Preventing unstable operation of a synthesis compressor using Dynamic System Simulation

Transcription

1 Preventing unstable operation of a synthesis compressor using Dynamic System Simulation Mr. J.P.M. Smeulers, M.Sc.* TNO Science & Industry T: F: E: jan.smeulers@tno.nl * Corresponding author Mr. D. Meulendijks, M.Sc. TNO Science & Industry T: F: E: dennis.meulendijks@tno.nl D. Meulendijks

2 Unstable operation of a synthesis gas compressor An example of unexpected dynamic response Results of an analysis using modeling of the complete compressor installation 1. Lay-out of the compressor system 2. Description of the problem 3. Measurements 4. Analysis by means of a computer model 5. Conclusions 2

3 Lay-out of the compressor system 3-stage compressor, with all stages on a single shaft Suction pressure speed controlled Suction pressure 23 bar; Discharge pressure 133 bar Nitrogen flow is controlled to have a constant 3:1 hydrogennitrogen ratio nitrogen ASC valve Reactor FC PC RPM control Hydrogen plant Recycle compressor 3

4 Problem description Compressor operated stable for rated capacity Compressor started surging far away from the surge point, limiting the operating envelope No operation possible at part load At 9% capacity pressure variations occurred that increase until surge occurs Temporary solution was to apply a large surge margin, resulting in a considerable reduction of part load efficiency 4

5 suction Analysis - Measurements discharge 1st stage discharge 2nd stage M e a s u r e d d y n a m i c p r e s s u r e s surge discharge 3rd stage P discharge [bar] T i m e [ s ] suction discharge 1st stage discharge 2nd stage P suction [bar] P interstage1 [bar] P interstage2 [bar] M e a s u r e d d y n a m i c p r e s s u r e s discharge 3rd stage P discharge [bar] T i m e [ s ] Pressure variations start to grow autonomously not a classical surge phenomenon, but a system instability 5

6 Analysis Measurements interpretation 1st stag e 2nd stag e 3rd stag e P [bar] T im e [s] Each stage has a phase shift of approximately 6 degrees Consumed power varies for each stage The sum of the power variations even out to a large extent: When stage 1 requires more power the other stages require less 6

7 Analysis Computer model A model has been built to analyse this using PULSIM, which has building blocks for: Fluid machinery: compressor stage, rotor, driver Pipe system: pipes, volumes, Controllers steamturbine Rotor 1 m PC 7 m 7 m 7 m 3 m ~ 8 m3 ~6 m3 artificial excitation 22 dp - flow 1st stage Approximated dp - flow curve Pressure difference [bar] surge point 15 RPM 11 RPM 2 P = AQ. + BQ. + C Flow [kg/s] A negative, B~ RPM and C~ RPM 2 7

8 Analysis - Response to artificial excitation Response curve flow suction Flow [%pp] 1,2 1,8,6,4 unstable range 1%load 95% load 9% load 85%load,2,1,2,3,4,5,6,7,8,9 1 Frequency [Hz] Conclusion: system is marginally stable for 85% load 8

9 Analysis - Causes of unstable behavior 1. Reduced compressor impedance at lower flows Idealised curve P=A.Q 2 +B.Q+C Compressor impedance Rc= -d P/dQ= -2.A.Q-B Surge for Rc< or load the compressor impedance is a factor of 2 smaller load Pressure difference [bar] overall surge point dp - flow 1st stage surge point Flow [kg/s] 15 RPM 11 RPM y = -.11x 2 + y x = -.11x x

10 Analysis - Causes of unstable behavior 1. Reduced compressor impedance at lower flows 2. Exchange of power between stages via rotor.15 Power 1, 2, 3 and total 1st stage.1 total 2nd stage 3rd stage Power [MW] Time [s] 1

11 Analysis - Causes of unstable behavior 1. Reduced compressor impedance at lower flows 2. Exchange of power between stages via rotor 3. Poor controllability of suction pressure control: small changes in suction pressure lead to large variations in flow 4. Variation of molecular weight caused by pulsations Flow variations at mixing point cause change of composition that travels with the flow velocity to the compressor. Travel time 7 seconds or ca. 3.5 period of instability cycle. nitrogen 1 m 7 m hydrogen 11

12 Solution - Stabilising the system 1. Improve characteristic of first stage (steeper) 2. Flow control instead of suction pressure control (cascade with suction pressure control) Difficult to achieve in an existing system 3. Reduce variation of molecular weight (mixing vessel or hydrogen flow control) 4. Dampen the resonance: orifice plate or control valve in hydrogen line Flow amplitude Original With orifice 12 The orifice was actually implemented, and the system operated stable Frequency [Hz]

13 Concluding remarks 1. Unexpected instabilities can occur in a compressor installation as a result of pipe system and compressor dynamics 2. Low pressure loss systems with multi-stage compressors are susceptible for instabilities 3. Compressor head-flow curves are not as smooth as presented here. Especially multiple wheel stages show deviations from the idealised quadratic curves. Differences exist between design, test and real curves. 4. Analysis of the compressor control during the design stage is recommendable. Powerful simulation tools are available to analyse the dynamics and control. However, this is no replacement for understanding what is going on in the system. 5. Surge cycles itself can also be simulated nowadays, i.e. the unstable behavior. 13