Operating the Buffer Dewar Controls

There are three valves directly connected to the 500 l buffer dewar:
EV9180 (supply JT), EV9178 (cold return JT), and EV9177 (warm return ball valve)

Incorrect operation of any of these valves is likely to disrupt the 4 K liquid helium distribution system in Hall C and could produce an ESR outage that would halt the experiments in all three halls for many hours. Changes in the status of any of these valves or the PID loops that control them should only be made after consultation with:

Mike Seely		Chris Keith		Dave Meekins
office: 269-5036		office: 269-5878		office: 269-5434
home: 223-1216		home: 596-3002		home: 874-4750
pager: 881-1194		pager: 888-4090		pager: 872-2442

The following conditions may indicate a problem with the buffer dewar system:

Incorrect liquid level in the buffer dewar

The PID loop is set to maintain this level at 65%. It will vary under normal operation from around 60% to around 70% during the fill cycle.

Vapor plume at the top of the buffer dewar

A pressure excursion in the cold return line may cause the relief valve on the buffer dewar to open. It may freeze open. A brief access to the hall is necessary to warm the relief valve.

Incorrect buffer dewar pressure

The cold return JT valve (EV9178) operates to maintain this pressure around 1300 mbar.

High return temperature

If the flow rate in the return line falls too low, the return will warm up. The return temperature TD9178 should never rise above about 6.5 K. It has been our experience that the return line will stay below this temperature as long as the supply valve EV9180 remains 35% or more open. PID loop EV9180A controls the minimum allowed setting for EV9180 to keep the return line at 5.8 K or below. However, attempting to maintain a return temperature that is TOO low will result in minimum setting for EV9180 that is too high. In this case the buffer dewar may overfill.

Oscillations in 4 K flow to the hall

Sharp spikes or oscillations on the 4 K helium flow to the hall frequently indicate that one or more branches of the 4 K distribution piping is warm.

Unusual valve settings

The supply JT valve EV9180 is normally open 35-70%. The return JT valve is normally around 50-70% open. The warm return valve is always closed. Excursions much beyond these valve positions will produce alarms.