The 3% rule for pressure relief valves (PRVs) is a guideline that recommends the total pressure loss in the inlet piping to the PRV should not exceed 3% of the valve's set pressure.
API 520 Pt II states that the 3% may be exceeded if “an engineering analysis is performed”. Historically many installations throughout the chemical and refining industries have not followed the 3% rule. In the past, it was not uncommon for operating companies to accept inlet pressure drops up to 5%.
The API and ASME guidelines and standards for emergency relief systems both state that total non- recoverable inlet pressure losses between protected equipment and a spring-loaded relief valve should be limited to 3% of the relief valve set pressure.
1910.111(d)(4) Safety Relief Devices
Safety relief valves shall be set to start-to-discharge at a pressure not in excess of the design pressure of the container and shall have a total relieving capacity sufficient to prevent a maximum pressure in the container of more than 120 percent of the design pressure.
The set pressure limit is the threshold that triggers the high-pressure relief valve action. For example, if the normal system pressure is 3000 PSI the engineer might determine the set pressure limit is 3300 PSI.
Both standards recommend the “3% rule” for the inlet piping. According to the “3% rule,” the pressure loss along the inlet piping should be kept below the 3% of the PRV set pressure (and at the rated flow capacity).
In simple terms, the 10/13 rule is a guideline for setting the design pressures in a shell and tube heat exchanger. The Rule: The design pressure of the side with the lower pressure (be it the shell or the tube side) should be set to at least 10/13 of the design pressure of the side with the higher pressure.
API 520 includes information on correction factors (more conservative) for back pressure and viscosity, therefore is the standards (code) mostly used for pressure relief valve sizing or installation.
API 521 is for fire case. API 520 is for other cases i.e. blocked discharge, control valve failure. You will need to define all of your relief cases to be able to adequately size a PSV.
Rule 3 - General Definitions
(a) The word "vessel" includes every description of water craft, including non-displacement craft, WIG craft and seaplanes, used or capable of being used as a means of transportation on water. (b) The term "power-driven vessel" means any vessel propelled by machinery.
ISO 4126-1:2013 specifies general requirements for safety valves irrespective of the fluid for which they are designed. It is applicable to safety valves having a flow diameter of 4 mm and above which are for use at set pressures of 0,1 bar gauge and above. No limitation is placed on temperature.
Multiple pressure relief valve should never be installed in series to the system lines. It may result in excessive loss of the fluid due to multiple valves releasing pressure at the same time.
Pressure relieving equipment
Do not use ring and/or doughnut cushions as these can cause further pressure damage. If you are assessed as being at risk of developing a pressure ulcer, your healthcare professional will advise you on: the appropriate pressure relieving equipment for chairs and/or your bed.
For initial certification, the maximum blowdown for valves set at or below 100 psi (700 kPa) is 4 psi (30 kPa). For valves used on high-temperature hot water boilers and forced flow steam generators, the maximum blowdown is 10%. For all other valves, the maximum is 4% of set pressure.
The maximum allowable backpressure for the conventional PSV is 10% of set pressure, for balance bellows is 30% of the set pressure. For pilot-operated valves, the allowable backpressure is 50% of set pressure, but depending on the valve manufacturer, the backpressure could be as high as 100% of the set pressure.
Vapors: Fluids that are or will become vapor should generally be on the shell side due to their larger volume and lower heat-transfer coefficient. Non-condensing vapors: If the vapor won't condense, it's often better on the tube side to prevent stagnation in the shell, which can reduce heat transfer efficiency.
The framework is simple: before you make a decision, ask yourself three questions: 10 minutes from now, how will I feel about this decision? 10 months from now, how will I feel about this decision? 10 years from now, how will I feel about this decision?
If valves are fitted they must normally be locked in the open position and ideally should be configured to prevent normal use of the system when the relief valve is out of circuit. This can be achieved, for example, by using a three-way valve which vents the main liquid line to the suction side.
Remember, the system pressure relief valve must be set about 200 PSI above the pump pressure compensator setting, or the two pressure settings will fight each other. This would result in the system acting as if it were a fixed displacement system.
PSVs require that the outlet of the valve be larger than the inlet, based on an understanding of engineering flow calculations of compressible gas/fluids. The pressure is often much lower downstream of the PSV, so when the compressible gas/fluid is relieved the gas/fluid will expand.
The pressure rating of the relief valve must be equal to or less than the MAWP of the vessel. Most water heaters have an MAWP of 150 psi; however, some are rated higher, typically 160 psi. Some storage vessels have a lower pressure rating; they are commonly rated 125 psi.
discharge to a floor drain, to an indirect waste receptor, or to the outdoors. not be directly connected to the drainage system to prevent backflow of potentially contaminating the potable water. discharge through a visible air gap in the same room as the water-heating appliance.
4. Set the Relief Pressure. The Relief Pressure is generally determined by the equipment being protected, and is calculated as Relief Pressure = Set Pressure + Overpressure. By default, ProMax uses the stream pressure as the Set Pressure, and a 10% Over Pressure, but these can be modified for your analysis.