[Esr-users] Cryo ESR operations feedback

[David Schleeper]

ESR Users,

For many years, the Cryogenics Department has predicted that the Halls would require cryogenic capacity far above the capabilities of the End Station Refrigerator (ESR) and would require constant support of the CHL transferline to meet the proposed loads. It was also published that recovering from ESR trips would require considerable more time for the third priority Hall because of the lack of capacity. The dependence on CHL also means that trips of the CHL1 plant might lead to problems for the third priority Hall depending on the duration of the down, because of the inability to support transferline flow during CHL recovery system. These predictions were made based on our knowledge of the CHL1 and ESR plants and the estimates of magnet heat loads provided by Physics Division.

During the 2017 summer Accelerator shutdown period, the Cryogenics Department and groups from Physics Division performed tests using a mixture of real and simulated Hall loads to determine where the operating points of the machines would move. During these tests, ESR appeared to handle the loads as predicted. After the tests, there were several instances where there appeared to be more actual load on the ESR than during the tests, thus requiring more CHL support. No explanation was ever found.

As predicted, it has been recently observed that under the current load configuration (low lead flows, no magnet current), the ESR is very near the maximum possible output. As the loads increase, the amount of assistance from CHL increases. The ESR dewar contains a heat exchanger that provides final cooling of helium flow to the Halls. Whenever the ESR loads exceed plant capacity, the dewar’s LHe inventory depletes and the boil-off increases the flow of gaseous helium, which must be processed at the main compressors which have a limited capacity.

There are two standard methods of utilizing the CHL1 transferline 4.5K flow to boost the capacity of the ESR that have been in use for many years. The first and preferred method is to route the 4.5K flow to the LHe dewar to help maintain liquid level. The second is to route the 4.5K flow directly to the primary 4.5K supply line to the Halls to help maintain supply pressure to the Halls. Both of these arrangements are highly sensitive to pressure oscillations at either ESR or CHL could result in lost capacity rather than added capacity if operating points drift and requires careful tuning. These scenarios were considered during the operational planning to support the Fall run.

It was recently observed that ESR must operate near its maximum discharge pressure to provide an adequate supply pressure to the halls.  Additionally, with 12 g/s supplied from the CHL transferline the ESR compressor system is nearly out of capacity to process Helium.  We were recently asked about two other possibilities to increase the ESR 4.5K supply flow to the halls:

Using the ESR Turbine 2 (T2) bypass valve:
This method was investigated many years ago and resulted in very unstable operation of the plant. Conceptually, the bypass valve is effectively a zero efficiency turbine in parallel to T2, thus the thermodynamic output condition of each device mismatch. Opening this valve presents a real risk of tripping the T2 turbine upon each opening resulting in lower plant reliability. Additionally, opening this valve results in higher dewar boil off rates increasing the gas flow back to the compressor system. The compressor system is already operating at maximum capacity (interstage pressure is ~2.6 and no longer able to be controlled by the bypass valve) and cannot process any additional gas.  Further, with the added dewar boil off, it will be necessary to use the CHL transferline at all times, implying a dependence on CHL as well.  This additional mass into the system will further strain the already limited compressor capacity.

Increasing T2 speed:
This method was also investigated many years ago and results in moving the turbine to a lower operating efficiency point which lowers capacity. In addition, the increased turbine speed will result in less energy extraction from the helium process which results in higher dewar boil off rates increasing the gas flow back to the compressor system. The compressor system is already operating at maximum capacity (interstage pressure is ~2.6 and no longer able to be controlled by the bypass valve) and cannot process any additional gas.

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Thank You,

David Schleeper, PE
Mechanical Engineer, Cryogenics Engineering
Thomas Jefferson National Accelerator Facility
757-269-6532