proj:  NEXT-100 XENON TPC
        Energy Plane, Tooling 	
 title:  Pressure Safety Note, Pressure Test Chamber   
DRAFT
This is an Engineering Safety Note for a small pressure vessel to be used for gas pressure testing prototype photomultiplier tube (PMT) enclosures for the NEXT-100 Xenon Time Projection Chamber (TPC). The PMT enclosures operate under external xenon gas pressure of 15 bara.
	The pressure vessel is a simple construction using Victaulic grooved pipe couplers with a section of 6" IPS schedule 40 pipe, 9 inch long, and two 1 inch thick caps. This results in a much lighter and easier assembling vessel than a similar one with ANSI flanges. One cap, the lid, is drilled and tapped for several 1/4NPT fitting to allow pressurization/vent, pressure relief and two service tubes from the PMT enclosure inside the vessel to be brought out. 
	The vessel will likely be used no more than 2 dozen times. 

Background 
NEXT is a collaboration between LBNL and other institutions to build a detector to observe neutrinoless double beta decay. The NEXT-100 TPC will contain 100 kg of enriched xenon gas at up to 15 bara pressure. The PMTs used inside this gas volume cannot withstand this pressure, so an enclosure has been designed to protect it. 
	Although not the subject of this Note, this PMT enclosure is designed in accordance with ASME PV code sec VIII div. 1, and sees external pressure only. It is fabricated from OFE (C10100) copper for high radiopurity, and is heavily built to provide shielding from background gamma rays.  The enclosure incorporates a single crystal sapphire window for the PMT to view light produced in the xenon gas; this is the critical strength component of the enclosure due to its brittle failure mode. There will be a total of 60 of these windows inside the TPC, and they have been designed using the methodology of the Weibull distribution, in combination with linear elastic fracture mechanics to provide a quantitative estimate of reliability. These windows will be pressure tested hydrostatically in a separate test chamber (described in a separate note) before using. 
	This pressure test chamber is for the purpose of demonstrating proper PMT operation under high pressure conditions. We are primarily interested in measuring Xenon permeation through O-ring seals, and the achievable vacuum inside the enclosure under actual external pressure, using different conduit tubing diameters. We want to verify the collapse resistance of the different tubing diameters (after bending to shape). We plan to use an aluminum alloy substitute "window" here for most tests to avoid any brittle fracture possibility. We may run a pressure test with a pre-tested sapphire window installed; we will first do this without a PMT inside the enclosure, and with a high flow filter or muffler installed on the outside tubing (open to atmosphere) so as to capture any fragments of sapphire should it fracture. 
	 The maximum pressure to be applied to the windows, for testing is 1.6 x 15 bara = 360 psia; therefore we set 350 psig as the MOP for the test vessel described below.   

Assembly and Operation 
The PMT enclosure prototype, (with cable conduit and vacuum sense tubes assembled) is placed inside the vessel, as shown above. The lid is placed over the tubes and the top clamp assembled. The through bored Swagelok tube fittings have plastic ferrules to avoid tube deformation that would disallow lid removal without cutting the tube. 
	The space between the vessel and the enclosure will then be pressurized with inert gas, thus applying an external pressure to the PMT enclosure. During this time, a vacuum will be applied through the cable conduit tube, and the vacuum inside the PMT enclosure will be measured with a vacuum gauge applied to the vacuum sense tube. In further testing the PMT will be assembled inside the enclosure, with electrical signals brought out through the cable conduit/vacuum tube; allowing  the PMT operation to be verified. the PMT operates at a modestly high voltage 1.5 kV and we are concerned about flashover on the pins and resistor base

ASME certified pressure relief valves are available in 400 psi (closest size)

Check , is MAWP > 110% MOP? (per PUB3000 recommendations)

The pressure vessel is simply a section of 6 inch schedule 40 stainless steel pipe, grooved on each end for Victaulic grooved pipe couplers (type 77, flexible). the ends are capped with stainless steel lids machined from plate. 
All components except the pipe, lid and cap, are manufacturer rated for working pressures higher than MAWP above. The lid and cap are designed and made here at LBNL.
Pipe Calcs
Pipe is a  6" schedule 40 IPS ( 6.625"OD x .28"thk) welded 304 stainless steel. It is likely ASME SA-312 (general purpose austenic stainless welded and seamless pipe), though provenance is unknown.
Maximum allowable stress, from 2009b ASME PV code sec II, part D - table 1A,

From UG-27  pressure rating would be:    

However, pressure rating is conjunction with the Victaulic type 77 flanges is given in Victualic publication # 17.09 "Pressure Ratings and End Loads for Victaulic Ductile Iron Grooved couplings on stainless steel pipe" :  

Therefore:  

pipe has a cut groove
This publication also states that a one time only overpressure of 1.5x the max. working pressure is acceptable for a field test. We will not exceed the working pressure for the hydraulic presure test
Lid and Cap Calculations
From UG-34 Unstayed Flat Heads and Covers

diameter:

attachment factor:

from fig. UG-34:

material : steel SA--283 carbon steel or better (plate is SA-240 304L stainless steel (UNS30403)

from table 1A ASME PV code,sec II part D

(weld efficiency)
from Table UW12

Minimum head thickness is then:

(1)

This completes the calculation for the (bottom) cap. The lid has four 1/4-NPT tapped holes in it:
UG39 Reinforcement Required for Openings in Flat heads
UG-39(a) General, rules in this section are exempted for openings that do not exceed size and spacing limits of UG-36(c)(3)
UG-36 (c) (3) Strength and Design of finished Openings:

<--no rapid fluctuations
<-- not applicable
<--holes <60mm dia
<-- check this below
<-- check this below
Lid drawing is shown below (excerpted):

holes do not form clusters so (d) is not applicable; closest hole spacing is 

These are 1/4NPT holes

Reinforcement is not needed, and in fact, 1/2-NPT holes could be used, where  d=.73in
Nevertheless we calculate here:

We have no nozzle, weld or reinforcement, reinforcement area is only available as extra shell thickness

Area or reinforcement required UG-39(b)(1), total:

t as defined in UG-34
UG39 (b)(2) holes may be reinforced individually as above, as no two holes have a distance between their centers less than 2x their avg. diameter ( 2d=1inch < min. distance)
from UG-40(b) Limits of Reinforcement, limit parallel vessel wall, from hole axis: 

We can see in the drawing above, that these limits will not overlap
from UG-40(c) Limits of Reinforcement, limit perpendicular  to vessel wall: 

This is greater than the nominal plate thickness, so all excess plate material can be counted for reinforcement.
Area available for reinforcement, on each side of opening axis

t, th as defined in UG-34
Check:

Cap and Lid material certs:

The remainder of the vessel will consist of Swagelok tube fittings, stainless steel, in tube sizes from 1/4 to 7/16. Swagelok pressure ratings are based on the size of the lowest rated end connection, which is 1/4-NPT. From Swagelok publication MS-01-140.pdf :