Transient Event Studies

The purpose of transient event studies is to ensure the system will not be adversely affected by a sudden event such as an emergency shutdown, upset condition, or sudden events that are part of normal plant operation. This applies to equipment such as compressors, pumps and piping systems.

Owners want to avoid reliability problems associated with these transient events.

Transient events can include

  • Sudden opening or closing of valves
  • Equipment start-up or shutdown
  • Sudden changes in flow conditions
  • Electrical power loss
  • Control system malfunction
  • Fluctuating pressures at Gas Turbine inlet

These transient events typically involve changes in the pressure, flow or boundary conditions (end points) in a system over very short periods of time. During the event, high dynamic pressures and forces can be created. These dynamic loads can lead to excessive vibration and stress, which can result in a fatigue failure and or damage to the system.

Start of transient event

Start of transient event

Features

  • Simulate flow conditions using a non-linear Time Domain software tool
  • Evaluate dynamic pressure and forces on piping system
  • Evaluate for all upset conditions
  • Determine pressure, vibration and stress amplitudes
  • Recommendations to modify piping system, or change control scheme or operating procedures

BETA pioneered the development of time domain (TD) simulation capability in 1998 to support a variety of transient studies. 

Examples

Case 1: What happens to a flare system during an ESD event?
The client was concerned about the design of the relief valve and flare system layout and support with respect to the dynamic forces generated when the relief valves open during an emergency shut down (ESD). The picture, above, is a model of the ESD piping system.

 

 

Plant ESD piping included in Transient Analysis
Case 1 ESD piping included in the transient event study

 

Model used in Case 2 example
Case 2 model used in transient analysis 

 
 

There are two main concerns with the flare system:

  • High frequency acoustic excitation (noise) generated during the relief event that can cause failures.
  • Lower frequency forces generated in the piping upstream and downstream of the relief valve due to the change in flow and pressure inside the piping when the relief valve opens.

The analysis of the low frequency forces in the flare system includes two parts:

First, a transient acoustical simulation is conducted to calculate the time varying pressure and forces in the piping. The flare system in this case is a closed system with the flare gas captured in a large vessel rather than being vented to atmosphere. There are several simplified approaches for the analysis of relief valves which vent to atmosphere, but the analysis of a closed flare system is more complex requiring a full system model.

Transient force in piping

Transient force in flare system piping

The second part of the analysis includes applying the calculated dynamic loads to a finite element model of the piping and supports to calculate vibration and dynamic stress.

Based on the analysis, a number of recommendations were made to minimize the excitation forces, as well as changes to the pipe layout, adding reinforcing pads, and modifying support designs to minimize piping response. These improvements will avoid fatigue failures on the piping system. 

 

Case 2: What happens to turbine driven power generators during a plant upset?
The scope of this study included a transient analysis to evaluate two reciprocating compressors supplying gas to two turbines used for power generation. A picture of the model used in this analysis is shown above. The purpose of the study is to determine the size of a vessel (buffer tank) required in the discharge system to meet strict pressure guidelines at the turbine inlet for normal operation as well as for a plant upset or unit shut-down.

The pressure pulsation at the turbine inlet must be controlled to very low levels for normal steady state operation to minimize the effect on the turbine performance; and the mean pressure at the turbine inlet must be controlled in very specific way during an upset condition to allow time for the control system and turbine to take appropriate action to ensure safe operation. Large volumes (buffer tanks) are required in the discharge system to ensure there is a sufficient volume of gas in the discharge system during this transient event.

The governing design criterion in this application was to ensure a maximum pressure change at the turbine inlet, 11 psi/sec or less. The pressure change at the turbine inlet was found to be well above the design specification for the original design, 54 psi/sec. Two buffer tanks of 58” ID x 178” s/s were required for this application to meet the design criterion.

This case study illustrates how a transient analysis study can be used to ensure safe operation for turbines when supplied by reciprocating compressors.

> Click here for the full case study examples.pdf

Customer Benefits
Avoid unexpected failures and damage to a system, and mitigate safety issues, due to fatigue failure or pressure fluctuations.

Improve reliability and safety of equipment.

Advantages
BETA has the expertise and software to perform the accurate dynamic and non-linear time domain simulation needed to effectively model transient events in gas or liquid systems.