Anyone planning to automate a handling system for cryogenic liquids will discover there is a variety of valves, actuators, and positioners available. The problem of which to choose is confounded by the fact that many are from different manufacturers and will require costly modification to provide the performance you require.
The best way to choose a system is to find out all the different components available and how the various combinations can affect the cost, size, weight, and performance of the whole package.
Valve types
The most popular valves for on-off control and flow modulation of cryogenic fluids are globe, gate, and plug valves in sizes up to 2 in (50 mm), knife-gate valves from 2 to 3 in (50 mm to 75 mm), butterfly valves from 3 in (75 nun) and up, and ball valves from ~A in (12.5 mm) to 6 in (150 mm) and larger.
Globe and gate valves will handle pressures to 5000 lb in-z (35 MN m -2 ) and temperatures to 5 K. They are suitable for use where some leakage can be tolerated. Should leakage occur in the closed position, it can usually be stopped by tightening the valve disc against the seat which is either metal or a soft material such as KeI-F. However, overdosing a globe or gate valve with a metal seat will make it difficult to open if the valve is allowed to warm up in the dosed position and often results in seat damage.
Globe valves are best for modulating flow both manually and automatically but provide the lowest flow rates of all the valves, Table 1. When the valves are properly installed, their seats are less affected by fluid velocities during flow control than those of the other valves.
Gate valves are usually full ported to provide straight-through flow and high flow rates. They are good for throttling fluids at high velocities but only withcorresponding high pressure drops across the valves. Their seats are less exposed to flow than those of the other valves and, hence, are less susceptible to wear.
Rising plug valves are normally contoured or characterized for control purposes and are considered too expensive for simple on-off applications. They will handle pressures to 1500 lb in-2 (19 MN m -z) and temperatures to 5 K and
The author is with the Engineered Products Group, Worcester Controls Corp, West Beylston, Mass, USA.