ES_BlowOut User Manual - Rev. 1

1. Introduction

2. Methods and Formulae

 

2.1 Compressible Steam Flow Analysis

 

2.2 Cleaning Force Ratio

 

2.3 Reaction Force

 

2.4 Filed Calculation Method

3. Major Screens

 

3.1 Input Screen

 

 

3.1.1 Design Calculation Input

 

 

3.1.2 Field Calculation Input

 

3.2 Menu

 

3.3 Iso-metric Screen for Permanent Pipe

 

3.4 Iso-metric Screen for Temporary Pipe

 

3.5 RTF Text Output Screen

4. Test Run Results


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1. Introduction   (TOC)

ES_BlowOut is the program to analyze steam blow-out piping system.   The program has two functions for calculation category.   One is Design Calculation and the other is Field Calculation.

The Design Calculation is to select the blow-out steam condition by analyzing the permanent and temporary pipes using the blow-out steam flow rate input by user.    Meanwhile, the Field Calculation is to calculate the actual cleaning steam flow and cleaning force ratio based on the temporary pipe exit pressure as well as the permanent pipe inlet pressure and temperature.

 

 2. Methods and Formulae   (TOC)

2.1 Compressible Steam Flow Analysis

Compressible steam flow analysis of permanent and temporary pipes 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, if 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.

2.2 Cleaning Force Ratio   (TOC)

The cleaning force of steam blow-out comes from friction.   That is, cleaning is achieved by the friction force when steam flows through the permanent pipe during steam blow-out at high velocity.   Therefore, the friction force generated during steam blow-out should be equal or higher than the maximum friction force during normal operation of the steam pipe being cleaned.

Friction force is same with friction loss and the friction loss is expressed as below (Darcy equation).

dP = C * f * L * W^2 * v / d^5

where,

dP

:  Friction loss = Friction force

 

C

: Constant

 

W

: Mass flow rate

 

v

: Specific volume

 

d

: Pipe inside diameter

Variables C, f, L and d is same in both normal operation and steam blow-out, and W and v vary.

Therefore, the cleaning force ratio during steam blow-out can be expressed by the following equation, which is the friction force ratio during steam blow-out to the friction force during normal operation.

R = W^2 * v / Wr^2 / vr

where,

R

:  Cleaning force ratio

 

W

: Mass flow rate during steam blow-out

 

v

: Permanent pipe inlet specific volume during steam blow-out

 

Wr

: Mass flow rate during normal operation, normally the maximum value

 

vr

: Permanent pipe inlet specific volume during normal operation of Wr

Basically cleaning force ratio more than unit is satisfactory.   But, normally 1.25 to 2 value is used depending on the engineers or companies involved.

 

2.3 Reaction Force   (TOC)

At the temporary pipe exit, a reaction force is exerted by the steam mass discharging at high velocity and calculated as below.   If a silencer was installed at the temporary pipe exit, the reaction force is exerted at the silencer exit plane, even though the reaction force is small for low discharge velocity by larger exit area.

F4 = W * V4 / g + (P4 - Pa) * A4

where,

F4

: Reaction force, kg

 

W

: Mass flow rate during steam blow-out, kg/sec

 

V4

: Steam velocity at temporary pipe or silencer exit plane, m/sec

 

g

: Gravity acceleration, 9.81 m/sec2

 

P4

: Steam pressure at temporary pipe or silencer exit plane, kg/m2 abs.

 

Pa

: Ambient pressure, normally standard atmospheric pressure, 10332 kg/m2 abs.

 

A4

: Area of temporary pipe or silencer exit plane, m2

The reaction force by the equation above is for steady state.    During actual blow-out there is a fluctuation of reaction force due to its high velocity and the magnitude of fluctuation is the function of physical condition of piping.    In order to express the maximum fluctuation, a dynamic load factor(D.L.F) is used.   The DLF should be analyzed by complicated method.    However, normally DLF of 2 is used for preliminary estimation of actual reaction force.

 

2.4 Field Calculation Method   (TOC)

 The preliminary blow-out steam condition is selected by analyzing the steam flow by calculation(Design Calculation) before actual steam blow-out.    However, the actual blow-out steam condition should be adjusted at filed by calculating the actual cleaning force ration at field by pressure and temperature measurement.

When the steam flow is choked, the temporary pipe exit pressure measured is higher than ambient pressure, and the steam velocity at the pipe exit is sonic velocity.     Using this fact, with the pressure and temperature of permanent pipe measured, the mass flow rate and cleaning force ratio during actual steam blow-out can be calculated simply at field.

The method is described below, but it is noted that the method is applied only when the temporary pipe exit pressure is higher than the ambient pressure so that the flow is choked and the velocity at the exit plane is sonic.

1)

Using the permanent pipe inlet pressure(P1) and temperature(T1) measured, get the enthalpy(H1), entropy(S1) and specific volue(v1) from steam table.

2)

Using S1 and the temporary exit pressure(P4), the isentropically expanded temporary pipe exit entropy(S4) and enthalpy(H4) from steam table.(S4 = S1)

3)

Using (P4 + 0.01 kg/cm2) pressure and S4, get specific volume(v4a) from steam table.

4)

Using (P4 - 0.01 kg/cm2) pressure and S4, get specific volue(v4b) from steam table.

5)

Calculated the sonic velocity at temporary pipe exit plane by the following equation.

V4 = (dP / dRo * g)^0.5 * 100

where,

V4

: Sonic velocity at temporary pipe exit plante, m/sec

 

dP

: Pressure change = 0.02 kg/cm2

 

dRo

: Isentropic density change for infinitesimal pressure change = Absolute value of (1 / v4a - 1 / v4b) kg/m3

 

g

: Gravity acceleration, 9.81 m/sec2

6)

Calculated the temporary pipe exit steam enthalpy by the following energy equation.

H4 = H1 - V4^2 / 2 / g / J

where,

H4

: Temporary pipe exit steam enthalpy, kcal/kg

 

J

: Joule constant, 427 kg-m/kcal

7)

If the discrepancy between two H4s got at Stage No. 2) and 6) is not acceptable, e.g. more than 1 kcal/kg, get a new temporary pipe exit steam entropy(S4) from steam table using P4 and H4 calculated at Stage No. 6), and then repeat the procedure from Stage No. 3) to Stage No. 6) till acceptable discrepancy is achieved, e.g. less than 1 kcal/kg.

8)

Using H4 and P4, get the temporary pipe exit steam specific volume(v4) from steam stable.

9)

Calculate the steam blow-out mass flow rate by the following continuity equation.

W = A4 * V4 / v4

where,

W

: Mass flow rate, kg/sec

 

A4

: Temporary pipe exit cross-sectional area, m2

10)

Using the maximum mass flow rate and specific volume during normal operation(Wr and vr) with W and v1 got above, calculate the actual cleaning force ratio by the following equation.

R = W^2 * v1 / Wr^2 / vr

The sonic velocity calculation equation for ideal gas(Vc = (k * g * R * T)^(0.5) = (k * g * P * v)^(0.5)) may be used in Stage No. 5) above.   However, since normally steam table is available at the field and the convergence is easily achieved, it is better to use the general sonic velocity equation as described in the method above.

It is recommended to install the temporary pipe exit pressure measuring device at 20 pipe diameter upstream from the exit as a minimum in order to prevent measurement fluctuation.

 

3. Major Screens   (TOC)

3.1 Input Screen

3.1.1 Design Calculation Input

 

Default input screen is of Design Calculation as shown above.    The user inputs for Design Calculation include ;

- Permanent pipe inlet steam condition during normal operation with mass flow rate

- Total steam enthalpy of blow-out steam

- Maximum allowable steam pressure at permanent pipe inlet(Loc. 1) during steam blow-out

- Discharge ambient pressure during steam blow-out

- Iso-metric pipe files(*.pip) of permanent and temporary pipes

- Mass flow rate of blow-out steam

In the pervious version, steam blow mass flow rate to be used in calculation was calculated based on ;

- Target cleaning force ratio input by user in [Calc Set] menu, and

- Specific volume at the maximum pressure and total steam enthalpy input by user

Then, the previous version of ES_BlowOut calculates steam conditions at permanent piping inlet and actual cleaning force ratio as output.   This concept was selected for user to iterate the software run till the calculated steam conditions at permanent piping inlet become close to the input.

However, this concept gives user confusion and is not in line with actual engineering calculation practice.   Therefore, in the latest version of ES_BlowOut, the mass flow rate of blow-out steam becomes one of the inputs and the output comes out as user wants.

We recommend the following step for the input of Blow-out Steam Conditions.

1) Get preliminary blow-out steam pressure, temperature and mass flow rate at permanent pipe inlet from the performance data provided by boiler manufacturer.

2) Input the mass flow rate.

3) Get total steam enthalpy by using Steam Table command button.

4) Increase Max. Press. with enough margin, e.g. two times of the pressure.    The Max. Press. is to provide the pressure range only for the software to search for the conditions satisfying the compressible flow equations, and has no influence on steam analysis.   During steam analysis, the Total Enthalpy is kept constant and divided into velocity head and static enthalpy.    Mass flow rate is also kept constant through the analysis.

5) Run ES_BlowOut

6) Compare the blow-out steam condition at permanent pipe inlet calculated by ES_BlowOut with those of preliminary input conditions.

7) Iterate ES_BlowOut run till the steam conditions of ES_BlowOut becomes same with those from boiler manufacturer.

 

* Tip for run of intermediate flow rate change :

Since steam blow scheme is diverse, there may be flow change in the middle of permanent or temporary pipes.   If two parallel main stop valves are installed for steam turbine, blow steam is divided into two, upstream and downstream of the stop valve.    The case can be analyzed by three run of ES_BlowOut as below.

First, run ES_BlowOut for common temporary pipe from the stop valve to the end of temporary pipe, with full blow-out flow.  In the run, a dummy pipe iso file shall be used for permanent pipe.   Dummy pipe iso file has the same pipe ND and Schedule, but its length is zero.

Second, run ES_BlowOut for branch pipe upstream and downstream of the stop valve with half of the blow-out flow.  In the run, the ambient press(Loc. 5) shall be the upmost upstream pressure of the first run.    The branch pipes shall be input as temporary pipe and a dummy pipe shall be input as permanent pipe as in the first run.

Third, run ES_BlowOut for the pipe from common piping from boiler outlet to the branch of the stop valve in the same way of the second run, but with full blow-out flow.   In this run, the ambient press(Loc. 5) shall be the upmost upstream pressure of the second run.

 

The permanent pipe inlet steam condition during normal operation should be input by the steam table command button and the default value of discharge ambient pressure is standard atmospheric pressure.

 

The input of permanent and temporary 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.

It is noted 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.

 

3.1.2 Field Calculation Input   (TOC)

The Field Calculation function can be selected in [Calc Set] menu, of which input screen is as shown above.

The permanent pipe inlet steam conditions during normal operation and steam blow-out should be input by each steam table command button, and the default value of discharge ambient pressure is standard atmospheric pressure.   It is noted that the conditions are measured values.

Also the permanent pipe inlet ID, and temporary pipe exit steam pressure and ID should be input by user.   The temporary pipe exit steam pressure is the measured value.   The permanent pipe inlet steam condition during steam blow-out is considered as static condition so that the permanent pipe inlet ID is used to calculate the total enthalpy after initial calculation.   The calculation is iterated twice for this purpose.

The ambient pressure is used only for checking of choked flow by assuring the temporary pipe exit pressure higher than the ambient pressure.

You may see that the Field Calculation with the output of Design Calculation shows the same result.

 

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.

[Set]-[Calc] item includes,

-

Two option buttons to select either Design Calculation or Field Calculation

-

A input box for target cleaning force ratio

-

A input box for design pressure selection multiplier in percent

-

A input box for dynamic load factor to be applied to temporary pipe exit reaction force calculation

-

  Two check items to show calculation details of permanent and temporary pipes

[Set]-[Text Output] has two check box sub-menu of [show details of permanent pipe] and [show details of temporary pipe].

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

 

3.3 Iso-metric Screen of Permanent Pipe   (TOC)

The Iso-metric window of permanent pipe is same with that of discharge elbow in ES_SVVent User Manual.

The iso-metric window can be printed.

 

3.4 Iso-metric Screen of Temporary Pipe

The Iso-metric window of permanent pipe is same with that of vent stack in ES_SVVent User Manual.

It is described in Ref. No. 1 that the ID of temporary pipe should be smaller than that of permanent pipe.   The description does not mean that the steam does not flow if the ID of temporary pipe is larger than that of permanent pipe.  It  merely means that if the case the analysis results performed by the method of Ref. No. 1 may  not be correct, because there is a chance of choked flow at the permanent pipe exit plane and the method of Ref. No. 1 does not provide with such function.   However, in ES_BlowOut there is no restriction for temporary pipe size and the increaser between the permanent and temporary pipe is analyzed using the method described in Clause 4.5 of Compressible Flow Analysis of Steam web page, checking the choked condition at increaser upstream.

The iso-metric window can be printed.

 

3.5 Text Output Screen   (TOC)

The Text Output window is same with that of ES_SVVent User Manual.

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

For Design Calculation, the followings are shown.

1.

Steam Condition during Normal Operation

2.

Initial Steam Condition for Blow-out Calculation

3.

Other Design Inputs

4.

Steam Blow-out Flow Rate

5.

Cleaning Force Ratio

6.

Selected Steam Condition during Steam Blow-out

7.

Permanent Pipe Calculation Details

8.

Temporary Pipe Calculation Details

 

For Field Calculation, the followings are shown.

1.

Steam Condition during Normal Operation

2.

Steam Condition during Blow-out Operation

3.

Other Design Inputs

4.

Temporary Pipe Exit Calculation Details

5.

Calculated Blow-out Steam Flow Rate

6.

Cleaning Force Ratio

 

4. Test Run Results   (TOC)

Test run results of ES_BlowOut for the sample calculation of Ref. No. 1 are as below.

Description

Ref. No. 1

ES_BlowOut

< Design Calculation Comparison Table >

VWO mass flow rate, lb/hr at 2520 psig, 1000 oF

2,079,066

Total enthalpy, Btu/lb

1200

 

Permanent pipe

Inlet pressure, psia

550

593.3

(12" ND, 200 ft L)

Inlet temp., oF

477

485.0

 

Exit pressure, psia

483

539.1

 

Exit velocity, ft/sec

428

400.5

 

 

 

 

Temporary pipe

Inlet pressure, psia

483

507.4

(10' ND, 200 ft L)

Exit pressure, psia

166.6

194.5

 

Blow-out mass flow rate, lb/hr

1,273,662

1,316,561

Cleaning force ratio

1.04

1.025

 

< Field Calculation Results with Ref. No. 1 Data >

Permanent pipe inlet steam pressure during blow-out, psia

 

550

Permanent pipe inlet steam temp. during blow-out, oF

 

477

Permanent pipe inlet pipe ID, inch

 

11.938

 

 

 

Temporary pipe exit steam pressure, psia

 

166.6

Temporary pipe exit pipe ID, inch

 

10.02

 

 

 

Blow-out mass flow rate calculated by ES_BlowOut, lb/hr

 

1,136,882

Blow-out cleaning force ration calculated by ES_BlowOut

 

0.833

The Design Calculation results of ES_BlowOut above are the result of 3 times program runs with the constant target cleaning force ratio of 1 and constant total enthalpy of 1200 Btu/lb, but with the new blow-out maximum pressure input replaced by the previously calculated blow-out pressure.   Starting blow-out maximum pressure was 750 psia.

It is seen that the results of ES_BlowOut are likely in line with that of Ref. No. 1, but anyway there are deviations.

Ref. No. 1 calculates first the blow-out mass flow rate based on 30 psia temporary pipe exit pressure and then calculate the blow-out mass flow rate at the other temporary pipe exit pressure by linearly proportioning the 30 psia mass flow rate to the pipe exit pressure ratio.   In ideal gas nozzle flow, the mass flow rate per nozzle throat area(W/A)  linearly proportions to the inlet pressure if choked with constant inlet steam temperature.   Ref. No. 1 likely uses this theory.     However, steam deviates much from ideal gas for its properties and behaviors.    Therefore, the calculation results of Ref. No. 1 could not be accurate.   The deviation also resulted from the use of Fanno Line equation for steam analysis in Ref. No. 1.

 

References :  (TOC)

1. Cleaning of Main Steam Piping and Provisions for Hydrostatic Testing of Reheaters (GEK - 27065D)", General Electric Co.


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