©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

Flow Transmitter Response Time IssueFlow Transmitter Response Time Issue

©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

100%

0

100%

0Pc

100%

Pd0

1 SEC.

Po

A

D

AD

A D

• The speed of approaching surge is high

• In only 400 ms, PO dropped by 14%, with a 2% change in Pc

• The transmitter type and brand should be selected based on two major factors:

– Reliability– Speed of response

• Desired rise time for p (flow) transmitters is 200 ms or less

– Pressure step is 100%– The first order response (63%) is less than 200

ms

Time

Actualpressure Transmitter

output

63% response1- (1/e)

1 is less than 200 ms

• Desired rise time for pressure transmitters is 500 ms or less

Response time of the FMD TransmitterResponse time of the FMD Transmitter

©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

• Knowing the flow is essential to determine the distance between the operating point and the SCL

• Damping the po (flow) transmitter destroys essential information

50

0-50

0

1.25 2.50 3.75 5Time (seconds)

FlowStart of Surge

Actual Flow

Damping the po (flow) transmitter can paralyze the complete antisurge control system!!!

The Effect of Damping the Dpo (flow) Transmitter

The Effect of Damping the Dpo (flow) Transmitter

©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

50

0-50

0

1.25 2.50 3.75 5Time (seconds)

FlowStart of Surge

Actual Flow

= 6 s

This shows that a very long transmitter response would miss the event completely

The Effect of Damping the Dpo (flow) Transmitter

The Effect of Damping the Dpo (flow) Transmitter

©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

50

0-50

0

1.25 2.50 3.75 5Time (seconds)

FlowStart of Surge

Actual Flow

= 16.0 s

= 1.70 s

A rise time of 200 milliseconds can track the surge cycles, which is what we need

The Effect of Damping the Dpo (flow) Transmitter

The Effect of Damping the Dpo (flow) Transmitter

©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

50

0-50

0

1.25 2.50 3.75 5Time (seconds)

FlowStart of Surge

Actual Flow

= 16.0 s

= 1.70 s

= 0.20 s

A rise time of 200 milliseconds can track the surge cycles, which is what we need

The Effect of Damping the Dpo (flow) Transmitter

The Effect of Damping the Dpo (flow) Transmitter

©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

50

0-50

0

1.25 2.50 3.75 5Time (seconds)

FlowStart of Surge

Actual Flow

= 16.0 s

= 1.70 s

= 0.20 s = 0.03 s

To track the actual flow we would need a rise time of 30 milliseconds, which is not commercially available. Thus the acceptable compromise is 200

milliseconds

The Effect of Damping the Dpo (flow) Transmitter

The Effect of Damping the Dpo (flow) Transmitter

©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

BarriersBarriers

• “We have barriers in our lines that introduce a 150 millisecond delay. How does that affect the flow transmitter rise time issue?”

©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

BarriersBarriers

• Barriers delay the transmission of data from the field transmitter to the controller, but they do not dampen the signal

• The transmitter 200 millisecond rise time resolution is still given to the controller, just 150 milliseconds later

• The residence time within the loop (cooler, knockout drum, piping, etc.) will allow a 150 millisecond delay without major penalty

• The key issue is that the flow signal is not degraded due to the delay, it is just delayed

©20

09 C

ompr

esso

r Con

trol

s Cor

pora

tion

BarriersBarriers

50

0-50

0

TimeResolution from Transmitter

50

0-50

0

TimeResolution into Controller

150 millisecond delay