Surge Explained

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Exit

Suction Pressure

Discharge Pressure

Suction Pressure

Discharge Pressure

Normal Pressure Profile

Suction Pressure

Discharge Pressure

Surge Pressure Profile

Balance Piston

Suction Pressure

Discharge Pressure

∆P  (1/2 ρ U22) ρ = gas density U =Impeller Tip Speed Frequency = 1/2 to 2 Hz

Pressure Pulsations Associated with Surge

Stall Cells

Incipient Surge

∆P = 0.05 (1/2 ρ U22) ρ = gas density U =Impeller Tip Speed Frequency = 0.3N to 0.6N

Pressure Pulsations Associated with Incipient Surge

Surge Control Basics Opening the Recycle Valve accomplishes two things which helps the compressor avoid surge:

• Recirculates flow back to the compressor inlet • Relieves resistance in compressor discharge network, allowing flow to increase

Surge Control Application The following factors make surge control a unique and difficult control application.

• • •

Accurately defining the compressor operating point Locating and defining the surge limit of the compressor Acceleration of surge due to compressor curve shape near surge • Interaction with other control loops • Special requirements for load sharing between multiple compressors

Compressor Performance Equations Hp =

P2 ( P1

[( )

k-1 kη

)

]

kη ZRT1 -1 (k-1) MW

h Z R T1 Q1 = MW P 1

Readily Measureable Variables Hp =

P2 ( P1

[( )

k-1 kη

)

]

kη ZRT1 -1 (k-1) MW

h Z R T1 Q1 = MW P 1

Simplification of Head Factor Hp =

P2 ( P1

[( ) Q1

2

=

k-1 kη

)

]

kη ZRT1 -1 ( k-1) MW

h Z R T1 MW P1

Simplification of Flow Factor P2 ( P1

[( )

k-1 kη

)

]

kη -1 ( k-1) h P1

The Universal Surge Curve

P2 P1 - 1

( )

h P1

Determining the Surge Limit Point A Surge Limit Point, in terms of

Pd ho & , is determined Ps Ps

by testing the compressor in the field or from using the predicted curves supplied by the manufacturer. The compressor should be tested for at least three surge points if possible, one at minimum speed, one at maximum speed, and one at 50% of the speed range. This establishes the relationship of speed vs. surge limit.

Increasing Surge Margin Hp

Surge Limit Line

Auto Increase of Surge Margin Open Loop Response or High Gain Surge Control Line New Surge Control Line 1

2

N1

N2

N3

Q2 (ICFM) If the controller is unable to prevent a compressor surge, it is desirable to automatically increase the surge margin so that the compressor does not continue to surge. The controller must detect a surge, increment the margin and alarm.

Instrumentation Considerations Flow Measurement is the most important signal for proper surge control. Close attention should be given to selecting and locating the flow device and transmitter.

• Location: Preferred in compressor suction for simplifying algorithm. Commonly found in compressor discharge piping and is compensated to inlet conditions. MUST be located to measure TOTAL COMPRESSOR FLOW inside the recycle loop. • Size: The flow measuring device and transmitter must be sized for maximum compressor flow. The required pressure differential corresponding to maximum flow should be 10” WC or greater.

Instrumentation Considerations (cont.) • Flow Transmitter: The length of tubing between flow device and the transmitter should be minimal. The transmitter must be reliable, repeatable, and have a speed of response (rise time) of 100 msec or less. • Pressure Transmitters: These transmitters should be located as close to the compressor as possible. For constant speed compressors with suction throttling valves, the suction pressure must be measured downstream of the valve. • Temperature Transmitters: Location is not as critical but should be located to provide correct temperature.

Instrumentation Considerations (cont.) Flow Measurement for Load Control The flow measuring device for load control must be located to measure "user" flow and not compressor flow. It must therefore be located outside the recycle loop. Also, most flow control applications are "mass flow", requiring temperature and pressure readings as well as flow. FIC

SIC

ST

FT PT PT

TT

TT

UIC

FY

FT

Flow Measurement Need to have a sufficient “signal to noise” ratio. Accuracy of the flow coefficient is not critical. Must be repeatable.

• Venturi Tube- best device- most costly • Orifice Plate- good characteristicsunrecoverable pressure loss- less costly • Annubar - widely used, least costly, least desirable - poor signal-to-noise ratio.

Transmitters • Flow transmitters- Rosemount 1151 analog and 3051 digital are most widely used for surge control. • Pressure transmitters- Acceptable to use “smarts.” • Temperature transmitters- Acceptable to use “smarts.” • Avoid Honeywell Smarts

Control Valve • Typically actuated with pneumatics. Some axials may be hydraulic. • Size and Speed are the two most critical factors. • Linear Valves are preferred. Others can be characterized.

Control Valve (cont.) • Size- Valve size should be checked at all operating conditions along the surge line • Speed: Stroking speed of 1 sec. for 6" and smaller valves. Stroking speed of 2 sec. for 8" and larger valves. Normally requires a volume booster in air supply

Valve Actuation Piping/Tubing for pneumatic actuator and volume booster.

Large Cv Volume Booster

SV

1:1

Out Supply In

Air Supply

SV

I/P

Control Features

Proportional Function Closed

0%

rPROPTM

Open

100%

rSUCLN

Pdischarge Psuction

rSULIN 7 r MAR 0

h orifice P suction

Setpoint Hover Feature Control Line Surge Line Pressure Ratio

Control Setpoint Current Operating Point

PD/PS

Hover Setting

Suction Flow (hx)