ES_CasDrain User Manual

1. Introduction

2. Methods and Formulae

 

2.1 Incompressible Water Flow Analysis for LCV Upstream Piping

 

2.2 Compressible Steam Flow Analysis for LCV Downstream Piping

 

2.3 Erosion at Piping Elbow

 3. Major Screens

 

3.1 Input Screen

 

3.2 Menu

 

3.3 Iso-metric Screen of LCV Upstream Piping

 

3.4 Iso-metric Screen of LCV Downstream Piping

 

3.5 Text Output Screen

4. Test Run Results


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1. Introduction

ES_CasDrain is the program to analyze cascade heater drain piping system.   Cascade heater drain piping system analysis is separated into two for discontinuity of analysis at LCV.   One is the upstream piping of heater level control valve(LCV) and the other is the downstream piping of LCV.   For proper operation of LCV and drain system, the upstream flow should not have flashing, while the downstream piping allows flashing.

ES_CasDrain uses different analysis method for the upstream and downstream pipings.  For the upstream piping, incompressible flow analysis is used while flashing is checked providing alarm to user for flashing.   For the downstream piping, compressible flow analysis developed by ENGSoft Inc. is used, which has good analysis results even for flashing (two phase) flow of saturated water as presented in Clause 3.2 Flashing Saturated Water of ES_StmNzl and ES_StmPipe Program Test Results web page.

If flashing occurs at LCV upstream piping, it may cause high erosion of LCV itself and unstable heater level control due to unsteady specific volume of flashing water at LCV inlet.  The flashing can be avoided by adding vertical pipe runs before LCV or increasing the upstream pipe diameter and consequently reducing pressure drop.

ES_CasDrain may be used for analysis of other flashing piping in power plant such as turbine casing drain piping system to condenser, in which flow control orifice acts as LCV in cascade heater drain piping system.

Analysis of a cascade heater drain system should be done for every suspicious operating conditions as well as rated condition.    Especially upstream heater bypass condition, if exists, should be analyzed.    Low load conditions should be also analyzed since the pressure drop allowable between heaters at turbine low load may not be sufficient.

 

2. Methods and Formulae   (TOC)

2.1 Incompressible Water Flow Analysis for LCV Upstream Piping

Incompressible water flow analysis of LCV upstream piping is performed by conventional method using Bernoulli and Darcy equation.   ESNSoft Inc. has a program to analyze incompressible flow, named ES_dPCalc of which user manual you may find in ES_dPCalc User Manual web page.    The LCV upstream piping of ES_CasDrain is analyzed using the same method with ES_dPCalc.   

ES_dPCalc has a function to check and give alarm for flashing in water analysis, and ES_CasDrain also use the function to check and give an alarm for flashing in LCV upstream piping.

2.2 Compressible Steam Flow Analysis for LCV Downstream Piping   (TOC)

Compressible steam flow analysis of LCV downstream piping is performed by the method developed by ENGSoft Inc. and presented in Compressible Flow Analysis of Steam web page.

Since the specific volume of compressible fluid is considerably big, the pressure variation due to elevation change is negligible so that all pipes with same internal diameter can be calculated as one pipe regardless of their elevations.   Using this concept, the program investigates the pipe data, sums the resistance coefficient(K) of the pipes with same ID and calculate the pipes as one pipe.   When pipe ID changes by increaser or reducer, the piping system is divided at the increaser or reducer and analyzed separately. 

Analysis is progressed from the piping exit to the inlet, inversely to the flow.   This is because the choked flow at the piping exit should be determined first and then the upstream condition is calculated progressively based on the downstream condition selected by comparison with critical pressure calculated.    For example, iIf there is no increaser or reducer in the piping system, the entire piping system is analyzed as a pipe, while analyzed in two pipes if there is one increaser or reducer in the piping system.

The program calculates the critical pressure of LCV for checking whether the inlet pressure of LCV downstream piping is less than the critical pressure so that LCV has good enough pressure drop for selection as well as operation..   LCV is analyzed as a nozzle and the critical pressure of LCV is calculated using the method described in Clause 5.1 Nozzle of Compressible Flow Analysis of Steam web page.

 

2.3 Erosion at Piping Elbow   (TOC)

When flashing mixture flows through a elbow or bend, erosion by momentum forces at the elbow or bend should be checked and evaluated.

According to Ref. No. 2, when saturated mixture of water and steam flows around elbow or bend, the increase in pressure at the outer wall, which results from the change in direction of flow, will cause steam bubbles to collapse, constituting the cavitation condition.   Ref. No. 2 recommends the momentum force of flashing flow calculated by the following equation should not be higher than 35 kg(75 lbs) in order to prevent erosion at elbow or bend.

Fmom = 1.414 * W * V / g

where,

Fmom

: Momentum force, kg

 

W

: Drain flow, kg/sec

 

V

: Velocity, m/sec

 

g

: Gravity acceleration = 9.81 m/sec2

The methods to mitigate the erosion at elbow or bend are the use of tees instead of elbow or bend for momentum forces to be dissipated against a blind flanges and the use of thicker cast iron elbow or bend.

 

3. Major Screens   (TOC)

3.1 Input Screen

The followings are required for user to input.

-

Pipe iso-metric files for LCV upstream and downstream pipings

-

Higher pressure heater shell pressure (P0)

-

Higher pressure heater outlet drain temperature (T1)

-

Drain flow rate (W)

-

Lower pressure heater shell pressure (P5)

Required is the drain condition at the inlet of drain piping for the analysis of cascade heater drain piping.   

However, normally the drain pressure at the inlet of drain piping is not available at design stage.   Instead, the higher pressure heater shell pressure is available from turbine heat balance diagram.    Actual drain pressure at the inlet of drain piping is the higher pressure heater shell pressure plus static head pressure by condensate level of heater drain chamber minus the frictional pressure drop from heater shell to drain piping inlet.   The latter two pressure is relatively small compared to heater shell pressure and normally the two pressures are offset because the pressure magnitudes are similar.   Therefore, the use of higher heater shell pressure instead of the inlet pressure of drain piping for analysis of cascade heater drain piping is acceptable.

The drain temperature at the inlet of drain piping is available from turbine heat balance diagram.   Therefore, ES_CasDrain requires T1 as an input.    T1 should be the drain temperature at drain cooler outlet, if installed, regardless of internal or external drain coolers.

While LCV upstream piping is analyzed for sub-cooled water at P0 and T1, LCV downstream piping is analyzed for saturated water at T1.   

The input of LCV upstream and downstream pipe iso-metric information should be performed using the sub-program of ES_PipeIso, of which user manual you may find in ES_PipeIso User Manual web page.   [Open] command button is used for opening a existing *.pip file, while [Edit] command button is used to open [ES_PipeIso] window for editing a already-opened *.pip file.    A existing *.pip file may be open in [ES_PipeIso}window using its own file open menu.

When *.pip file is open by [Open] command button, the units of the *.pip file keep their original values, while the calculation in the mother program is performed in the mother program's units.    When *.pip file is open in [ES_PipeIso] window, the units of *.pip file are converted into the mother program's units, and then if user saves the *.pip file the units of *.pip file are saved as converted.

Please note that the *.pip file has to be saved before exiting [ES_PipeIso] window in order to apply the changes made in the window to the calculation in  mother program.

 

3.2 Menu   (TOC)

[File] menu has [New], [Open], [Save], [Save As] and [Exit] items with four file items lately used.

[Run] menu has only [Start] item which uses function key [F5] as a short key.

[Set] menu has [Title], [Unit], [Calc], [Text Output] and [Graph Output] items.

In [Set]-[Title] item, two titles may be input as [Title 1] and [Title 2], which also are shown in Text Output and Graph Output.

In [Set]-[Unit] item, the unit of calculation is set.

In [Set]-[Calc] item, LCV critical pressure and reference momentum force can be input by user.

[Set]-[Text Output] has two check box sub-menu of [show details of LCV upstream piping] and [show details of LCV downstream piping].

[Set]-[Graph Output] item shows a form to set the parameters of graph output.

 

3.3 Iso-metric Screen of LCV Upstream Piping   (TOC)

When calculation result does not exist, the pipe iso-metric screen of LCV upstream piping shows only iso-metric drawing, while shows inlet and outlet pressure also when calculation result exists.

The program checks and gives users an alarm for flashing in LCV upstream piping.   The user alarm is given in this screen as well as in the text output screen described below.

The iso-metric window can be printed.

 

3.4 Iso-metric Screen of LCV Downstream Piping   (TOC)

When calculation result does not exist, the pipe iso-metric screen of LCV downstream piping shows only iso-metric drawing, while shows inlet and outlet pressure also when calculation result exists.

The program checks and gives users an alarm for the inlet pressure of LCV downstream piping lower than the critical pressure of LCV analyzed as a nozzle..   The user alarm is given in this screen as well as in the text output screen described below.   The experimental equation used in Ref. No. 1 for calculation of LCV critical pressure(Pcv) is also the equation for a nozzle and provides the same results with this program.

The momentum force at the exit condition of LCV downstream piping is calculated and compared with the reference momentum force for preventing erosion at elbows and bend.    If the momentum force calculated is higher than the reference momentum force, then a user alarm is given in this screen as well as in the text output screen described below.   Default reference momentum force is 35 kgf and can be modified by user at [Set]-[Calc] menu.

The iso-metric window can be printed.

 

3.5 Text Output Screen   (TOC)

[Text Output Screen] shows the calculation result details and can be printed.

The followings are shown.

1.

Discharge drain condition of higher pressure feedwater heater

2.

LCV upstream piping

3.

LCV inlet condition

4.

LCV outlet condition

5.

LCV downstream piping

6.

Lower pressure feedwater heater pressure

 

4. Test Run Results   (TOC)

Test run results of ES_CasDrain for 8 in. cascade heater drain pipe of Example Calculation, Appendix B of Ref. No. 1 and are well in line with each other.

Description

Ref. No. 1

ES_CasDrain

LCV exit critical pressure(Pcv), psia

10.6

10.85

LCV downstream piping inlet pressure(P3), psia

9.6

9.795

LCV downstream piping exit pressure(P4), psia

8.0

8.0

LCV downstream piping exit critical pressure(P4*), psia

6.2

6.397

 

References :

1. Analytical Approach for Determination of Steam/Water Flow Capability in Power Plant Drain Systems by G.S. Liao and J.K. Larson, ASME Publication 76-WA/Pwr-4

2. Flow of a Flashing Mixture of Water and Steam through Pipes by M.W. Benjamin and J.G. Miller, Transactions of the ASME, Oct., 1942

 

 


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