Purging Calculations

Find flow rate through the blow-off valve by using the formula for critical velocity, Method A – Finding the flow rate through the blow off valve(s) using a modified Weymouth formula for critical velocity. Method B where pipe section is assumed to be filled with air prior to purge. Blow off pressure should be between 18-20 psi and maintained. Blow off coefficient of valve must be known.

𝑄 = 𝐾(𝑃₂)

where

𝑄 − Flow Rate[MSCFH]

𝐾 − Blowoff coefficient for standard sizes[MSCF/(h x psia)]

𝑃₂ − Pressure just upstream of blowoff valve(recommended 18−20)[psia]

  From rearranged Weymouth formula to find an estimate pressure value of necessary to maintain this flow rate:

𝐿 − Length of Purge Section[mi]

𝐷 − Internal Pipe Diameter[in]

𝑃₁ − Purge Gas Injection Pressure[psia]

  𝑃_𝑀−Average Pressure[psia]   Method A Recommended purge time is 2T. The minimum purge time in minute is

𝑇_𝑎 − Minimum Purge Time[min]

𝑇_𝑟𝑎 − Recommended Minimum Purge Time[min]

Where

𝑉_𝑎 − Volume of Gas Lost − min time[MSCF]

𝑉₁ − Actual Volume of Pipe Section−assume filled with air before purge[MCF].

𝑉_𝑟𝑎 − Volume of Gas Lost−recommended time[MSCF]

  Method B Recommended purge time is 2T. The minimum purge time in minute is

𝑇_𝑏 − Minimum Purge Time[min]

𝑇_𝑟𝑏 − Recommended Minimum Purge Time[min]

Where

𝑉_𝑏 − Volume of Gas Lost−,min-time[MSCF]

𝑉₁ − Actual Volume of Pipe Section−assume filled with air before purge[MCF]

𝑉_𝑟𝑏 − Volume of Gas Lost−recommended time[MSCF]

Input Parameters

1. To create a new case, click the “Add Case” button
2. Select the Purging Calculation application from the Pipeline Testing module.
3. Enter Case Name, Location, Date and any necessary notes.
4. Fill out all required fields.
5. Make sure the values you are inputting are in the correct units.
6. Click the CALCULATE button.

 

• Blow-off Pressure, just Upstream of the Blow-off Valve (Recommended 18-20) (psi)
• For standard blow-off size(in) (dropdown)
• Internal Diameter of Pipe Section(in)
• Length of Pipe Section(ft) (dropdown)

Outputs/Reports

1. View the results.
2. If an input parameter needs to be edited be sure to hit the CALCULATE button after the change.
3. To SAVE, fill out all required case details then click the SAVE button.
4. To rename an existing file, click the SAVE As button. Provide all case info then click SAVE.
5. To generate a REPORT, click the REPORT button.
6. The user may export the Case/Report by clicking the Export to Excel/PowerPoint icon.
7. To delete a case, click the DELETE icon near the top of the widget.

 

• Flowrate at Critical Velocity (MSCF)
• Purge Gas Injection Pressure(psi)
• Actual Volume of Pipe Section (assume filled with air before purging)(MCF)

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