Case 4 – Subsonic Flow in a Pipe Comparison of RELAP5 to Crane Example Problem 4-22
Would like more test cases with measured data I would be interested in doing more comparisons to measured data – especially flow down pipes – sonic and subsonic. This would be for noncondensable gases, not steam. 8
Basic Problem - Modeling Approach The components being protected by the relief valve are downstream
Relief Valve Modeling We can not simply treat a RV as an orifice unless the discharge is directly to atmosphere. This is because the RV is a differential pressure device. Most RVs open and stay open based on the differential between upstream and downstream pressure rather than the difference between upstream and atmospheric pressure. Therefore the back pressure from the relief exhaust line must be taken into account. This is especially important for the Advanced Test Reactor (ATR) installations where the exhausts are routed long distances to HVAC ducts. Backpressure is roughly translated into an increase in pressure at the RV inlet. “Roughly” because the relief valves do have a blowdown pressure, which keeps the valve open at a certain percentage below the set pressure – typically 10%.
Relief Valve Modeling (cont) So, once open, a 100 psi relief valve would require at least 90 psi differential to stay open. This is important because a RV can be massively oversized but if the exhaust line is too small or too long, it will not properly protect the system.
Relief Valve Modeling (cont) Valve is modeled in RELAP5 using vendor supplied orifice area and then discharge coefficient is adjusted to achieve vendor’s published flow capacity Relief valve is modeled as a Motor Operated Valve (MOV) with trip open at 5% over set pressure and trip closed at 5% under set pressure (i.e., 5% blowdown which is conservative)
ATR Experiments – Relief Protection of Downstream Low-Pressure Components
Effect of heat transfer assumption Case Relief valve Inlet temp. (°F) Heat transfer? (isothermal) Flow rate (SCFM) Pressure (psig) 3118CSS30Yes58.038.3 4118CSS100Yes57.738.1 5118CSS30No60.438.2 Table 3. Calculated results for the supply line to the actuator Taken from C. Davis calculations supporting INL internal doc, ECAR-1464 100F Almost no effect
Effect of RELAP junction e flag At large discontinuities the user manual recommends setting the e flag equal to “1”. * from to area f loss r loss 1050101 100010000 110000000 1.576e-6 20800. 20800. 1000000 This turns out to be very important for the restriction orifice in a typical ATR experiment supply system 17
Effect of RELAP junction e flag With e flag set to 0 temperature downstream of orifice is much too low. 18 This does not seem to be a result of Joule-Thompson cooling since RELAP calculates a deltaT about the same for both helium and argon which have dramatically different J-T coefficients Helium is the fluid
Effect of e flag on flow through straight pipe Another look at test Case 4.0858 lb/sec.0793 lb/sec Matches Crane almost exactly.0851 lb/sec Air, 10 ft of ½” Pipe
A look at temperature change across individual nodes – Case 4 20 First Law e = 0 e = 1 -.464 o F -.387 o F -6.97 o F -32.9 o F First Law e = 0 e = 1 +.37 o F +2.54 o F First Law: Air, 10 ft of ½” Pipe
Isothermal Modeling RELAP5-3D does not have an isothermal flag However, noncondensable gas models can be made to run isothermally by incorporating heat structures, i.e., by setting the pipe wall to a fixed temperature and using it to keep the fluid at approximately the set temperature Another (less elegant) way is to switch the e flag to 1 judiciously, i.e., turn the e flag on and off at various junctions down the length of a pipe. This is done by trial and error, but for small problems it may be easier than the heat structure approach Suggest incorporating an isothermal option in RELAP5. COMPFLO has this and it is handy for noncondensable gas problems such as those described in this presentation 21
Conclusions RELAP5-3D is an effective tool for modeling pressure relief systems for noncondensable gases Care must be taken in modeling relief valves to ensure sufficient pressure drop is taken across the valve If choking is possible at the end of any pipe the node density should be increased at that end The thermodynamics behind the junction e flag are not clear to this user Suggest incorporating an isothermal option in RELAP5. 22