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Double Isolation Valves On Pressure Instrument Inlet

p&id pressure instruments double isolation

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#1 abcd63b

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Posted 17 February 2020 - 02:08 AM

Dear Sirs,

 

I can't share the P&ID but in a high pressure reactor circuit I found two gate valves on the inlet of pressure instruments and there was a note which said that "Pressure Transmitters to be located at top of reactor with self draining leads.Use 1" welded pipe for pressure instruments"

 

Does anyone know the reason for this?

 



#2 abcd63b

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Posted 17 February 2020 - 02:08 AM

I forgot to mention. It also says "Do not use tubing"



#3 Chemitofreak

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Posted 19 February 2020 - 08:37 AM

Hi,

 

Please provide the following information so that the group members will be able to help you

 

1) Fluid Handled

 

2) Pressure & Temperature of the system

 

3) % of toxic components present in the stream

 

Prima facie I think this isolation arrangement is provide as the system is high pressure  



#4 abcd63b

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Posted 28 February 2020 - 12:03 AM

Hi,

 

Please provide the following information so that the group members will be able to help you

 

1) Fluid Handled

 

2) Pressure & Temperature of the system

 

3) % of toxic components present in the stream

 

Prima facie I think this isolation arrangement is provide as the system is high pressure  

Hello Sir. Thank you for replying. 

 

1) Fluid Handled: Hydrocarbon (Waxy Lubricant)

2) P: 169 kg/cm2g T: 320°C

3) Hazard: Non-toxic but flammable

 

Is draining of pressure instruments a problem in such cases? Why is that so? Because the P&ID instructs to locate the instruments at top of reactor with self draining leads and to use pipe instead of tubing. 

 

Thank you for your valuable time.



#5 Chemitofreak

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Posted 28 February 2020 - 01:47 AM

Hi,

 

1) I think the double valve isolation arrangement is due to high pressure and high temperature

 

2) Pressure Instruments are always installed on the vapour space. Self draining leads are provided as the material handled in the reactor is waxy and there is a possibility of that clogging the instrument.

 

Regards,



#6 Technical Bard

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Posted 24 March 2020 - 10:36 AM

Double block valves on instruments are common in high pressure, high temperature and/or toxic systems because there is always a change that a valve no longer seats properly.  This way there are TWO valves between maintenance and the process if you have to take the instrument out.

 

The draining aspect is to prevent the waxy lubricant from plugging the leads.

 

The use of pipe instead of tubing is for mechanical integrity.  While tubing an handle the internal pressure, it is more prone to damage from physical movement (i.e. people holding onto it).



#7 salam801

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Posted 11 April 2020 - 02:39 PM

I think that double isolation valve on the pressure transmitter as per standards in order to prevent the leak or passage . In some cases these instrument pressure taping also uses to determine the overall pressure differential pressure across reactor and bed differential pressure of the reactor. It may be one of the reason that the pressure transmitter have self draining leads to give reliable readings.






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