GEA Grasso - NH Circuit

OPERATING INSTRUCTIONS
CO
2
/NH
3
CASCADE UNIT
250 - 1000 KW
38 _551511_om_kas_gbr_1_.doc
• Safety device to prevent exceeding of the
maximum shutdown pressure in the CO
2
liquid
separator, pressure switch (5900)
The auxiliary refrigeration unit (6800) is switched
on if the maximum allowable shutdown pressure
is exceeded.
3.5.2 NH
3
circuit
• Security devices against exceeding of the final
discharge pressure
— Pressure transducer (105)
The compressor control switches off the
cascade if the limit pressure is exceeded.
Limit value final pressure alarm (max) = see
parameter list
However, before the limit value is reached
the output level of the NH
3
compressor is
lowered to prevent the limit value from
being exceeded.
— Safety pressure switch (350)
The NH
3
compressor is switched off if the
limit value is exceeded.
Limit value p = 0.9 x p
max
bar
(p
max
= maximum allowable operating
pressure on the high-pressure side)
— Pressure relief device (overflow valve 340)
The back-pressure independent overflow
valve fitted to the oil separator discharges
on the compressor's intake side when p
max
is
reached.
• Pressure relief device of the pressure vessel on
the low-pressure side (discharging safety valve
2230)
The safety valve discharging into the outside
opens the low-pressure side of the plant if p
NDmax
= 16 bar g.p. is reached.
• Safety device to prevent the suction pressure
falling below the allowable limit (pressure
transducer 100)
The compressor control switches off the cascade
if the pressure falls below the limit value.
Limit value p = p f(to-10 K)
However, before the limit value is reached the
output level of the NH
3
compressor is lowered
and the hot gas bypass is opened on the ND
(low-pressure) side to prevent the pressure
falling below the limit value.
• Safety device to prevent the final discharge
temperature from being exceeded (resistance
thermometer 120)
The compressor control switches off the cascade
if the limit pressure is exceeded.
Limit value t = 110°C
• Safety device to prevent the oil temperature
from being exceeded (resistance thermometer
125)
The compressor control switches off the cascade
if the limit pressure is exceeded.
Limit value t = 90°C
• Safety devices of the compressor drive motor
— Rated current limitation (016) which is
realised by the respective compressor
control.
If the rated motor current is exceeded, the
output level of the NH
3
compressor is
lowered until the motor current is back
within the allowable range.
The normal output control then comes back
into force.
— Thermistor (017) which shuts down the
compressor drive motor if its winding
temperature limit has been exceeded.