PIPELIQTRAN, Thermo-Hydraulic Transient Simulation

Description

PIPELIQTRAN is a toolkit designed to analyse thermo-hydraulic transients including water hammer and pressure surge phenomena in liquid (non-compressible) piping networks.

The toolkit can be used for different applications for example:

  • Sizing and optimization of pipe networks (diameters, pressure losses, etc)
  • Selection of pumping systems (energy optimization, NPSH analysis, etc))
  • Selection of surge control devices
  • Selection of control valves
  • Design and tune the control system
  • Pressure transient analyses

Features

  • The toolkit enables the user to start the transient calculation from a steady state situation
  • Transients can be generated due to events in time or events in the system
  • Effects caused in the piping system due to cavitation and the release of gas dissolved in the fluid can be taken into account
  • The speed of sound is calculated taking into consideration the elasticity of the pipe and the release of gas in the fluid. Moreover, wave forces can be also calculated.
  • The working fluid is considered incompressible and of constant composition
  • The calculation of thermodynamic properties takes into consideration the dependence on temperature
  • The process control system can be easily included in the simulation model using the standard Control library supplied with EcosimPro/PROOSIS
  • The specification of the pipe elevations and the pressure losses along the pipes is easily managed by the EcosimPro/PROOSIS input data editors
  • The library includes a wealth of piping system components and boundary condition components (pipes, pumps, valves, headers, vacuum breaker, water box, surge tanks, etc)

Circulating water system of a combined cycle power plant

This user case involves the fluid transient analyses in a circulating water system of a combined cycle power plant. The schematic diagram of the simulation model is depicted in the following figure.

Example for the PIPELIQTRAN library

The case consists of the sequential startup of both pumps with the system full. The purpose of this analysis is to check the correct opening time of the pump relief valves so that system pressures do not exceed the maximum allowable pressure. The opening time of the valves is 10 seconds. The relief valve of pump B1 starts to open at 20 s and that of pump B2 at 85 s. The first pump, B1, is connected during the first instant, whereas pump B2 is connected after 65 s. Vacuum breakers C1 and C2 have an initial air volume of approximately 5 m3.

The following plots illustrate the evolution of the discharge pressures of both pumps and the mass flow circulating through them.

Example for the PIPELIQTRAN library

Example for the PIPELIQTRAN library

Fluid Transients in Heavy Fuel Oil (HFO) Storage and Distribution System

The objective of this study is to check the need for hydraulic accumulators in the Heavy Fuel Oil Storage and Distribution System.

There may be three reasons to the installation of hydraulic accumulators in the HFO system:

  • Dampen the pressure pulsations caused by the positive displacement pump.
  • Maintain the pressure of the oil guns and the boiler fuel flow after a pump trip during the time needed to start up the standby pump.
  • Make it easier to control the pressure at the pump discharge and to reduce pressure disturbances at boiler inlet.

The model represents the following hydraulic elements:

  • Suction pipe
  • Three HFO screw pumps
  • Recirculation valve and recirculation lines.
  • Series of pipes from the pump discharge collector to the boiler TP (the HFO heater is represented by a pipe).
  • Fuel flow control valve inside the boiler.
  • Oil guns inside the boiler.

The model also represents two optional hydraulic accumulators: one accumulator connected to the pump discharge and other accumulator connected to the boiler TP. The user can override both accumulators specifying a very large pressure drop coefficient for the connecting pipe of the corresponding accumulator.

The model represents two PID controllers, one PID adjusts the position of the recirculation valve to maintain constant the pressure at the pump discharge and the other PID adjusts the position of the fuel control valve in response to the fuel flow demanded by the boiler.
A new component type was specifically designed to represent the oil-guns.

The scope of the model is shown in the following figure:

Example for the PIPELIQTRAN library

Simulation Cases
The model was used to analyze different possible designs of the HFO system:

  • Design 1: System without hydraulic accumulators
  • Design 2: System with hydraulic accumulator in boiler side
  • Design 3: System with hydraulic accumulator in pump side
  • Design 4: System with hydraulic accumulators in pump side and boiler side

The following six transients have been analyzed for each of the designs:

  • Transient case 1: Pump trip at 100 % Load
  • Transient case 2: Pump trip at 44% Load with only one operating pump
  • Transient case 3: Pump transfer at 44% Load with 1 operating pumps
  • Transient case 4: Pump transfer at 44% Load with 2 operating pumps
  • Transient case 5: Load Step Changes 100%-90%-100%
  • Transient case 6: Boiler Trip with fast closure of boiler valve

According to the calculations, the HFO system requires two accumulators: one in the pump side, as emergency energy storage during pump trips and to facilitate the control, and one in the boiler side as shock arrester.

The following figure shows the pressure results corresponding to the system without hydraulic accumulator during a Master Fuel Trip at 100% Load in Boiler.

Example for the PIPELIQTRAN library

It can be seen that when maximum pressure is 44 bar(a), there are two contributions to the high pressure: (1) the water-hammer overpressure due to the flow detention, (2) the pressurization of the pump discharge collector because of the delay in the opening of the recirculation valve.

The water-hammer over-pressure can be reduced by placing an accumulator at the boiler TP- With respect to the pressurization of the pump discharge collector, a feed-forward signal in the control can alleviate the second source of pressurization.

The following figure shows the pressure results corresponding to the system with accumulator in both sides, i.e., Pump and Boiler,which includes an improved pressure control with feed-forward signal.

Example for the PIPELIQTRAN library

With the final design, the maximum pressure in Boiler TP remains below 28 bar(a) during the Boiler Trip. This value is 16 bar lower than the one obtained in the system without hydraulic accumulators. The feed-forward signal to the recirculation valve based on the number of operating pumps and the flow demand by the boiler or the measured flow entering into the boiler significantly improve the pressure control in the system.

Analysis of the operation of Seawater Intake Pumping Station

The objective of this study was to optimize the start-up and stop operation sequence of the Intake Pumping Station of a desalination plant fulfilling the operational requirements. The following figure depicts the model schematics of the system:

Example for the PIPELIQTRAN library

The model includes all the relevant components for the transient simulation of the Intake Pumping system, in particular, pipes, check and discharge valves, pumps, surge tank, accumulators, raw water tanks and overflowing pre-chamber for the correct simulation of the level behaviour. The model also includes the level and flow control loop of the system.

The study has allowed to optimize the control and operational parameters (valve positions and pump speeds) and to confirm the design of the anti-waterhammer devices.

The results shown below correspond to the start-up sequence of the intake pumping system at summer ambient conditions. The following figure shows the upward sequence followed in this scenario. It has been considered that the mode changes every 2500 seconds .

Example for the PIPELIQTRAN library

The evolution of the flow-rate supplied by each pump and the run-out flow and the rotational speed are shown in the following figures:

Example for the PIPELIQTRAN library

Example for the PIPELIQTRAN library

In some operation modes, the pumps are forced to operate close to their run-out curves for the maximum operating conditions.

The levels in the raw water tanks remain within acceptable limits as shown below. The oscillation of the tank levels throughout the startup sequence is less than 0.5 m.

Example for the PIPELIQTRAN library

The position of the discharge valves of the intake pumps considered in the start-up simulation scenario are depicted below.

Example for the PIPELIQTRAN library

Finally the following figure shows the evolution of the total mass flow at different locations of the system (shore, before and after the surge tank) and the downstream demanded flow (p_from_uf).

Example for the PIPELIQTRAN library