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9.2 Clogging by vaporization
Since MV vaporizes a liquid material and supplies the vapor material, the particles in the
material and product generated by thermal decomposition of the material may cause
clogging in MV. If clogging develops to completely block the passage, the inside of MV
cannot be purged. We recommend you to consider taking the following measures to
monitor the clogging state and conduct periodic inspection and overhaul.
(1) Provide a pressure sensor between the MV carrier gas supply port and the MFC for
carrier gas to monitor the pressure while the carrier gas is being supplied. Then, you
can check for internal clogging based on changes in pressure loss of the carrier gas in
MV. However, when the operating conditions, such as the carrier gas flow rate, liquid
material flow rate, secondary pressure and MV temperature, change, also the pressure
being monitored changes. It is necessary to compare the pressure under uniform
conditions.
(2) The valve travel can be monitored by checking the supply voltage to the control valve
(valve voltage monitor output). Since MV is normal open, when it is fully closed, the
control valve voltage increases up to approx. 120 V. However, when the flow rate is
controlled, the voltage reduces so that the liquid flows at the specified flow rate. By
comparing the voltage while the flow rate is controlled, clogging of the valve can be
checked. Like the carrier gas pressure, the voltage is affected by the carrier gas flow rate,
vaporization flow rate, secondary pressure, MV temperature and liquid supply pressure.
Therefore, it is necessary to compare the voltage under uniform conditions. When the
voltage is 0 V, the valve is fully opened to the limit travel. Therefore, as the voltage
becomes closer to 0 V, the valve travel has less allowance. If the voltage keeps lowering
under the normal working conditions while the flow rate is being controlled, purge the valve
before it is completely blocked, and replace it with a new one.
