1. The document discusses different methods for controlling parallel gas compressors to prevent surge during varying load conditions. 2. The base load method operates one compressor at maximum flow while the other swings based on demand, but is inefficient and requires frequent intervention. 3. The suction side speed control and equal flow balance methods aim to control both compressors independently using a master pressure controller and additional elements, but have disadvantages related to complexity and control dynamics. 4. The equidistant to surge line method coordinates anti-surge and load sharing controllers to keep the operating points of both compressors equally distanced from the surge line to handle varying loads while preventing surge.

1. SURGE CONTROL FOR PARALLEL GAS COMPRESSOR OPERATIONS
BASE LOAD OPERATION OF PARALLEL
COMPRESSORS
M
KO Drum
Suction Block
Valve
Anti-Surge
Valve (ASV)
Compressor A
(BASE LOAD)
CoolerUIC
A
SC
M
KO Drum
Suction Block
Valve
Anti-Surge
Valve (ASV)
Compressor B
(SWING LOAD)
CoolerUIC
B
SC
CommonDischargeHeader
PIC
HIC
Set Point
(SP)
BASE LOAD METHOD
1. Compressor A Speed is manually set (HIC) for
maximum flow & termed as ‘Base Load’ while
Compressor B Speed via speed controller (SC)
is swung based on Master Pressure
Controller’s (PIC) Set Point (SP) to attend to
remaining swing in flow variations (loads).
2. During periods of low process demand,
Compressor B (Swing Machine) can be
recycling & sometimes even close enough to
the Surge Control Line (SCL) causing the
Swing Machine to Trip. Hence an inefficient
configuration.
3. Base Load Operating Style requires frequent
operator intervention
4. A configuration that is not in vogue anymore &
not recommended.
Discharge
Block Valve
Discharge
Block Valve

2. SURGE CONTROL FOR PARALLEL GAS COMPRESSOR OPERATIONS
SUCTION SIDE SPEED CONTROL OPERATION OF
PARALLEL COMPRESSORS
M
KO Drum
Suction Block
Valve
Compressor A
Cooler
SC
M
KO Drum
Suction Block
Valve
Compressor B
Cooler
SC
CommonDischargeHeader
SUCTION SIDE – SPEED CONTROL
METHOD
1. No Base Load Exists, but instead the Master
Pressure Controller (PIC) is shifted to the
suction side.
2. Master Pressure Controller (PIC) provides set
point (SP) to speed controller (SC).
3. Both Compressors would not necessarily
operate at the same speed/flow during a
recycle since UIC A/B are acting independently
of each other.
PIC
Anti-Surge
Valve (ASV)
UIC
A
Anti-Surge
Valve (ASV)
UIC
B
Set Point (SP)
CommonSuctionHeader
Remote Set
Point (RSP)
Remote Set
Point (RSP)
Discharge
Block Valve
Discharge
Block Valve

3. SURGE CONTROL FOR PARALLEL GAS COMPRESSOR OPERATIONS
EQUAL FLOW BALANCE OPERATION OF
PARALLEL COMPRESSORS
M
KO Drum
Suction Block
Valve
Anti-Surge
Valve (ASV)
Compressor A
FT
CoolerUIC
A
SC
M
KO Drum
Suction Block
Valve
Anti-Surge
Valve (ASV)
Compressor B
FT
CoolerUIC
B
SC
CommonDischargeHeader
PIC
Set Point
(SP)
EQUAL FLOW BALANCE METHOD
1. The Master Pressure Controller (PIC) on the
common discharge header determines the total
load demand and alters Compressors A & B
speeds (SC). The individual flow control signal
to each speed controller is achieved by scaling
the total load demand (BIAS A & BIAS B) to the
individual flow controller (FC) on each
compressor.
2. Both Compressor Operations are independent
of the Anti-surge Valve (ASV) Operation.
3. Flow Balancing method requires additional
control elements thereby increasing CAPEX
costs.
4. Since Flow Element & Transmitter (FT) is
installed on the compressor discharge,
additional pressure drop occurs representing
energy losses & corresponding costs.
5. For the cascaded control used, PIC  FC 
SC, the inner loop (FC) must respond faster
than the PIC outer loop causing PIC to be
sluggish. The faster FC loop also means, the
compressor speed would increase faster than
required often reaching maximum speed &
hence does not offer the best control strategy.
FC
BIAS
A
BIAS
B
Remote Set
Point (RSP)
Remote Set
Point (RSP)
FC
Discharge
Block Valve
Discharge
Block Valve

4. SURGE CONTROL FOR PARALLEL GAS COMPRESSOR OPERATIONS
EQUIDISTANT TO SURGE LINE METHOD
OPERATION OF PARALLEL COMPRESSORS
M
KO Drum
Suction Block
Valve
Anti-Surge
Valve (ASV)
Compressor A
Cooler
UIC
A
SC
M
KO Drum
Suction Block
Valve
Compressor B
Cooler
SC
CommonDischargeHeader
PIC
Set Point
(SP)
EQUIDISTANT TO SURGE LINE METHOD
1. Based on the principle - The Deviation/Distance
between the operating points & the surge
control (SCL) line in both trains is equidistant.
2. For this configuration N1  N2 & Q1  Q2, but
Dev1 = Dev2 (i.e., Dev = Distance between Op.
Point & Surge Line)
3. Both UIC A/B (Anti-surge Controller) & LSIC A/B
(Load sharing Controller) would coordinate
their responses in Real time.
4. Applicable also for parallel compressors that
have dissimilar / asymmetrical performance
curves.
5. An effective configuration for varying loads,
that ensures both compressors independently
adjust operations to stay away from surge.
LSIC
A
Remote Set
Point (RSP)
Remote Set
Point (RSP)
Anti-Surge
Valve (ASV)
UIC
B
LSIC
B
Discharge
Block Valve
Discharge
Block Valve