High-pressure safety valves sizing

Schmidt, J., Peschel, W., and Beune, A. (2009) Experimental and theoretical studies on high pressure safety valves sizing and design supported by numerical calculations (CFD). Chem. Eng. Technol., 32 (2), 252-262. 

Sizing of High-Pressure Safety Valves for Gas Service... 

What if area wide electrical power fails. (UPS instrumentation power remains) Runaway reaction through loss of agitation. Indicated by agitator motor off, low coolant flow, high reactor pressure, and high reactor temperature. Runaway reaction causes reactor overpressure and loss of containment. Add shortstop and burp reactor to stop runaway. Depressurize reactor - SIS (Pressure safety valve sized for this event). Use LOPA to determine required SIL ... 

For normal extraction times, two manifolds with a corresponding set of valves located on the top and bottom of each extractor are adequate. For short extraction times three manifolds may be necessary. Small and medium-sized plants are equipped with control valves for discontinuous operation steps. From the technical- and economic points of view, high-pressure control valves are limited with KVs values between 6 and 10, especially for pneumatically driven valves. If such valve sizes are too small, high-pressure ball valves must be used, thereby substantially increasing the costs for the interlocking system and the safety requirements. 

As normally designed, vapor flow through a typical high-lift safety reliefs valve is characterized by limiting sonic velocity and critical flow pressure conditions at the orifice (nozzle throat), and for a given orifice size and gas composition, mass flow is directly proportional to the absolute upstream pressure. 

For example, increasing a valve size or installing a larger pump could result in high pressure in a vessel, thus increasing the risk of a release. Sanders (1993) presents a number of examples of changes affecting the safety of a plant. 

The secondary relief device is usually sized to a much lower discharge rate than the primary. The secondary relief device can be a safety valve, a bursting disk, an automatic pressure-controlled vent valve, or a high-pressiu-e switch that opens an automatic vent. If the secondary device is a relief V2dve or a bursting disk, credit for its rate of discharge can be teiken when sizing the primary relief device. 

One of the most important safety components on any high pressure compressed gas system is the flow restrictor. There are two t5q)es of flow-restricting devices an excess flow switch and a flow restrictor. The excess flow switch can be either mechanical or electromechanical and has an excess flow sensor which trips a valve, shutting off gas flow when a preset limit is exceeded. The restrictor is a passive device (limiting orifice) which is sized to limit the flow of gas to a predetermined rate. 

Overall, the sizing method according to EN-ISO 4126-7 is based on very simple equations mainly for ideal gases. In the literature, the validity of this sizing method was shown by numerical calculations up to pressures of about 10 MPa (100 bar) [3,4]. No limits of application are formulated in the standards for high-pressure valves. In addition, the discharge coefficients for safety valves are generally measured at pressures between 0.1 and 5 MPa - the application limit of current test facilities amounts to about 25 MPa (250 bar). The question arises whether the application of the current standard beyond a pressure of more than 10 MPa is allowed. 

In practice, often an ideal behavior of gases is assumed at moderate pressures when sizing a safety valve for gas service. Real gas behavior is only assumed at a very high pressure, for example, at a pressure of more than 100 bar. In general, the real gas behavior is rather determined from the proximity of the thermodynamic critical point. With the reduced thermodynamic pressure and the reduced thermodynamic temperature, the deviation from ideal behavior can be described much better than with the absolute values of pressure and temperature. If the reduced pressure and the reduced temperatures at the entrance of the nozzle exceed p/pc > 0.5 or T/Tc > 0.9, the deviations from the ideal behavior are usually no longer tolerable. 

Some technological problems involved in building large LDPE production units are process operation, size of compressors, reactor structure, high-pressure valves, and safety problems [7]. Due to high exothermicity of the polymerization reaction, the removal of reaction heat is a critical design problem. Factors that affect the heat removal include reactor surface/volume ratio, reaction mixture and feed ethylene temperature difference, thickness of the polyethylene layer at the inner wall of the reactor, reaction mixture flow rate, and reactor material heat conductance. It should be noted that the thickness of the laminar layer at the reactor wall is affected by the reaction mixture flow rate. 

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