Safety Aspects and Design of the Vent Isolation Chamber Prepared: W.M. Snow Checked: H. Nann Approved: W. M. Snow 1. Scope This document addresses the safety aspects and design of the vent isolation chamber which is part of the safety hydrogen relief system of the NPDGamma liquid hydrogen target. 2. Components covered The documents cover the vent isolation chamber up to its input flanges and relief devices inside the chamber, see Fig. 1. The dimensional calculations for the fill/vent line between the hydrogen vessel/cryostat and the vent isolation chamber are given in document Designreliefvent.doc. In this document we describe the strength calculations for the vent isolation chamber and how the relief devices have been sized. 2. Arrangement The purpose of the vent isolation chamber is to couple the hydrogen vessel and isolation vacuum lines to the single line vent stack through the relief and rupture disks. The vent isolation chamber contains a large relief valve of the hydrogen vessel and three rupture disks. The rupture disks are the primary safety devices used to achieve Code compliance for the high mass flow accident scenario situation. The safety plan assumes that the rupture disks will open only when there is a catastrophic failure of the isolation vacuum or rupture of the hydrogen vessel. The second disk in parallel configuration on the isolation vacuum is for additional redundancy. In addition to these two rupture disks is a parallel small throughput spring-loaded relief valve, RV201, with opening pressure of 3 psid which is designed to vent small amount of gas from the isolation vacuum into vent stack to protect the isolation vacuum rupture disks from unnecessary rupture. During a long target run a small amount of degassed gas can be condensed to the cold surfaces in the cryostat and during a warmup the pressure in the isolation vacuum could raise over the pressure outside the cryostat and needs to be relieved. Note that the replacement of the broken rupture disks in the vent isolation chamber requires a major effort. The vent isolation chamber contains the relief valves and rupture disks for the hydrogen vessel and the isolation vacuum chamber. The 1.5 in diameter hydrogen line and the 6 in diameter vacuum line are installed coaxially. This coaxial line is called the fill/vent line and it runs from the target cryostat to the vent isolation chamber where the two lines are separately connected to the inlets of the vent isolation chamber, see Fig. 1. The outlet of the vent isolation chamber is connected to the vent stack. The chamber is mounted inside of a vent isolation
cabinet along with valves and other components of the gas handling system. All the demountable joints are made from CF flanges with copper gaskets and the bolts of the joints are tightened to the recommended torque by manufacturer. Coxial fill/vent line From isolation vacuum Vent isolation chamber Hydrogen line Fig. 1 The model view of the vent isolation chamber inside the ventilated vent isolation cabinet when the cabinet cover has been removed Inside the vent isolation chamber (a cut view is shown in Fig. 2) are the pressure relief devices for the LH 2 target vessel; relief valve RV104 (set point: 20 psid) and in parallel burst disk RD101 (set point: 30 psid), and the burst disks for the isolation vacuum, RD201 (set point: 7 psid) and in parallel RD202 (set point: 7 psid). Parallel with these rupture disks is also the low throughput relief valve, RV201 with a setpoint of 3 psid. RV201 is not connected to the vent isolation chamber but to the vent stack.
Fig. 2. A cut view of the vent isolation chamber showing the relief valve, RV104 and rupture disks, RD101, RD201, and RD202. The vent isolation chamber is a cylindrical vessel, constructed of 10-inch, Schedule 10 NPS stainless steel pipe of thickness 0.165 inch, with flanged, flat Conflat heads on either end. It is 28 inches long. The inlet to the vent isolation chamber is the 1.5-inch pipe from the LH 2 target vessel through a flanged head. The flanged outlet at the other end is for the 6.0-inch diameter vent stack which conducts the relieved hydrogen to outside of the Target Building. The thickness of the two end flanges is 1.120 inch. The end of the large relief valve RV104 is stabilized against transverse motion relative to the chamber axis by a spider alignment fixture see attached drawing 315340-001. Additionally, there are two 10-inch full inlet joints on the side of the cylindrical shell, which are connected to the target isolation vacuum chamber. Lifting lugs are provided for motion and positioning. The design drawings of the vent isolation chamber are in attached set of drawings #315337. 3. S trength Calculation of the Vent Isolation Chamber The following calculations were performed to evaluate the mechanical strength of the designed vent isolation chamber. The cylindrical part has a length of L = 28 inch, a diameter of D 0 =10.75 inch, and a wall thickness of t = 0.165 inch (10-inch NPS, Sch. 10S). Thus L/D 0 = 2.60 and D 0 /t = 65.2. The thickness of the flat end heads is 1.120 inch. The required minimum wall thickness of the straight pipe according to Paragraph 304.1.2 of ASME B31.3 for an internal pressure of p = 150 psi is
( SE + py) 150( 10.75) [ 20000( 1) + 150( 0.4) ] pd0 tmin = = = 0.0402 < 0. 165inch 2 2 The required thickness for the flat heads on either end is according to UG-34 of ASME VIII, Div.1.: ( 0.33) 150 ( ) Cp t req = d = 10.42 = 0.518 < 1. 120inch SE 20000 1.0 In summary, the maximum internal pressure in the vent isolation chamber is defined by the mass flow and the flow impedance of the vent stack. The calculated maximum pressure in the vent isolation chamber during the worst possible incident is 4 psig, which is significantly below the 150 psi used in the calculations. 4. S pecifications for R elief Valves and R upture Dis ks 4.1 H 2 vessel relief valve The specifications for the LH 2 vessel relief valve, RV104 are: 1. vacuum tight to the level of 10-9 bar cc/s, allowing the LH 2 vessel to be evacuated for leak checking and held evacuated without leakage. 2. no leakage under pressure up to 85% of the set pressure. 3. withstand the maximum operating pressure of 32 psia and the maximum allowed working pressure of 40 psia 4. handle the mass flow under an accident scenario In choosing the LH 2 target vessel relief valve, RV104, we assume that the mass flow (maximum H 2 mass flow rate = 0.009 lb/s) is caused by loss of the isolation vacuum. The pressure difference through the 1.5 fill/vent line up to the relief valve is 21.7 psid for a resistance coefficient of K = 40. The calculations behind this estimate can be found in the document Designreliefvent.doc. The H 2 relief valve, RV104, is an Anderson Greenwood (AG) direct-acting spring valve model 83 with a J orifice (1.287 in 2 flow area) and a 20 psig set pressure. The ASME rated mass flow capacity at set pressure is 0.306 lb/s which is larger than the assumed maximum H 2 flow rate of 0.009 lb/s. 4.2 H 2 vessel rupture disk The specifications for the LH 2 vessel rupture disk RD101 are: 1. vacuum tight to the level of 10-9 bar cc/s, allowing the LH 2 vessel to be evacuated for leak checking and held evacuated without leakage. 2. no diffusion of gas from the vent isolation box into the main vacuum 3. withstand the maximum operating pressure of 32 psia and the
maximum allowed working pressure of 40 psia 4. handle the mass flow under an accident scenario We use Fike Corporation model SR-H rupture disks that have a molded-on gasket and fit Tri-Clover flanges. A 2 diameter rupture disk model 2 SRL/Ni/50#/72F/STD with SRL-GI/150#/CS/CS holder with range of +0 10% was chosen. The pressure set point is 30 psid, which is greater than the RV104 relief valve set point of 7 psid. The Fike catalog gives a flow resistance coefficient K =1.88 for a ruptured SR-H disk which indicates a rather free opening after the rupture of the disk. To satisfy the requirement #2 we designed a double o-ring seal for the rupture disk flange which did not require any modification of the disk mechanical structure and therefore preserves the integrity of the manufacturer s pressure rating. In the inner grove we use indium wire to prevent diffusion of gas to isolation vacuum. Fig. 20. Design of the double seals for the rupture disks. For details see drawing SNEUT-Y-212-015 Main Vent Assy.pdf, available on web page www.iucf.indiana.edu/u/lh2target/export-files/. The design incorporates an indium groove inside of the viton o-ring groove so that there is no diffusion of gas from the relief pipe into the isolation vacuum. This design produces no mechanical modification to the burst disks themselves which are certified by the manufacturer. 4.3 Vacuum rupture disks
Specifications for rupture disks RD201 and RD202: 1. Must be vacuum tight to the level of 10-9 bar cc/s. 2. Pressures: a. Normal operation = vacuum b. Vacuum chamber design pressure = 32 psia c. Maximum allowable working pressure = 29 psig d. Pressure set points = 7 psid and 7 psid. We chose the Fike model 4 SRL/Ni/30#/72f/std rupture disks with pressure tolerance of 0 10% and burst pressures of 7 psid. The holder is 4 SRX- GI/150#/CS/CS. 4.4 Vacuum relief valve Specifications for the small relief valve, RV201: 1. Must be vacuum tight to the level of 10-9 bar cc/s, which is a low enough leakage rates that vacuum chamber can be isolated from pump system for weeks. 2. Essentially no leakage under pressure up to 85% of set pressure 3. Pressures: a. Normal operation = vacuum b. Vacuum chamber design pressure = 32 psia c. Maximum allowable working pressure = 29 psid d. Pressure set point = 3 psid We chose Swagelok valve SS-4CA-VCR-50 for this small relief valve. Backpressure on Rupture Disks RD201 and RD202 In the case of a failure of the two refrigerators (e.g. power failure) with no vacuum failure the LH 2 target vessel will slowly warm up causing the liquid hydrogen to boil off. This will cause a slow pressure increase in the 1.5-inch relief line up the pressure relief valve RV104, situated in the vent isolation box, till RV104 opens at its set pressure of 20 psid. Then hydrogen gas will flow into the vent isolation box causing a pressure increase in the box. Assuming a normal atmospheric pressure of 14.7 psi at Oak Ridge, then the maximum pressure in the vent isolation box is 14.7 psi with respect to vacuum, provided that the gas flow into the vent isolation box is not larger than the gas flow out of the box into the atmosphere.