Principia Cryogenica

Principles of mechanical design of large scale refrigeration systems

Component selection

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. The pilot should relieve into the main body of the relief (instead of having a separate exhaust to atmosphere).

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, most 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