Principia Cryogenica

Principles of mechanical design of large scale refrigeration systems

Intended to expand upon the mechanical design considerations given in the Cryogenic Seminar (Rao Ganni, 2011) and other key resources.

As this article grows, it may be split into separate pages.

Mechanical design considerations from a process basis

Maxims / universal truths

1. All valves leak

Chronic diseases of large-scale helium refrigerators

Contamination entry due to air ingestion

Concern

The cryogenic helium process is extremely sensitive to air contamination. At even the parts per million level, this air contamination will ruin our process over a relatively short period of time.

Air within the cryogenic refrigerator will freeze out, typically blocking small flow passages in heat exchangers. This leads to increased pressure drop and poor flow distribution, lessening the heat exchange efficiency. This can cause increased electrical power consumption at the main compressors, increased LN2 consumption in the cold box. In addition, the lower temperature of helium returning to the compressor suction presents a major risk of equipment damage if the decreased temperature causes the compressor oil to become much more viscous.

A subatmospheric (vacuum) helium process is especially vulnerable to contamination by air ingestion, as the pressure differential across any leak between the process and the atmosphere will cause air to enter the system.

Mitigations

The primary mitigation is to build leak-tight systems and aggressively fix leaks that appear during operation.

Positive-pressure equipment such as the suction side of the warm helium compressors are controlled at a pressure just above 1 atm (typically 1.08 atm).

Seals in subatmopheric piping are protected by a guard vacuum. The guard vacuum gives us enough protection to safely handle the failure of any one sealing component. Generally there are two concentric o-rings, with the space between the seals evacuated to the guard vacuum pump. If the outer o-ring fails, air will leak into the guard vacuum system and will be removed by the guard vacuum pumps. If the inner o-ring fails, the subatmospheric helium process gas will also leak into the guard vacuum and will be removed by the guard vacuum pumps.

Helium inventory loss due to leakage to atmosphere

Concern

The cryogenic helium refrigerator operates on a fixed inventory of helium, and any losses due to leakage must be replaced. Helium is a non-renewable natural resource, and as cryogenic operators we have a responsibility to be good stewards of this resource.

Mitigation

The primary mitigation is to build leak-tight systems and aggressively fix leaks that appear during operation.

Material selection

Weld & braze consumable materials

Welding

The choice of what filler material to use should come from the engineer either on the drawing, sketch or DPF if it is a pressure system. If there is a difference of opinion between the shop and the engineer, then the shop should make every effort possible to contact the engineer, or supervisor, to resolve this difference before an arc is struck. Any updated filler material will need to be edited into the documentation.

For the vast majority of welding of stainless steel, we can use ER316L-low ferrite rod. We must use it for any cryogenic application below 77F and it's probably easiest just to say any piping that would normally see cryogenic temperatures. There are other options available but we bought a lot of this rod several years ago for this application.

We can also use this rod for non-cryogenic applications if this makes things simple and reduces the chance that we will use a rod not suited for a cryogenic application. However, ER308L can be used for most non-cryogenic applications such as vacuum jackets if engineering and the shop agree to this policy and we can demonstrate that this won't jeopardize future cryogenic welding.

We also use ER309L for welding low alloy and austenitic stainless steels together--this is usually done when adding a SS pant leg to a carbon steel vacuum shell.

Brazing

AWS 5.8 BAg-7 silver brazing filler metal should be used for copper-to-stainless brazes designed for high thermal conductivity at cryogenic temperatures. Products include Harris Safety-Silv 56 and Lucas-Milhaupt Silvaloy 560. This material gives the best combination of low melt temperature, high thermal conductivity at cryogenic temperatures, and good flow/wetting as reported by the brazers.

Alternatively, AWS 5.8 BAg-5 silver brazing filler metal can be used for structural connections between copper and stainless, but where where thermal conductivity does not matter.

We typically use Harris Stay-Silv white brazing flux. This seems to work for a wide range of silver-bearing braze materials. Since the flux residues are soluble in warm water, we clean away excess flux by steam-cleaning. Alternatively, you can break away flux residues mechanically with a mild abrasive.

We use Tempil Anti-Heat to protect portions of complex assemblies from overheating during braze and weld operations.

Surface coatings

Carbon steel

Carbon steel exposed to atmospheric conditions should be sandblasted and painted. Apply one coat of a rust-inhibitive primer, followed by a finish coat of high-build, controlled chalking, alkyd resin-based white paint to a minimum dry film thickness of 4 mils.

Galvanizing should be avoided, especially for structural components where it's conceivable that we may need to weld to it in the future.

Carbon steel exposed to vacuum (for example, inside surfaces of gas storage tanks and large insulating vacuum shells) should be sandblasted and not painted.

Stainless steel

Stainless steel should be cleaned in a way that will not induce stress corrosion cracking. The use of carbon steel wool or brushes to clean should be prohibited by an adequate control process in the fabrication shop. The surfaces should be passivatated per ASTM A380. Do not paint.

Soft seals

O-rings

O-rings are typically a GLT type fluorocarbon (Viton) in accordance with AMS 7287. This material has an improved low temperature resistance vs typical commercial grade FKM. This material has a 75 Shore A hardness.

In higher radiation environments, it is recommended to substitute EPDM for Viton.

Gaskets

Flanges on warm helium piping may be sealed with spiral-wound semi-metallic PTFE-filled gaskets such as Flexitallic style LSI.

Flanges on cooling water piping may be sealed with gaskets. Any compressed vegetable fiber gasketing material (that doesn't contain asbestos) may be used. Typical gasketing material is green Texcel Tex-Seal 6010, 1/8" thick. Comparable Garlock style 3000 is also acceptable.

Component selection

General notes

Typically, components installed in helium piping are to have socket weld connections to mitigate potential leaks.

Ball valves

Ball valves of size 2 NPS and smaller are to have "oval" handles.

Check valves

Care should be taken that when check valves are added to cryogenic piping, the piping downstream of the check valve is protected against overpressurization by a thermal relief valve.

Relief valves

Process relief valves should be ASME type. Reliefs should be sized such that the effective orifice area K*A is sufficient for the flow to be relieved. The flow regime may be either critical or sub-critical.

The mounting design should take into account the reaction forces due to the momentum change of the relieved flow, evaluated at the steady-state relief condition. In most cases, it is wise to install a device on the exhaust that redirects or (attempts to balance) the exhaust flow. Reliefs should not be exhausted towards where personnel might stand or walk. It is imperative to design the exhaust spool such that any moisture can drain out by gravity, especially if it is attached to a cryogenic relief that may attract moisture from the air when cold. The exhaust spool must also prevent entry of creatures who might like to live in it (birds, wasps, etc.).

To minimize time to replace, reliefs should have flanged interfaces to the inlet piping. Where they are exhausted remotely by some lengthy outlet piping, the outlet piping interface should be flanged as well.

Pilot relief valves should have a modulating pilot to minimize blowdown, thus mitigating helium inventory loss during a relief event. The pilot should relieve into the main body of the relief (instead of having a separate exhaust to atmosphere). In the event that the process goes subatmospheric, the pilot must not allow air to flow back into the inlet of the main relief.

Design for manufacturing

Pipe bending

A bending process is sometimes used to fabricate warm and cryogenic pipe spools in lieu of welding an assembly of various fittings and short lengths of straight pipe.

A significant fraction of the JLab cryogenic fabrication shop technicians (welders and pipefitters) know how to use the tool and most of them have completed pressure system projects using it. There is an experiential learning process that is required before people become proficient at creating correctly formed parts. To a project manager, that means you must budget time and material to allow trial and error to occur. Whenever we train a new user we probably generate 40℅ usable parts for the first few dozen bends, and 80-90℅ thereafter.

One reason you would choose to use a bend as opposed to a welded pipe elbow is because you want to increase flexibility in your assembly. That is, you want your piping spool to conform to the dimensions needed to satisfy the loads on the system.

It takes 3-4 parameters to fully define a typical bend. As you can see, none of these are very tightly controlled.

• radius
• bend angle
• position along pipe
• (if necessary) rotation angle w/r/t other features

Bend radius

The bend radius is fixed by the tooling chosen for the bend. When you purchase bender tooling you must specify pipe diameter and bend radius. For our bender, there is a very small set of available radius options for each given pipe size. Generally thinner-walled pipes must be bent using larger-radius tooling or else the intrados will collapse.

This is not to say that the bend radius matches the tooling radius. Because of springback (discussed next), the actual bend radius always exceeds the tooling radius by some small amount.

Bend angle

Because of material elasticity, pipes in their loaded condition in the bender will spring back some amount when they are returned to a free state. Because of this, some "overbend" is required to achieve the desired bend angle. An experienced operator will have a feel for this, and an inexperienced operator will iterate until he finds a solution. I believe our machine comes preloaded with a program that helps estimate the overbend but I don't know how effective it is. Often I have seen the technicians intentionally underestimate the overbend, then creep up on the intended angle by adding increments of bend.

The exact required overbend is a complex function which would be really fun to characterize. It appears to be dependent on

• nominal bend angle
• wall thickness
• choice of material
• choice of lubrication

Bend position along pipe

This becomes important when putting 2 or more bends on the same spool. This is critical for building a U-tube, for example.

On our machine, this is left entirely up to the operator. Our technicians usually make a mark on the pipe and line it up with one of several notches/pointers on the frame of the bender. It takes iteration and experience to get this right, but once a solution is found it is generally repeatable.

With the addition of a couple roller stands and a string potentiometer, it's possible to gain a digital readout of axial pipe position. In fact on our machine this is an option that we haven't yet purchased. Clearly the footprint of the machine would increase if we did this, and for the moment we value our bender for its easy portability.

If you add motors to drive the pipe along its axis, you can gain full CNC control over this parameter.

Rotation angle

This is important only when making 3-dimensional pipe spools. Like the previous parameter this is also left entirely up to the operator on our machine. Unfortunately there are no indicators to allow the technicians to repeat the setup, so it must be reestablished each time. Usually they level the top of the machine, then use an angle finder on the previous bend to set the orientation of the spool.

You also have the option of buying machines with NC or CNC control over this parameter.

There's also mandrel bending machines, which use an expanding tool to press against the ID of the pipe at the business end of the bend. This prevents collapse of the intrados and allows you to achieve tighter radii with thinner wall material vs non-mandrel bending. Mandrel benders have a necessarily larger footprint because the mandrel must enter through the end of the pipe.

It's worth noting that when you cold work 304 stainless steel, you'll induce some small amount of magnetism. It would be fun to measure this on our machine, but we haven't had the opportunity.

There's a soft brass wear component (called the wiper if I remember correctly) as part of the tooling. The manufacturer recommends heavily lubricating this part, as well as the pipe, with a white lithium grease before bending. Since it takes effort to degrease afterwards, we typically skip this step for pipe sizes less than 1.5" although this is probably not good practice. The ungreased wipers are wearing faster (although we have not had to replace them yet). Potentially worse, they're probably leaving behind brass particles which could lead to weld contamination if they're present in enough volume.

Project documentation

P&I development

The JLab P&I design "style" originates from the following principles:

1. Equipment in a thermodynamic process should be arranged on the P&I in generally the same location and orientation as they are found on a TS diagram. For example, warm compressors are shown on the P&I with the flow direction from right to left, just as they are on a TS diagram. Likewise, expansion processes are shown happening from left to right. Also, the 'main process' of the P&I will generally be oriented so the the piping with the warmest temperature located at the top of the drawing, and the coldest at the bottom.

2. Piping layout and vessel shapes should loosely resemble the actual geometry, within reasonable artistic interpretation (e.g. that jog in the line may actually represent a "thermal stability loop").

JLab P&IDs are traditionally created in the ME10 drafting system. Thus, much of our "style" results from normal behavior of ME10. There are considerable limitations; for example, it's especially tough to maintain consistency between drawings.

Very rarely we feel the need to create color drawings. More often we use only two or three different line thicknesses to distinguish the main process piping from the auxiliary systems. When these lines cross, we use a "jump" to make it clear that they don't connect. It's sometimes hard to decide which line should jump the other line, but typically the auxiliary piping will give way to the main process.

Other "best practices" of JLab P&I development include:

• maintain no more than one vacuum enclosure per drawing
• instrument valves (e.g. isolation valves, equalization valves, bayonet purge valves, etc.) fall below the threshold of the drawing scope and are not shown
• flags (which work like GOTO statements) are used to join far-away groups of piping
• reducers are never assumed; a reducer is shown explicitly wherever there's a reducing tee
• identical, repeating components are shown once; the example cases are labeled with variables in their names

Uncategorized

Branches should be oriented vertically down off piping headers.

Cryogenic valves should have vertical piping on either side to allow the entire valve to warm up when shut off.