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Pressure Surge Calculation

Pressure Surge Calculation

Avoid damages to piping systems and components caused by pressure surges

Findings from previous studies of feedwater systems

What has been regarded as state-of-the-art technology for plant design in recent years is no longer sufficient to deal with the effects and risks posed by the high operating pressures in the feedwater system or boiler in order to ensure the best possible performance and optimum levels of efficiency.

In our studies of various feedwater systems in operation we have discovered:

  • that pressure surge load cases (pump failure) are not sufficiently considered in the design
  • to some extent serious load exceedances to supports, components and piping
  • that in some power plants damage has already occurred
  • plastic deformation of components, which could lead to costly damages and maintenance issues

In order to specifically identify problem areas, we can support you with our expertise and our fluid dynamics and structural mechanics calculation methods.

 


EC-Flow Fluid program

The “ec-flow” fluid program was developed by Envi Con for the calculation of piping systems of any configuration. The program calculates pressure surge loads on the basis of switchover operations of valves, pump failures and transient condensation shocks. Furthermore, pressure losses and mass trimming can be determined in complex piping systems.

The following can be calculated:

  • Water (liquid, vaporous)
  • Oils
  • Gases

A big advantage of ec-flow is that individual adjustments can be made for each calculation:

  • Special modeling of boilers, cooling towers, basins, non-return valves and safety valves

 

FSI

Another important feature of the program is a module that takes into account the FSI (fluid structure interaction). With FSI the movement of the structure is linked with the movement of the fluid and the interactions between them are taken into consideration.

This means that ec-flow (calculated movement of the fluid) and the structural program ROHR2stoss (calculated movement of the structure/pipes) run sequentially as well as in parallel. A special interface program takes the current fluid forces from ec-flow and returns the currently-calculated structure accelerations from ROHR2stoss for each time step. The consideration of FSI immensely improves the accuracy of the fluid analysis. It is particularly effective when the flow velocity of the medium is the same scale as the velocity of the moving structure.

The amplitudes abate faster due to the impulse division between fluid and structure (see illustrations). The FSI thus has a significant influence on the pressure wave spectrum and normally reduces the maximum fluid forces, as shown in the following example from a fluid analysis in the feedwater system:

 


On the basis of our experience with a number of coal-fired units newly put into operation from 2012 to 2014, pressure surge analyses without FSI, which are often still commonly used, lead to significant exceedances of structural attachment loads or to pump and preheater nozzles, as this method is too conservative. ed units newly put into operation from 2012 to 2014, pressure surge analyses without FSI, which are often still commonly used, lead to significant exceedances of structural attachment loads or to pump and preheater nozzles, as this method is too conservative.  

  • Failure of both electric feedwater pumps (2x50%) at the same time or of one turbine feedwater pump
  • Failure of one of the electric feedwater pumps (2x50%) while the second keeps running
  • Switching process of HP preheater protection

Subsequently: Transfer of the fluid forces for the examination of supports, components and piping in a structural dynamics program (Example: Rohr2, Caesar). 

The stress ratio S2 without FSI for a load case “Failure of a turbine feedwater pump” can be seen see in Illustration 1. The maximum utilization is 109.9% without FSI. With FSI the utilization drops to below 100% at this point. The maximum stress ratio with FSI occurs at a different point and is about 91% (see Illustration 2).

Illustration 1: Maximum stress ratio S2 without FSI

Illustration 2: Maximum stress ratio S2 with FSI


 

The tapping points on intermediate and low-pressure turbines should likewise be more precisely considered in large conventional thermal power plants. Due to a significant increase in operating parameters in recent years, damage could occur to non-return valves in the bleed lines.

The non-return valves are installed in order to prevent return flow to the turbine in the event of turbine trip. The closing operation takes place in failsafe quality, whereby two valves are switched in series, one of them usually with a hydraulic drive triggered by the turbine protection.

The goals of the pressure surge calculation are:

  • To determine piping loads
  • To show the maximum velocity and acceleration at non-return valves

Through the turbine trip the pressure before the valve in the turbine drops quickly while the pressure behind the valve (e.g., pressure of the feedwater tank) often remains almost constant during the observation period. The valves thus close very quickly in up to 200 ms.

The valves in the bleed lines are often not designed for such high velocities and accelerations and could therefore be damaged. In addition, leakages may occur as a result (which have already been observed a number of times).

The valve manufacturer must then determine whether a valve is suitable for the ascertained loads.

 

The work steps and calculations will of course be individually adjusted to your systems conditions and needs. 

As overall designer we would like to inform you of any possible negative consequences of the problems presented and thereby help protect your staff and your systems.

We hope that this leaflet has been able to provide you with helpful information.

We and our team of highly-qualified experts would be happy to answer any questions you might have or to provide you with additional information. 

 

Contact

###CONTACTBUTTONTITLE###

Rainer Teichgräber

Rainer Teichgräber

Sales Director

Phone: +49 911 48089-143
Fax: +49 911 48089-129

EFFECTS OF SWITCHOVER OPERATIONS AND FAILURE
OF PUMPS IN THE FEEDWATER SYSTEMS

- Damage to support
- Recalculation ec-flow

Advantages of ec-flow

- Proprietary development
- Input of various fluids
- Individual modifications possible in the program
- Structural movements taken into account