Dynamic
simulations are used to understand the dynamic behaviour of plant and process systems employing centrifugal compressors. These compressors are prone to process instabilities and
surging and this is highly damaging to the integrity of the machines. Why
Do Compressors Surge? Normal flow of gases is accompanied by a pressure drop; flow through compressors on the contrary results in pressure rise. Compressors deliver gas from low pressure to high pressure with the expenditure of external energy - power. Compressor blades and passages are aerodynamically designed for maximum efficiency but have a finite operating range. The flow varies typically between design (100%) flow to minimum flow (typically 70-75%) when the flow becomes unstable as it approaches surge.
Further reduction in flow results in flow reversal through the machine, ie gas flows from the discharge (high pressure side) to the inlet side. This is accompanied by flow pulsation and "hammering" sounds and high vibrations which trip the machine.
Stable operation requires avoidance of compressor surge or discharge
- to ensure a plant operates without instabilities, surging of compressors should be avoided. Surge
Control Systems and Control Lines Since
a compressor will surge if the flow falls below surge lines, anti-surge systems are designed with a control line set at a flow (typically 10%) higher than
the surge flow. The surge control system reacts to prevent flow falling below the control line by opening a recycle or anti-surge valve. So long as flow remains well to the right of the control line, the surge control valve remains closed and the machine will operate without surging. However, if
the flow through the machine drops and the operating point moves towards surge it will first cross the control line; the control system will act and open the recycle valve. This action recycles sufficient volume of gas and the flow to ensure that the actual flow through the compressor,
i.e. process flow plus recycle flow, never falls below the control line. Theory
vs. Practice That is the theory.
In practice, the surge events occur at high speeds (typically in milliseconds) when opening and closing of the recycle valve which are mechanical devices may be just too slow to prevent a fast surge. Dynamic
simulation is the technology which has been developed and successfully used by MSE to model surge control strategies and determine a cost effective system which will prevent the surging of compressors and maintain process stability for all foreseeable operating conditions. Dynamic
simulation has been used for: Optimising surge control valve size, opening times and valve characteristics,
e.g. linear, quick opening and equal percentage Tuning process control system parameters (set points,
PID) Investigating the interaction of process control and the surge control system Start-up and shut down events System failures Process
instabilities
Uses of Dynamic Simulations Plants suffer from low availability due to process
instabilities arising from surging of compressors. This leads to frequent shut
downs. A dynamic simulation study is a cost-effective way of identifying the cause.
Once the reasons for low availability are understood, corrective
action is identified and rigorously tested using our dynamic simulator plant model. The corrective actions can be
simple such as changing the control system algorithms. This approach
is highly cost-effective as it doesn't require plant shutdown.
Another solution can be changing the valve trim or stroke times. In all cases, MSE examines the alternative strategies and rigorously tests the schemes before making recommendations. Online
Dynamic Simulation Demonstration
For
more details on how MSE can help you with the design, modification
and upgrading of your gas compression systems, please contact us on
+44 (0)20 8773 4500 or Further
Information
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