Minutes of the meeting on the status of the VELO system held on
October 17/18 in Amsterdam
Participants: Jo van den Brand, Martin Doets, Luc Jansen, Michel Riet,
Eddy Jans, Hans de Vries (NIKHEF), Pierre Strubin, Adriana Rossi, Delio
Duarte Ramos (CERN).
Abstract:
A delegation from the CERN AT-VAC group has visited NIKHEF in order to
discuss the progress of the VELO vacuum system of LHCb. Various procedures
were reviewed and the general opinion is that significant progress was
obtained. A follow-up meeting has been planned on November 14, 2005 at
CERN.
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The presentation of Hans de Vries on the status of the VELO system was
followed by questions and discussions. The VELO team then showed the
functioning of the VELO pumping-venting system.
A summary of the whole meeting follows:
GENERAL
1) Measurements of surface outgassing from the different materials that
will be contained inside the RF box (including the Torlon coating on the
inside of the thin foil) have been carried out. The pressure expected
in the detector vacuum will be of the order of 10-4 mbar.
2) A prototype (about 250x250mm) of rectangular bellows compensator (new
design with folded undulations and welds only on the corners) has been
leak tested and cycled up to 15000 times and yet no failure was observed.
Two full-size rectangular bellows compensators have been made; they
behave as expected: they are leak-tight, and they move very smoothly.
3) The cooling capillaries for the detector planes have been tested to
150 bar. The operational pressure be 100 bar maximum. After completion,
the system will be tested at 150 bar.
4) NEG coating of the first prototype RF box has been performed at CERN.
CERN will write a report on it.
VACUUM
5) The gas injection and pump-down system for the VELO vessel (beam
vacuum) + detector volume was presented and discussed.
6) A pair of membrane switches OS412 (measuring differential pressure)
is used to activate, via a PLC, a system of process valves PV301, PV302,
PV303, PV304 and PV305. During the evacuation and venting procedures,
these valves close the corresponding conductances between the pumps and
the detector or the beam vacuum, as soon as the pressure difference
exceeds 5 mbar.
If the system is regulating around atmospheric pressure, another
procedure is used: the corresponding process valves will be opened if
the pressure difference exceeds 5 mbar.
7) In addition, a safety valve SV421 connecting directly the two volumes
is hard-wired to two additional pressure switches OS413. Whenever the
pressure difference overcomes 10 mbar, this valve is opened (the pressure
at which the thin foil of the RF box undergoes plastic deformation is
above 15 mbar).
In principle the safety valve will open only if something is seriously
wrong!
Its state is recorded and can be retrieved also in the case of a power
failure so that one always can determine if the valve was opened and the
beam vacuum was contaminated.
CONTROL OF THE INJECTION-EVACUATION SYSTEM
Part of the logic of the control system was explained:
8) Before venting or evacuation of the vessel is started, a "prepare"
procedure is performed. The state of all pressure gauges in front of the
gate valves GV302, GV112 and GV212, valves and pumps is checked. The pumps
are switched on. If no errors occur, the pumps are tested on a minimal
obtainable under-pressure before the check out is concluded.
9) During venting or evacuation, the process valves are activated in
order to maintain the pressure difference between detector and beam vacuum
below 5 mbar. As soon as the 5 mbar limit is exceeded, the corresponding
process valve will be closed, and it will be kept closed for a certain
period such that the other volume can catch up.
10) When either the venting or the evacuation process is started, the
system will use a fixed closure time for the process valves during the
first 5 minutes to allow the pressure between the two volume to equilibrate.
During injection these 5 minutes will also be used to adjust the flow
of gas into the system.
11) After these 5 minutes, the system will perform an optimization
procedure for the closing time. As soon as one membrane switch is activated,
a timer is started, and the corresponding process valve will be kept
closed until the other switch is activated. The time interval is determined,
the process valve related to this second switch will be closed, the
other valve is opened, and the time until the first switch is activated
again is determined.
These two time intervals give the time for the system to go from
delta-P = 5 mbar to delta-P = -5 mbar and visa-versa with only one
venting/evacuation line active.
During the further evacuation or venting procedure, the closing time of
the process valves is chosen as 80% of the measured time intervals. With
this choice, the system will operate around the equilibrium pressure,
and the number of switching actions is minimized.
12) The venting is stopped when the pressure switches OS411 between the
beam vacuum vessel and the outside atmosphere indicate a differential
pressure smaller than 3 mbar. After an adjustable time interval (presently
10 s) it is checked if the differential pressure is still between -3
mbar and +3 mbar. After that, the switches OS411 are no longer used for
the control, while the pressure between the detector and beam volumes is
regulated by injecting/pumping gas into either volume.
13) Before one is allowed to open the detector volumes, the valve PV306
venting the volume to atmospheric air shall be opened (but this is part
of a procedure, nothing can prevent people from unbolting one of the
detector halves if this valve is not opened!). The main reason is to
equilibrate the pressure with the outside atmosphere. The valve is interlocked
with the 3 mbar pressure switches, so it cannot be opened if the
pries re difference between the outside atmosphere and the beam vacuum is
larger than 3 mbar. Should that be the case (if for example the system has
been left for long enough and the outside pressure has undergone too
large variations), the operator should restart the venting process (in case
of atmospheric under-pressure he first has to pump the VELO for a short
time), so that the vessel will be refilled at the right differential
pressure.
14) At the end of the evacuation period, the TP301 for the beam
vacuum is switched off and isolated between two valves GV301 and GV302,
and the ion pumps IP431 and IP441 on the vessel take over.
15) The pressure compensation system is ON at all times, also when the
beam is on. In case of emergency, the hard-wired safety valve SV421
will be opened.
DEMONSTRATION
The VELO team demonstrated successfully the venting and evacuation
procedures for the VELO system (the vessel + one external volume simulating
the detector volumes).
16) Several "accidents" were induced to test the control system:
- Reset of the process (venting) before completion.
- Valve disconnecting
- Manual opening (which is password protected!) of one valve followed
by "accidental" injection of gas into the detector volume. This
resulted in opening of the safety valve.
- The UPS takes over if there is a power failure.
Note that the UPS can work for about 30 min, after which a diesel power
supply shall operate. AR has to make sure that the services necessary
for this to work are in place.
Few minor bugs were found in the process which will be fixed by Luc
Jansen. Note that the ion pumps are in self-protection mode so they switch
off if there is an over pressure (this might happen if the other ion pump
is switched on). In order to switch them back on one can reset the pump
at the touch screen, or one has to reset the system and restart the
evacuation process.
DISCUSSIONS
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VELO SCHEDULE: The VELO vessel will be shipped and installed at CERN in
Feb. '06. AR pointed out that this schedule leaves very little time for
the vacuum people to test and get accustomed to the system.
GRAVITY VALVE: AR asked what happened to the gravity valve that was
studied in the past as hardware interlock in case of failure of the pressure
compensation system. The VELO team replied that the concept has been
dropped due to the fact that a valve with a large enough conductance to
work, would have a large leak rate which would cause NEG contamination
from the detector to the beam vacuum.
SAFETY VALVE: AR suggested that the safety valve should have a
redundancy (i.e. a second identical valve), since the gravity valve is no
longer in place. PS pointed out that before taking the decision, a safety
analysis should be rerun to check if this is a real advantage or not.
Massimiliano Ferro Luzzi will be asked to carry out this analysis if possible
before the next meeting (see below).
RECTANGULAR SEALS: Between the rectangular bellows and the VELO vessel
on the one side and the RF box on the other side, there are VITON seals.
AR asked if, as discussed in a previous meeting, it would be possible to
replace those VITON seals with metal seals. NIKHEF replied that they
do not plan to do so mainly because they believe this would bring more
complications, in particular given the fact that it is not possible to
perform a leak detection on the seal between the vessel and the rectangular
flange (since the maximum pressure difference that can be used in such
an operation is only 15 mbar).
Ne CONTAMINATION:
1) AR fears that during the system evacuation, the pure Ne in the beam
vacuum could be contaminated by the back-stream through the turbopumps
when its rotational speed is slowed down to 10% at the beginning of
pump-down from atmospheric pressure. Moreover the rotational speed does not in
crease to 80% before at least 2 hours. People around the table thought
that in viscous regime, the dragging should be strong enough for the
contaminant not to reach the vessel. AR shall try to establish if this is
the case. If not, it was checked that is would be possible to add a filter
(NEG cartridge type) between the TP and the vessel that should be able
to pump the contaminants.
2) In order to reduce the risk of corrosion of the current leads to the
ion pumps (which is almost the only cause of failure of these pumps, as
experienced at CERN), it is advisable to heat the current leads
constantly at about 30C. This way the humidity condensation is reduced and
so is the risk of corrosion.
3) PS noted that during evacuation, in the beam vacuum line, at the
moment when the process valve PV302 to the 3 mm diameter conductance is
closed and the gate valve GV302 to the TP is opened (presently at 3.5 mbar
in the vessel), the rotational speed of the TP slows down from 100% to
below 70%. This is actually the critical moment when back-streaming
impurities could very well contaminate the NEG. The pressure at which
this operation is done will be adjusted to such a value that the TPs
rotational speed does not go below 80%.
4) At the end of the evacuation procedure, when the ion pumps take over,
the beam vacuum TP is isolated between two valves and is switched off.
In principle this should not constitute a problem since the roughing
pump is always running, and the TP will be started and tested before the
valves are opened again.
Nevertheless, the procedure should be checked to find if there is any
instance that could lead to contamination of the beam vacuum through the
TP before it is at its nominal rotational speed.
5) At present, turbo-molecular pumps are not baked. PS pointed out that
TP301 (the one connected to the beam vacuum) shall be baked (together
with the gas injection-pumping line). Therefore, it was agreed that TP301
will be equipped with a heater jacket.
6) AR shall provide the maximum flow delivered from the pure gas
injection system.
CONDUCTANCES: MD said that the fixed conductances for the pressure
control system depend on the inlet flow (during venting) and on the exact
vessel volumes. This means that they may have to be re-adjusted to the real
case. AR pointed out that this should be done with equivalent volumes
(1.7 m3 for the beam vacuum and 0.5 m3 for the detector vacuum) at NIKHEF,
since once the VELO is installed it will be difficult to perform any
complicated commissioning. MD mentioned out, that the volume of the beam
pipe should be known. Furthermore, he expects that final adjustment of
the restrictions can be done at CERN.
RISK ANALYSIS: Previous risk analysis studies were made for a different
system composition (for example with the presence of a so-called gravity
valve). Note that various systems have redundancy (such as the switches
set at 5 and 15 mbar), while others (bypass valve between LHC beam
vacuum and detector vacuum) have no redundancy. The risk analysis should be
re-addressed.
MEETING AT CERN in NOV. '05.