to protect any pressurised system from the effects of exceeding its design pressure limit. 1 746 S a fe t y R e l i e f Va l ve s INTRODUCTION 746 Safety Relief Valves The effects of exceeding safe pressure levels in an unprotected pressure vessel or system, can have catastrophic effects on both plant and personnel.
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