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Pepperl+Fuchs Blog

Calculating Purge Times on Purge and Pressurization Systems in a Few Easy Steps

Posted by Brent Dean on Tue, Jul 17, 2012

Calculations for Type X, Type Y, Type Z, or Ex pz purge systems

calculating purge times resized 600

Calculating purge times on our purge and pressurization systems is, basically, just a matter of following a formula. But it's important to understand what type of system you have and the technical specifications of that unit to ensure that the numbers you are using in those formulas are correct.

We have a range of purge and pressurization systems for virtually every kind of pressurization type and rating requirement. Let’s discuss those systems, their features, and how you can reliably calculate the purging time on each system in this blog.

Q: How do I calculate the purge time with a Pepperl+Fuchs system 1000 Type Y and Z purge and pressurization unit?

A: The 1000 series includes Type Y and Z systems.

  • A type Y purge system reduces the classification within the protected enclosure from Division 1 to Division 2.
  • A type Z purge system reduces the classification within the protected enclosure from Division 2 to non-hazardous.

General-purpose equipment can then be operated within the protected enclosure.

Enclosure volumes on these units range from ≤2 ft3 to ≤450 ft3 and these volumes are used to calculate the purge time.

A: To find the purge time, take the size of the enclosure in cubic feet that you want to purge and multiply it by 4. (This represents 4 volume exchanges). For ATEX applications multiply the enclosure size by 5 volume exchanges, and for applications involving a motor, multiply the enclosure size by 10 volume exchanges.  

Next, divide that number by the Exchange Flow Rate (SCFM) for that system. (The SCFM can be found in the System Specifications Table.) This will give you the purge time in minutes. Round up to the nearest minute, and that is your purge time. Two minutes minimum, whichever is greater.

The 1001 model is an exception; it does not have a certified flow rate. You have to measure that with a flow meter and calculate the purge time manually.

Another option to calculate the purge time would be to take the size of the enclosure in cubic feet and divide it by the “exchange time” listed for the model.

Examples:
1.) Below are the specifications for our 1003 purge system.

  • The Exchange Flow Rate is 10 SCFM / 600 SCFH.
  • The Exchange Time is 1 minute per 2.5 ft3.

If your enclosure is 30 ft3
The purge time would then be: (30 ft3) / 2.5 ft3 per minute = 12 minutes

2.) Another way to calculate your purge time using the 1003 specifications below:

  • The Exchange Flow Rate is 10 SCFM / 600 SCFH.
  • The Exchange Time is 1 minute per 2.5 ft3.

If your enclosure is 30 ft3 and you want 4 volume exchanges:

The purge time would be: ((30 ft3) X 4 volume exchanges) / 10 SCFM = 12 minutes

System 1000 Type Y and Z purge and pressurization unitQ: How do I calculate the Pepperl+Fuchs purge time with a system 2000 Type X purge and pressurization unit?

A: The 2000 series are Type X systems.

  • A Type X purge system reduces the classification within the protected enclosures from Division 1 to non-hazardous. General-purpose equipment can be operated within the protected enclosure.

Enclosure volumes on these units range from ≤2 ft3 to ≤450 ft3 and these volumes are used to calculate the purge time.

To find the purge time, take the size of the enclosure in cubic feet that you want to purge and multiply it by 4. (This represents 4 volume exchanges).  For ATEX applications multiply the enclosure size by 5 volume exchanges, and for applications involving a motor, multiply the enclosure size by 10 volume exchanges.  

Next, divide that number by the Exchange Flow Rate (SCFM) for that system. (The SCFM can be found in the System Specifications Table.) This will give you the purge time in minutes. Round up to the nearest minute, and that is your purge time. Two minutes minimum, whichever is greater.

Once you have your purge time, look up the corresponding setting for the rapid exchange timer (RET) in the system 2000 manual. Set the RET to the corresponding setting. The RET enables a time delay after rapid exchange pressure is detected to allow the volume exchanges prior to energizing the enclosure power relays. Another option to calculate the purge time would be to take the size of the enclosure in cubic feet and divide it by the “exchange time” listed for the model, then set the RET accordingly.

Examples:

1.) Below are the specifications for our 2002 purge system.
The Exchange Flow Rate is 4 SCFM / 240 SCFH.
The Exchange Time is 1 minute per cubic foot.
If your enclosure is 10 ft3

The purge time would then be: (10 ft3) / 1 ft3 per minute= 10 minutes

2.) Another way to calculate your purge time using the 2002 specifications below :
The Exchange Flow Rate is 4 SCFM / 240 SCFH.
The Exchange Time is 1 minute per cubic foot.
If your enclosure is 10 ft3 and you want 4 volume exchanges

The purge time would be: ((10 ft3) X 4 volume exchanges) / 4 SCFM = 10 minutes

2000 Type X purge and pressurization unitQ: How do I calculate the Pepperl+Fuchs purge time with a system 3000 Type Y, Z, and Ex pz purge and pressurization unit?

A: The 3000 series includes Type Y, Z, and Ex pz* systems.

  • A type Y purge system reduces the classification within the protected enclosure from Division 1 to Division 2.
  • A type Z purge system reduces the classification within the protected enclosure from Division 2 to non-hazardous.
  • A type Ex pz purge system reduces the classification within the protected enclosure from Zone 2 and 22 to non-hazardous.

General-purpose equipment can then be operated within the protected enclosure

* Our Ex pz purge systems are designed according to European requirements—ATEX directive 94/9/EG.

Enclosure volumes on these units range from ≤2 ft3 to ≤450 ft3 and these volumes are used to calculate the purge time.

To find the purge time, take the size of the enclosure in cubic feet that you want to purge and multiply it by 4. (This represents 4 volume exchanges). For ATEX applications multiply the enclosure size by 5 volume exchanges, and for applications involving a motor, multiply the enclosure size by 10 volume exchanges.  

Next, divide that number by the Exchange Flow Rate (SCFM) for that system. (The SCFM can be found in the System Specifications Table.) This will give you the purge time in minutes. Round up to the nearest minute, and that is your purge time. Two minutes minimum, whichever is greater.

Another option to calculate the purge time would be to take the size of the enclosure in cubic feet and divide it by the “exchange time” listed for the model.

Examples:

1.) Below are the specifications for our 3004 purge system.
The Exchange Flow Rate is 30 SCFM / 1800 SCFH.
The Exchange Time is 1 minute per 7.5 ft3 (this is the 4 volume exchange time listed below).
If your enclosure is 150 ft3

The purge time would then be: (150 ft3) / 7.5 ft3per minute = 20 minutes

2.) Another way to calculate your purge time using the 2002 specifications below :
The Exchange Flow Rate is 30 SCFM / 1800 SCFH.
The Exchange Time is 1 minute per 7.5 ft3 (this is the 4 volume exchange time listed below).
If your enclosure is 150 ft3and you want 4 volume exchanges

The purge time would be: ((150 ft3) X 4 volume exchanges) / 30 SCFM = 20 minutes

3000 Type Y, Z, and Ex pz purge and pressurization unit

Q: How do I calculate the Pepperl+Fuchs purge time with a system 6000 purge and pressurization unit?

A: The 6000 Ex px/ Type X purging system is a fully automatic system with an enclosure volume of 250 ft3. First you need to enter the size of the enclosure that you wish to purge and the number of exchanges: the NEC/NFPA-496 standards require 4 volume exchanges 5 for ATEX applications, and 10 for applications involving a motor. There are two options for the purge rate. The first option would be to manually select the purge rate from:

  • The NEC/NFPA-496 standards that require four volume exchanges5 SCFM @ 1.3” w.c.
  • 12 SCFM @ 2.5” w.c.
  • 20 SCFM @ 3.1” w.c.
  • 30 SCFM @ 3.4” w.c.

Once the purge has been initiated the purge time will be displayed in minutes.

The second option would be to select “Dynamic Purge Time,” which is based on the maximum flow and is displayed in percentage complete.

Questions about Purge Systems? Get the Engineer's Guide

Topics: Troubleshooting, Purge/ Pressurization Systems, Ask an Expert

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