During selection, SizeMaster recommends the optimum pressure relief valve for each Farris valve series that is applicable to your process parameters. SizeMaster includes catalogue information for Farris , , , , , , , and series pressure relief valves, including their black and white drawings. While you are selecting a Farris type number for a tag, SizeMaster ensures that you don't pick a valve that is not applicable to your process parameters such as a liquid-service valve for a vapor scenario , and on-line help is available to assist in the determination of each of the digits of the Farris type number, based on your process scenarios.
SizeMaster manages the entire sizing and selection work-flow process for jobs and tags, from request for quotation through order generation. From its Quick Size and Capacity Wizards through to its consistently standard Windows user interface, SizeMaster makes the job of sizing and selecting pressure relief valves easy, and its worksheet management and workflow auditing facilities let you ensure that your results are correct.
For selecting relief-valve setpoints, two limits are considered. The lower limit is based on the maximum operating pressure of the system. The purpose of this margin is to avoid any immature activation of the relief valve. Backpressure is the pressure that exists at the outlet of a pressure relief device as a result of the pressure in the discharge system. The total back pressure is the sum of superimposed and built-up backpressures.
Superimposed backpressure is the static pressure that exists at the outlet of a pressure-relief device at the time the device is required to operate. This is the pressure before the relief valve opens. Superimposed backpressure is the result of pressure in the discharge system coming from other sources and may be constant or variable.
Built-up backpressure is the increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief valve opens Figure 2. This type of backpressure is caused by fluid flowing from the pressure relief valve through the downstream piping system. As built-up backpressure varies with the shape and size of the discharge piping, it's always variable.
Blowdown is the difference between set pressure and reseating pressure. It refers to how much the pressure needs to drop, below the set pressure, before a valve reseats. Blowdown happens because, when a relief valve lifts, larger disc area is exposed to system pressure, and it will not be possible for the valve to close until the system pressure has been reduced to below the set pressure.
The design of the control chamber, or huddling chamber, determines at what pressure the closing point will occur. Proper blowdown helps in reducing the chances of chatter or seat leakage. Test facilities may not have sufficient capacity to accurately verify the blowdown setting.
The estimated inlet piping pressure drop should be also stated on the relief valve specification sheet. The valve will close when the inlet flange pressure drops to psig — 7. Now, assuming that inlet losses are psig, the valve will close at psig, which is 3-psig higher than the set pressure. The valve will immediately try to open again, resulting in chattering and potentially damage to the valve.
It's worth mentioning that this inlet pressure loss criteria alone isn't sufficient to predict PRV stability. Additional factors that need to be considered include blowdown, relieving pressure and overpressure.
However, relief devices are typically set to open at the design pressure, instead. In some cases, the design pressure is equal to the MAWP — but it will never exceed it.
In cases where the MAWP is not well-established, the design pressure may be used for the set pressure. The Set Pressure is usually given in terms of gauge pressure, therefore any Back Pressure is added to the set pressure and overpressure to calculate the Relief Pressure in absolute units. The Back Pressure includes both the constant superimposed downstream pressure and any built-up backpressure due to the discharge of the fluid from the relief device through the downstream piping and treatment system.
The Over Pressure is usually expressed as a percentage of the Set Pressure. The default value for this parameter is the mass flow of the stream in the simulation, but it can be set to the desired value for a specific scenario. This value is used to select the appropriately sized Pressure Relief Valve. Although an orifice is often used to describe the minimum flow area constricted in the valve, the geometry and relief area calculations are more appropriately modeled based on an ideal isentropic nozzle.
Mass balance at any point in the nozzle dictates that the mass flow rate is constant: In this equation, u n is the fluid velocity at the nozzle throat, A n is the throat area, and p , u , and A are the density, velocity, and flow area, respectively, at any given point in the nozzle.
The fluid density decreases as it flows through the nozzle due to the decrease in pressure. Additionally, the flow area decreases as the nozzle restricts, reaching a minimum value of A n at the throat.
The velocity u , then must increase, and reaches u n at the throat. The rate of increase in velocity is greater than the rate of decrease in density, therefore the mass flux reaches a maximum at the throat.
For a given mass flow rate, determined for the particular emergency scenario, the minimum required area A min is calculated at the maximum mass flux, which was determined to occur at the nozzle throat. Real valves are not ideal nozzles, so a discharge coefficient, K D , is used to account for the difference between the predicted ideal nozzle and the actual mass flux in the valve. The discharge coefficient, K D , can be estimated by ProMax or specified directly from vendor literature.
Single and two-phase flows are both frequently encountered in various relief scenarios. Due to the large number of variables associated with the fluid properties, distribution of fluid phases, interaction, and transformation of the phases, sizing a two-phase relief scenario is considerably more complex than single-phase. The Mass Flux calculation varies depending on the relieving fluid type:.
For liquids with constant density, Bernoulli's Equation reduces to This equation is valid for fully turbulent flow, where the flowrate is independent of the fluid viscosity. For low Reynolds number high-viscosity flows, values can be multiplied by a correction factor.
For a vapor flow, the equation used depends on whether the flow rate is critical or subcritical. When the downstream pressure is reduced, the velocity and mass flux increase at the throat; eventually the mass flux reaches a maximum value at the choked, or critical, flow pressure.
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